FUNDAMENTALS OF MACHINE DESIGN MIR PUBLISHERS MOSCOW P. ORLOV II. H. OPJIOB OCHOBM KOHCTPmPOBAHHH H3AATEJ1BCTBO «MA LUIIHOC TPOEHHE* MOCKBA FUNDAMENTALS OF MACHINE DESIGN P. ORLOV TRANSLATED FROM THE RUSSIAN BY A. TltOITSKY AIIR PUBLISH MRS . MOSCOW First published 1977 Revised from the 1972 Russian edition The Greek Alphabet A a Alpha 1 1 Iota Pp Rho BP Beta K x Kappa Sct Sigma r x Gamma A X Lambda Tt Tau A 6 Delta M p Mu r» Upsilon Ee Epsilon N v Nu (D cp Phi Z£ Zeta E| Xi Xx Chi Hi] Eta Q o Omicroti Yif Psi e# Theta II n Pi ) is presented in Fig. \a. The housing and its cover, as Fi «- F Assembly of gear shaft into housing well as the bearing bushings accommodated therein, are solid. The shaft is inserted into the housing axially and locked by the cover which is centred with respect to the housing by means of a cylindrical shoulder. In the case of the radial assembly (Fig. \b) the housing and the bushings are parted along the longitudinal axis. The shaft is fitted into one half of the housing and covered by the other half. Both hal¬ ves of the housing are located with respect to eacli other by adjus¬ ting pins and clamped by transverse bolts. Figure lc illustrates a combined radial-axial assembly. In this case the housing is split and the cover, solid. A multi-step centrifugal pump (Fig. 2) may be taken as an example to show the advantages and shortcomings of the axial and radial assembly. In the design consistently following-the principle of axial assem¬ bly (Fig. 2a) the housing of the pump is made up of a number of sec¬ tions carrying diffusors 1 and diaphragms 2 with guide varies 3. The unit is assembled by slacking impellers oil the shaft (preliminarily 1.1. Axial and Radial Assembly 13 placed into the bearing of the rear cover) conseculively in all the sections, the sections being individually bolted together. The as¬ sembly ends with the tightening of the impellers by a nut on the free end of the shaft in the front-cover bearing. With a purely radial assembly (Fig. 26) the housing consists of two halves parted in the plane of the shaft. The casings of the bea¬ rings and guide vanes 3 are cast integral with the pump housing. Diffusors 1 are also parted. The diffusor and guide vanes are butted together in the parting plane of the pump housing. The pump is as¬ sembled in the following order. The impellers are stacked and clam¬ ped on the shaft, the assembled shaft is installed in the bearings in the lower half of the housing and covered by the other half after which the housing halves are tightened by internal and side bolts. A comparison of the axial and radial assembly patterns leads us to the following conclusions, common to multi-step units. In the case of the axial assembly it is easy to cast a sectional hou¬ sing and its machining is convenient. The surfaces being machined are open to view, accessible for cutting tools, and can easily be meas¬ ured. Since the machining is performed on continuous cylindrical surfaces, high-speed methods can be employed to make individual compartments. The design as a whole is highly rigid, and its internal cavities are sealed off well. The shortcomings of the axial assembly are as follows. 1. Complicated assembly of the unit. It is difficult to check and adjust axial clearances, particularly the face clearances between the impellers and the back surfaces of the diaphragms primarily because the shaft is secured only in one bearing at all assembly sta¬ ges, including the final stage. Correct clearances can be maintained either by means of special fixtures or by increasing the accuracy of the axial dimensions of the structural elements. 2. Complicated inspection of the internal members, because all the preceding stages have to be dismantled to open any one stage. The radial assembly is opposite in its advantages and shortcom¬ ings to the axial assembly. It is difficult to make the housing com¬ prising two massive castings, and its machining is intricate. The internal cavities are machined either by an open method, i.e., sepa¬ rately in each half of the housing with their subsequent matching, or by a closed method when the halves of the housing are assembled by means of set pins, with the mating surfaces being finish machi¬ ned earlier. Either method requires special tools, measuring instru¬ ments and highly skilled personnel. Since the housing sections are not symmetric, the housing has unequal rigidity. The rigidity is less in the jointing plane and lar¬ ger in the direction perpendicular to it. As the structure is weake¬ ned by the longitudinal parting, the sections of the housing walls 14 Chapter 1. Assembly have to be increased, which makes the unit heavy. The housing cavi¬ ties are in need of careful sealing along the shaped jointing plane without disturbing the cylimlrieity of the internal machined surfa¬ ces. This is usually attained by lapping together the mating surfa¬ ces and using jointing compounds. The diffusor and guide vanes have to be matched in the jointing plane, or one has to use sets of stacked vanes which are individually installed into the annular re¬ cesses of the housing. On the other hand, assembly and disassembly are extremely con¬ venient. During assembly the shaft with the impellers fitted the¬ reon previously is placed into the bearings of the lower half of the housing. The axial clearances can thus be easily measured and pro¬ perly adjusted. The internal cavities of the unit can likewise be easily inspected. The removal of the upper half of the housing re¬ veals the internal spaces of the unit and provides free access to all the parts installed in the housing. It follows therefore that, the axial assembly is more suitable when a strong and light design is required {transporting machines) and a few operational inconveniences may be allowed. If the mass of the construction is not important and higher manufacturing costs may be allowed to make assembly and operation more convenient, the radial assembly is used. Various combinations of the elements of the axial and radial as¬ sembly patterns are in common use. In the radial assembly (Fig. 2c) to make the casting process easier the housing halves are assembled of separate half-rings clamped by fitted longitudinal bolts 4. The housing halves thus assembled are machined together on the parting surfaces and further the clamping holts are not, removed. The shortcomings of the design are the increa¬ sed volume of machining operations and a larger number of bulls perpendicular to each other. In the design shown in Fig. 2 d, diaphragms 5 are made separa¬ tely, each of the two halves being bolted together with the use of set pins and fitted into the split housing. In the combined radial-axial assembly (Fig. 2c) the middle por¬ tion of the housing consists of two halves that can be detached along the axis of the shaft. Front (d) and rear (7) covers carrying the bea¬ rings are attached to the end faces of the housing. During assembly the shaft with the impellers is placed into the lower housing to which the covers are afterwards attached, and the shaft is centred in the bearings. Then, the upper half of the housing is mounted and the upper bolts of the covers are tightened. During disassembly for inspection the covers remain screwed to the lower half of the housing. With such a design the manufacture of split housings is simpler and assembly and disassembly are as convenient as before. 1.1. Axial and Radial Assembly 15 In the combined assembly (Fig. 2/) each diaphragm 8 is made up of two halves and inserted into the solid housing together with the shaft and the impellers according to the axial assembly principle. The assembly paflerns of a single-step reduction gear in which the axes of the gears are arranged in a horizontal plane are illustra¬ ted in Fig. 3. In the axial assembly (Fig. 3a) the presence of the base does not allow the housing to be split along the axis of symmetry. The gears Fig. 3. Assembly of single-step reduction gears are mounted in the wall of the housing on one side, and on the other in its detachable cover 1 located on (he housing by set pins. The design provides for the convenient machining of the housing. As distinct from multi-step units, installation is also convenient. Ins¬ pection hole 2 is used to check the meshing of the gears and inspect the interiors of the reduction gear. In the radial assembly {Fig. 3b) the housing consists of two parts joined in the plane of the gear axes, the parts of the housing being fixed with respect to each other by set pins. Like other radial as¬ semblies this design is difficult to machine. The seating holes to re¬ ceive the shaft bearings are machined in the assembled bousing, the Chapter 1. Assembly 46 mating surfaces of the housing halves being machined previously, or individually in both halves with the subsequent finish machi¬ ning of the jointing surfaces. The'latter method is more complica¬ ted than the former one. The sealing of the joint involves some difficulties. Elastic gas¬ kets must not be used lest the cylindricity of the bearing seats should be spoiled. The mating surfaces should be lapped together and sealed with jointing compounds. It is especially difficult to seal off simultaneously the flat joint and the external cylindrical surfaces Fig. 4. Detaching the housing of rolory machine of the bearings (if the bearing bushings are solid). An inspection IioIb should be provided in the housing lest the joint should be distur¬ bed during operation. In this case the axial assembly is preferable. It allows easy ma¬ chining and good installation. In the combined radial-axial assembly (Fig. 3c) the shafts of the gears are supported in the walls of the housing provided with a cover having its parting plane arranged above the bearing seats. The assembly takes the following course: the gears are introdu¬ ced into the housing, the shafts are passed through one of the bea¬ rings and through the gear hubs (the shafts should be stepped) and the gears are fastened to the shafts. This design is much better than the previous ones because of simpler machining and more stable position of the shafts in the housing though the installation is more difficult. Figure 3 d-f shows a reduction gear with gears arranged in a ver¬ tical plane. The axial (Fig. 3 d), radial (Fig. 3c) and radial-axial (Fig. 3/) assemblies have respectively the same advantages and short¬ comings as the designs shown in Fig. 3a, b and c, the only difference being that the shortcomings of the radial assembly arc here more evident due to the presence of two butts. Sometimes the pattern of assembly is unambiguously defined by the design of the unit. Thus, the axial assembly (Fig. 4 a) is out of the question in the case of a stationary rotary machine mounted on a foundation, because it would be necessary to remove the machine from its foundation to inspect its internal mechanisms. Only the 1.1. Axial and Radial Assembly 17 radial assembly (Fig. 4 b) or the limitedly combined assembly (Fig. 4 c and t?) is possible in this case. It is practically impossible to use the axial assembly for crank¬ shafts of multi-cylinder piston engines because of the shape of the shafts and the installation conditions of the split ends of the con¬ necting rods. The radial assembly is not always possible for cup-type parts such as impellers (Fig. 5). The design illustrated in Fig. 5 a can be assembled only by the axial method because the radial assembly of the hou¬ sing is impeded by the projection (by amount m) of the impeller disk with respect to the housing hubs. For the radial assembly the hub should be shortened (Fig. 5b) and an axial clea¬ rance s left between the impeller and the hub. In most cases several assembly ver¬ sions may be utilized. The task of the designer is to select the one most sui¬ table for the given conditions of opera¬ tion. Let us discuss the methods of the radial and axial assembly of a standard gearbox (Table 1). All the radial assembly versions (drawings 1-4) fully ensure uni- lized assembly, allow convenient gear engagement checking and adjustment of the gear positions with respect to. the adjacent parts. However, manufacture is more complicated. The joint between the housing halves must be thoroughly machined and the seating surfaces and their end-faces machined conjointly in the assembled housing halves. Soft sealing gaskets in the joint must never be used test the fit of the bearings in their seats should be spoiled. The par¬ ting weakens the housing, and its rigidity has to he increased by making the walls thicker, employing ribs, etc. The pattern can only be applied if the axes of the other gears of the drive are also arran¬ ged in the parting plane. The axial assemblies (drawings 5-19) are more simple to manu¬ facture. The strength and rigidity of housings are as a rule higher. In mechanisms with multiple gears the gear axes may be located in different planes. The centre distance between the adjacent gears is restricted in some designs (drawings 8-11). Mounting is more complicated in the systems of axial assembly. In both systems inspection holes ensure convenient servicing du¬ ring operation (drawings 2-4, 8-19). (a) (b) Fig. 5. Assembly of enclo¬ sed impeller 2-01658 18 Chapter 1. Assembly Cluster Gear Assembly Patterns Table 1 Radial assembly 1 The parting plane of the housing passes through the axis of the cluster. The bearings of the shaft with assemb¬ led gears are placed on the seating surfaces of the lower half of the hou¬ sing and covered by the upper one which is located with respect to the lower half by set pins. The left-hand bearing is fixed by cover a, the right- hand bearing is floating. The halves of the housing arc loca¬ ted with respect to each other by the outer bearing races and rings b. The right-hand hearing floats on the shaft. The bearing seating surfaces can be through-pass machined. The upper half of the housing is located with respect to the lower one by the outer bearing races. The right- hand bearing floats on the shaft. The shortcoming of this design is that it is impossible to through-pass machine bearing seating surfaces. The halves of the housing are lo¬ cated with respect to each other by the bearing races and covers c. The design may be applied when the dis¬ tance between the bearings is not too large. ■?.?. Axial and Radial Assembly 19 Table 1 ( continued ) Axial assembly The detachable wall d is located with respect to the housing by set pins. During assembly the cluster is installed with its right-hand bearing into the housing and covered by the detachable wall (lock ring e of the bearing should first be removed) af¬ ter which the cluster is secured by cover /, The shortcomings of the design are the reduced rigidity of the housing and the position of the sealing gasket below the oil level. Another design version (suspended housing). The housing (drawing 5) has a hole with a diameter exceeding that of the larger gear. The cluster is installed in cover g and inserted into the housing (drawing 9). The centring surfaces in the housing are machined in one operation. The diameter of the cover restricts the arrangement of adjacent gears in the gearbox. Chapter 1. Assembly 1.2. Independent Disassembly 21 4 Table 1 ( continued ) The cluster complete with the bearings can also be assembled by the same meth¬ od (drawing IS), if the bearings are mounted in intermediate bushings j (dra¬ wing 19) and the upper hole is somewhat enlarged. 1.2. Independent Disassembly The assembly pattern should be selected so as to ensure a conve¬ nient inspection, checking and adjustment of the units. The remo¬ val of a part or unit should not disturb the integrity of the other units to be checked. The gear shown in Fig. 6a is obviously mounted unhappily. The gear is locked by nut 1 also serving to fasten the stud shaft in the housing. The entire unit has to be disassembled to remove the gear. 22 Chapter J. Assembly In the improved design (Fig. 66) the shaft and the gear are secured separately, and the gear can be taken off without removing the shaft. In the fastening unit of a bearing (Fig. 6c) the cap and the bot¬ tom member are clamped by through bolts. The bearing falls apart as soon as the cap is removed. In the design shown in Fig. 6d the cap and the bottom member are disassembled separately. Figure 6e shows a bevel transmission to a camshaft. The bottom members of the bearings are made integral with the frame and the Fig, 6. Assembly patterns caps form a single whole with the housing of the frame. When the housing is removed, the shaft remains in the lower half-liners, and it is impossible to check the operation of the unit. It is better to make the housing of the frame independent and fasten each cap to the bearings separately (Fig. 6/). After the hou¬ sing is removed, the entire mechanism is open to inspection. Apart from convenient disassembly, this design makes it easier to accu¬ rately machine the bearing holes. 1.3. Successive Assembly When several parts are successively mounted on a single shaft by an interference fit, one-diameter fits should be avoided (Fig. la, c and e). The mounting and dismantling grow in complexity beca¬ use the parts have to be moved over the seating surface, and there is a hazard of damaging it. In such cases it is more expedient to em¬ ploy stepped shafts with the diameter of the steps increasing suc¬ cessively in the direction of assembly (Fig, lb, d and /). It is especially difficult to assemble a large number of parts on long shafts with a heavy drive fit (Fig. 8a). The assembly can be facilitated by heating the parts to be fitted on to a temperature that allows them to be freely mounted on the shaft (although this ope¬ ration complicates the assembly). This cannot be done during disas¬ sembly. A correct design with a stepped shaft is shown in Fig. 86. If there are many steps, the standard shaft diameters have to be relinquished and individual dimensions introduced to prevent ex¬ cessive increases in the diameter of the last steps of the shaft. The 24 Chapter I. Assembly difference between the diameters of the steps is reduced in this case to the minimum (about several tenths of a millimetre) enough to fit the parts on easily. It is better if the assembly is effected from both ends of the shaft (Fig. 8c). In this case the shaft and the hubs can be machined much Fig. 9, Installation with two seating surfaces easier. The number of nominal diameters and the range of special cutting tools (reamers, broaches) and measuring tools (snap and plug limit gauges) are halved. If parts are mounted on a shaft by a slide or easy slide fit, it is good practice to use a smooth shaft. This also refers to spline-fit¬ ted connections (Fig. 8 d): stepped diameters make the manufacture of the unit much more difficult since each hub requires special bro¬ aches, and special hob cutters are needed for each step of the shaft when centring is done from the infernal diameter of the splines. When assembling parts having two seating surfaces, the parts should be fitted into their seals locating in a proper sequence. If the part first fits into the first seat (in the direction of motion) and a clearance m (Fig. 9a) remains between the end face of the part and the second seat, the inevitable skewing of the part hampers its pro¬ per installation, and even makes it altogether impossible when hea¬ vy drive fits are employed. All the seating surfaces of a part (Fig. 9fc) should never come into contact with their mating surfaces simul- 1.4, Withdrawal Facilities 25 taneously. Correct designs are illustrated in Fig. 9c. The part should first fit into the second seat to a distance n (2-3 mm) enough to guide it properly, and then into the first seat. 1.4. Withdrawal Facilities Such facilities must be provided without fail in interference-fit¬ ted connections, in connections using sealing compounds or having (a) (£» (c) ( d > Fig. 10. Withdrawal facilities parts difficult of access, and also in connections operating under cyclic loads when cold hardening and frictional corrosion may oc¬ cur. Disassembly is made much easier if parts are designed with beads, flanges, threaded surfaces and boles, etc. Figure 10 show's a bushing interference-fitted into a frame. The design shown in Fig. 10a is difficult to disassemble. The dismant- Fig. 11. Withdrawal facilities lor tightly fitted hubs ling process can be facilitated by increasing the height of flange m (Fig. 106), by introducing annular clearance h (Fig. 10c) or recess q for a withdrawal tool (Fig. lOd) between the flange aud the hou¬ sing, or by providing threaded holes for puller screws either in the bushing (hole s in Fig. lOe) or in the housing (hole t in Fig. 10/). At least three threaded holes spaced (at 120°) are necessary to remove the part without skewung. Figure 11 shows withdrawal facilities employed to pull tightly fitted hubs off cylindrical surfaces. 1.4. Withdrawal Facilities 27 The hub in the designs in Fig. 11a and b is provided with a thread for a puller. In Fig. lie and d the circlips introduced into the hub serve as pullers. A system of differential threads is shown in Fig. lie and f. The clamping nut has two threaded surfaces each with a different pitch. As the nut is unscrewed, the hub is removed from the shaft. Figure 12 illustrates some examples of withdrawal facilities (des¬ ignated by figure J) incorporated into the design. it is practically impossible to replace the press taper-fitted valve seat in the design shown in Fig. 12a. The joint can be made deta¬ chable if the hole in the body is enlarged with respect to the seat edges (Fig. 12ft) or the seat is provided with an internal ta¬ per (Fig. 12c), Then, it beco¬ mes possible to press out the seat by applying a force to the top of the seat. Stuffing-box glands (F ig. 12 d) frequently jam because the packing is forced into the clearance between the gland and the shaft. A stuck gland can be removed from the box only if a withdrawal means, in the form of a flange (Fig. 1 2e) for example, is provided on the gland. The best method is to install a lock ring in the gland nut (Fig. 12/). In such a design the gland leaves the box as the nut is unscrewed. A bushing press-fitted into a hollow shaft is shown in Fig. V2g and ft. In Fig. 12g the bushing can be pressed out only by damaging it, for example, if a threaded taper rod is screwed into it. In Fig. 12ft the bushing can be forced out by pressing against its end face. Other examples of wrong and correct designs are illustrated in Fig. 12j and / (press-fitting of a pin) and k and l (installation of a swirler in an injector). Some methods to ease the dismantling of hubs are presented in Fig. 12m. and n. The hub in Fig. 12m is provided with holes for a puller. In Fig. 12ra (a hub mounted on centring cones) the flange of the clamping nut is inserted into an annular groove in the split cone. When the nut is unscrewed, it first, draws out the cone which then thrusts against lock ring 2 and takes off the hub. Lips (Fig. 12o) or holes (Fig. 12p) for tongs are used to facilitate the removal of circlips fitted into holes. Figure 12<7 shows a feather key provided with a tapped hole for a puller screw. Fig. 13. Hydraulic withdrawal 28 Chapter t. Assembly Of late, parts assembled by heavy drive and wringing fits are ta¬ ken apart by a hydraulic method (Fig. 13a) whereby oil at a pres¬ sure of 1,500-2,000 kgf/cm 2 is supplied to the mating surfaces. The hydraulic method of forcing a bushing out of a blind hole is illustrated in Fig. 13f>. A plunger is introduced into the bushing bore previously filled with oil. When a force produced by a press is applied to the plunger, the pressure developed in the oil layer for¬ ces the bushing out of its seat. 1.5. Dismantling of Flanges Considerable difficulties are frequently met with when disassem¬ bling large-diameter flanged joints Fig. i4. Withdrawal facilities for flan¬ ges sealed off by means of gaskets or jointing compounds, or operating at increased tempe¬ ratures, because the jointing surfaces stick together. The simplest withdrawal means for such flanges are illustrated in Fig. 14. One of the flanges (Fig. 14a-c) is provided with projections or recesses (usually three, spaced at 120 c ) which allow axial forces to be applied to detach the flanges. Figure 14 d-f shows designs with pro¬ jections or recesses on both flanges which can be taken apart with a screw driver in¬ serted between the flanges. Fig, 15. Separation of flanges by means of pressure bolts Better withdrawal facilities are presented in Fig. 15. Three threa¬ ded holes spaced at 120° are made in one of the flanges. The flanges are separated by means of pressure bolts (Fig. 15a) screwed into the holes. The crushing of the jointing surface (especially in parts made of fight alloys) is prevented by hardened inserts installed under the pressure bolts (Fig. 15&). The holes for the bolts are reinforced with threaded bushings. 1.6. Assembly Locations 29 1.6. Assembly Locations The position of parts during assembly should unambiguously be determined by assembly locations. Any uncertainties in design when Fig. 16. Locking of parts during as¬ sembly the fitter has to carry out the assembly according to his own ideas should never be permitted. Undesirable are also designs re¬ quiring adjustment or matching of parts during assembly. As¬ sembly errors committed in ma¬ nufacture can be revealed through quality control. But in actual service, especially if the machine is bandied by unskilled personnel, there is no guarantee of its being assembled correctly. Any uncertainty in assembly involves more labour and time to correct the faults and reduces the efficiency of the assembly. The quality of the assembly in this case depends mainly on the skill of fitters. An example of a wrong design is shown in Fig. 16a. The gear is tightened on the shaft from both ends with two annular nuts 1. In this design there is no location determining the axial position of the gear and the shaft. Additional time is needed to adjust the posi- 30 Chapter 1. Assembly tion of the gear when the unit is built or reassembled. An unskilled or careless fitter is likely to assemble the unit wrongly. In the design shown in Fig. 166 a poor attempt is made to secure the position of the gear. Locating bearing 2 is tightened against the shaft collar m. The gear is tightened so that it rests against the inner race of the bearing. If the locating bearing is tightened first and then the gear, the position of the gear is quite definite, but it is-also possible that the gear will be tightened first through bearing 3 , and then through bearing 2. In this case the gear may be displaced from its nominal position. The correct design in Fig. 16c has a rigid location in the form of shoulder n against which the hearing and the gear are tightened independently. The position of the gear and the shaft is properly secu¬ red and may vary only within the machi¬ ning tolerance limits. In Fig. 16d an overhung gear is moun¬ ted in radial-thrust bearings clamped in the housing at both ends with annular nuts. There is no location and the posi¬ tion of the gear in the unit may vary within the stroke of the nuts. In the correct design shown in Fig. 16c the gear is fixed in position by means of a location (bolted-on washer 4). The radial position of the blades on the rotor of an axial compressor (Fig. 17a) is uncertain. The unit can be assembled correctly only with a spe¬ cial fixture used to adjust the blades to the same distance from the centre of the rotor. In the design in Fig. 176 the position of the blades is fixed by a location although it is unilateral. The con¬ centricity of the blades is maintained during assembly by thrusting their bases against the outer cylindrical surface of the rotor. The best designs are those in "which the blades are rigidly fixed in both radial directions (Fig. 17c). 1.7. Prevention of Wrong Assembly Not infrequently errors in mounting parts, negligible at first sight and difficult to detect, may derange the operation of the as¬ sembled unit and even cause its breakdown. In such cases the cor¬ rect position of the parts in the assembly should never be indicated hy means of marks, notches, etc. The only correct solution is to take proper design measures to ensure the assembly of the parts in the required position only. W (b) (c) Fig. 17. Mounting the bla¬ des of an axial compressor 1.7. Prevention of Wrong Assembly 31 In the bearing unit shown in Fig. 18 the cap is located with res¬ pect to the housing by two set pins 1 {Fig. 18a). The error lies in the symmetrical arrangement of the pins: the cap may be turned thro¬ ugh 180° relative to its initial position and then installed; this will impair the cylindricity of the seat and the alignment of the end faces attained during previous machining on the assembled bearing. An asymmetrical arrangement of the pins (Fig. 186 and c) prevents: wrong assembly. In the sliding contact bearing presented in Fig. 18d the shells are installed in a split housing, the upper shell being held by oil-feed sleeve 2 and the lower one, by set pin 5, both having the same diame¬ ter. During assembly the lower shell may be erroneously installed at the top and the upper one at the bottom. The error can be preven¬ ted if the sleeve and set pin 3 have different diameters (Fig. 18e). In the bearing unit shown in Fig. 18f-i the bush should be ins¬ talled so that the oil-feed hole in the housing coincides with the hole in the bush. In the design in Fig. 18/ the bush may be turned by mis¬ take through 180° in which case the oil-feed hole will be shut off. In the design in Fig. 18g wrong assembly is prevented by a check pin 4. A flat is provided at the oil-hole entrance in the bush to allow for its lower positional accuracy. This can also be done by means of two diametrically opposite holes with flats made in the bush (Fig. 18b). The bush in the design in Fig. 18t is provided with an annular groove that feeds oil with the bush in any position. Figure 18/-I shows cover 5 with a recess connecting two oil holes in the housing. The design in Fig. 18/ is wrong because the cover may, by mistake, be mounted on the fastening bolts so that the ho¬ les in the housing will be shut off. The unit will operate properly if the recess is made in the housing (Fig. 18&) and not in the cover, or if the recess in the cover is made cylindrical (Fig. 180- Figure 19 a-c shows the installation of a flange with an inner mo¬ unting pad m. When the fastening bolts are arranged symmetrically (Fig. 19a) the pad is likely to be displaced from the required angu¬ lar position. This can be prevented either by locating the flange with a set pin (Fig. 196) or by placing the fastening bolts in an asym¬ metric order. The displacement of a single bolt through an angle a = 5-10° (Fig. 19c) will be sufficient to ensure correct assem¬ bly. Figure 19d-i shows studs screwed into a bousing. In the design shown in Fig. 19<2 the ends of the studs have the same thread, but the lengths of the threaded portions are different, and the studs may be screwed into the housing with their wrong ends. Various distinctive features such as different shapes of the stud ends, for example, making one end spherical (Fig. 19e) or elimina¬ ting the smooth portion on the other (Fig, 19/) have little effect. 1.7. Prevention oj Wrong Assembly 33 Assembly will always be correct if the stud threads have diffe¬ rent, pitch (Fig. 19 g) or, better still, different diameters (Fig. 19ft). Besides, the ends of a stud may be imparted the same shape and the same axial dimensions (Fig. 19i) in which case the position of the stud in assembly is indifferent. The principle of foolproof assembly precludes the possibility of errors, increases the efficiency of assembly operations and saves Fig. 19. Prevention of wrong assembly the fitter’s time otherwise necessary to find the proper position of the part. Figure 19/ shows a bush press-fitted into a housing. The bush has a slow entry chamfer c at one end for installing a rolling-contact bearing. In the case of wrong assembly the chamfer will be on the opposite side making it difficult to install the bearing. In the de¬ sign shown in Fig. 19ft where both ends are chamfered the position of the bush during assembly is indifferent. Fastening nuts with only one chamfer (Fig. 19I, n) are imprac¬ ticable because the fitter must see to it that the nut is placed cor¬ rectly. In mechanized assembly such nuts delivered to the nut-run¬ ning tool must be properly oriented. Preference should be given to nuts with chamfers on both sides (Fig. 19m, o) which can be fitted by either side. Also, it is not advised to employ washers of asym¬ metric shape (Fig. 191, m). In the oil-seal unit with split spring rings (Fig. 19p) seal 1 is asymmetric and must be installed in one position only. The unit will not operate if the installation is wrong (Fig. 19 q). In the de¬ sign in Fig. 19r the seal is symmetric and the unit will function pro¬ perly irrespective of its position. 3—010SS 34 Chapter 7 , Assembly 1.8. Access of Assembly Tools Fasteners should be easily accessible for assembly tools to facili¬ tate mounting and dismantling. A poor design is illustrated in Fig. 20 a (mounting of a V-belt drive pulley with a sealing gland). A wrench can approach the bolts of the neck bush only after the pul¬ ley is removed from the shaft. In the design shown in Fig. 20b the error is corrected by removing the pulley to a distance s enough to apply a box-end wrench to the bolt heads. Fig. 20. Access of assembly tools a, a, t —wrong; 6, c, /, g —correct The disk of the pulley in the design in Fig. 20c is provided with holes n to admit a socket wrench to tighten up the bolts of the neck bush. Figure 20 d-g shows the cylinder fastening of an air-cooled engine. The design in Fig. 20d is wrong: clearance b 1 between the lower rib and the ends of the clamping studs that remains after the cylinder is fitted onto the studs is less than thickness h of the fastening nuts. This unit can only be assembled by a single, highly inefficient, meth¬ od: the cylinder is raised up by the studs (Fig. 20a) and the nuts fitted on the ends of the studs are then tightened up in succession. For an effective assembly clearance h t should be provided between the lower rib and the end of the stud exceeding thickness h of the nut (Fig. 20/) or recesses m for the nuts made in the lower ribs (Fig. 20 g). Generally, it is recommended that the design should permit the use of socket wrenches for screwing nuts and bolts, because these are more convenient to handle, improve the efficiency of the assembly work, damage the nut flats to a lesser degree and enable the tighte¬ ning force to be increased. In mechanized assembly nuts and bolts are usually screwed with electric or pneumatic tools equipped with socket-type work heads. Some examples of design changes in fastening units to make them suitable for mechanized assembly are presented in Fig. 21. 1.8. Access of Assembly Tools 35 In the design in Fig. 21a the nuts can only he tightened with an open-ended wrench. Clearance s in the design in Fig. 216 permits the use of a socket wrench. The most convenient for assembly is the design in Fig. 21c where the nuts are arranged on unobstructed sur¬ faces of the part. In the bracket fastening unit (Fig. 21 d) socket-wrench tightening is only possible if the bolts are at a distance s (Fig. 21e) from the Fig, 21. Tightening up of nuts bracket boss or if they are arranged on the side op¬ posite to the bracket (Fig. 21/). It is difficult to reach the inner nut iu the faste¬ ning unit of an elbow pipe (Fig. 21g) and it is impossible to use a socket wrench to tighten the nut. In the design in Fig. 2 lh the error is corrected by turning the flange through 90° with respect to the pipe axis. The design in Fig. 21 i where the nuts are arranged above the pipe surface is still better. When nuts are arranged in confined places, minimum clearances for wrench application should be assigned that will suit the dimen¬ sions of st andard nut-runners and their replaceable socket work heads. The heads of bolts should be locked against rotation during ti¬ ghtening, for example, by butting the hexagon against a shoulder (Fig. 22a and b) by means of flats (Fig. 22c), nibs (Fig. 22d), etc., so that the head need not be held by a wrench when the nut is tightened. It is just as important to prevent the axial displacement of bolts being tightened and prevent them from falling out especially if the assembly is carried out vertically. It is unpracticable to lock the bolts by an annular stop (Fig. 22e) since the groove for the stop weakens the bolt. The designs in Fig. 22/ and g are better. Slow entry chamfers on the ends of fasteners will make nut enga¬ gement, much simpler when the tightening is done mechanically. 38 Chapter 1. Assembly Fig. 22. Locking oi bolts against rotation and axial motion. 1.9. Rigging Devices Large, heavy machine components and units should be provided with some rigging devices to enable them to be easily handled during assembly and transportation. Fig. 23. Suspension of parts in load handling If the shape of the machine permits, lifting slings and grips are attached to lugs or projections {Fig. 23a), flanges (Fig. 236), holes (Fig. 23c) or bars passed through the holes (Fig. 23d) available on the machine. If the machine has no such elements it must be equipped with eye-bolts. A machine or a large part may be suspended from one point, only if its centre of gravity is low and the axis of gravity passes through 1.10. Spur Gear Drive. 37 the suspension point, i.e,, when the part is high and has a small cross- section (Fig. 23e). If a wide part is suspended from one point (Fig. 23/) it may get out of balance and topple over. Parts of such shape should be suspen¬ ded from at least two points (Fig. 23g). Low, wide and long parts must never be suspended from one or two points (Fig. 23ft, i). Such parts should generally be suspended from three or, better still, four points (Fig. 23/). Cylindrical shaft-type parts are handled by means of rigging bolts screwed into threaded holes usually located at centre holes (Fig. 24c). Ring bolts (Fig. 24 b) are most commonly used. Standard ring bolt sizes are selected to suit the acting load. Cylindrical cantilevered rigging bolts with necks for slings and grips (Fig. 24c) are employed for side mounting. Figure 24 d shows a cantilevered sling bolt intended to carry a heavy load. Extreme care should be taken when designing non-standard rigging bolts since their poor design may cause a machine to fall from the pulley blocks, thus breaking the machine and possibly injuring personnel. Rigging bolts should have large margins of safety. The use of east rigging bolts should he avoided. The portions where the bolts contact the lifting slings should be smoothly round¬ ed. 1.10. Spur Gear Drives During manufacture, the quality of gears is controlled either by checking individual gear elements determining the correctness of engagement (tooth thickness, pitch, runout, tooth profile, etc.), or by testing the gears as a whole against a master gear in a double- or single-profile meshing (without or with backlash, respectively). In the latter case subject to evaluation are the kinematic accuracy of the drive, smoothness of run, backlash, and contact between the teeth. The master gear is made to drive the gear under test, which is being slightly braked, first in one direction and then in the other. 38 Chapter 1. Assembly A recorder registers on a profilograph the deviations in the run of the gear under test as compared to that of a calibrated reference gear also meshing with the master gear. The kinematic accuracy is determined by the value Ashowing the maximum variations in the angular velocity of Fig, 25, Engagement chart the gear during one revolution (Fig, 25). This value primarily indicates the run¬ out of the pitch cylinder with respect to the locating surfaces of the gear (journals, seating, holes). The smoothness of mn is estimated by the arithmetic mean value of the cyclic errors during one revolution of the gear _ ^t + ^e+ ■ ■ ■ +Q n . n showing the composite error in the tooth thickness pitch and tooth form. The change in the backlash depending on the angle of the gear rotation is expressed by the distance c between the extreme points of the profilographs for the right- and left-hand rotations, which are separated from each other by distance e 0 equal to the mean backlash value. Contact between the teeth is checked by applying a thin layer of a marking compound (for example, Prussian blue) onto the teeth of the master gear, rota¬ ting the gears and then measuring the gear-contact patterns on the teeth of the gear being tested. Another method consists in coating the teeth of the gear under test with soot and measuring the bright spots on the teeth after rotation. Tooth contact is characterized by the relative size of the gear-contact pat¬ terns (Fig. 26a): over the face width ~ 100 % over the depth of tooth 100 % where a — mean width of the gear-contact patterns (minus interruptions) B — face width h — mean depth of the gear-contact patterns H — depth of tooth The displacement of the patterns towards the tooth tip (Fig. 266) shows that the diameter of the pitch cylinder is decreased,’and'their shift to the root of tooth (Fig, 26c) shows that the diameter is increased. Contact near the edges (Fig. 26. Given: the working temperature of the gears—10O°C and of the housing—50°C. The centre distance is 200 mm. Heating changes backlash by Ac = A.4 tan a (1.1) where AA = difference between the increase of the centre distance and that of the radii of the gears a = pressure angle (for a standard gear system a. — 20', tan a = 0.365) For the cast-iron housing A A = 200 X 11 X 10(50 — 100) = —0.11 mm Ac = —0.365 X 0.11 = —0.04 mm i.e., backlash is appreciably diminished. For the aluminium housing A.4 = 200 (25 X 10-» X 50 — 11 X 10-* X 100) = 0.03 mm Ac = 0.365 X 0.03 = 0.011 mm i.e., backlash is slightly increased. 40 Chapter 1. Assembly Possible variations in backlash resulting from the inaccurate centre distance may be found from the relation. A'c = AM tan a where AM is the tolerance for the centre distance. In the case of ordinary accuracy (AM = ±0.05 mm) A'c = 0.05 X 0.365 = 0.018 mm Thus, in an unfavourable case (cast-iron housing and the centre distance to the minus tolerance) backlash may become smaller than the nominal value by 0.04 -f 0.018 « 0.06 mm. Except for the thermal ones, most of the other factors affecting the operation of gears are accounted for by checking the backlash between the teeth of the gears mounted in pairs in the housing. Backlash is commonly checked with a thickness gauge inserted into the spaces between the meshing teeth with the gears in several positions (within one revolution of the gear wheel). With this me¬ thod, free access to the engagement area must be ensured. If the access is difficult, backlash is determined by swinging one of the gears, with the other being fixed, with the aid of an indicator the contact point of which is applied to one of the accessible teeth in a direction tangential to the pitch circle. Measurements are taken with the gear wheel in several angular positions. In designs having gears difficult of access backlash is measured by an indicator with a pointer secured to the free end of the gear wheel shaft. Backlash in this case is found by multiplying the mea¬ sured values by the ratio of the pitch cylinder radius to the arm of measurement. For a rough check-up, a thin lead strip is passed between the teeth, the thickness of the strip then being measured in sections correspon¬ ding to the engagement areas. The minimum amount of backlash determined by one of the above methods should exceed the possible backlash reduction due to hea¬ ting, on average, by not less than 0.05 mm, USSR State Standard TOCT 1643-56 establishes backlash values for each grade of accuracy. For medium-accuracy general-purpose drives backlash may be determined from the formula c — (0.04 to 0.06) m where m is the module. Contact between the gear teeth is checked with a marking compound. The check-up will only be effective if it is carried out under a load equal to the working load. The possibilities for adjusting the engagement parameters of spur gears are limited. Should the check-up reveal too small a backlash or unsatisfactory contact, an individual selection of gears is practi¬ cally the only method of obtaining the needed parameters. This 1.11. Bevel Gear Drives 4i complicates tlie assembly. For this reason, when designing gear dri¬ ves it is important to select such gear accuracy grades, size tole¬ rances, and form of the bearings as would ensure the interchangea¬ bility of the gears without compli¬ cating excessively the production process. Diiferent hardness is frequently impa¬ rted to the teeth of meshing gears to increase their durability and improve running-in. The pinion teeth are harde¬ ned, carburized (58-62 He) or nitrided (i,000-1,200 VPH) while the gears are structurally improved (30-35 Rc) or medium-temper hardened (40-45 Rc). In such drives the pinions should he made wider than the gears (Fig. 27c) so that the pinion teeth overlap the gear teeth whatever are the variations in the axial position of the gears. If the width of the pinions and gears is the same (Fig. 27a), a displacement of the gears (because of manufacturing and mounting inaccuracies) will cause a stepwise wear of the softer teeth (Fig. 276) and in the case of subsequent changes in the axial position of the gears this will disturb the engagement. 1.11. Bevel Gear Drives A frequent error in designing units with bevel gears is that the gears are only fixed in one direction, namely, in the direction of the acting axial forces (Fig. 28a), assuming that the gears are fixed in the reverse direction because they thrust against the teeth of the mating gear. Gears should always be fixed in both axial dire¬ ctions (Fig. 28 b) for the drive to operate reliably and noiselessly, especially under dynamic load conditions. Provision should be made for the adjustment of the axial position of both gears, for otherwise it will be impossible to match the api¬ ces of the pitch cones and obtain the required backlash and satisfac¬ tory contact between the working faces of the teeth. The design in Fig, 28c is wrong, -while that in Fig. 28d is correct. Engagement is usually checked with a marking compound by rotating the drive under a load as near as possible to the working load. Engagement is satisfactory if the gear-contact patterns on all the teeth extend to 0.6-0.8 of the face width and are located in the middle of the tooth (Fig. 29a) or closer to the thickened end of the tooth (Fig. 29ft). The concentration of the gear-contact patterns near the edges of the teeth (Fig. 29c and d ), and especially at the edge of the thinned portion of the tooth (Fig. 29d) must not be per¬ mitted. The design of a drive should allow for an easy inspection of the gear teeth to dispense with disassembly during each check-up. The method of adjustment by moving the gears until the end faces of the teeth are matched (on the outer side of the gears) is less accu- 1,11. Bevel Gear Drives 43 rate. With this method the end faces of the teeth in the engagement area should he open to view. Because of the smaller accuracy in the manufacture of bevel gears the backlash in such gears is made slightly greater [ (0.06 to 0.1)m, where in is the module]. The backlash between meshing gears is che¬ cked either with a thickness gauge introduced into the spaces bet¬ ween the meshing teeth from their ends (on the outer side of the gears) or with an indicator the contact point of which is applied to one of the teeth or to a pointer secured on the gear shaft. There are two methods of adjusting the axial position of gears. With the first method the position of the gear on the shaft is chan¬ ged. The shaft secured by its bearing surfaces remains in place. This method can be only applied if the gear is not made integral with the shaft. With the second method the gear is shifted together with the shaft. This method may he applied if the change in the axial position of the shaft within the adjustment range (usually 0.5-Imm) does not affect the operation of the parts mated with the shaft. Otherwise it is necessary to divide the shaft into two portions, one ©f which can be shifted axially while the other is fixed in the axial direction, and connect both portions by means of a compen¬ sator (for example, a splined compensator). This is the only possible method for gears made integral with the shaft. It is also frequently employed for fitted-on gears. Some methods of adjusting the axial position of gears mounted in rolling contact bearings are illustrated in Fig. 30. The axial position of a gear on a shaft is commonly adjusted by means of changeable calibrated washers 1 (Fig. 30a). For adjustment the gear has to be taken off the shaft in which case the unit must be disassembled. In order to make the adjustment easier, the calibra¬ ted washers are manufactured in the form of half-rings 2 (Fig. 305) inserted into a recess made in the gear. In this case it is enough to shift the gear on the shaft to a distance equal to the depth of the recess after which the half-rings can easily be removed and replaced by other ones, Tbe gear can be made to shift together with the shaft by replacing thrust washers 3 (Fig. 30c — gear made integral with the shaft; Fig. 30d — fitted-on gear). Figure 30c-/ shows the adjustment by shifting the bearing housing. In the design in Fig. 30e the adjustment is done by means of a set of shims 4 made of metal foil and placed under the housing flange. The shortcoming of the method is that the unit has to be disassembled. In the design shown in Fig. 30/ the adjustment is done by repla¬ cing calibrated half-rings 5 fitted into a recess made in the housing flange. It is enough to move the housing forward to a distance equal to the depth of the recess to replace the half-rings. HI. Bevel Gear Drives 45 In the design in Fig. 30g the adjustment is carried out without disassembling the joint with the aid of pressure screws 6 (usually three in number). In order to move the gear towards the centre of the drive it is necessary to slacken the screws by the required amount and then tighten up the fastening bolts. To move the gear away from the centre of the drive one should unscrewjthe fastening bolts Fig. 31. Adjustment of axial position of gears and then screw in the adjusting screws. An essential shortcoming of this design is that it is difficult to locate the housing from three points simultaneously; as a result, there is a possibility of the housing being skewed when tightening the bolts. In the design shown in Fig. 30ft the axial shift is effected by tur- ning the housing which is thread-fitted in the bed (with a smooth centring portion). The adjusted housing is secured by means of a lock nut. In the design in Fig. 30i the housing is shifted in the axial di¬ rection by means of annular nuts installed on both sides of the hou¬ sing. All these methods slightly impair the centring of the shaft becau¬ se the housing must be installed by a slide fit. Convenient adjustment is shown in Fig. 30;'. Here, the housing is shifted by rotating annular nut 7 screwed onto the housing and fixed in the axial direction by washer 8. The rotation of the bearing 46 Chapter 1. Assembly housing is prevented by screw 9. The joint will inevitably have an end play equal to the sum of the clearances in the thread and on the faces of the annular nut. As distinct from the designs shown in Fig. 30e-s, the housing is not tightened, an undesirable feature under dynamic loads. In the design in Fig. 30& (radial assembly) the adjustment is done with the aid of half-rings 10 (a loose joint), and in Fig. 30/, by means of half-rings 11 tightened with a nut. The methods of adjustment for gears mounted in sliding contact bearings are presented in Fig. 31. In the designs in Fig. 31a-c the gear is shifted along the shaft and in the designs in Fig. 31 d-f, toge¬ ther with the shaft. 1.12. Spur-and-Bevel Gear Drives As distinct from bevel gear drives in which the generatrices of the active surfaces of the teeth converge at the point of intersection of the gear axes (Fig. 32a), in the combined spur-and- bevel gear drives one of the gears (pi¬ nion) has straight teeth (Fig. 32 b). In the mating gear the tooth spaces correspond to the pinion tooth profi¬ les, i.e., the generatrices of the spa¬ ces are mutually parallel and the teeth become thinner towards the ce¬ ntre of the gear to a larger degree than in ordinary bevel gears. Friction without sliding over the Fig. 32. Gearing diagrams t0 ^ h wi £ th . . observed in . . J* , t ordinary bevel gear drives is absent a-hevel drive; 6-spur-and-bevel drive ifl tJ)e apur . and .g evel ge ar drives. In many cases this fact is immaterial. Pure (rolling friction in any involute gearing is only observed in tooth sections close to the pitch circle; sliding friction is added to rolling friction Fig. 33. Spur-and-bevel drives at the root and the top of the tooth. Sliding over the tooth width (also occurs in drives with skew gear axes, but nevertheless this does not prevent these drives from operating reliably for a long time. 1.12. Spur-and-Be.uel Gear Drives 47 In the combined spur-and-bevel gear drives sliding diminishes as the angle ip between the gear axes becomes smaller (Fig. 33fc-d). When q> = 0 (Fig. 33a and e) a spur-and-bevel gear drive becomes a purely spur gear drive. Sliding is reduced with a smaller face width with respect to the diameter of the gear, and with a higher gear ratio. Spur-and-bevel gear drives are manufactured on the gear-cutting machines used for spur gears. Pinions 1 with straight teeth are machined by the usual shaping and milling methods, and the mating gears with wedge-shaped teeth are generated using a gear cutter whose shape corresponds to that of the spur pinion. Both can easily be ground, and a high surface hardness can be imparted to their teeth. Helical teeth are cut by the usual shaping methods with a helicalfgear cutter. The spur pinion (with straight teeth) is not subjected to axial pressure and does not require any axial adjustment if its teeth overlap those of the bevel gear. Spur-and-bevel gear drives can be engaged and disengaged by moving the spur pinion, in the same way as the ordinary spur gear drives. Spur-and-bevel gear drives are employed with small and medium torques and with gear ratios from 1 and higher. Such drives are known to be used in high-power installations. Chapter 2 Convenience in Maintenance and Operation When designing units, assemblies and machines one should pro¬ vide for their convenient maintenance, operation, disassembly, reassembly and adjustment, make them easily accessible for inspec¬ tion, and prevent their possible breakdowns due to unskilled or careless handling. Also, the machine should have an attractive external appearance. 2.1. Facilitating Assembly and Disassembly Let us consider some examples of how to facilitate the assembly and disassembly of connections which have to be frequently dis¬ mantled when in use (Fig. 34). Fig. 34. Ways to facilitate assembly It is difficult to fit a flexible hose onto the pipe shown in Fig. 34a. In the design in Fig. 34 b the guiding portion with rounded-off edges makes the process much more easier. In seals with split spring rings (Fig. 34c) the assembly is simplified if the housing is provided with a slow entry chamfer of diameter D exceeding the diameter d of the rings in their free state (Fig. 34 d). 2.1. Facilitating Assembly and Disassembly 49 In the case of hard-to-reach joints, especially with blind assembly, it is good practice to provide the male parts (Fig. 34e) with a taper, and the holes, with guiding cones (Fig. 34/). The inner spaces and ducts of oil systems should periodically be cleaned to remove dirt and the products of thermal decomposi¬ tion of oil. Oil ducts should preferably be plugged up (Fig. 35 c, d) and not sealed permanently as shown in Fig. 35a, ft. Figure 36a illustrates an irrational design of the oil space in the neck of a crankshaft. The space is sealed by end caps made of sheet steel and press-fitted into the crankshaft webs. The space can be Fig. 33. Sealing of oil ducts Fig. 36. Sealing of oil spaces in a crankshaft cleaned only by injecting a washing solution into the interiors of the shaft, The design with detachable caps (Fig. 36ft) is far more better. Joints which are frequently disassembled and assembled when in use should be made readily detachable. Figure 37 shows the tip of an ignition system conductor. In the design in Fig. 37a the faste¬ ning nut of the contact screw has to be unscrewed completely to remove the conductor. In the design shown in Fig. 37ft where the conductor has a split tip it will be enough to unscrew the nut to the height h of the fixing flange on the tip to remove the lat¬ ter from the screw. Figure 38 illustrates a quick-acting clamp with a swing bolt (fre¬ quently used to fasten the covers of autoclaves). Tho nut is unscre¬ wed to the height ensuring its free passage over the comer of the cover and the holts are then swung back to release the cover. The fastening of a cylindrical part in a spring U-clamp is illu¬ strated in Fig. 39. Quick-acting connections widely employ clamps with a swing arm. The clamp operating on the toggle principle consists of arm 1 (Fig. 40a) swinging on pin 2. Stirrup 3 engaging the hook of part 4 being tightened is attached to the arm. When the arm is swung to the position shown in Fig. 406 it. tensions the hook. By the well- known property of the toggle mechanism the tension reaches its maximum at the dead centre. Beyond the dead centre (angle a) the 4—0 1658 50 Chapter 2. Convenience in Maintenance and Operation ninii Fig. 37. Tip of a conductor Fig. 38. Swing bolt Fig, 39. Spring clamp Figure 42 illustrates the adjustment of the axial position of a shaft in a split sliding-contact bearing by means of adjusting rings (radial assembly). In the design in Fig. 42a the adjusting rings 1 » are solid. To carry out the adjust- J ment, it is necessary to take off /i a bearing cap 2, remove the shaft •' and take off the fitted-on part 3. 4 3 I rig. 40. rast-acling lock Fig, 41. Tubular part fixed by fast-acting lock In the design shown in Fig. 42b where the adjusting rings are split (half-rings 4, 5) it is only necessary to remove bearing cap 2 2.1. Facilitating Assembly and Disassembly 51 and then, leaving the shait in place, remove half-rings 4 and then, half-rings 5 after turning them around the shaft axis through 180°. 1 1-* iji , x-l « ■1 t HH 1 Si 9 wA B m Fig. 42. Adjustment of axial position of a shaft If the operating conditions require a full hearing surface not inter¬ rupted by splits, additional solid rings 6 are introduced {Fig. 42c). During adjustment, these rings remain in place. Adjusting half-rings 7, 8 can be taken off without disassembling the shaft. For a more convenient disassembly and reassembly the detachable covers of housing-type components should be provided with parti¬ tions (Fig. 43) to form several compartments for the taken-off fas¬ teners, each compartment accommodating fasteners of a definite size and type. Handles, handw'heels, hand nuts, etc,, should have convenient shape. Figure 44a presents an irrational design of a jack screw handle. Improved designs are illustrated in Fig. 44 b, c, d. 4* 52 Chapter 3. Convenience in Maintenance and Operation Knurled nuts (Fig. 45a) cannot be tightened forcibly by hand. In the design in Fig. 45 b the sharp edges of the nut may cut the fin¬ gers. Besides, a dirt trap is formed in the upper hollow of the nut. Fig. 45. Hand-driven nuts Correct designs that permit good tightening by hand are illustrated in Fig. 45c and d. If hand nuts are to be forcefully tightened, use is made of additional elements in the form of flats or hexagons (Fig. 45e). To accelerate and simplify the assembly of joints which are fre¬ quently disassembled while in use it is good practice to employ the so-called non-losable nuts which are held in the part being attached, for example, by means of circlips (Fig. 46). Each single nut is held with a minimum axial clearance m. Such nuts are used as pullers. In a joint with several nuts the axial clearance m should slightly exceed the length n of the bolt thread. Otherwise, it will be difficult to screw the nuts on and off (as all nuts would have to be turned in succession by a small amount each time to avoid the misalignment and pinching of the part). Hand nuts, handwheels, etc., should be designed to provide free access to the hand and a firm grip. The handwheel design shown in 2J. Facilitating Assembly and Disassembly 53 Fig. 47a is wrong. The small clearance m between the handwheel and the fastening bolts does not admit the hand. In the design in Fig. 47 b the handwheel rim is farther from the housing wall. If sunk hexagonal bolts are used (Fig. 47c) the clearance m is increased by the height of the bolt head h. The minimum clearance m necessary to conveniently grip a hand¬ wheel is equal to 20-25 mm. It should never be less than 35-40 mm for machines operating in open air, especially if gloves are worn. (a) (b) W (0 (hi (i) Fig. 49. Designs of nuts and bolts Handwheels and handles intended for rapid rotation (for example, the gear shifting handles of metal-working machines, handwheels used in worm drives, etc.) should possess an increased flywheel mass which makes it easier to overcome the nonuniform torque of the drive. Drive handles (Fig. 48a) should carry counterweights (Fig. 4 8b) or be made in the form of handwheels with massive rims (Fig. 48c). Hand-operated controls should be polished to the 11th or 12th class of surface finish to prevent injury to the hands, improve exter¬ nal appearance and avoid corrosion. For general-purpose screw joints one should use nuls chamfered on both faces, which may be installed by either side. For screw joints which have to be frequently taken apart while in use, it is advisable to apply thicker nuts and bolts with taller heads [ H — (1 to 1.4) d], as in Fig. 49 b, e, instead of ordinary nuts and bolts 1 h — (0.7 to 0.8) <2], as in Fig. 49a, d, and increase their hardness (35-40 Re) in order to prevent the crushing of their flats. A collar at the base of the hexagon (Fig. 49c, /) makes the nut application easier for it prevents wrench slip. However, this design 54 Chapter 2. Convenience in Maintenance and Operation is not suitable for mass production (as the nuts cannot be manufac¬ tured from hexagonal rolled stock). Whenever the design permits, box or socket wrenches should be used. As a rule, the hexagon dimensions should be unified as much as possible to reduce the size range of wrenches. But if the bolts are Fig. 50. Distinction marks tor fasteners with left-hand thread secured by lock nuts, it is advisable to use different tools for the bolts and the nuts (Fig. 49 k, i). In the case of identical hexagons (Fig. 49g) one has to keep duplicate wrenches in his tool set. Nuts and bolts with left-hand threads should be marked to pre¬ vent unscrewing them in the wrong direction, as this may cause Fig. 51. Structural attachment of parts a, c—wrong; 6, d —correct damage to the clamped parts. Such marks for fasteners with left- hand threads are illustrated in Fig. 50a~h. Individual parts belonging to the basic outfit of a machine should be structurally attached to it as loose parts may be lost when the machine is transported or repositioned. Examples are shown in Fig. 51 a, b (an inspection cover) and c, d (a leg with a self-aligning shoe). 2.2. Protection Against Damage Measures should be taken to safeguard the brittle elements of ma¬ chine components and their precision surfaces against careless handling. ■ ; Let us take, by way of example, the head of an air-cooled engine cylinder made of an aluminium alloy (Fig. 52a). The thin ribs can 2.2. Protection Against Damage 55 be safeguarded against breakage by making the lower rib thicker (Fig. 52 b) or press-fitting a steel rib on the cylinder (Fig. 52c). The end faces of splines will be effectively protected from dents, in the case of chance impacts, dropping, etc., by chamfers of diame¬ ter D that exceed the major diameter D 0 of the splines (Fig. 52 d) Fig. 53. Protection against the consequences of breakdown One example is the valve of an internal-combustion engine (Fig. 53a). Should the valve spring break, the valve hangs in the guide and hits the piston crown, and if, in addition, taper valve retainer blocks 1 leave their seats the valve drops into the cylinder. The result is a serious breakdowu^because of the valve stem butting against the combustion head. In the design shown in Fig. 535, the breakdown is prevented by retainer ring 2 fixed on the stem at a distance h from the end face of the guide, the distance somewhat exceeding the valve stroke. 56 Chapter 2. Convenience in Maintenance and Operation Two (Fig. 53c) or three concentric valve springs will practically exclude the possibility for the valve to fall into the cylinder. The coils of the adjacent springs are oppositely inclined so that if one spring breaks, its coils do not get into the spaces between the coils of the adjacent intact spring. Figure 53d shows torsion spring 3 used for an elastic transmission of torque from shaft 4 to shaft 5. As in all other springs, higher design stresses are adopted for torsion springs and, as a result, their failure in the case of overloads, for example, when torsional oscillations develop, cannot be excluded. To prevent overloads, the spring is enclosed in splined sleeve 6 meshing with the same splines as the spring but with a larger backlash. The torsion spring operates in its normal conditions. When the ra¬ ted torque is exceeded the load is taken up by the sleeve, which pre¬ vents the spring failure. If the spring breaks the torque is trans¬ mitted by the sleeve, although with reduced elasticity. 2.3. Interlocking Devices Machines and their units should be reliably protected against damage that may be caused by careless or clumsy handling. The machine must be designed so as to exclude any possibility for its wrong operation. In machine tools this is achieved by means of switching of push buttons a —wrong; correct automatic interlocking devices which cut out the machine orbits mechanisms in the case of overtravels. In change-over mechanisms provision should also be made for devices that will not allow simul¬ taneous engagement. Figure 54a show's the hand drive of directional control valves. The conditions of operation require that each valve be turned only when the other one is in a definite position. This is done by means of lock pin 1 controlled by disks 2 rigidly attached to the actuating handles. When handle 3 is turned, handle 4 is held in place by the 2.4. External Appearance and Finish of Machines 57 lock pin. Handle 4 can only be turned when handle 3 is in a defini¬ te position. An interlocking device widely applied in gearboxes in which the- gears are shifted by means of selector bars is illustrated in Fig. 54b. Bar 5 can only be moved when bars 6 and 7 are locked, bar 6 can be shifted when bars 5 and 7 are locked, and bar 7, when bars 5 and 6 are locked. Thus, this device allows for each gear to be engaged only when all the other gears are brought out of mesh. The problem can often be solved by introducing mechanical links between the elements to be shifted, the drive being effected in a centralized manner by means of a single handle {single-handle control). The design of hand-operated push buttons should be such as to prevent accidental switchings. Protruding push buttons (Fig. 55a) are not safe because they may be accidentally depressed. Sunk buttons (Fig. 555) are the best design. 2.4. External Appearance and Finish of Machines The machine as a whole and its structural elements must have smooth outlines. This is a very important pro¬ vision for facilitating the maintenance of the machine and keeping it tidy. Undesirable are high ribs, sharp corners and cavities which accumulate moisture, dirt and dust, making it difficult to wipe and wash the machine. It is more practicable to replace outer ribs (Fig. 56a) by inner ones (Fig. 56 b). Fasteners should never be arranged in recesses (Fig. 56c). It is better to place them beyond the surface of the faste¬ ned part, (Fig. 56d). Figure 56 e shows-a poor design of a trough-shaped lug. It is diffi¬ cult to clean the trough of dirt accumulating between the ribs. A better design closed on the top is presented in Fig. 56/, but the closed box-shaped design shown in Fig. 56g is the best. 58 Chapter 2, Convenience in Maintenance and Operation Recessed covers (Fig. 56/t) should be avoided. Flat (Fig. 56i) or slightly convex (Fig. 56;') covers are more preferable. In the sight glass fastening (Fig. 56ft) the protruding heads of the bolts impair the general appearance and make it difficult to wipe the glass clean. The design in Fig. 561 is better because the holts are replaced by countersunk screws. In the best design shown in Fig. 56m the outer surface is smooth and the glass frame is fastened from the internal side of the housing by means of studs resistance- welded to the frame. Aesthetic aspects are as important. Smooth, streamlined contours are undoubtedly pleasant to the eye. The aesthetic aspect of a machine in the first place is determined by its engineering reasonableness. When a rational compact layout — 1 ■ ■ 1 m Ml ■ 1 1 1 1 (a) (b) Fig. 57. Machine housing shapes a —irrational, b —rational is combined with an effective power scheme the machine always have a beautiful appearance. A machine with slap-dash units, with open operating members, with openings and hollows between the ■structural elements loses much in its appearance. For all their compact layout and smooth external appearance machines should never take the form of mere box-like structures. It is expedient to adhere to a definite architectural pattern agreeing with the shape of the machine and accentuating its general hori¬ zontal or vertical design. Such a pattern can be produced by using cornices, ribs, abutting welts, etc., emphasizing in relief the prin¬ cipal structural elements. A machine with a box-like shape and having smooth joints (Fig. 57a) produces an impression of a heavy block of metal. The machine assumes a much lighter and well-proportioned appearance if the alternating horizontal components are made of slightly different length and width with welts along the contour of the joints (Fig. 576). The welts have not only a decorative, but also a practical purpose. They ■can be filed to correct casting inaccuracies and match the contours of the con¬ tacting surfaces. 2.4. External Appearance and Finish of Machines 59 It is advisable to enliven long surfaces, panels and shields by a simple and austere reliefed pattern that conforms to the shape of the machine, for example, in the form of parallel ribs directed ho¬ rizontally or vertically, depending on the general design of the ma¬ chine. Besides, reliefs increase the rigidity of the shields. Much attention should be given to the arrangement, external appearance and finish of control members. They should be mounted near the operator’s station, in a place convenient for manipulation and inspection, and, as far as possible, on a single panel. It is good practice to polish metal parts or coat them with chromium or colou¬ red enamels. Glittering coatings (decorative chrome plating) should be avoided because they fatigue and even blind the eyes with bright illumination. Lustreless chrome plating is most effective. All sorts of trade marks, tables indicating the parameters, diag¬ rams, etc., should be imprinted on massive plates in clear and large characters by means of phototype or engraving (but not punched on thin tin sheets). The plates should be positioned in a place convenient for reading, and, if necessary, illuminated (if installed in recesses or boxes). A beautiful finish of a machine will without any doubt make the personnel treat it with greater care. A machine should never be excessively beautified. Abundant glittering surfaces, diversity of colours, bright and flashy hues in the finish will impair the external appearance of the machine. The finish of a machine should be technically justified, correspond to the functional purpose of the parts and make control and servicing easy and convenient. The forms should be simple and austere, and the colours, serene. It is good practice to paint machines operating in enclosed pre¬ mises with light colours (pale blue, light green, light grey) which possess a higher reflection coefficient and intensify the illumination of the premises. Where sanitation is the prime demand (food indust¬ ry, medicine) preference should be given to milky-white or ivory colours. Machines operating in open air and subjected to the action of dust, soot, exhaust gases, etc., should preferably be painted with dark colours. A coating should be durable and wear resistant, proof against atmospheric effects, possess good adhesion to metal surfaces and reliably protect the metal against corrosion. Oil varnishes are now being ousted by new, more stable synthetic coatings (nitrocellulose enamel, escapone varnishes, alkide, phenolic and {epoxy coatings, etc..). Organo-silicon coatings are the best. They effecti¬ vely repel water, dust and dirt and are stable against light and heat. Chapter 3 Designing Cast Members Casting is widely used for making shaped parts ranging from small elements to very large beds and housings. In many machines- (internal-combustion engines, turbines, compressors, metal-cutting machine-tools, etc.) the weight of cast parts comes to 60-80 per cent of the total machine weight. Casting can produce most intricately shaped parts which cannot be made by any other forming method. The casting process is highly productive and inexpensive. Characteristic of cast parts are reduced strength, differences in mechanical parameters between their different portions and liabi¬ lity to the formation of internal defects and stresses. The quality of a casting depends on its design as well as manufacturing process. For this reason the designer must know the basic casting practices and the methods for obtaining high-quality castings at the minimum production costs. The following casting methods are commonly in use. Sand mould casting. This is the most widespread and universal method and practically the only one used to make large-size castings. Moulding is done- to wooden or metal patterns in flasks packed with sand-clay mixtures. The internal cavities in castings are formed by means of cores moulded from sand mixtures with hinders in core boxes. The dimensional accuracy of a casting depends on the quality of the mould manufacture and properties of the casting alloy (average deviation from nomi¬ nal dimensions is rfc7°/oo). The surface finish is within class 3-4. The efficiency of the casting process and tho quality of castings arc appreciably improved by using mehcanical moulding when the flasks are packed with squeeze moulding, jolt moulding and sand-throwing machines. Critical and large-size parts are cast in core moulds the external and internal surfaces of which are formed by blocks of cores connected mechanically or by bonding. Shell mould casting. The moulds in the form of shells 6-15 mm thick are prepared to metal patterns from a mixture of sand with a thermosetting resin (bakelite) which is then set by heating to 150-35Q°C. This method is mainly employed'to cast open (through- or cup-shaped) parts with a size of up to one metre. The dimensional accuracy is ;±5°/cc and ike surface finish, up to the 6th class. Chill casting. Metal is poured into permanent iron or steel moulds (chills). For; small-size and nonferrous castings the internal cavities are formed by metal 3.1. Wall Thickness and Strength of Castings m cores and in tlie case of medium- and large-size castings, by sand cores (semi¬ permanent mould casting). This method provides for increased strength of the castings, an accuracy of d=d°/oo and surface finish of up to the 6th class. Centrifugal casting. This method is utilized to cast hollow cylindrical com¬ ponents such as pipes. Metal is poured into revolving cast-iron or steel drum¬ shaped moulds where it is compacted by centrifugal forces. The casting accuracy (wall thickness) depends on the accuracy of metering the metal feed. Small parts are cast on centrifugal machines with permanent metal moulds. Investment casting. Patterns are made of easily fusible materials (paraf¬ fine, stearine, wax, colophony) by pressure casting into metal moulding dies. The patterns are joined into blocks, coated with a thin layer of a refractory material (quartz powder with ethylsilicate or liquid glass) and moulded into unsplit sand moulds w r hich are then heated to 850-900 a C with the result that the patterns are completely removed. The remaining cavities are filled with metal at normal pressure or under a pressure of 2-3 atm. Thi3 method is used to cast small- and medium-size parts of arbitrary shape. The high dimensional accuracy (±2°/oo) and surface finish (up to the 7th class) in many cases make it possible to dispense with subsequent machining, and for this reason this method is frequently applied for making parts from difficult- to-machine materials (for example, turbine blades from heat-resistant alloys). Cavityless (full-form) casting. Patterns of foam polystyrene (density 0,01- 0.03 kgf/am 3 ) are moulded into unsplit sand moulds. When metal is poured in, the patterns are gasified, the vapours and gases escaping through the overflows and ventilation holes. Sublimation {heating t.o 300-450°C without access of air) and dissolution of the pattern in dichloroethane or benzene are another two methods employed to remove the moulded patterns. The lull-form casting makes it possible to obtain accurate castings of prac¬ tically any shape. Pressure die casting. Metal is poured into permanent steel moulds under a pressure of 30-50 atm. This method is highly productive and ensures accurate dimensions (±l°/oo) and good surface finish {up to the 8th class), and generally does not require any further machining. The method is used for the mass pro¬ duction of small- and medium-size parts predominantly from easily fusible alloys (aluminium, copper-zinc, etc.). The moulding dies for steel and iron ■castings must be manufactured from heat-resistant steol. Now we consider the most widespread method —sand mould casting. Many of the design rules for sand castings are also applicable to castings obtained by other methods. 3.1. Wall Thickness and Strength of Castings The walls of cast members feature unequal strength in their cross section because of the different conditions of crystallization. The strength is the highest in the surface layer where the metal, as a result of the increased cooling rate, gets a fine-grained structure and where residual compressive stresses favourable for the strength de¬ velop. In the surface layer of iron castings there prevail pearlite and cementite. The core which solidifies at a slower rate has a coarse¬ grained structure with the predominance of ferrite and graphite. Dendritic crystals and shrinkage cavities and porosity often deve¬ lop in the core. The thicker the wall, the greater the difference in strength between 62 Chap ter 3. Designing Cast Members the core and the skin. For this reason an increase in the wall thickness is not accompanied by a proportional increase in the strength of the entire [casting. The dependence of strength on the sample diameter is illustrated in Fig. 58. For these reasons, and also to reduce weight, it is advisable to make the casting walls to the minimum thickness permitted by casting conditions. The required rigidity and strength should be ensured by ribbing, using rational profiles, and imparting convex, vaulted, spherical, conical and similar shapes to castings. This always results in lighter structures. The quality of the shape of a casting may be approximately estimated by the ratio of its surface to the volume, or when the length is known, by the ratio of its perimeter S to cross section F 0 = 4 - (3M> 10 20 JO MO SO Bmm ... roc, ,i i .. i, Figure 5 Sa-c specifies the values of £i Hg. 58. Strength oi casting alloys for J veral eql / va i ent sections with different wall thickness. Massive shapes (Fig. 59a, b) are impractical as to their strength and weight. Thin-walled shapes greatly developed on the periphery (Fig. 59c) are the correct casting shapes. 3J2. Moulding 63 Figure 59d shows irrational designs of cast parts in the shape of massive castings while their rational designs in the shape of thin-walled struct'™" are presented in Fig. 59«. The machining of cast parts should he minimized not only to reduce the manufacturing costs but also for strength considera¬ tions. The machining results in the removal of the strongest surface layer from the casting. The surfaces to be machined are reinforced by making the adjacent walls thicker. 3.2. Moulding The design of a casting must ensure simple and convenient mould manufacture. This condition is broken down into the following particular ones: (a) the pattern must be easily extractable from the mould; (b) the cores must be easy to mould in core boxes; (c) the shape and fastening of the cores must not hamper the as¬ sembly of the mould. (a) Elimination of Undercuts A pattern can easily be removed from the mould if its surface carries no undercuts — projections or recesses perpendicular or inclined to the direction of withdrawal — which are liable to cut off some portions of the mould when the pattern is extracted. A scheme of undercutting is illustrated inJFig. 60a. The part has inclined ribs. When the pattern is withdrawn (the withdrawal direction is shown by the hatching perpendicular to parting plane A-A of the mould) the ribs cut off the mould portions shown bla¬ ckened in the drawing. The undercutting can be eliminated if the pattern portions hampering the extraction are made detachable or movable. Before the pattern is extracted these portions are taken away or drawn inside the pattern, after which the pattern can freely leave the mould. In another method the pattern is made so that the portions subject to undercutting are completely filled. This pattern takes the form shown in Fig. 60b. The required shape is obtained by €4 Chapter 3. Designing Cast Members means of cores installed in the mould after removing the pattern (Fig. 60c). All these methods make moulding more complicated and expen¬ sive. It is better to shape the part so as to exclude undercutting. When the ribs are parallel to the pattern withdrawal direction (Fig. 60tf) the pattern can easily be taken out of the mould. When designing a casting one must have a clear idea of the arran¬ gement of the parting plane and the position of the part in the mould during pouring. As a rule, parts are cast with critical surfaces down, Fig. 61. Undercuts in moulding the bosses (d: m (f) Fig. 62. Elimination of undercuts since the metal in the lower portions of the casting is denser and better than in the upper portions. After establishing the parting pla¬ ne, all the elements of the design must be inspected in succession and undercuts eli m inated. The rule of shadows is helpful in this case. Imagine that the part is illuminated by parallel rays normal to the parting plane (Fig. 60a). The shadowed portions show the presence of undercuts. Figure 61a presents examples of undercuts when moulding bosses (the direction in which the pattern is extracted is shown by arrows). Figure 616 shows how the undercuts can be eliminated. Examples of typical undercuts and the methods for their elimina¬ tion are presented in Table 2. Undercuts are not always seen clearly on drawings and can easily be overlooked by the designer. An example of an unapparent under¬ cut is presented in Fig. 62a (the unit is shown in the moulding po¬ sition; the parting plane is designated by the letter A). The box fillet forms a dead volume (shown blackened in the dra¬ wing) in the bottom half mould. This corner can be moulded if the vertical wall of the box is conti¬ nued to the parting plane (Fig. 626), or if the parting plane is trans¬ ferred to the section where the fillet merges with the wall. In this case the lug must be extended to the parting plane (Fig. 62c). 32. Moulding 65 Table 2 Elimination of Undercuts _ . . , . Corrected design and I . , , Corrected design and Original design method of elimination [ Original design method of elimination Handwheel Changing the shape of the part Pipe connection Arranging the flange axes at right angles Tubular part Housing Eliminating the flange by changing over from the bol¬ ted to studded fas¬ tening Housing Extending the bosses to the hous¬ ing topj Changing the boss shape Fan impeller Eliminating the blade overlap Enlarging the in¬ ternal cavity of the housing Housing Extending the bosses to the hou¬ sing bottom Handwheel spokes Turning tl spoke 1-seetioa through 90° I Merging the bos- | ses together 5—01658 66 Chapter 3. Designing Cast Members Table 2 (continued) In the cup-shaped part (Fig. 62 d) the surface of recess m is too close to the adjacent rough wall. The machining allowance n provided in the pattern (Fig. 62s) forms an undercut (blackened portion). The undercut can be elimi¬ nated, if the recess is deepened with respect to the rough surface by the machining allowance value (Fig. 62/). (b) Mould Parting The parting of moulds along inclined or stepped planes should be avoided as this complicates the mould manufacture. Fig. 63. Elimination of stopped parting of a mould A stepped parting is required to mould a lever with offset T arms (Fig. 63a). The moulding will be easier, if the arms are arranged in one plane (Fig. 63f>). 3.2. Moulding 07 The moulding of a curved pipe connection (Fig. 6da) can be simpli¬ fied, if the connection axis is made straight, Iho position of the attachment points being slightly changed (Fig. 64b), or even kept unaltered, if necessary (Fig. 63c). Fig. 64, Moulding of curved pipe connections Figure 64 d-f illustrates a change in the design of the outlet con¬ nection of a centrifugal pump. The most rational design is that in Fig. 64/, which simplifies the casting process and reduces the hyd¬ raulic losses in the pump because the fluid flow turns only once, and not twice as in the designs shown in Fig. 6d(f and e. (c) Open Castings. Cored Castings Open castings should preferably be moulded to patterns without the use of cores. In this case the pattern is shaped so as to conform Fig. 65. Moulding of internal cavities accurately to the shape of the final product. When a pattern is moul¬ ded a negative imprint of the cavity (cod) is obtained. This method can only be used if there are no undercuts on the internal surface of the part. An example of internal undercutting is schematically shown in Fig. 65a. The part has a flange projecting into the cavity. When the pattern is removed the cod is damaged. 5 * 68 Chapter 3. Designing Cast Members In the presence of internal undercuts the use of cores is the only way of moulding the cavity. In this case a solid pattern leaves in the mould the impression shown in Fig. 65b, The internal cavity is formed by means of a core (Fig. 65c). Fig. 66. Cored and careless moulding a }, cover; c, d—'bracket: e, f—lever; g. A—housing; di-adapter; h, I-rotor, m,n—bearing shell The part can easily be modified to suit coreless moulding by placing the flange on the outside (Fig. 65d). Examples of adapting standard parts to coreless moulding are illustrated in Fig. 66. The requirements of simple and inexpensive production do not alway coincide with the demands for the proper strength and rigidity of parts and their convenient operation. The open design of a cover (Fig. 66b) is simpler to manufacture than the design in Fig. 66a, which requires core moulding. F>ut the design shown in Fig. 66a has a more attractive appearance. I The open design of a rotor (Fig. 661) is simpler and can be made at a lower cost. But the box-like design shown in Fig. 66*, that requires the use of cores, is much stronger and sliffer. In other cases, conversely, a less expensive desigD proves stronger and more convenient. Thus, the bearing body cast without cores (Fig. 66 n) is stronger and more attractive than the one cast with cores (Fig. 66m). The moulding of internal surfaces by means of cods is limited by the maximum permissible height of the latter. With the usual composition of moulding mixtures the height of bottom cods should be H < 0.85 and that of the top ones, h < 0.3s where 5 and s are the 3.2. Moulding 69 mean cross sections of the cods, respectively {Fig. 67). In the case of reinforced moulds (moulds made from mixtures with bentonite or binders, skin-dried moulds, chemical-set moulds, etc.), and also when the moulding is done mechanically, the height of the cods can be increased by 30-50 per cent as against the above values. The designs of cast elements should be devoid of narrow cavities, deep pockets of small cross section, etc. (Fig. 68a). Such cavities Fig. 67, Determining the height of Fig. 68. Strengthening of weak ele- cods meats of a mould are poorly filled with the moulding mixture and form in the mould weak pillar- or band-type protrusions m which crumble when the pattern is extracted and are easily washed away by the pressure'of liquid metal. The methods of eliminating these faults are illustra¬ ted in Fig. 685. ( d ) Cores When designing internal cavities the core should be given such a shape as ensures its easy extraction from the core box. Fig, 69. Moulding a core Figure 69a illustrates a core used to form in a part a cylindrical cavity with internal ribs. The shape of the core allows the box par¬ ting to be made in plane ^1-^1 only (because of the presence of annu¬ lar rib m in the cavity). The ribs form undercuts in the box. In such 70 Chapter 3. Designing Cast Members cases the cores have to be made up of separate parts bonded toge¬ ther, which complicates the core manufacture and reduces the casting accuracy. In the good designs shown in Fig. 69 b arid c the ribs are arranged in the parting plane or perpendicular to it, and the core is easily extracted from the box. Particular difficulties arose when moulding cores for structures with skew axes. Figure 70a shows a manifold in the form of cylindrical header m with drop-shaped branches n the axes of which are offset with respect to the header axis. In this design the core cannot be moulded. With any arrangement of the core box parting plane—horizontal (plane A-A, Fig. 706), vertical (plane B-B , i Fig. 70. Moulding a drop-shaped cylindrical header Fig. 70c) or, the more so, inclined—undercuts are formed (shaded and blackened portions on the drawing). Undercuts are also formed when the pattern is moulded into a mould parted along plane A-A (Fig. 7(W). The mould cannot bo assembled. The core makes it impossible to join the top and bottom half moulds (sections o, p in Fig. 70e). Bringing the header and branch in line (Fig. 70/, e) makes it possible to mould the core in a box parted along plane A-A or B-B. The pattern can be moulded in if the mould parting passes through plane A-A. If the skew axes are to be maintained the shape of the manifold should be changed according to Fig. 70A. In this case the core can he moulded if the core 3.2. Moulding 71 box parting is in plane A-A or B~B, and the pattern moulded in if the mould parting is in plane A-A. In the design shown in Fjg. 70 i the branches are given a rectangular cross- section, The core and the mould can be made with the parting along plane A-A or along any other plane passing along the branches and arranged within the limits oi the straight portion of the side walls of the branch. In this case the manifold retains its assigned shape. (e) Installation of Cores in a Mould The shape of the internal cavities in a mould must permit the easy installation of cores. The design of the drop-shaped manifold in Fig. 70eis an example of a mould that cannot be assembled. Figure 71a shows the head of an internal combustion engine with a spark plug well formed by a sus¬ pended core 7. When assembling the mould, the core installed in the top half mould comes (in section m) against core 2 forming the water jacketof the head and installed pre¬ viously in the bottom half mould. In the correct design (Fig. 716) the well is shaped so that the top half mould can be easily mounted. (/) Escape of Gases The design of internal cavities should permit the escape of the gases evolved from the cores when the metal is poured in. (a) (b) (O Fig, 72. F.scape of gases from a core An unsatisfactory design is illustrated in Fig, 72a. The gases ac¬ cumulating in the upper part of the core form blowholes in sec¬ tions m. Provision should be made for holes n (stopped up afterwards) for the escape of gases (Fig. 726). The vaulted shape of the upper portion 72 Chapter 3. Designing Cast Members of the casting (Fig. 72c) ensures the escape of gases through the top core print. Blowholes can be prevented by using core mixtures with a low gas formation. (g) Band Cores Slender cores are usually reinforced with a wire frame in order to increase their strength. When removing the core from the casting the frame has to be taken out, and this limits the minimum cross section of the core and calls for a well thought-out arrangement of holes for core prints. For castings of small and medium size the thickness of cores reinforced with wire should be at least 6-8 mm. The core thickness can be reduced to 5 mm in local nicks. The width of cavities should be not less than b = S -f- s where S and s are the thicknesses- of the walls forming the cavity (Fig. 73). It is better to make the cores as thick as the overall dimensions of the castings permit it. ( h) Unification of Cores When designing castings with several cores of about the same shape, it is advisable to unify the cores in order to shorten their type list. An example of the unification of cores for the crankcase of an in¬ line reciprocating engine is shown in Fig. 74. In the design in Fig. 74a Fig. 73. Determi¬ ning the minimum width of core-mo¬ ulded cavities Fig. 74. Unification of cores the internal cavities of the crankcase are formed by cores 1, 2 and 3 of three different types. A slight change in the shape of the rear crank¬ case wall (Fig. 74h) makes it possible to reduce the number of the core types to two (7, 2). 3 2. Moulding 73 The number can even be reduced to one (Fig. 74c). However, this entails the shortening of the middle crankshaft bearing which in engines of this type is loaded more heavily than all the other bearings and therefore must be longer than they are. In the design in Fig. lAd all the large cores are unified and the middle bearing is made longer by means of an additional core m that imparts a box-like shape to the middle crankcase partition^ (i) Fastening of Cores in a Mould In castings with open lower cavities the cores are installed with their bases in the bottom box (Fig. 75 a). The cores forming the upper cavities are suspended in the top box from an inverted cone (Fig. 755) (a) (bJ (c) (d) Fig. 75. Installation of cores or from a wire (Fig. 75c) attached to a bar resting against the box. It is good practice to make the top core rest upon the bottom one through a hole in the horizontal wall of the casting (Fig. 1 75d). (a) U) (m) (n (o) (p) Fig. S3. Methods of stopping up the casting holes until the last thread is forced into the threaded hole (Fig. 83 h). The threads are coated with sealing compounds. Heat-resistant compounds (siloxane enamel) are used for parts operating under high tempera¬ tures. To improve the external appearance, the tightening means on the plugs are usually made sunk (Fig. 83c and d). The tightening hexagons and tetrahedrons arc cut flush with the hole edges after screwing the plugs home (Fig. 83b). Large-size or shaped ports are closed with bolted plates or cast covers (Fig. 83g). Screw plugs are locked by embossing or flaring (Fig. 83l). In castings made of plastic metals (steel and non-ferrous casting) the plugs are secured by rolling in the casting surface (Fig. 83/). Centre holes in must be provided in the plugs to centre the rolling tool. 78 Chapter 3. Designing Cast Members Spherical deformable plugs (Fig. 83A") are made of plastic low- carbon steel. During installation the plug is flattened and its edges cut into the walls of the hole, forming a strong and tight seal. Use is also made of plugs flared cither from the outside (Fig. 831) or from the inside (Fig. 83m). In steel members the plugs are fastened by soldering or welding (Fig. 83»)- The plugs can also be fixed in place with epoxy adhesives (Fig. 83o and p). Out of all these methods preference should be given to designs requiring minimum machining, for example, in the case of small holes, to screw plugs with a taper thread. A smooth surface is very important for the holes arranged on the outside. Recesses and pockets which accumulate dirt are undesirable (Fig. 83e, /, l and n). In this respect the designs in Fig, 83m, o and p are preferable. 3.3. Simplification of Casting Shapes The shape of castings should be simplifed to reduce the costs of production and increase the casting accuracy. The outlines of parts and inner cavities should be formed by simple straight lines, circular arcs, etc. The bracket shown in Fig. 84) i.e., volume shrinkage is about three times greater than linear shrinkage. Shrinkage is one of the main casting properties of a material and, alongside other properties (castability, thermal capacity, heat con¬ ductivity, oxidability, liability to segregation), shows whether the given metal is suitable for casting. The smaller the shrinkage, the higher the dimensional accuracy of the casting and the less the hazard of shrinkage stresses, cavities, cracks and warpage in the casting. The linear shrinkage values for the main casting alloys are as follows: Material Linear shrinkage, % Phosphoric iron 0.7-0.8 Grey iron 1.0-1.2 High-tensile cast iron 1.5-1.8 Carbon steel 1. 8-2.0 Alloy steel 1.8-2.5 Phosphor bronze 0.6-0.8 Tin bronze 1.3-1.6 Aluminium bronze 2.0-2.2 Aluminium-copper alloys 1.4-1.5 Aluminium-magnesium alloys 1.2-1.3 Aluminium-silicon alloys 1.0-1.2 Magnesium alloys 1-5-1 .7 3.7. Internal Stresses 83 These figures refer to the case of free shrinkage determined from samples cast in open horizontal moulds. The actual shrinkage depends on the resistance to the contraction in the dimensions of the casting offered by the internal por¬ tions of the mould ( restricted shrinkage). With rigid cores, shrinkage may be 30-50 per cent less than free shrinkage, but in this case higher shrinkage stresses develop in the casting walls. The shrinkage of a casting is taken account, of by correcting the dimensions of the mould, using for the manufacture of patterns and core boxes shrinkage ( patternmaker's ) rules with dimensions increased by the amount of shrinkage as compared with normal ones, 3.7. Internal Stresses Internal stresses arise in the casting walls whose shrinkage is restricted because of the resistance of the mould elements or the action of the adjacent walls. Shrinkage cavities and porosity appear in those parts of the casting that solidify last, i.e.,in thick and solid portions from which heat withdrawal is difficult ( hot spots). Increased internal stresses make the casting warp and may lead to the development of cracks. In the course of time, internal stresses are redistributed and partly disper¬ sed as a result of slow diffusion processes ( natural ageing). After two or three years the part changes its original shape, which in precision machines (metal- cutting machine tools, for example) is impermissible. Shrinkage stresses develop only during those stages of cooling when the metal loses its plasticity (within 500-600°C for cast iron and 600-700°C for steel). At higher temperatures the change in dimen¬ sions is compensated for by the plastic flow of the metal and the shrinkage manifests itself only in the thinning of the walls. In the box-shaped casting of length L and width I (Fig. 92a) the internal partition (shown black in the drawing) cools at a slower rate than the horizon¬ tal walls. Assume that at the given moment the partition has a temperature corresponding to the temperature at which the metal passes from plastic into elastic state, and the walls have a lower temperature < 2 at which the metal is already elastic. While cooling further, below t lt the partition material hardens and, con¬ tracting, undergoes tension. Since the contraction occurs in two directions (along axes x and y), by the end of cooling, biaxial tensile stresses develop in the partition and compressive stresses of reaction, in the walls. If, conversely, the partition temperature at the initial moment is below the temperature of the walls (Fig. 926), by the end of cooling, biaxial compres¬ sive stresses will arise in the partition and tensile stresses, in the walls. As a rule, the portions of a casting which cool first undergo compression , and those cooling later are subjected to tension. Let us find the shrinkage stresses for the case when the partition cools later (see Fig. 92a), considering deformation along axis x only. By the end of cooling, the partition would have shortened by the amount J.j = al (t 1 — ( n ) and the walls, by a smaller amount = al (( 2 — /„), where l is the length of the walls along axis z, and t a is the final temperature. The dif- 6 * 84 Chapter 3. Designing Cast Members fecenee = Ai — — E (tj fg) determines the magnitude of the stresses in the casting. ftM-J Fig. 92, Appearance of shrinkage stresses According to Hooke’s law PI Pt k.'k^al (ti — 1 2 )= + E p^ where P = force developing in the system E = mean modulus of elasticity within the temperature range t 1 - F, and F 3 = cross-sectional areas (normal to axis x) of the partition and walls, respectively (F l = s x L, F 2 — 2s „L) Force P is Ea{ti~h) X,_L Ft ^ Ft The tensile stress In the partition «‘=T7 = - Ea (ti — to) <+£ The compressive stress in the walls P E Walls should never be connected at an acute angle (Fig. 95 k). If this is inevitable, the conjugation radius should not be less than (0.5 to 1) % Figure 951, m illustrates the recommended ratios for T-cormoct ions, and Fig. 95n, o, for connections of walls with flanges. Walls of different thickness (Fig. 96a) should be connected by wed¬ ge-shaped transitions with tapers of from 1 : 5 to 1 : 10 (Fig. 965- Fig. 96. Joints between casting sections of various thickness and c). It is good practice to reinforce the transition section with ribs (Fig. 96b). Figure 96 e-p illustrates connection of walls -with bosses. In the- profile projection the bosses are linked with the walls by radii R = 2s (Fig. 96e and t) or by tapers of from 1 : 1 to 1 : 5 (Fig. 96/, g , /, k) reinforced with ribs (Fig. 96 h, l). In the plan projection, connection is made with radii R = (3 to 5) s (Fig. 96m-p). The radii found from these tentative ratios are rounded off to the nearest standard dimension (R = 1, 2, 3, 5, 8, 10,15, 25, 30, 40 mm). Since a slight change in the conjugation radii affects hut little the quality of casting, it is recommended to unify these radii. The predominant transition radius is as a rule not marked at each position on the drawing of a part, but is indicated in a drawing margin (or in specifica¬ tion) by an inscription such as: Unspecified radii 6 mm. In the case of curved external corners the main radius is indicated by an inscription, such as: Unspecified outer fillets R3. ( b ) Elimination of Massive Elements Cast members should be free from local metal accumulations, and thick, massive elements forming hot spots. When designing a casting, one must carefully examine all places of material accumulation and 90 Chapter 3. Designing Cast Members account for machining allowances which to a large degree affect metal distribution. Figure 97 shows how massive elements (designated by the letter m) in a cast fastening flange (Fig. 97a~c), mounting pad (Fig. 97d-/), frame (Fig. 97g-i) and engine jacket (Fig. 97/ and k) can be eliminated. Fig. 97. Elimination of massive elements Rapid cooling should be provided in the sections where massive elements are inevitable. It is useful to enlarge the surface of contact with the moulding mixture by ribbing the walls. To improve the filling of the mould, the connection of massive elements with the nearest walls should be reinforced with fillets (Fig. 98a), wedge-shaped transitions (Fig. 98£>), 3.10. Design Rules 91 bell mouthings (Fig. 98c) and ribs (Fig. 98d). It is advisable to use corrugated (Fig. 98e) and box-shaped (Fig. 98/) walls. Fig. 98. Reinforcing tile sections conjunct with bosses These types of connection improve casting conditions and increase the rigidity and strength of castings. (c) Reduction of Shrinkage Stresses The shape of castings should facilitate shrinkage. Figure 99 illustrates a large-diameter gear wheel whose rim is connected with the hub by spokes. The design with straight spokes Chapter 3. Designing Cast Members H2 ■ It is more expedient to use tangential (Fig. 99b), spiral (Fig. 99c) or conical (Fig. 99d) spokes. In a disk-type sheave with a massive rim (Fig. 100a) the disk solidifies before the rim and retards the shrinkage of the latter. Com¬ pressive stresses develop in the disk and tensile stresses in the rim. If the rim solidifies first (Fig. 100c) the disk, wTiile contracting, un¬ dergoes tension, and compressive stresses develop in the rim. In either case shrinkage stresses can effectively be diminished by making the disk conical (Fig. 100 b and d). In a cast frame (Fig. 100c) the partitions m located in one plane with massive flanges retard the shrinkage of the latter. The shrinkage conditions will somewTiat be improved if the partitions are displaced from the plane where the flanges are arranged (Fig. 100/). But most advisable it is to make the partitions conical (Fig. lOOg) or spherical. Vaulted and convex shapes reduce shrinkage stresses, improve casting conditions, and increase the strength of parts because the resisting moments of cross sections of such shapes are greater. The rigidity of structures is also enhanced, which is especially important for castings made of alloys vrith a low modulus of elasticity (grey iron, light alloys). 3.10. Design Rales 93 (d) Prevention of Blowholes The shape of a casting must provide for the rising of nonmetallic inclusions and the escape of gases which emerge as the casting cools down, because of their solubility in metal decreasing with tempera¬ ture. When a sump is cast with its bottom up (Fig. 101a) gas bubbles accumulate at the tops of the ribs and appreciably weaken them. It is better to make the bottom inclined and transfer the ribs onto Fig. 101. Escape of gases the internal surface (Fig. 1016). Such parts are recommended to be cast with the ribs down (Fig. 101c). In this case the blowhole porosity is concentrated in the riser on the flange, -which is removed in subse¬ quent machining. Casting with the inclination of the mould is like¬ wise used. For cylindrical parts {Fig. 101d) it is good practice to make the upper walls conical (Fig. 101c) or slightly spherical (Fig. 101/). In disk-shaped parts (Fig. lOlg) the disks and ribs should be inclined (Fig. lOlfe, t). The internal partitions (Fig. 101/) should preferably be vaulted. Gas bubbles and nonmetallic inclusions can best of all be with¬ drawn by means of lugs (Fig. 101/c) or bosses (Fig. 101 i) in theupper part of the partitions, or with the aid of risers (dashed lines). Casting under vacuum and addition of gas absorbing substances (cerium) to the casting metal are the process methods used to prevent blowhole porosity and cavities. 94 Chapter 3. Designing Cast Members (e) Rimming The external outlines of cast parts are usually rimmed (Fig. 102a, b) to obtain proper rigidity, uniform solidification and prevent chilling spots (in iron castings). In parts joined by their end faces (Fig. 102c) the rims help to uni¬ formly distribute tightening forces. With such rims it is much (cz) T (b) (C) (d) (?) Fig. 102. Rimming easier to remove irregularities and projections formed in joints due to inaccurate casting, and to match the outer contours of the joints. As a mle, the lightening and process holes in the casting walls (Fig. 102d and e) should be rimmed to increase the strength of the casting and improve its cooling conditions. Approximate dimensions of rims are given in Fig. 102a and d. (/) Flanges The thickness of flanges to be machined on one side (Fig. 103a) is made equal to (1.5 to 1.8) s, on the average, and that of flanges to be processed on two sides (Fig. 1036), (1.8 to 2) s, where s is the thickness of optr] | v —k the adjacent wall. In order to increase their strength p-r t K and rigidity, flanges are connected j , j I - with walls by ribs (Fig. 103c) or are I \ J 1 ) Fig. 103. Determining the thickness of Fig. 104. Elimination of heavy flanges sections in flanges made box-shaped. Methods of eliminating heavy sections in thick flanges are illustrated in Fig. 104a-c. 3.10. Design Rules 95 ( g ) Holes Long holes of small diameter should be avoided in castings. The minimum diameter of holes in castings may be found appro¬ ximately from the formula d = d 0 -f- 0.11, where l is the length of the hole in mm (Fig. 105). For aluminium alloys and bronze d 0 =5, for cast iron d 0 = 7 and for steel d n = 10 mm. Holes of smaller diameter are to be drilled. It is better to make long holes (such as oil ducts) by drilling or by casting-in tubes, or replace them with de¬ tachable pipelines. The shape of cast oil ducts and cavities should allow' their surfaces to be cleaned completely of burnt-on sand and other conta¬ mination. After careful cleaning the surfaces should be coated with oil- and heat-resistant compounds (bakelite or siloxane enamels). Fig. 105. Determining the diameter of holes- in castings (h) Ribs Ribs are used to increase the rigidity and strength of cast parts and to improve casting conditions. A rational arrangement of ribs improves the feed to casting elements and prevents shrinkage cavities and internal stresses. The shapes of ribs are illustrated in Fig. 106. Ribs arranged in a plane perpendicular to the mould parting should have casting drafts- Fig. 106. Shapes of ribs The thickness Sj of the rib at the top is its basic dimensional para¬ meter (Fig. 106) about twice. Dimensional deviations of horizontal surfaces occur as a result of inaccurate installation of cores in the vertical direction, because of foreign matter getting on the parting planes of moulding boxes and cores, etc. As a rule, the surfaces moulded in the bottom box are more accu¬ rate than the ones moulded in the top box. Surfaces moulded by the pattern are more accurate than those moulded by inner cores. Among the other causes of inaccuracy are the deviations in the dimensions of the pattern set from the nominal sizes, the change in the dimensions of cores upon drying, cracking of the patterns in storage, the charge in the dimensions of the mould occurring when rapping the patterns during extraction, variations in shrinkage due to the different pliability of cores, and warpage of the casting under the action of shrinkage stresses. Variations in the dimensions of castings are reflected in the sys¬ tem of machining allowances according with the USSR State Stan¬ dards TOCT 1855-55 (grey irons) and 2009-55 (steels). The amount of allowance depends on the accuracy grade and dimensions of Fig. 115. Appearance of inaccuracies in casting in two moulding boxes 106 Chapter 3. Designing Cast Members castings, the nominal distance of the given surface from the loca¬ tion, the position of the surface in pouring (bottom, top, side) and the type of casting alloy. Figure 116a-c illustrates averaged values of standard allowances for grey iron castings of various grades of accuracy, depending on the maximum overall dimension A of the casting for various distan¬ ces L of the surface from the location. The diagrams show allowances for upper surfaces of type m (Fig. 117) which have the maximum values because such surfaces are less accurate mainly due to the accumulation in the top stock of nonmetal inclusions, slag and other admixtures which are subject to removal in machining. The machining allowances for bottom surfaces n and side surfaces o are 20-30 per cent less than those for the top surfaces. Allowances for steel castings are 25-40 per cent greater than for iron castings. Variations in casting dimensions are especially important in sections where rough walls join work surfa- Fig. 117. Determining the amount ces. Machining accuracy is many of allowance times higher than casting accuracy. A cast part, may schematically be considered a rigid frame made up of work surfaces surrounded by a “floating” envelope of rough surfaces. Let us denote the magnitude of possible displacements of rough surfaces by the letter k. The following rules should be observed when designing castings: (1) protruding work surfaces must lie above the adjacent rough surfaces by the amount k (Fig. 118a), this preventing the tool from cutting into the adjacent rough surfaces (Fig, 1186); (2) sunk work surfaces must lie below the rough surfaces by the amount k (Fig. 118c) to allow a full reach by the tool (Fig. 118d) and prevent rough spots; (3) the thickness of walls adjoining work surfaces (Fig. 118e) must he greater by the amount k than the thickness m required by the •design. Otherwise, the walls may be impermissibly thinned should the cast surfaces he displaced (Fig. 118/). Figure 119 shows how these rules are applied to hubs (Fig. 119a), bosses (Fig. 1196 and c) and flanges (Fig. 119 d and e). Jointing pianos should be connected with the nearest rough walls by surfaces perpendicular to the machining plane, the height of such surfaces being not less than k (Fig. 120), otherwise the contour of the joint may be distorted. Fig. 118. Transitions between machined and rough surfaces Fig. 119. Transitions between machined and rough surfaces ; -assigned shape; £•—shapes that can be obtain¬ ed with casting errors; 3 —shapes accounting for the displacement h of cast surfaces 108 Chapter 3. Designing Cast Members Mounting pads on housing-type components (Fig. 121a) should be designed with reserve k over the contour (Fig. 121c) so that the moun¬ ted part does not overhang (Fig. 121b). The value of k depends on the accuracy aud overall dimensions of the casting and the distance of a given element from the casting and Fig. 122. Diagrams for determining the value of k (A is the maximum overall dimension of casting) a—iron casting; b ~steei casting machining locations as well, and, in the general case, is determined by calculating dimensional chains. Practical designing, however, requires a simpler method. The value of k can be found from the machining allowances (see Fig. 116) since the latter are determined by the same parameters as k (the maximum overall size of the casting, distance from the casting locations, grade of casting accuracy). To dispense with calculating the distances to the locations, the upper limits of allowances (dashed lines in Fig. 116) may be taken, which will go into the safety margin. Accounting for the fact that the diagrams give the maximum allow¬ ance values (for the top surfaces), a reduction factor of 0.7 should be introduced. Figure 122n, b shows the values of k calculated by this method for iron and steel castings of the 2nd and 3rd grades of accuracy. The values of k may be directly utilized to find the required distance between rough and workjsurfaces. The wall thickness of bosses may easily be found from the relation S = as where s is the mean wall thickness of the casting and a is a coefficient equal to 1.5, 1.7 and 1.8 for the 1st, 2nd and 3rd grades of accuracy, respectively. These relations practically assure against excessive reduction in the wall thick¬ ness. 3.13. Dimensioning 109 3.13. Dimensioning The dimensions of cast parts on drawings must specify the position of casting and machining locations, and also account for size devia¬ tions. The principal rules for the dimensioning of cast parts are as follows: (1) rough surfaces must be related to a casting location either directly or by means of other dimensions; (2) the initial machining location must be related to a casting location, all the remaining dimensions of the work surfaces being related to the machining location either directly or by means of other dimensions. Casting dimensions must never be related to the dimensions of work surfaces or vice versa, except for the case when the casting and machining locations coincide (the case of axial locations). These rules must be observed for all three coordinate axes of the casting. Dimensioning of a cast part, is illustrated in Fig. 123. Dimensioning accor¬ ding to Fig. 123a is wrong. The distance between the work surfaces, related to the rough surfaces by the sum of the dimensions 15, 175 and 10 mm, varies in this case within wide limits together with the variations in the dimensions of the rough surfaces. The same error is made in the design in Fig. 1236, where the distance bet¬ ween the work surfacos is specified by the sum of the dimensions 185 and 15 mm. When dimensioning is as in Fig. 123c the distance between the work surfaces (200 mm) is maintained within the necessary narrow limits (within the machi¬ ning tolerance). The error lies in that the rough surfaces are related to the adjacent work surfaces (dimensions 15 and 10 mm). It is practically impossible to maintain this coordination. The position of the rough surfaces varies within the casting accuracy limits, entailing variations in the distance to the work surfaces. 3.13. Dimensioning 111 In Fig. 123d the error is aggravated by the lact that the thickness of the upper horizontal wall (directly specified in the previous eases by the dimension 5 mm) is determined by the height of the inner cavity, which is related to the lower work surface (dimension 185 mm). This introduces one more source of inaccuracy. The thickness of the wall will vary within wide limits. In the dimensioning system according to Fig. 123? the position of the lower work surface is specified by two dimensions—one measured from the upper rough surface of the part (dimension 190 mm) and the other, from the upper rough surface of the flange (dimension 15 mm). It is practically impossible to preserve this coordination. Figure 123/ shows a correct system. The casting location is the top rough surface of the flange (blackened lozenge). The initial machining location (the bottom surface of the flange, marked by a light lozenge) is related to the casting location by the dimension 15 mm. The top work surface is related to the machi¬ ning location (dimension 200 mm). The top rough surface is coordinated from the casting location (dimension 175 mm), and from it, the thickness of the upper wall (dimension 5 mm ). The distance k between the top work surface and the upper rough wall is the closing link in the dimensional chain and serves as a compensator for the deviations in the position of cast surfaces. Since the value of k is not specified on the drawing, it is not accounted for when checking the part. It stands to reason that the nominal value of k must be larger than the maximum possible displacement of the upper wall caused by casting inaccuracies. Examples of wrong and correct dimensioning of cast parts are il¬ lustrated in Figs. 124 and 125 (wrong dimensions are given in squa¬ res). Chapter 4 Design of Parts to Be Machined Machining is one of the most laborious and expensive methods of manufacture and amounts to 70 per cent of the cost of a product. The principal production methods of increasing the machining efficiency "are as follows. 1. Reduction of machining time {intensification of cutting proces¬ ses), These methods include high-speed cutting (increasing the main cutting speed), heavy-duty cutting {increasing the cutting feed and depth), and high-productivity processing (machining with multipoint tools; internal and external broaching; turn-milling; etc.). 2. Reduction of handling time (the use of quick-acting appliances; automatic feed, mounting, fastening and removal of the blank; machining to preset operations; automatic readjustment of the machine set-up; and automatic control). Another form of this method is the consecutive machining of blanks in multistation fixtures. 3. Matching of process operations in time (proper sequencing of operation elements). This method includes machining with com¬ bination tools and multiple-tool machining (multi-cutter turning and planing; milling with a set of milling cutters). The method is most fully embodied in unit-head machine tools in which several surfaces of a blank areTmachined simultaneously. 4. Simultaneous machining of several blanks (parallel and paral¬ lel-consecutive machining in multi-station fixtures; continuous machining on rotary and drum-type machine tools and on vertical turret lathes). 5. Rapid transfer of blanks from machine to machine (mechanical transportation of blanks; rational arrangement of equipment). Automatic and semi-automatic transfer lines, especially those of rotary type, are most productive. Mass and stable production and all-round unification of designs with few models are requisite for the application of highly productive machining methods, special manufacturing riggings and special- purpose machine tools. When designing parts to be machined the labour required by the machining process should be reduced to the minimum, and high Chapter 4. Design of Parts to Be Machined 113 quality, reliability and durability of machines ensured at the same time. When designing parts for machining the following rules are to be observed: reduce the length of work surfaces to the design minimum required; decrease the machining allowances to the minimum; manufacture parts by the most productive methods which do not involve chipping (forging, cold upsetting, coining, etc.); widely use shape steel rolled stock, leaving most of the surfaces in the as-rolled condition; make parts from blanks having their shape as close as possible to that of the final product; use composite structures to make easier the manufacture of labour- consuming parts; avoid unnecessary precise machining. In each particular case use the lowest grade of accuracy ensuring proper functioning of the unit and meeting interchangeability requirements; provide for the use of the most effective machining methods (with calibrated multipoint tools, etc.); provide as far as possible for through-pass machining, which is the principal condition for increasing productivity and obtaining high-accuracy and finish standards of the machined surfaces; if through-pass machining proves impossible, ensure that the tool overtravel is sufficient 1o obtain well-finished and accurate surfaces; ensure convenient approach of the cutting tool to the work surfaces; make it possible to machine the maximum number of surfaces dur¬ ing one operation on one machine in a single setting and with one and the same tool; shape parts of repeated and mass application so as to make them suitable for group machining with the use of combination tools; provide for the machining of accurate coaxial and parallel holes in a single setting to obtain good alignment and precise centre dis¬ tances; assure a clear distinction between the surfaces machined in diffe¬ rent operations, by different tools and to different accuracies; provide for the distances between the work and nearest rough sur¬ faces which will make machining possible with the maximum varia¬ tions in the blank size; avoid joint machining of assembled parts, which disturbs the continuity of the production process, impairs interchangeability and makes it difficult to replace parts during operation; reduce the range of the tools employed by unifying the size and shape of the elements to be machined; in piece and small-lot production reduce the number of special cutting tools to a minimum, using standard tools as far as possible; 8—018 a 8 114 Chapter 4. Design of Parts to Be Machined impart to the work surfaces such a shape as will make the tool operate smoothly without impacts; relieve cylindrical multipoint tools {drills, reamers, counterbores, etc.) from a unilateral pressure in operation; impart to the portions to be machined a high and uniform rigidity ensuring an accurate machining to good finish and making for the use of efficient processing methods; provide convenient datum surfaces for size control with the use as far as possible of universal measuring tools. 4,1. Cutting Down the Amount of Machining The examples in Fig. 126 show how superfluous machining can be eliminated. In the fastening unit of a guideway (Fig. 126a) it is <9> W Fig. 126. Cutting down the amount of machining advisable to reduce the depth of the locating slot in the housing (Fig. 126&) to an amount sufficient for reliable locking. In cast parts (pit for a fastening bolt, Fig. 126c and d\ cover. Fig. 126e and /; liousing.Fig. 126g and h) the surfaces to be machined should be arranged above the adjacent rough surfaces. In an antifriction bearing unit (Fig. 126i) precision machining should be applied to strictly limited portions of the working surfaces (Fig. 126;)- Figure 126A and 1 shows the shortening of the press-fitted portion of a bushing in a housing, and Fig. 126m and n, the reduction of the centring portion of a dowel bolt. For parts made of round rolled stock the labour required for ma¬ chining and the amount of chips removed can be reduced mainly by decreasing the difference between the diameters of the parts, espe- 4.1. Cutting Down the Amount of Mackining 115 cially the largest diameters which determine in the first place the amount of the cut-off material. The shoulder on a stepped shaft (Fig. 127a) increases the diameter D of the blank and sharply increases the amount of the cut-off metal. The large difference between the step diameters requires in turn more machining. The volume of the cut-off metal amounts to 135 per Fig. 127. Parts made of round rolled stock cent of that of the final product. The coefficient of utilization of the material of the blank is 0.43, i.e., more than half of the blank metal is rejected as chips. In the shaft design without shoulder and with a smaller difference in the step diameters (Fig. 1276) three times less metal is removed as compared with the previous case, thanks to the smaller diameter D. Most of this reduction to diameter D i (80 per cent) is due to the eli¬ mination of the shoulder. The coefficient of utilization of the material is increased to 0.7. Figure 127c illustrates a further reduction in the amount of the metal removed, made on account of the part being manufactured from a cold-drawn bar with a diameter equal to the maximum diame¬ ter Z) 2 of the shaft. In this case the coefficient of utilization of the material is increased to 0.8. Examples of cutting down the amount of machining by reducing the maximum diameter of parts are illustrated in Fig. V21d-f (pressure screwy, Fig. 127g, h (tommy bar head), Fig. 127i, j (cap) and Fig. 127&, 1 (leg). The diameter of a product should correspond to the standard diameters of round rolled stock. The maximum diameter of a product should be less than the nearest standard diameter of the bar by an amount equal to the diametral machining allowance a. The value of a may be found from the ratio a = 5yl>L 8 * 116 Chapter 4. Design of Parts to Be Machined where D — diameter of the surface to be machined, mm L = length of the blank, mm b — coefficient equal in various types of machining to: i 1 Machining Total Operation rough finish allowance Turning 0.5 0.4 0.9 Grinding 0.2 0.1 0.3 It is better to make mass-produced fasteners from sized rolled stock, leaving the maximum possible portion of the blank surface un¬ machined. Figure 128a and b shows how labour input can be reduced by making a stud from a cold-drawn sized bar. The design of a hexagon nut with an annular collar (Fig. 128c) is unsuitable for mass production. Such nuts can be manufactured only by the piece. The nuts Fig. 128. Mass-produced fasteners made of rolled stock in Fig. 12 8d and e are made of hexagon bar steel. A cylindrical serrated nut in which the serrations come onto the surface of the cylin¬ der (Fig. 128/) is unsuitable for mass production because such nuts have to be milled individually. Correctly desig¬ ned nuts which can be manu¬ factured from cold-drawn sized bar are shown in Fig. 128g and h. Much less machining will be required if pipes are used to make hollow cylindrical parts. Figure 129a presents a hollow pillar made from solid bar sleel. The amount of machining will be less if the pillar is made of seam¬ less pipe and the internal surface left rough (Fig. 1296), and still less, if the collar diameter is reduced (Fig. 129c). Figure 129cf shows the shell of an antifriction bearing. It takes much labour to make part 1 (Fig. 129c) from a cylindrical blanks; besides, 85 per cent of the blank volume is wasted in chips. In Fig. 129/ the shell is divided into three parts. The side cheeks are made of plate steel and the middle portion, of thin-walled pipe. In mass production part 1 is preferably forged (Fig. 129g). When making machine parts by slitting cylindrical blanks (Fig. 129 h-k), the angular dimensions of the parts should be assigned 4.2. Press Forging and Forming 117 so as to fit an integral number of them within the blank circum¬ ference, with due regard for the slitting cutter thickness, thus making the maximum use of the blank. The parts in Fig. 129h and j are designed without allowance for the slitting cutter thickness s. Therefore in the first case about half Fig. 129. Manufacture of cylindrical parts, and in the second, one third of the blank is wasted. In Fig. 12 ( M and k the dimensions of the parts are selected with due regard for the slitting, and the entire bla nk is completely utilized. 4.2. Press Forging and Forming It is most advisable to make parts from blanks having their shape close to that of the final product, obtained by hot forging in closed- impression dies. This reduces the amount of machining and increases the strength of the part, thanks to the compaction of the metal, for¬ mation of fibre structure and fine equiaxial grains resulting from recrystallization which occurs as the blank cools down. All other conditions being equal, forgings are stronger and lighter, and require less machining than composite parts. The use of dies is economically justifiable in mass production where the initial investments in the manufacture of dies are rapidly recouped because of increased output and reduced machining. However, thanks to the high strength of forged parts, the method is often used in the manufacture of important machi¬ nes irrespective of the scale of output and manufacturing costs. 118 Chapter 4. Design of Parts to Be Machined The highest accuracy and surface finish standards are provided by cold sizing (coining) applied as a final operation after hot forging. In some cases coining completely dispenses with machining. Figure 130 illustrates methods of making a cup-shaped part (shown on the drawing by thin lines). Much labour is required to turn the part out of a cylindrical blank (Fig. 130a). Besides, the part is weakened because the metal fibres are cut. Figure 130f> shows a blank obtained by hammer forging in open dies with a profiled lower die and flat upper die; Fig. 130c and d illustrates the same blank made with the use of profiled lower and upper dies. When the blank is forged in a closed single-impression die (Fig. 130e) most of the surfaces take the final shape except for the surfaces to be machined. The hole is marked by recesses 1. The flash in the hole is removed by machining or subsequent forging operations. Forging in a finish impression (Fig. 130/) provides a higher wall accuracy and in this case smaller machining allowances can be as¬ signed. The partition in the hole is cut out by a punching die. A blank with pierced hole obtained on a horizontal forging ma¬ chine is presented in Fig. 130g. 4.3. Composite Structures 119 Cold sizing (coining) imparts the final shape to all surfaces (Fig. 130ft) except for the surfaces which require a most precise machining (seating hole, centring recess, end face of flange). Flat shaped parts are advisably made of plate material. The laborious contour machining of the part shown in Fig. 131a can be simplified by making the parts of plate material (Fig. 131ft) with gang form milling or shaping of the external contour. The requi¬ red section can also be obtained by extrusion. The parts in this case are produced by cutting the extruded section to the required length (Fig. 131c). The clamp shown in Fig. 131 <2 requires arduous contour machining or die forging followed by all-round trimming. If the design is slight¬ ly changed (removal of lug projections m) such clamps can be made from plate (Fig. 131e) with form milling of the external contour. 4.3. Composite Structures Composite structures are used in small-scale production when the manufacture of dies is economically unjustifiable. Some examples of dismembering parts as a means of reducing the amount of metal wasted in chips are illustrated in Fig. 132, 1, 2 (plug cock), 3, 4 (piston) and 5-7 (pillar fastening). Parts are often dismembered to reduce the labour required for machining. In the unit comprising a labyrinth seal and a self-expanding ring seal (Fig. 132, 8) it is practically impossible to make part, a because the cutting tool cannot approach the crests of the inner labyrinth and the spring-ring grooves. When the part is separated into two elements (Fig. 132, 9) it can easily be machined. Figure 132, 10, 11 shows the simplification of the machining of an annular T-shaped slot by separating the part into two elements. The part with an internal hub (Fig. 132, 12) can be machined to the required accuracy only with the aid of a cup-shaped grinding wheel (Fig. 132, 13). With the composite design (Fig. 132, 14) the detachable hub is ground externally. Figure 132, 15-34 shows examples of separating intricately shaped parts—pipe union (Fig. 132, 15, 16), cup-shaped part with an inter¬ nal spherical surface (Fig. 132, 17, 18) and hollow shaft with an internal partition (Fig. 132, 19, 20). It is difficult to machine cylindrical and spherical projections whose axes do not coincide with the rotation axis of the part. They can be machined on lathes only with the aid of special attachments (offset centre fixtures) and ground by means of cup-shaped wheels. Such parts are preferably made detachable. The design of the carrier with rings made integral with the carrier housing (Fig. 132, 21) is not sound. It is more practical to fit the pins in holes {Fig, 132, 22 , 23) which can be accurately manufactu¬ red and coordinated with ease. Projections may be made integral with the part if there are not more than two projections arranged on different sides of thfe part 4.4. Elimination of Superfluously Accurate Machining {21 (for example, frontal cranks. Fig. 132, 24). The composite structure (Fig. 132, 25) is however more practical although it is inferior in strength to the integral one. Other examples of composite structures are presented in Fig. 132, 26, 27 (cross-shaped carrier) and 28, 29 (level?with a spherical stri¬ ker). In the latter case, another design (Fig. 132, 30) wherein the striker head is replaced by a spherical cup is just as valid. External threads on the projecting members of housing-type com¬ ponents (Fig, 132, 31) have to be cut manually. This is unsuitable for mass production, and it will be more practicable to make these parts detachable (Fig, 132, 32). Centring from external shoulders on housings (Fig, 132, 33) should preferably be changed to centring from holes (Fig. 132, 34). 4.4. Elimination of Superfluously Accurate Machining Close-tolerance dimensions should be applied only when absolutely necessary. One should always select the lowest grade of accuracy permissible from the standpoint of interchangeability of parts and reliable operation of the given unit. Surfaces whose manufacturing accuracy does not affect the opera¬ tion of the unit as a whole should be made lo lower grades of accuracy than the working surfaces. Figure 133d shows a shaft, mounted in rolling-contact, bearings. The seating surfaces for the bearings conform to the 2nd grade of accuracy. The centring surfaces of intermediate bushings 1, 2 and 3 and of grooved seal body 4 are machined to the same accuracy al¬ though rougher tolerances (to the 3rd or 4th grade of accuracy, Fig. 133b) can safely be assigned for these surfaces. It is not necessary to assign close-tolerance dimensions for the internal diameter of the seal body 4 and for the external diameter of bushing 3 because a radial clearance of 0.5 mm exists between these surfaces. These dimensions may be given without tolerances. When a ball bearing is locked with rings on the shaft and in the housing (Fig. 133c, d) it is not necessary to install the locking rings into grooves by a slide fit. and machine them to the 2nd grade of accuracy since the fit of the rings and the accuracy of the bearing location are determined only by the overall dimension 24 A 3 between the extreme end faces of the grooves and the total thickness of the parts included in this interval (locking rings, bearing race). In order to simplify machining it is advisable to fit the locking Ti ng s into the grooves with an axial clearance of about 0.3 mm (Fig. 133d). Figure 133c shows axial locking of a ball bearing in the bousing by means of checks 5. To ensure clearance-free installation the end faces of the housing are machined to the 2nd grade of accuracy to a dimen- 122 Chapter 4. Design of Parts to Be Machined sion equal to the width of the outer bearing race (20S). The manu¬ facture of the unit can be simplified by machining the end faces of the housing without keeping to close tolerances, the clearance-free installation of the bearing being ensured by means of a sized ring 6 Fig, 133. Elimination of excessively accurate machining {Fig. 133/). Another, and much simpler method is to make the housing wall thickness 0.1-0.2 mm smaller than the width of the bearing (dimension 19.8 in Fig. I33g). When fastening bolts 7 are tightened up the cheeks are elastically deformed and securely lock the bearing -axially. 4.5. Through-Pass Machining 123 4.5. Through-Pass Machining The through-pass machining where the cutting tool freely appro¬ aches and leaves the work surface is of great value for raising produc¬ tivity and improving surface finish and accuracy. The housing design in Fig. 134n is not good since the traverse of the cutting tool (face milling cutter) along the work surface is limited by the housing walls. The cutting conditions vary with different portions of the surface. At first the blank is brought to the cutter axially, then the cutter & in a (a) 0 ) Fig. 134. Through-pass machining of frames bites into the metal, in which case it is difficult to obtain fine sur¬ face. Several cuts are required to obtain more or less identical finish over the entire length of the machined surface. Such productive methods as high-speed cutting, machining to preset operations and also gang machining cannot be applied in this case. Each workpiece has to be machined individually, much time being wasted to feed the milling cutter in and back it out, and adjust the setup to size. In the correct design with the protruding work surface (Fig. 1346) the milling cutter operates with through feed and cuts the surface to the same finish at a high productivity. Figure 134c shows a plate design unsuitable for mass production. The work surfaces are arranged at different levels, each surface requiring individual machining. Due to the presence of internal bosses, the contour of the upper flange m has to be machined with a combined cross and longitudinal feed of the work. Tho bracket with a transverse hole, which protrudes below the lower surface of the plate, makes it difficult to machine this surface and mount the plate properly when machining the upper surfaces. It is inconvenient to drill the transverse hole in the bracket, especially if the hole is far from the external edges of the plate. 124 Chapter 4. Design of Parts to Be Machined In the good design (Fig. 134^) all the work surfaces are brought to the same level. The bracket is made detachable. The machining is done in two stages: first the upper surface is cut and then the lower one. Figure 135 shows examples of machining accurate holes. In design 1 the bearing is installed in a split housing (radial assembly), in a recess limited on both sides by walls. It is extremely difficult to machine the seating surface of the recess. Design 2 of axial assembly (a bearing mounted in a solid housing) is likewise unsuitable. Accurate machining of the seating surface is hampered by the shoulder that locks the bearing in the axial direction. The designs where the seating surface is through-pass machined are correct. In this case the bearing is secured axially by locking rings (design 3) or intermediate bushings (design 4) one of which is fastened in the housing and the other serves to tighten up the bearing race. Figure 135, 5, 6 illustrates irrational (5) and rational (6‘) moun¬ ting of a rolling-contact bearing. The mounting of rolling-contact bearings in a gear with a collar used to lock the bearings (design 7) is unsuitable. In this case it is especially difficult to ensure the concentricity of the seating surfaces machined in different settings. When the collar is replaced with a locking ring (design 8) the hole can be through-pass machined. If a ram is fitted into a blind hole (design 9), it is difficult to machine the hole and lap-in the ram. In this case a through hole is required (design 10). In a cover with a shaped flange m machined by milling (design 11) it is better to make the flange of such a shape as will allow through- pass machining (design 12). In design 13 the nut-seating surfaces are face milled individually. Changing the shape of the seating surfaces (design 14) makes it possible to machine them all in one go, thereby appreciably increa¬ sing the efficiency of machining. Slots (design 15) should preferably be made open (design 16) as in this case their machining is simplified and their side faces can be made more accurately. Some changes in design that allow through-pass machining are illustrated in Fig. 135, 17, 18 (fitting a bushing into a housing); 19, 20 (torque-unit transmitting in a flanged connection); and 21, 22 (fastening a shaft by means of a pin). Figure 135, 23, 25 shows wrong designs of housings with holes arranged in line. If there are solid walls the holes must be machined with an end cutting boring bar whose end is unstable and deflects under the effect of the cutting force. In Fig. 135, 24, 26 the housings are provided with holes for passing the boring bar and in this case the end of the bar can be steadied with a rest. 126 Chapter 4. Design of Paris to Be Machined Figure 135, 27, SO shows how machining can be simplified by ar- ranging. the work^surfaces in one plane. In the design of an eiigipe head (Fig. 135, 27) the machining is done on plane b where the head joins the cover, on plane a where the camshaft bearings are mounted, and on the seating surfaces of the fastening nuts. A good design is the one in which all the three surfaces are brought to the same level and machined in one go (Fig. 135, 28). Fig. 136* Through-pass machining of holes and surfaces In the crankcase bearing unit (Fig. 135, 29) the bearing cap is loca¬ ted by means of shoulders, which prevents the through-pass machi¬ ning of the jointing surfaces of the crankcase and bearing. In the design shown in Fig. 135, 30, the cap is located with set pins, and the through-pass machining of the surfaces is thus made possible. The design of the gear pump in Fig. 136a is unsuitable for mass production. The seats for the gears are blind and are arranged in different halves of the housing. In such conditions it is difficult to coaxially align the seats. A better design is presented in Fig. 1365 where the seats are situated in one half of the housing. The best design is the one where the housing is composed of three parts (Fig. 136c). The seats in the middle portion of the housing and the working sur¬ faces of the housing cheeks are through-pass machined. Figure 136d shows an irrational design of a flat slide valve. The working surface m of the housing is in a cylindrical recess, and it is impossible to grind this surface to the required accuracy. The conditions of grinding the working surface of the slide valve are likewise unfavourable. Even a slight out-of-squareness of the surface with respect to the valve axis may disturb the tightness of the seal. In the design shown in Fig. 136c, the working surfaces of the hou¬ sing and the slide valve can be through-pass machined on a surface¬ grinding machine. 4.6. Overtravel of Catting Tools 127 This design also incorporates other improvements. Tile slide valve is con¬ nected with the shaft by splines which makes for an easy self-alignment of the slide valve with respect to the housing and increases the reliability of the seal. The spring that presses the slide valve rests against the cover of the housing via a spherical joint. This uniformly distributes the pressure force acting on the slide valve and reduces friction when the slide valve rotates. 4.6. Overtravel of Cutting Tools Sometimes through-pass machining is impossible for design con¬ siderations. In such cases provision should be made for an overtravel of the cutting tool with respect to the work surface to a distance suf¬ ficient to obtain the specified finish and accuracy. Fig. 137. Grooves for the over travel of cutting tools When machining accurate stepped cylindrical surfaces the over¬ travel of the tool is ensured by means of grooves several tenths of a millimetre deep cut at the section transitions. If a cylindrical surface alone is subject to precision machining, use is made of cylindrical recesses (Fig. 137n), and when end faces are to be accurately machined (Fig. 137fr) end recesses are cut. Diago¬ nal grooves are made when a cylinder and the adjoining end face are to be precision machined (Fig. 137c). The shapes of grooves for the overtravel of a grinding wheel are illustrated in Fig. 137d (cylindrical grinding), e (face grinding) and / (cylindrical and face grinding). The dimensions of the grooves (in mm), depending on the diameter do of the cylinder, are given below. do . . up to 10 10-50 50-100 over 100 b . . . . . 2 3 5 8 h . . . . . 0.25 0.25 0.5 0.5 R . . . . 0.5 1.0 1.5 2.0 Ri ■ • . . — «s2 h - — Figure 138 presents the shapes of adjoining surfaces cf standard parts used in mechanical engineering. 128 Chapter 4. Design of Parts to Be Machined It is impossible to finish machine the portion of a stepped shaft {.Fig. 138, 1) where the cylindrical surface adjoins the end face of the (31) (32) (33) (3k) (35) (36) Fig. 138. Adjoining surfaces collar. To ensure tool overtravel, a groove should be provided at the point of transition (Fig. 138, 2). This method is not recommended for heavily loaded parts because recesses act as stress concentrators. 4.6. Overtrave.l of Catting Tools 129 In such cases a filleted transition (Fig. 138, 3) is required, made with a round-nose tool in turning, and with a round-face wheel in grinding. To obtain accurate inner surfaces (Fig. 138, 4), it is necessary to introduce undercut grooves (Fig. 138, 5) or, better still, to ensure through-pass machining (Fig. 138, 6). Designs inTwhich threads on cylindrical stepped portions are cut close to the end faces of the steps (Fig. 138, 7, 13) are practically im¬ possible. Threads should terminate at a distance l 45 from shoul¬ ders or end faces (Fig. 138, 8, 14), where S is the thread pitch, or separated from the adjacent surfaces by a groove (Fig. 138, 9, 15) with a diameter d, d — 1.55 for external threads and d 2 5=- d + -f 0.25 for internal threads, where d is the nominal thread diameter in mm. When cutting external threads with threading tools or dies the width of the grooves is, on the average, 6 — 25, and when cutting internal threads, b — 35. It is advisable to observe this rule also in the case of smooth shafts (Fig. 138, 10, 11) and holes (Fig. 138, 16, 17). Surfaces adjacent to threads should preferably be arranged lower (Fig. 138, 12, 18) to allow through-pass machining. The diameters d 1 and d a of such surfaces are determined from the relations iven above. When cutting longitudinal slots in holes, provision should be made for tho slotting tool exit, for example, into a transverse bore m (Fig. 138, 19) or into an annular groove (Fig. 138, 20) of radius /?>■ j/" h 2 H- ~ (where h is the distance from the slot bottom to thejcentre and c, the slot width). It is better for the adjacent surface to be located below the slot bottom (Fig. 138, 21). The design of a blind hole with splines machined by broaching (Fig, 138, 22) is wrong: the width b of the groove beyond the splines is not enough for the overtravel of the broaching tool. In the design shown in Fig. 138, 23 the length of the splines is reduced and the groove is made of greater width b'. The lowering of the adjacent sur¬ face (Fig. 138, 24) enables one to broach the splines more effectively and accurately. Figure 138, 25, 28, 31 shows unsuitable shapes of tapering surfaces which do not allow overtravel and infeed of the tool. Correct designs are illustrated in Fig. 138, 26, 27, 29, 30, 32, 33. Figure 138, 34, 35 shows irrational and Fig. 138, 36, rational designs of spherical sur¬ faces. Let us discuss examples of wrong and correct designs of standard units and parts used in mechanical engineering. In the design of a splined shaft with straight-sided splines (Figu¬ re 139, 1) it is impossible to grind the working faces and the centring surfaces of the shaft. To permit overtravel of the grinding wheel the 9—010 58 Fig. 139. Overtravel of cutting tools 4.6. Overtravel of Cutting Tools 131 surface of the shaft should be lowered at the base of the splines (Figu¬ re 139, 2), or grooves should be made (Fig. 139, 3). Figure 139, 4 , 5 shows wrong and correct designs of an inverted V-guideway, respectively, and Fig. 139, 6, 7, those of a snap limit gauge. The internal space of a step ball bearing (Fig. 139, 8) can be ma¬ chined easier if a groove is made at the base of the space (Fig. 139,9) or if use is made of composite structures (Fig. 139, 10, 11). In the free wheel (Fig. 139, 12) the spiral active surfaces of teeth (usually worked on relieving grinding machines) should be provided with undercuts to allow for overtravel of the grinding wheel (Figu¬ re 139, 13). It is impossible to mill the slots in the slotted bushing (Fig. 139, 14) because the cutter comes against the bushing wall. If four instead of three slots are used (Fig. 139, 15) they can be through-pass milled. It is very difficult to machine the end slot in the shaft (Fig. 139,f76). If the cutting tool overtravel is permitted into a transverse bore at the base of the slot (Fig. 139, 17), the shaft end then can be drilled at the slot edges (dashed lines) and the partition between the drilled holes removed by planing. A composite design comprising a rim press-fitted onto the slotted portion of the shaft requires still sim¬ pler machining (Fig. 139, 18). End slots on a shaft. (Fig. 139, 19) can only be formed by upsetting. Separating the slots from the cylindrical surface of the shaft by an annular groove (Fig. 139, 20) enables one to make them by planing. In the composite design (Fig. 139, 21) the slots can be machined more accurately and efficiently by through-pass milling. In the cup-shaped part (Fig. 139, 22) the neck of the shaft can be ground only by a very expensive and inefficient method using a cup wheel mounted eccentrically with respect to the shaft (Fig. 139, 23). To make cylindrical grinding possible the shaft journal should pro¬ trude beyond the cup to a distance s sufficient for overtravel of the wheel (Fig. 139, 24). In another cup-shaped part. (Fig. 139, 25), the grinding of the internal surface is hindered by the projecting end of the hub. The design in Fig. 139, 26 is also wrong because the end of the surface being ground coincides with the end of the hub, and a burr appears on the extreme portions of the surface. In the correct design shown in Fig. 139, 27 the end of the hub is displaced relative to the surface being ground to a distance s thus ensuring a good finish of the entire surface. In the cluster gear (Fig. 139, 28) the teeth of the pinion can'be cut if the distance a (Fig. 139, 29) is made sufficient for overtravel of the gear cutter (Fig. 139, 30). The minimum value of a (mm) as against the tooth module m is given below. m 1-2 3-4 5-7 8-iO 12-14 a 4-5 6-7 8-9 10 14 9 * 132 Chapter 4, Design of Parts to Be Machined When teeth are formed by a hob cutter much larger distances are required, determined by the diameter of the cutter (Fig. 139, 31) and the plan approach angle with, respect to the shaft axis. If.the rims have to be close together, composite designs are used (Figu¬ re 139, 32). To prevent the hob cutter from cutting into the thrust shoulder of the shaft (Fig. 139, 33) when the splines are machined by the genera¬ ting method, the shoulder must be positioned at such a distance from the shaft end as will permit the machining of the splines without the tool cutting into the shoulder (Fig. 139, 34). The best way is to through-pass machine the splines and replace the shoulder with a circular stop (Fig. 139, 33). Figure 139, 36 shows a conical valve with a guiding shank. The valve chamfer and the centring surfaces of the shank are plunge- cut ground with a form wheel. In this design it is impossible to finish grind the portion where the chamfer adjoins the shank. The design with a recess (Fig. 139, 37) is also wrong because the diameter d of the shank is equal to the smaller diameter of the chamfer and a burr may appear on the chamfer. In the correct design shown in Fig. 139, 38 the diameter d x of the shank is smaller than the minor diameter of the chamfer, and the surfaces of the shank and the chamfer being ground are overlapped by the grinding wheel. 4.7. Approach of Cutting Tools To increase the efficiency and accuracy of the machining process the cutting tool should have an easy approach to the work surfaces. For this reason one must have a clear understanding of the machining operations, know the dimensions of the cutting tool and its fastening elements and the methods of mounting and clamping the work. Figure 140, 1 presents a sheave of a V-hclt transmission with a threaded hole n in the hub for the fastening screw. The shape of the part allows the hole to be drilled and threaded only through the bore m in the rim (Fig, 140, 2) which should be provided in the design. Some methods of making the hole n in a bracket (Fig. 140, 3) are shown in Fig. 140, 4-6. When determining the inclination angle of a skew hole (Fig. 140, 5 ), the drill chuck dimensions should be considered. In the design of a pin-type fastening a cup-shaped part on a shaft (Fig. 140, 7) it is impossible to drill and ream hole m for the pin and also insert the latter. In this case it is necessary either to provide hole m in the sheave rim (Fig. 140, 3) or to change the position of the hub (Fig. 140, 9). (23) m) (25) Fig. 140. Approach of cutting tools 134 Chapter 4. Design of Parts to Be Machined Hole n (Fig. 140, 10) in the lug between the flanges of a cylinder can be drilled through hole m (Fig. 140, 11) or recess q in one of the flanges (Fig. 140, 12). When knurling the knob of the dial in the design shown in Figu¬ re 140, 13, the knurling roller cannot reach the base of the knob. The knob should be displaced from the dial to a distance s == 3-4 mm (Fig. 140, 14) sufficient to let pass the cheek of the roller holder. When the dial is large in diameter a composite design (Fig. 140, 15) is preferable, allowing the use of a short and rigid roller holder. Shaped slot t in the face cam (Fig. 140, 16) cannot be formed as it is impossible for an end mill to approach the slot because there is a gear made integral with the cam. To make the machining possible, thB cam must be made detach¬ able from the gear (Fig. 140, 17). In the design’of a gear with an internal splined rim (Fig. 140, 18) the splines can be cut only by slotting. The more efficient and accu¬ rate generating method can be employed, if thB splined rim is brought out beyond the huhd Fig. 140, 19), or if the hub is displaced (Fig. 140, 20), or else if a composite design is employed (Fig. 140, 21). The internal faces of the disks in the one-piece turbine rotor (Fig. 140, 22) can be machined if the disks are arranged farther apart by increasing distances b and reducing the width of the disk rims (Fig. 140, 23), or if a split design (Fig. 140, 24) is employed. It is possible to mill the impeller blades of a centrifugal machine (Fig. 140, 25) if the radius at the base of the blades is increased to an amount that permits approach of a milling cutter (Fig. 140, 26). Figure 141 shows examples of changes in design making the ma¬ chining of hard-to-reach surfaces easier. The machining of inner space m of a stop valve housing (Fig. 141, 1) can he simplified by in¬ creasing the diameter of the threaded portion of the housing (Figu¬ re 141, 3). In this case, ordinary or core drilling may be used instead of turning on a lathe. Figure 141, 3-5 shows the methods applied to facilitate the machi¬ ning of internal space n of a turnable pipe connection. The threads in holes should not he too deep (Fig. 141, 6), but made as close as possible to the upper end face of the part (Fig. 141, 7). It is simpler to machine a labyrinth seal (Fig. 141, S) if the ridges are made outside of the seal housing (Fig. 141, 9). It is practically impossible to cut the thread on the rod of a cup¬ shaped part (Fig. 141, 10). The machining can be done if the thread is cut beyond the cup (Fig. 141, 11) or if a composite design (Figu¬ re 141, 12) is employed. The grinding of a deep hole in a shaft is illustrated in Fig. 141, 13. The deflection and rnnout of the cantilever spindle carrying the grinding wheel make it impossible to obtain a well finished and Fig. 141, Methods of making the machining easier 136 Chapter 4. Design of Parts to Be Machined accurate surface. In the correct design shown in Fig. 141, 14 there is a through hole and the spindle now can be mounted on two sup¬ ports (the shaft rotates in a chuck arranged eccentrically with res¬ pect to the spindle). With this design the grinding may be replaced by fine boring, reaming or broaching. Figure 141, 15 shows difficult-to-machine surfaces t for fastening holts in a bracket with a base connected by an H-section rib with a bushing. Milling (Fig. 141, 16) is impossible in this case because the ribs hamper approach of the milling cutter (dashed line). Planing (Fig. 141, 17) is difficult since overtravel is not provided for the tool. Inverse spot facing (Fig. 141, 18) can be applied only if the hole diameters are large. The boss raised above the surface of the base can be planed (Figu¬ re 141, 19) or the base can be secured with bolts (Fig. 141, 20) moun¬ ted on the other side of the housing (in this case it is not necessary to machine the upper side of the base). In the case of high-precision casting (for example, casting into metal moulds) the surface for nuts may he left rough (Fig. 141, 21). However, the bearing surfaces in critical joints should be machined to prevent the skewing of the bolts. It is extremely difficult to machine surfaces in deep cavities (pad for mounting part v, Fig. 141, 22). The internal surfaces may be left unmachined, if the part is mounted on external pads and passed through a hole in the wall (Fig. 141, 23). If it is impossible to make the hole of the required size, the part is introduced into the cavity and fastened on bushings 1 (Fig. 141, 24, 25) flange-mounted on the outer pads of the housing, and the part being located in the bushings from set pins 2. Transverse holes arranged in housings at a considerable distance from the edges (Fig. 141, 26) or in recesses (Fig. 141, 2$)canbema¬ chined only with an extended tool, a ratchet drill or, an angular drilling head, etc. In such cases it is more practical to use detachable brackets mounted on pads in the housing (Fig. 141, 27, 29). 4,8. Separation of Surfaces to Be Machined to Different Accuracies and Finishes Surfaces to be machined with different tools and to different accu¬ racies and finishes should be designed with some separating elements between them. In a forked lug (Fig. 142, 1) the surfaces of the slot and the base coincide. In the correct design (Fig. 142, 2) the bottom of the slot is raised above the base surface to a distance s (at least by several tenths of a millimetre). 138 Chapter 4. Design of Parts to Be Machined The design of a shaft with a square shank for a fitted-on part (Fig. 142, 3) is wrong: it is practically impossible to machine the end face / of the shaft steplessly when the faces of the square are milled. In the design shown in Fig. 142, 4 the faces are raised above the end face to a distance s. The face is undercut when the cylindrical surface of the shank is turned. On the fitted-on part a recess is pro¬ vided to overlap the cylindrical shoulder. The square of the shank can be separated from the shaft end face by an annular recess with a diameter slightly smaller than the dis¬ tance between the square faces (Fig. 142, 5). In the wrong gear design (Fig. 142, 6) the root surface of teeth coincides with cylindrical surface g of the gear rim. In the correct design shown in Fig. 142, 7 the root surface is raised above the hub surface to a distance s that ensures overtravel of the gear-cutting tool and prevents it from cutting into the rim surface. It is practically impossible to manufacture a connecting rod end (Fig. 142, 8) whose merging surfaces are machined by different ope¬ rations. In the design shown in Fig. 142, 9 the surfaces machined with dif¬ ferent tools are separated. The external surface h of the H-section rod, which is machined with a plain milling cutter, is raised to a distance s relative to the connecting rod end. The internal spaces i of the rod, machined with a face cutter, are removed from the rod end to a distance s r The rod-end cantilevers, worked by turning, are separated from the rod by a distance s. 2 . In the cam design (Fig. 142, 10) the accurate surface of the cam merges with the cylindrical surface of the shaft which is machined to a lower accuracy. It is impossible to grind the back surface l of the cam flush with the shaft cylinder. In the correct design shown in Fig. 142, 11 the surface of the cam is raised above that of the shaft to a distance s ensuring the required machining of the cam. In the dog plate design (Fig. 142, 12) surfaces m and n of the dogs are turned together with annular sections q and r of the disk end face, and portions t are milled. It is impossible to match these surfaces. In the correct design shown in Fig. 142, 13 the surface to be milled is raised above the adjacent surfaces of the disk end face to a distan¬ ce s. Similarly, in the ridged plate design (Fig. 142, 14, 15) surface u to be milled should be higher than all the other surfaces of the end face which are turned. It is difficult to machine the block with cylindrical pins (Fig. 142, 16). It is necessary to turn surfaces v adjoining the pins in two cuts so that the surfaces are matched precisely. The design with cylindri¬ cal bases w raised to a distance s (Fig. 142, 17) is correct only if the surface v of the block between the pins can be left rough, because it is difficult to machine this surface. 4.8. Separation of Surfaces of Different Accuracy 139 If the surface adjoining the pins is to be machined, it should be shaped as shown in Fig. 142, 18. The bases w of the pins are turned on a lathe and the surface v is through-pass milled. In hexagons adjoining cylindrical surfaces (Fig. 142, 19) the faces should be arranged above the cylindrical surface (Fig. 142, 20). In the design shown in Fig. 142, 21 it is impossible to merge the ground working faces of the slot with its drilled base. The precision- and rough-machined surfaces should be separated (Fig. 142, 22) or the base of the slot drilled to a diameter larger than the slot width (Fig. 142, 23) to ensure overtravel of the grinding wheel. Examples of wrong and correct merging of accurate and rough surfaces are illustrated in Fig. 142, 24, 25 (push rod with a spherical head) and 26 , 27 (dowel bolt). The design of the joint between the crank pin, main journal and webs of a crankshaft (Fig. 142, 28) is erroneous: the ground fillets of the journals pass directly into the milled webs. In the correct design shown in Fig. 142, 29 the fillets are separated from the web surfaces by shoulders s. In the bevel gear (Fig. 142, 30) the ground bearing surface z passes into the turned fillet of the end surface of the teeth. It is practically impossible to obtain the smooth mating shown on the drawing. In the correct design (Fig. 142, 31) the surface to be ground is separated from the rough surface by step s. In the disk valve (Fig. 142, 32) the guiding surface of the rod, machined to a high accuracy and finish, gradually forms the fillet of the head. This fillet can be obtained in practice only by filing manually the transition section. In the correct design shown in Fig. 142, 33 the surface of the rod is separated from the fillet by a recessed portion s. It is expedient to separate cylindrical surfaces of the same diame¬ ter machined to different classes of finish (Fig. 143a) by a shallow groove (Fig, 143f>) or to through-pass machine the entire surface to the same finish. Surfaces having the same nominal diameter, but machined to dif¬ ferent tolerances so as to ensure different fits (Fig. 143c) should pre¬ ferably have their seating sections separated by a groove (Fig. l43tf), or one of the sections should be made of a smaller diameter than the other (Fig. 143e). If the nominal diameter of the seating surface on a shaft is equal to the major diameter of the adjacent thread (Fig. 143/), an increase in the thread diameter (due to the threads’ “rising” during cutting) often makes it impossible to install the fitted-on part on the shaft. In such cases the major diameter of thread should be through-pass machined together with the seating surface, a special note being made for the purpose on the drawing. But it is better to reduce the thread diameter (Fig. 143 g). 140 Chapter 4. Design of Parte to Be Machined Figure 143 h shows wrong and Fig. 143i, /, correct designs of separa¬ ting internal cylindrical surfaces machined to different classes of finish. in "ni A-It (a) (b) ' i 1 O-l c £ ■S3 LO V) 51 & LL _ <0 (d) . ‘O' 11 ___i iitiSRj (e) ■*k] " 6t Ql 1st E '///A mm\ (J) Fig. 143. Separation ol surfaces machined to different finish for various fits 4.9. Making the Shape of Parts Conformable to Machining Conditions The shape of parts to be machined must conform to the type of machining, the shape and size of the cutting tool, and the sequence of operations. Figure 144 shows a connecting rod end joined to an H-section rod. Fhe design shown in Fig. 144a can he obtained only by cloSed-im- pression die forging and cannot be machine cut. With the shape shown Fig. 144. Joining a connecting rod end to an H-section rod on the drawing, the recess m between the flanges cannot be milled. The contour machining of the external surface n of the end and the sections q where the flanges pass into the end is likewise impossible. The recess can be milled with a plain cutter (Fig. 144b) or with a face cutter (Fig; 144c). Both methods fully determine the shape of the joint, which must be shown on the drawing. 4JO. Separation of Rough and Machined Surfaces 141 Heavy sections t (Fig. 1446) and u (Fig. 144c) at the joint between the rod and its end are eliminated by face milling the transition portions (Fig. 144d, e). Ends x of the flanges are milled with a face or plain milling cutter up to surface y which is undercut when turning ends z of the bushings. The conjugation of a round bar and a forked lug (Fig. 145«) cannot be machined and is only obtainable by closed-impression die forging. In the design in Fig. 1456, the bar is turned, and the lug, milled. In the esign in Fig. 145c, the lug takes a cylindrical shape, and only Fig. 145. Machining ol a forked lug faces m and n are milled. In the design with the lug tapering towards the bar (Fig. I45rf) the taper and cylinder surfaces are turned, and the side faces and rounded end q, milled. In the most rational design (Fig. 145e) the lug having the shape of a sphere with a taper towards the bar is turned on a lathe and only side faces t are milled. 4.10. Separation of Rough Surfaces from Surfaces to Be Maehined work On blanks produced by casting, stamping, forging, etc., the work surfaces must be separated from the nearest rough surfaces by a dis¬ tance k exceeding the amount of possible displacement of the rough surfaces. Figure 146 illustrates the application of this rule to work surfaces arranged above (Fig. 146a) and below (Fig. 1466) rough surfaces, and also to those adjacent to rough walls (Fig. 146c). If the distance k is insufficient, an upward displacement of the rough surface in casting (Fig. 146a) will cause the tool to cut into the wall, and in the case of a downward displacement the tool will fail to reach the wall leaving it rough. In Fig. 1466, if the rough sur¬ face is displaced downwards, the tool may not reach the metal. The displacement of side walls (Fig. 146c) may cause the tool to cut into the wall metal. 142 Chapter 4. Design of Parts to Be Machined Figure 146d-/ shows this rule as applied to separating the work surfaces on fastening flanges. Sometimes, dimensions do not allow rough walls to be removed from the work surfaces. In such cases the required distance ft can be maintained by making local recesses, cavities, etc. in the walls (Fig. 146g, i —wrong designs, Fig. 146ft, /—correct designs). The value of k mainly depends on the manufacturing accuracy of the blank and its overall dimensions. The values of k for cast parts can be found from Fig. 122. For parts made by smith forging the values of k are about the same. In the case of die-forged parts, k varies within 0.5 to 2-3 mm, depending on the forging accuracy and dimensions of the blank. Figure 147a shows a case of lacing a boss on an internal wall of a cast housing, effected through a hole in an external wall. The diameter of the hole in the external wall is equal to the diameter d of the boss. If the boss is displaced from its nominal position in eas¬ ting, an unmachined burr may appear on the boss. In this design the end face can be machined only with the aid of a boring bar with an extensible tool. The correct design is illustrated in Fig, 147ft. The diameter of the hole in the external wall is made larger than the boss diameter by the amount 2ft of possible displacements. 4.10. Separation of Rough and Machined Surfaces 143 In the design shown in Fig. 147c the faced surface of the boss is arranged below the rough surface, and the diameter of the boss is increased. As a result, the facing tool cuts a correct cylindrical sur¬ face in the boss. Figure 147ci shows the spot facing of a boss in a pit with rough walls. The size of the pit does not permit the use of a spot facer of such a Fig. 147. Facing of bosses diameter as is required to correctly machine the boss and keep at the same time proper clearance k between the spot facer and the walls of the pit. In the design shown in Fig. 147e the diameter of the pit is increased so that the boss is overlapped by the spot facer. In the design in Fig. 147/ the work surface is sunk in the bottom of the pit. Figure 147 g-i illustrates the facing of a boss adjoining the wall of a part (Fig. 147g—wrong design, Fig. 147 h, i —correct designs). 144 Chapter 4. Design of Parts to Be Machined 4.11. Machining in a Single Setting Surfaces which require precise mutual coordination should be machined in one setting. In the speed reducer with overhung gears (Fig. 148«) the holes for the input and output shafts are machined from different sides of the Fig. 148. Machining in a single setting housing. In this case it is difficult to maintain centre distance A and make the hole axes strictly parallel. In the good design shown in Fig. 1486 provision is made for an ad¬ ditional hole m which makes it possible to machine the seating holes from one side. 4.11, Machining in a Single Setting 145 In the speed reducer with stepped holes for the doubly-supported gears (Fig. 148c) the hole steps are wrongly arranged and cannot be machined from one side. In the correct design shown in Fig. 148d an idle bushing n makes it possible to machine the holes from one side. It is difficult to align to holes in the housing (Fig. 148e) because the small diameter of the middle hole hampers the through-pass ma- chining of the holes. Holes of the same diameter (Fig. 148/1 or stepped holes of a diame¬ ter diminishing in the direction of the cutting tool run (Fig. 148g) are preferable for housings. The latter design is simpler and the ef¬ ficiency of machining in this case is higher. If the difference s bet¬ ween the radii of the adjacent holes is larger than the machining allowance, the stroke of the boring bar with respect to the work is reduced to a magnitude slightly greater than the maximum width m of the holes being machined, and all the holes are machined simul¬ taneously. In the design with holes of the same diameter (Fig. 148/) the boring bar stroke is many times longer and must exceed the distance l between the extreme points of the surfaces being machined. Holes of the same diameLercan effectively be machined by means of boring bars with extensible tools which are set to the required size after introducing the boring bar into the blank. In the unit with hushes mounted in a housing (Fig. 148fe) the sea¬ ting surfaces for the bushes can be machined only from the different sides of the housing because the diameter d of tho interme¬ diate hole is small. It is diffi¬ cult to obtain proper axial alignment of the holes. In the improved design shown in Fig. 148i the diame¬ ter c?! of the intermediate hole is increased to the size which allows the press-fitted bushes to be reamed simultaneously. The design in Fig. 148/ is most advisable. Here, the diameter d 2 of the intermedia¬ te hole is increased to such a size as makes it possible to through-pass machine the seating holes for the bushes and ream them together. Figure 149 shows the centring of parts 1 and 2 arranged on the dif¬ ferent sides of a housing. In the design shown in Fig. 149a the centring surfaces m are made in the form of collars on the housing, and it is practically impossible to align them. 10—01658 Chapter 4. Design of Parts to Be Machined 146 In the design in Fig. 1496 the centring is effected from holes in the housing which are machined in a single setting, (his ensuring complete alignment of the parts being centred. When machining the housing for rolling-contact bearings (Fig. 150) it is necessary to keep the alignment of the centring surface m of the housing and the seating surfaces n for the bearings to the given strict tolerances. This can be attained by either of the following two methods: (1) the housing is located on a mandrel from surface n finish ma¬ chined in advance and then surface m is machined; (2) the housing is clamped in a chuck on finish machined surface m and then surface n is machined. Neither method can be applied with the design shown in Fig. 150a because the thrust shoulder o is arranged wrongly. Such a possibility occurs if the shoulder is transferred to the right-hand side of the housing (Fig. 1506) or replaced by a stop ring (Fig. 150c). Surfaces m and n can be made concentric more simply and accura¬ tely, if the part is clamped in a chuck on surface p machined previous¬ ly and the surfaces then machined in a single setting. In this case it will be wrong to arrange the thrust shoulder o on the right (Figu¬ re 150(f). For correct machining the shoulder should be transferred to the left (Fig. 150e) or replaced by a stop ring (Fig. 150/). 4.12. Joint Machining of Assembled Parts The joint machining of assembled parts should be avoided, for this complicates and splits the flow of production and spoils the inter¬ changeability of parts in a given design. 4,12. Joint Machining of Assembled Paris 147 Exceptions to this rule are the cases when the joint machining is the only method that can ensure the operating ability of the design. Thus, for example, in the case of multiple-hearing crankshafts, the splitting of the crankcase along the bearing axis is a prerequisite for assembly, and the joint machining of the bearing seat halves in the assembled crankcase js the only method to ensure the alignment of the bearings. The housings of rotory-type machines are frequently made split along the axis to facilitate assembly and disassembly and simplify inspection. The joint machining of the internal surfaces and end faces of the bearing seats is required in the gear drive housing split along the shaft axis (Fig. 15 la). Prior to the machining of the bearing seals, Fig. 151. Combined machining in assembly the jointing faces of the housing halves must be finish machined and the halves positioned properly with respect to each other by means of set pins. The sealing of the joint with a gasket in this case is impermissible, and the butt-jointed surfaces are ordinarily lapped- in, the design losing its property of interchangeability of parts. Only the jointly machined housing halves can he accepted for as¬ sembly. It is impossible to replace a housing half during operation as this disturbs the cylindricity of the bearing seats and the align¬ ment of their cncl faces. The parts of the housing split in a plane perpendicular to the shaft axis (Fig. 151b) can be machined separately. The manufacture of the housing is greatly simplified, and the housing parts are interchangeable. Figure 151c shows the cylinder of a rotary filler mounted on a tank. The cavities of the cylinder and tank communicate through by-pass hole k. Two errors are committed in this design: (1) the hole is drilled simultaneously in the cylinder flange and the tank body; and (2) cover 1 enclosing the by-pass holes is mounted at the joint between the cylinder flange and the tank wall. It is necessary to machine the hole and the joint surface together when the cylinder is assembled with the tank. The cylinder cannot he replaced during operation. In the correct design shown in Fig. 151d the holes in the tank and the cylinder can be drilled separately. The joint surface is provided on the tank wall, and the cylinder can be replaced even when machi¬ ned to ordinary accuracy. 10 * 143 Chapter 4. Design of Paris to Be Machined 4.13. Transferring Profile-Forming Elements to Male Parts Internal surfaces are much more difficult to machine than external ones, and for this reason it is good practice to arrange profile-for¬ ming element on external surfaces. Figure 152a, b illustrates a laby¬ rinth seal. The ridges made on the male part (Fig. 1526) are much simpler to manufacture than those in the hole (Fig. 152a). The needle bearing in which the retaining shoulders are provided on the inner race (Fig. 152d) is better from the viewpoint of manu¬ facture than the one with the shoulders on the outer race (Fig. 152c) since the hole in the outer race in this case is through-pass machined. Fig. 152. Transferring profile-forming elements to male parts The design of the unit for fastening a spring cap on a valve rod by means of split tapering blocks centred by the outer cylindrical surfaces A of the ridges (Fig. 152e) is irrational. The sound design is the one in which the accurate centring surfaces B are through-pass machined in blocks (Fig. 152/). In a roller overrunning clutch the profiled elements (usually having the shape of a logarithmic spiral) should not be arranged on the outer race (Fig. 152g). They can he machined only by broaching and only when the hole in the race is a through one. In the design shown in Fig. 152 k the external profiled elements can easily be processed, for example, on a relieving lathe. Long threads in holes should be avoided (Fig. 152i). A long thread is good on a bar and a short one in a hushing (Fig. 152/). 4.14. Contour Milling Complex and irregular profiles are more difficult to mill than flat or cylindrical surfaces. The lever design requiring an all-round machining (Fig. 153a) is bad. The re-entrant, angles do not permit the external contour of the 4.14. Contour Milling 149 part to be machined with a plain milling cutter. It is also very dif¬ ficult to machine surfaces m confined within the cylindrical walls of the bosses. In the design shown in Fig. 1536 the external contour is described by straight lines and circumferences and can be form milled. Sections n between the bosses, which are bordered by straight lines, can be through-pass milled. One side of the lever (surface p) is made flat to simplify machining. It is practically impossible to mill the contour of the flange (Fig. 153c) because the fillets at the base of the bosses are too small. & (0 Fig. 162. Machining of holes of the tap starting section. In finishing taps the length of the starting section is, on the average, l = (0.3 to 0.4) d where d is the thread diameter. It is bad practice to drill holes at an angle a < 70° to the surface (Fig. 163a). This method requires preliminary drilling (Fig. 163ft) or milling (Fig. 163c) of the hole entrance portion, which complica¬ tes manufacture. Machining will be easier if the hole is arranged at an angle larger than 70° to the surface (Fig. 163d). It is better to drill a hole at right angles. Some methods of straigh¬ tening out the work elements for skew bores in cast parts (Fig. 163c) are illustrated in Fig. 163/-A. Examples of wrong and correct arrangement of holes are given in Fig. 163i, j (pinning a handwheel) and in Fig. 163 k-m (pinning 4,24. Redaction of the Range of Catting Tools 161 a cylindrical part on a shaft). The designs in Fig. 163/, l, m are correct. Figure 163n-p presents methods of drilling holes in a crankshaft, the holes being intended to feed oil from the main journal to the (fj) <°> o» Fig. 163. Drilling of skew holes crankpin. Most rational is the design with a straight hole through the web (Fig. 163p). Holes obtained by means of ordinary helical drills should never be more than 6-8 diameters deep for otherwise the hole may be mi¬ saligned and the drills broken. It is advisable to reduce the drilling depth to the minimum per¬ mitted by the design. Long and thin boras (Fig. 164a) should be replaced by stepped ones (Fig, 1646). The long and narrow oil duct (Fig. 164c) connecting the bores in the shaft is not just as good as the duct of a larger 'diameter (Fig. 164c?)* If the cross-section of the duct has to be reduced (for example, for faster oil feed during starting), this can be done by means of insert / (Fig, 164e). 4.24. Reduction of the Range of Cutting Tools The range of cutting tools can be reduced if the diameters of accurate surfaces are unified. This is especially important for holes machined by such tools as drills, counterbores, reamers and broaches. 1 1 - 01658 162 Chapter 4. Design of Part a to Be Machined One and the same tool is preferred for the maximum number of operations so that time is not lost in resetting and replacement. It is good practice to make the transitions between steps and shoulders on turned shafts, which do not serve as bearing surfaces (Fig. 165a, c ), tapered at an angle equal to the plan approach angle Fig. 165. Reduction of the range of cutting tools of the cutting edge of a turning tool (usually 45°) and with a fillet at the base equal to the standard tool top rounding R — i mm (Fig. 165&, d). This makes it unnecessary to change the cutting tool and undercut the step ends. Figure 165 (f> W Fig. 174. Machining a bracket with a set of milling cutters of the impeller machined by turning is slightly tapered. The surfa¬ ces s between the blades are milled. Machining productivity can appreciably be increased by the use of combination tools which simultaneously machine several surfaces (core drills, block cutters, sets of milling cutters, etc.). 4.28. Multiple Machining 171 The bracket (Fig. 174a) processed over the external m and inter¬ nal n side faces of the eyes and also over the surfaces o of the faste¬ ning bosses is machined with a set of plain milling cutters in two settings. The first setting is used to machine the side faces m and n of the eyes with a set of three milling cutters (Fig. 174 d). Then, the part is swivelled through 90° and the boss surfaces o are milled with a set of two cutters (Fig. 174e). Dislocation of the bosses in relation to the eyes (Fig. 1746) allows the part to be machined in a single setting with three milling cut¬ ters. The cutter side faces (Fig. 174/) cut the surfaces m. and n of the eyes, and the peripheries of the two outer cutters process the surfaces o of the bosses at the same time. In the very compact design c, the fastening bosses are arranged between the eyes and are machined by the periphery of the internal cutter (Fig. 174g) at the same time as the internal side faces n. 4.28. Multiple Machining In large lot and mass production, the tendency is to machine parts in groups to a preset operation with establishment of the blanks in quick-acting machining fixtures. Consecutive machining (Fig. 175a) reduces handling time (the time needed to mount the blank and adjust the machine tool). Fig. 175. Diagrams of group machining Parallel machining (Fig. 1756) reduces machining time in propor¬ tion io the number of blanks being simultaneously machined. Parallel-consecutive machining (Fig. 175c) is the most, productive. For all these methods through-pass machining is obligatory. Figure 176a illustrates a circular nut with radial wrench slots which are located below the thread by the amount m. The slots are machined by non-productive indexing method (only by planing or slotting). The shape of the part does not permit milling. 172 Chapter 4. Design of Parts to Be Machined In the design b in Fig. 176 the slots are milled, but as in the pre¬ vious case the part cannot be group machined. If the slots are located higher in relation to the thread by the amount n (Fig. 176c) a number of nuts mounted on a mandril can be consecutively machined together in groups by the generation method with the aid of a hob. The lug (Fig. 176c?) with a slot profiled to a circumferential arc is suitable only for piece machining. A straight slot (Fig. 176c) permits consecutive group through-pass machining. Figure 176/ shows plates 1 and 2 clamped by distance bolts 3 . The bolts can be turned only individually. The manufacture of the bolts is complicated because an accurate distance l has to be main¬ tained between the shoulders. In the design g in Fig. 176 the plates are lightened up against bushing 4. The centring shoulders make group machining of the bushings impossible. In the design h in Fig. 176 the distance bushing 5 has flat end- faces and the plates and bushings are mutually centred by means of dowel bolts 6. In this design the distance l between locating surfaces of the bushings can easily be maintained by machining the bushings in groups on a surface grinding machine, the bushings being clamped on a magnetic chuck. Bushings can be machined much more quickly on a rotary table grinding machine. Parts intended for consecutive and parallel-consecutive group machining should have datum surfaces that will ensure their correct mutual positioning during machining. When milling, datum surfa¬ ces may be the bases of the parts and their side faces. When cylind¬ rical parts are machined, the datum surfaces are usually centre holes. The parts are mounted on a mandril and machined in a group. The sections of workpieces intended for machining should be durable enough to withstand deformation under the action of the cutting forces. Gears in which hub faces protrude in relation to rim faces (Fig. 177a) are not suitable for group machining as the gear rims are not secured 4.2S. Multiple Machining 173 rigidly during machining and can deform and vibrate under the cutting force. It is preferable to make hubs flush (Fig. 177 b) or with a small (0.1-0.2 mm) clearance s (Fig. 177c) in relation to the rim. Fig. 177. Elimination of deformation of blanks in group machining It is good practice to clamp blanks using not the hubs but special end disks resting against the rims. Figure illd-f shows a lever requiring milling over its external contour. The protruding hub faces (Fig. Hid) do not allow the set to be clamped tightly. Design e allowing the parts to be clamped in pairs is better but the best design / for group machining has all faces arranged in one plane. Chapter 5 Welded Joints In mechanical engineering, welding is extensively employed to manufacture structures from plate rolled stock (reservoirs, tanks, hoppers, coverings, linings, etc.) and from pipes and shaped rolled stock (frame structures, trusses, columns, pillars, etc.). Nowadays housings and base members are also made by welding, including the most massive and stressed parts (for example, the beds of pres¬ ses and hammers). To simplify the manufacturing process it is sometimes expedient to separate intricate forgings and castings into simpler elements and connect them by welding (weId-forged and weld-cast structures). In individual and small-lot production welded structures are used instead of one-piece forgings when the manufacture of dies is not justified by the scale of production, and also as a means to make the manufacture of complicated parts less expensive. Low-carbon steel (<0.25 per cent C), low-alloy steel with a small content of C and nickel steel weld very well. High-carbon, medium- and high- alloy steels are more difficult, to weld. It is difficult to weld nonferrous metals (copper and aluminium alloys) in view of their high heat conduction and easy oxidation (formation of refractory oxide spots), which makes the use of flux necessary. The strength of welds is inferior to that of solid material because of the cast structure of the welded joints with its dentritic and aci- cular crystallites typical of cast metal. A coarse crystalline struc¬ ture forms in the metal adjacent to the weld seam and in the affected zone. The strength and resilience of the material in a weld are impaired by penetration of slag, formation of pores and gas bubbles and also because of chemical and structural changes in the weld (alloying elements burn-out, formation of carbides, oxides and nitrides). If the material of a weld is saturated with air nitrogen even in small quantities the weld will lose much of its plasticity (Fig. 178) and will become much more brittle. Metal contraction during solidification causes internal stresses in the weld and in the adjacent area with possible warping of the product. Chapter 5. Welded Joints 175 The reduction of strength in parts made of low-carbon steel (whose plasticity prevents the appearance of internal stresses) is not large, and is almost immaterial in structures operating under a static load and under moderate stresses. However, this reduction is very cf% 25 20 15 10 r r V y \ '- 6t.Sa2,K gf/mn? 6 D Kof/mm 0.05 OJ 0.15 N, % Fig. 178. Effect of nitrogen on the mechanical properties of low-carbon steel ?4 50 22 40 20 W 30 16 20 lit >2 10 W 10 = 10 6 s u — 1 i I mu "s s 1 I 1 mu li! ■IBB j _ mu 1 1 nv. 1 L IV ■ i l mu 1 fei 1 in llll f >fi mi ! 111 mi n IV ng i in niB ■ Id ■ 1 1 !!! in n J 1 in ■ II 1^1 ■ 1 in I? « in Ill II iK hi u in 3 hi in 1 HI II HI ■ i i in 1_ III IHH 1 _ r . in 10 7 Number of cycles tf Fig. 179. Fatigue curves 1 —solid specimen; 3—specimen with a circular weld tangible in structures loaded cyclically, especially if they are made of high-strength steel sensitive to stress concentration. The effect of welds on cyclic strength is plotted on the diagram in Fig. 179 illustrating the test of a solid cylindrical specimen made of a low-alloy steel (curve 1) and a specimen of the same steel with a circular V-weld (curve 2). The presence of the welded joint reduces the fatigue limit more than twice (from 20 to 9 kgf/mm 2 ). A stress of 15 kgf/mm*, safe for a solid specimen, is liable to destroy a welded specimen already at 3 X 10 s load cycles. Submerged arc welding or welding in the atmosphere of inert or reducing gases is employed to prevent chemical transformations in the welded metal. Welding causes warping of parts, which is more severe the greater the heat-affected zone (gas welding) and the greater the length and cross section of the welded joints. Warping can be prevented if a part is welded in rigid holding fixtures and by special methods (intermittent, multilayer or multi¬ pass and step and step-back welding). The warping can be removed 176 Chapter 5. Welded Joints The most widespread and universal method of welding. It is performed by means of an arc struck between a fusible metal electrode 1 (direct arc) and metal surface. The weld is protected against oxi¬ dation by thick-coated electrodes with the first coat liberating liquid slag and reducing gases (CO, H a ) when the arc burns. Welding by carbon electrodes with a direct (6) or an indirect (c) arc with the rods 2 is mainly reserved for thin-walled parts made of nonferrous alloys. Carbon electrodes are very popular for arc cutting (especially of alloy steels) Automatic submerged arc welding | Used in large-scale production to join parts by straight and circular welds. This method implies using hare wire 1 as electrode and the wel¬ ding is conducted under a layer of flux. The productivity of the process is 5-10 times higher than that of the manual electric arc welding, and the weld has a high quality. Shaped (in plan), short and scat¬ tered welds are accomplished by semiautomatic welders in which the welding wire is fed through flexible hoses. Chapter 5. Welded Joints 177 Table 4 (continued) Welding method Descri ption Gas-shielded welding Welding is done hy nonconsumahle (a) or consumable (tungsten) electro¬ des (6) in a flux of inert gases (argon, helium). The method is used to join parts made of high-alloy steel, titanium, nic¬ kel, aluminium and magnesium alloys. Carbon steel is welded with a less expensive carbon dioxide gas. Atomic hydrogen welding M? Welding is done by an indirect me with the use of nonconsumahle elec¬ trodes in a hydrogen flux which, being an active reducing agent, effec¬ tively prevents oxidation of the weld. Electroslag welding jut?, r f- - £/ Used to connect large blanks (fra¬ mes of large machines, high-pressure reservoirs). The weld is formed in the clearance between the parts being joined hy the fusion of laminated electrodes 1 under a layer of syn¬ thetic slag. The outflow of molten metal and slag from the clearance is prevented by water-cooled slide blocks or ceramic linings 2. Resistance welding -CT3 11— I. ■ mg i 0 Resistance butt welding (a) is em¬ ployed to join parts with small cross section. The end-faces of the parts are compressed hy a hydraulic press, and the current is switched on to bring the metal in the joint to a plastic state. In the case of flash welding the joint is first compressed hy a small force and then the current is switched on. This generates a large number of microarcs in the joint which fuse the metal ( b ). 12—0K15S 178 Chapter 5. Welded Joints Table 4 (continued) Welding method Description' After fusion the joint is compressed by a hydraulic press (c). Flash wel¬ ding is employed to join parts of Lar¬ ge cross section and also parts made of heterogeneous materials. O-vyacetylene welding When spot welding is used for lap joints ( d ) the plates are drawn bet¬ ween a stationary I and a movable 2 electrodes which periodically com¬ press the plates forming a spot weld. Strong-tight lap joints are formed by seam welding with roller electro¬ des 3 (e). Thin sheets are joined to massive parts by means of projection welding. First flutes are punched on the sheet (/). The parts are then compressed between copper electrode plates resul¬ ting in fusion and welding of the projections. Performed in the reducing flame of an injector burner. The addition agent is metal wire or rods similar in composition to the metal of the parts being welded. The quality of the joints is lower than in arc welding. Oxyacetylene welding is predominantly employed to join parts made of carbon steel in small-lot production. Oxyacetylene cutting is applied on a wide scale and noted for its high efficiency and better quality of cut¬ ting than electric arc cutting. Chapter 5. Welded Joints 179 Table 4 (continued) Welding method Description Gas-pressure welding The edges to be connected are hea¬ ted by oxyacetylene flame and pres¬ sed together by an up-setting mecha¬ nism. The method is widely utilized to weld pipelines on site, the joint being heated by burners arranged in. a circle. Thermit welding This method is mainly employed to wold structures on site. The source of heat is the exother¬ mic reaction of reduction of iron oxi¬ des by aluminium (aluminium ther¬ mits). The cleaned joint of the parts being welded together is enclosed in a detachable ceramic mould i((a) with thermit which is ignited by a phos¬ phorus primer. The reaction produces aluminium oxide that floats up^ in the form of slag, and molten iron which fills the gap in the joint. Welding is completed after the joint is compres¬ sed. S? 1 An improved method consists , in burning the thermit in a separate mould 2 and filling the joint with molten iron (ft). Power transmission lines are con¬ nected by muffle welding with mag¬ nesium thermit (mixture of irpn oxi¬ des with magnesium). The ends of conductors are inserted into muffle 1 (c) and are pressed together with a screw clamp. Friction welding Performed by the heat liberated when one of the parts (i) - being wel- / ? ded is rotated in relation to the B other stationary part (2) under an axi¬ al force. The method is used for butt-weiding of small, mainly cylin¬ drical, parts. 12 * 180 Chapter 5. Welded Joints Table 4 ( continued ) Welding method 'Description Explosion welding Li Used to join thin sheets to massive ones (plating of steel with copper, brass, titanium alloys, etc,). A layer of explosive 3 (ammonite) is placed on the surface of the parts to be welded and is exploded by a detona¬ tor. .The explosion pressure joins the sheet tightly to the base material. Furnace welding rf Used to join parts on cylindrical shoulders (connection of flanges to pipes, or of pipes in frame structu¬ res). A bronze or brass ring 1 (a) is fit¬ ted in the joint, or the joint is gre¬ ased with a paste of powdered bronze and flux ( b). Prepared products are heated in an electric furnace in a reducing atmosphere (natural gases) up to a temperature of 1100- 1150 °C. Press cold welding Used to connect plastic metals (Cu, Ni, Al, Zn, Cd, etc.). The cleaned and degreased joint surfaces (a) are compressed by a pressure exceeding the yield point of the material. The surfaces are strongly joined due to the diffusion and re crystallization processes occurring in the compres¬ sion zone. Lapped sheets are welded under a pressure of round or straight dies (spot welding, 6) or by roll welding (c). Parts made of nonferrous metals (contact points, seats) are welded to steel parts by pressing them into co¬ nical seats. Chapter 5. Welded Joints 181 Table 4 (continued)/ Welding method Description Induction welding oo 111 'i A 3 oo oo FFFF oo oo ■b} Done by heating the edges to joined with on inductor 1 (a) through which posses a high-frequency current (5-20 kHz), the edges being after- whrds compressed by an upsetting mechanism. When pipes are welded by the arc resistance method the ends of the pipes are heated by means of oppo¬ site directed current in the inductors 2, 3 (i). The currents induced in the joint form a rapidly revolving annu¬ lar arc which fuses the metal. Wel¬ ding is completed by compressing the- joint. Induction welding is widely applied in automatized pipe production (c). A blank rolled into a pipe is drawn through inductor 4 which heats the joint and the pipe edges are compres¬ sed. Diffusion welding The joint, of parts 2 and 4 being welded is bested by inductor S and compressed by ram 1 in a high-vacu¬ um chamber (10M0 8 mm Hg) or in atmosphere of inert gases (argon, he¬ lium). Heating to 750-800 and reliable joint. r C makes a good This method can bo applied to weld refractory and heat-resistant alloys, cermets and ceramics. Currents with a radio-frequency range of 50-200 kHz are employed to weld thin parts made of copper, aluminium and nickel al¬ loys and also stainless steel. 182 Chapter 5. Welded Joints Table 4 ( continued ) Welding method Description Performed in vacuum by a current of electrons emitted by a tungsten spiral 1 under a high voltage of 250 kV and passed through a circular anode 2. The current of electrons is focussed by electromagnetic coils 3. The temperature at the focus point is from 3000 to 10,000 °C; the hea¬ ting spot ranges from 2-3 mm to seve¬ ral hundredths of a millimetre. Electron-beam welding This method can be employed to weld parts (with a thickness of se¬ veral microns) arranged in enclosed spaces (vessels, housings) permeable hy electron beams. Plasma arc welding Effected by a jet of an inert gas (nitrogen, helium, argon) ionized by putting it through an electric arc struck between a tungsten electrode 1 and a water-cooled copper nozzle 2, The temperature along the axis of the jet is 15,000-18,000 °C. In plasmatron welders the gas is ionized by a high-frequency electro¬ magnetic field. The jet of plasma is formed with the aid of electromag¬ netic coils. The temperature of the jet is up to 40,000 °C. This method can be utilized to weld and cut most refractory mater¬ ials (including ceramics). Chapter 5. Welded Joints 183 Table 4 {continued) Welding method Description Ultrasonic weldi ng This method (with frequency 20-30 kH 2 ) is applied to join nonferrous metals and plastics. The parts are compressed by a vibrating contact jaw 1 connected by a waveguide 2 with a magnetostrictive oscillator 3. High-frequency oscillations heat the joint and cause diffusive interpene¬ tration of tho atoms of the materials being joined. In radio-electronics ultrasonic wel¬ ding is employed to connect parts up to several microns thick. Laser welding ■+ * Effected by a concentrated light beam produced by laser 1 (ruby or neodymium crystal). The tempera¬ ture of the axis of the beam is up to 10,000 °G; the heating spot ranges from several microns to several hun¬ dredths of a millimetre. In radio-electronics laser welding is used to connect parts up to seve¬ ral microns thick. after welding by stabilizing hoat treatment (low annealing at 600- 650 °C). The mechanical properties of welded joints depend on the welding process and in manual work on the skill of the welder. Careless welding and improper methods will cause defects impairing the life of the weld and its strength. In manually w r elded joints the strength characteristics vary within the weld, the product or a group of the products. Important welded joints are tested by magnetic, X-ray and gam¬ ma-ray methods. The ultrasound test is the most sensitive and accu¬ rate. Large lots of welded products are tested selectively by cutting up of specimens, by tensioning, bending and flattening them and by investigating their microstructure and chemical composition of the metal in the weld. The principal welding methods are illustrated in Table 4. 184 Chapter 5, Welded Joints 5.1. Types of Welded Joints The main types of joints made by arc and gas welding are as follows: butt (C), comer (Y), lap (H) and tee (T). Fillet welds of triangular profile are made straight (Fig. 180a), convex (Fig. 180b) and concave (Fig. 180c). The most common is a straight ( normal ) weld. Convex welds (also called reinforced welds) have a ten¬ dency to form undercuts (poor penetration at the points m where the weld adjoins the walls of a part) and possess Fig. 180. Fillet welds a lowered cyclic strength. Concave welds are the stron¬ gest but their manufacture is more difficult and less productive. The design leg K is the principal dimensional characteristic of fillet welds. When thin sheets (less than 4 mm) are welded the leg of welds in lap joints is made equal to the thickness $ of the sheet (Fig, 181a). For thicker materials (4-16 mm) tho leg of a weld can he found from the relation K = 2 + 0 As mm (5) When materials of various thickness (Fig. 1816, c) are welded the leg is made equal to the thickness s of the thinner material, hut not larger than indi¬ cated in formula (5). Tn this case a concave weld is preferred. In corner joints with the same thickness of the walls (Fig. 181d) the length of the leg depends on the thickness of the edges. In corner and tee joints Fig. 181. Dimensions of fillet welds (Fig. 181e, /) where the dimensions of a weld may be arbitrary the leg is equal to the thickness s of the elements being welded together, hut not larger than the values in formula (5). When members of various thickness are tee-welded (Fig. 18'lg) the leg is equal to the thickness s of the thinner element. It is preferable to make concave welds. Lapping is the most simple and reliable method of joining plates (Fig. 182a, ft). The shortcoming of this method is that lap joints subjected to tho action of tensile and compressive forces are bent by a moment 5.1. Types of Welded Joints 185 approximately equal to the product of the acting force and the sum of the half-thicknesses of the plates being welded (Fig. 182a), and are therefore deform-ed (Fig. 1826). The two welds drastically reduce Fig, 182. Operational diagrams of lap joints the welding productivity, and the weight of the joint is great er than in the case of butt joints. Lap joints also include slotted (plug) welds formed by fusing up round (Fig. 183a) or elongated (Fir. 1836) prearranged holes in one of the plates to be connected (these joints are sometimes called rivet welds). The laborious manufacture, low strength and poor tightness of the weld make this joint one of the worst which may be employed only when the design requirements do not allow welding by other more productive methods. If one of the members being welded is less than 6-8 mm thick slotted wel¬ ding is replaced by the simple and effective opera¬ tion of spot penetration (Fig. 183c) of the thinner element ( poke welding) or seam, transfusion welding (Fig. 183d). When thin (<3 mm) sheets are butt-welded at an angle the edges are flanged (Fig. 184a, t). The edges of plates with an average thickness of <8 mm for manual arc welding and <20 mm for automatic welding are made straight (normal to the plane of the plate). For weld penetration through the entire cross section, the parts to be welded are assemb¬ led with a clearance m — 1-2 mm (Fig. 1846, /) filled with molten metal during welding. In the case of a greater thickness the edges should be prepared mainly by chamfering to produce a weld pool and ensure penetra¬ tion through the entire cross section. fa) (b) (c) ((f) Kig. 183. Slotted [a, b) and transfusion (c, d) welds 186 Chapter 5. Welded Joints The principal types of preparation are illustrated in Fig. 184c-h (butt joints), k-m {corner joints) and n-p (tee joints). The sharp cor- ners’are broken, leaving belts with a height of h = 2-4 mm (Fig. 184c). Fig. 184. Preparation of edges ^Round chamfers are turned, and straight ones—milled or planed. If the thickness of the edges is over 15-20 mm chamfers are removed by automatic gas cutting. Preparation with curved bevels (Fig. 184g, h) is mainly employed for straight and circular welds. A complicated milling operation to a templet is required to prepare edges having an irregular shape in plan. 5.2. Welds as Shown on Drawings According to Soviet standards, welds arc shown on drawings by solid basic lines which coincide with the edges of parts to be welded together. Invisible welds (arranged on the reverse side of the projec¬ tion) are designated by dash lines. Welds of spot and seam resistance welding as well as welds obtai¬ ned by transfusion are shown by dash-and-dot, lines drawn through the centres of the welded sections. A wold is designated by an inclined extended line with an arrow pointing to the line of the weld. The horizontal wing is used for the basic symbol of the weld including: (1) designation of the kind of welding (Russian letters) (P—ma¬ nual, A—automatic, II—semiautomatic); (2) letter index of the type of welding (3—arc welding, T—gas welding, CD—submerged arc welding, 3—gas-shielded welding. III — electroslag welding, Kt —resistance welding, Ya—ultrasonic wel¬ ding, Tp—friction welding, X—cold welding, Tl3-plasma arc wel¬ ding, 3 ji— electron-beam welding, diffusion welding, II—in¬ duction r welding, Tn—gas-pressure welding, Tm— thermit welding, JIa —laser welding, B3—explosion welding); (3) graphical symbol of the typo of weld (with the dimensions of the weld when necessary). 5.2. Welds as Shown on Drawings 187 Welds are usually designated on the drawings of welded joints in an abbre¬ viated form. Tbe letters P, A and n are omitted and all tbe pertaining problems are solved by the process engineer of the welding department depending on the scale of production a'nd available equipment. The letter (9) designating electric welding is also omitted because it is the most widespread type of welding. The letters Kt (resistance welding) are not written since the kind of welding ia here fully determined by the symbol of the weld. The other letters are given only if a joint should be formed by a cer¬ tain method of welding. Thus, most frequently, the designation of a weld consists only of a graphical symbol. Some symbols arc illustrated in Table 5. Table 5 Type of weld Fillet weld fJT — design leg of weld) Symbol Joint Type of weld Single-V butt weld symbol Joint Lap spot weld Single-V butt weld blunted Double-flanged butt weld Double-bevel butt weld Square butt weld Double-V butt weld Single-bevel butt weld Single-J butt weld Single-bevel blunted butt weld 1 * Single-V butt weld Convex (reinforced) weld Concave weld Remove reinforce¬ ment to the surface of edges being welded O _CL. \/ Process the weld to a smooth transition to base metal The symbols 4-7 mm high are drawn by thin lines. The angle a m 45° and the distance between the adjacent parallel lines of the symbol is not less than 0.8 mm. 188 Chapter 5. Welded Joints The extension lines are drawn as a rule on the visible welds on the projection where the weld is most clear (ordinarily on a plan projection). Extension lines should never be repeated simultaneously on several projections (for example, in plan and in cross section). Fig. 185. Designations on extension lines Symbols are marked above the wing if the extension line is drawn from the face side of the weld (Fig. 185a) and under the wing (in an inverted position) if the extension line is drawn from the reverse side of the weld (Fig. 1856). The symbols for two-side symmetric welds are written in the middle of the wing (Fig, 185c), (a) (b) Fig. 186. Designation of intermittent welds The designations of intermittent welds include the length l and the pitch t of the wmlded portions (the diameter d and the pitch t of the spots are indicated for spot welds) separated in the case of chain welds by a skew' line (Fig. 186a) and for staggered welds by the sign 7. (Fig. 1866). Designations of welded joints arc illustrated in Tables 6-10. Figure 187 shows some additional symbols. Welds made to a clo¬ sed contour are designated by a circle at the intersection of the extension fine and the wing (Fig. 187a). Welds done during assembly are marked by the symbol ~| (Fig. 1876). Double-flanged butt joint One-side square butt joint Two-side square butt joint Square butt joint with de¬ tachable strap Square butt joint with permanent strap Lock joint One-side single bevel butt joint Two-side single¬ bevel butt joint Depiction of weld on driiwinge Lap Joints Table 7 Corner Joints Table 8 iJepicuon of weld on drawings Type of Joint A Veld Symbol in plan face side | reverse side 1 section Table 10 Joints Formed by Electric Resistance Welding Type ol Joint Weld Symbol Projection wel¬ ding lan i i ~n — ! d> o o d Nonfusion butt welding ■ T JL — Fusion butt wel¬ ding m t — 52. Welds as Shown on Drawings 195 This symbol is employed only for the simplest assembling units. In the case of intricate joints assembling drawings should he separately provided for each unit. Welds intended for machining are marked by the finish symbol written on the extension line (Fig. 187c). The symbol is used only in the simplest cases (cleaning of a weld). If machi¬ ning changes the shape of the weld and affects the adjacent portions of the^base metal a separate drawing (weld as¬ sembly) is provided which shows the product after welding with all neces¬ sary machining allowances, as well as the machining drawing (mechanical assembly) showing the product in its final form. Welds of the same type and size are designated only once indicating the total number of welds of a given type (Fig. 187d), the other welds being marked only by extension lines. If the welds are to be numbered according to the table on the drawing the ordinal number is written after the symbol (Fig. 187e). The figure should be 1.5-2 times higher than other symbols. The length l of triangular fillet welds is marked as shown in Fig. 187/. The design thickness a of the sheets to be welded is also marked for other fillet, welds (Fig. 187g). Additional data (for examp¬ le, for strengthening processes) are written under the wing (Fig. 187 h) or indicated by symbols which should be interpreted on the drawing or in technical documents. Technical specifications use special denominations consisting of a letter indicating the mode of a welded joint (C, H, Y, T— denoting butt, lap, corner and tee joints, respectively) and a figure specifying the tvpe of a weld according to the USSR State Standard (rOCT 8713-58). The method of designating the welds by a basic line is inconvenient for welds formed on separate portions of edges since the line of weld merges with the line of the contour arid it is impossible to determine the length l of the weld (Fig. 188a) and coordinate the weld from the datum surface (size s) without additional explanations. A weld can be shown by straight or slightly curvedfdash lines (Fig/1881) with the height approximately equal to the width of the weid (to the scale of the drawing). The necessary dimensions are marked on the projection. Invisible welds are depicted by spaced lines. Another method is to show the contours of a weld by thin lines, solid for visible welds and dash lines for invisible ones (Fig. 188c), The drafting process is retarded if the welds are marked by bold lines (Fig. 188 Q « 2 ±. / / 7 1 r (a) ' (b) ' (C) Kl\-1 Kl\L \/a*l KK / , //, (e) (/) rg) (h) Fig. 187. Additional symbols 13 * 196 Chapter S. Welded Joints The methods shown in Fig. 1886-rf make it possible to indicate the dimen¬ sions of intermittent chain and staggered welds directly on the drawing (Fig. 188e) and also specify the distance $ of the weld from the datum surface which is not shown by the symbols. Fig. 188. Depiction of partial welds The symbols of welds for which the edges are prepared must be explained in the technical specifications for welded joints and refe¬ rences to respective standards. For nonstandard welds, drawings should be prepared indicating the dimensions of all weld elements and edges (angle of edge preparation, clearance between edges, amount of edge blunting, height of reinforcement, etc.). The drawings of lap joints should specify the width of lap, the distance of welds from longitudinal and transverse edges, and the dimensions and coordination of holes for plug joints. 5.3. Drawings of Welded Joints The documents for welded joints usually include drawings of blanks, an assembly drawing of a welded joint (weld assembly), a machining drawing (mechanical assembly) and a drawing of the welded part in its final form. An example of complete drawings of a welded structure is illust¬ rated in Fig. 189. Blanks (Fig. 189a, b) are drawn in the form in which they are delivered for welding with all the necessary allowances for subse¬ quent machining of the joint. Machined surfaces of blanks untouched by the machining of the welded joint are drawn in their final form indicating the needed tolerances and finish symbols. On the weld assembly drawing (Fig. 189c) the product is shown as it should be after welding. Only parameters that are necessary for welding are specified: dimensions, type, length of welds, the dimensions showing the mutual arrangement of parts (without locating datum surfaces), and also the dimensions required to make welding jigs. Superfluous dimensions (repeating the dimensions of the blanks, the dimensions being self-evident after connecting the parts by the locating datum surfaces) will only complicate the drawing and divert the attention of the worker. Fig. 18t). Drawings of welded] joinLs r/, b—blanks; c— weld assembly; ■ 3 tapered portions of length l >■ 5 (5 — s); P>3 (5 — 4;) are introduced Welding a flange to a thin-walled pipe The flange is given a thin- walled annular transition portion f Welding a pin to a plate The pin is given a thin-walled flange Cutout is provided in the pin in the welding 5.5. Increasing the Strength of Welded Joints 203 Table Jl (confirmed) ' Design poor improved Welding disks to a gear rim Rim is given tli in-walled transi¬ tion rings Arrange simple fixing of parts so that welding jigs are dispensed with Head is centred on the bar Welding a flange to a pipe Flange is centred on the pipe arid held in the axial direction Welding a boss to a plate Boss is fixed axially by shoulders Seam welding a partition to a shell Partition is held in the axial direction by a flnte Chapter 5. Welded Joints 204 Table 11 (continued) Design poor | improved Avoid laborious edge preparation. Form welding pools by part displacement Welding of edges 3 : Comer joint Connecting shaped parts to plates r lj fj .L t Lj _n 11. _k Welding pipes to a coupling iHi tC-i Process the parts which are simple to machine Plug is machined g ' v i 5.5. Increasing the Strength of Welded Joints 205 Table 11 (continued) pior Design improved Eliminate fitting of preformed parts to complete joint contours. Simplify the preformed parts Welding a preformed rib to a trough-shaped profile Gusset plate The curved cut in the gusset plate is replaced by a stra¬ ight one Rib is cut out at the fillet Unify the blanks Welded sheave t __ — *r Sheave is made of two identical | -- parts : jjl Tank Tauk halves are identical For thin-walled materials make wide use of bent and die-forged elements to increase the rigidity Welding of a flange :- /> -- .A : n 3 r j =, The composite flange is re- it j placed by a formed one H 1 L 206 Chapter 5. Welded Joint- Table 11 (continued) Reinforcing pipe corner joints Separate flat gusset plates are replaced by one Lent plate Reinforcing a trough-shaped profile Connecting a flange to a pipe Prevent burn and fusion of thin edges in the welding zone 5.5. Increasing Ike Strength of Welded Joints 207 Table 11 ( continued ) Design poor improved Welding a bushing to a lever Burn of thin edge k is prevented by increasing its cross section Welding a flange to a ferrule Fusion of the edge of hole u> is pre¬ vented by removing the weld away from the hole. Another method is to drill the hole after welding Remove machined surfaces from the welding zone. Machine accurate surface after welding Welding of a threaded fitting j n \ Thread is removed from the > ““P \ F l weld to a distance l sufficient to L i 3 M t prevent fusion of the thread J —e X- 1 Welding of a pin. 1. Weld is removed from the machined surface 2. Slock on the pin is removed after welding 2r>8 Chapter 5. Welded Joints ’ Table 11 {continued) Design poor improved Welding of a bushing 1. To prevent warping of the hole the weld is moved away from the body of the bushing 2. Hole is finish machined after welding When parts with different cross section are welded, use heat buffers to prevent thermal stresses caused by nonuniform cooling Welding a jacket to a cylinder When welding closed cavities, prevent warping of walls caused by the formation of vacuum during cooling Welding an annular rigid profile 10 to shell 11 5.5. Increasing the Strength of Welded Joints _ 209 Table 11 (continued) Design poor i improved Do oot weld together hardened and chemically heat treated parts (the effect of heat treatment is lost in heating) Connecting a hardened tip to a tubular rod 1. Tip is connected by rivet welds 2. Welding is replaced by press-fitting 3. Head is stellitized Figure 191, 1-3 shows consecutive strengthening of a torsionally loaded unit with a welded flange by increasing the diameter of the circular weld. The resistance to shear (proportional to the square of the joint diameter) with the same weld cross section is seven times larger in the design 2 and eighteen times larger in the design 3 than in the design 1. If the design of the weld is correct the additional fasteners (the thread, Fig. 191, 4, the heavy drive fit. Fig. 191, 5, etc.) may be dispensed with. In centring joints the parts being welded are located by clearance fits usually with a class of accuracy not above the 3rd one (fits Se s , Se±, R 3 , if 4 , Rl 3 ). If more accurate centring is required use is made of slide fits S sa , S 3 and wringing fits W.a. U'•>. Welds should be relieved by transferring the load to sections with solid material, the welds being intended only to join the parts. Some examples of relieving the welds of loads are shown in Fig. 191, 6, 7 (a bar loaded with an axial force) and in Fig. 191, 8 , .9 (bearing flange). In the unit fastening the cover to the shell of a cylindrical reservoir subjected to internal pressure (Fig. 191, 10) the welds of the cover and the shell are bent and shorn off by the pressure forces. In the improved design 11 the weld of the shell is relieved of internal pres¬ sure by introducing the shell into the flange and the weld of the bottom is relieved by clamping the bottom between the flanges of the shell and the bottom. 14—01658 55, Increasing the Strength of Welded Joints 211 Power welds should preferably be loaded by shearing and ten¬ sile forces whereas bending load should be eliminated. Figure 191, 12 shows an irrationallyjwelded-on bar loaded with transverse force P. The force P rotates the bar about point O and produces high tearing stresses in the area opposite this point. Besi¬ des, the weld is subjected to shear. Figure 191, 13 shows a better setup. The*bar is centred in a seat of the part, and the weld is not subjected to shear. But the critical cross section of the bar is weakened by the weld. In Fig. 191, 14 bending and shear caused by force P are acting upon the solid cross sections of the bar which are not weakened by welding. The weld is virtually relieved of the stress and is used only to secure the bar in the part. It is better to reinforce with a rib the welded-on wall which ia subjected to bending by force P (Fig. 191, 15, 16). The bending of the butt-weld (Fig. 191, 17) can be eliminated by using a strap (Fig. 191, 18) whose welds are mainly in tension. In this design the butt weld is in compression. The butt-weld of the angle bars (Fig. 191, 19) is not strong enough. It is more reasonable to weld them over the plane of the flanges (Fig. 191, 20) and strengthen them by corner plates for arduous operation conditions (Fig. 191, 21). It is better to join the corner plates not by butt welding (Fig. 191, 22) but by lap welding (Fig. 191, 23). Welded-on ribs should be positioned so that they work in comp¬ ression (Fig. 191, 25) and not in tension (Fig. 191, 24). This practi¬ cally relieves the welds of all load. Figure 191, 26-29 presents a consecutive strengthening of a sheet joint loaded by tensile force P and bending moment Mi IL , nd . The strength of various joints is compared in Table 12. Table U Joint Strength tensile bending Butt joint (Fig. 191, 26) 1 1 Lap joint (Fig. 191, 27) 2 4 Lap joint with welded-on reverse side (Fig. 191, 28) 3 5 Single-V lap joint (Fig. 191, 29) 2.5 5 The strength of the butt joint shown in Fig. 191, 26 is assumed as a unit. Besides all-round welding over the contour of long and thin pla¬ tes, straps, corner plates, etc,, should preferably be connected with 14 * 212 Chapter 5. Welded Joints the basic member by further spot welding (Fig. 191, 30) so that the plates may not come off when the system is deformed. Skew welds of a lap joint (Fig. 191, 31) subjected to tensile stres¬ ses are also affected by additional stresses emanating from shear along the line of the weld. In the balanced single-V joint (Fig. 191, 32) the welds are relieved of shear. Figure 191, 33-36 illustrates the weld designs of channel bar as¬ semblies. In the joint with the channel legs arranged upwards (Fig. 191, 33) the sections m of the vertical welds are subjected to high tearing stresses resulting from the action of force P. When the channel bar has its legs downwards (Fig. 191, 34) the load is taken by the long horizontal weld n and the weak end sec¬ tions of the vertical welds are subjected to compression. When the channel bar is connected by a tongue (Fig. 191, 35) the welds are relieved of bending stress caused by force P. The bending moment is taken by longitudinal welds and the transverse weld t is in shear. Fig. 191, 36 shows a joint strengthened by a corner plate. Out-of-centre force application causing a weld to bend should be avoided. Flanged welds in units subjected to tension (Fig. 191, 37) are bent. Butt-weld designs are better (Fig, 191, 3S). In the unit where a bottom is welded to a cylindrical reservoir with a flange (Fig. 191, 39) internal pressure bends the weld. The butt weld (Fig. 191, 40) is mainly subjected to rupture. Welds should not be arranged in highly stressed zones. In the case of welded beams subjected to bending it is good prac¬ tice to arrange the welds not at the flanges (Fig. 191, 41) but at the neutral line of the cross section (Fig. 191, 42) where the normal stresses are the lowest. In joints subjected to cyclic and dynamic loads, welds should not be made in sections where stresses are concentrated, for example in the transitions from one section to another (Fig. 191, 43). In these conditions the weld is highly stressed and is also the source of an increased stress concentration due to the heterogeneity of its structure. An improved design is illustrated in Fig. 191, 44. If it is impossible to move the weld beyond the section of stress concentration, concave welds should be used (Fig. 191, 45) with deep penetration being attained by welding with a short arc. The profile of a weld should be, as far as possible, symmetrical to the load action. Two-side welds (Fig. 191, 47) are very effective in tee joints subjected to tension (Fig. 191, 46). Butt joints (Fig. 191, 49) should be used in preference to lap joints (Fig. 191, 48). It is expedient to prepare the edges in butt joints on both sides 5.6. Joints Formed by Resistance Welding 213 (Fig. 191, 51) since the force lines are distorted in joints with an asymmetric weld (Fig. 191, 50), with sharp stress variations. The cyclic strength of welds can appreciably be increased by machining which imparts a rational form to the weld and reduces stress concentration. ' It is good practice to machine corner welds radially with a smooth transition into the surfaces of the parts being joined (Fig. 191, 52). Butt welds are machined flush with the surface of the product, the weld metal being removed both on the side of the basic weld and on the opposite side (Fig. 191, 53). For a smooth connection between a weld and the walls of a product it is necessary in most cases to undercut the walls simultaneously with the machining of the weld (dash lines on Fig. 191, 52, 53) providing for this purpose allowance c. •Figure 192 illustrates the cyclic strength curves for a strengthened butt joint (lower curves) and after the reinforcements arc removed by machining (upper curves). Thin lines show the cyclic strength of the joint without heat treatment, and thick lines—after stabilizing heat treatment (anne¬ aling at 670 °G). The diagram shows that the removal of the reinforcements increases the cyclic strength approximately twice and the heat treatment by 15-20 per cent. A smoothing fusion of welds with a tungsten electrode in argon medium considerably increases (by 30-40 per cent) the cyclic strength. Fig. 192. Effect of heat treat¬ ment and machining of "welds on cyclic strength. Steel OX12HHJI. According to Zaitsev G. Z. and Ponoma¬ rev V. Ya. Plastic deformation in the cold state (roll burnishing, shot blas¬ ting, coining with pneumatic tools) makes it possible to raise the cyclic strength of the weld to the strength of the base metal. 5.6. Joints Formed by Resistance Welding As a rule parts joined by butt resistance welding are not centred in relation to each other (Fig. 193a) because they are mutually fixed when mounted between the clamps of the welding machine and the upsetting mechanism. When the parts are centred (Fig. 1936) one of them should float in the clamps. When thin parts are welded to thick ones transition sections corresponding to the form of the thin part being attached should be provided on the thick part (Fig. 193c-e, /, g). < 214 Chapter 5. Welded Joints If increased stability is to be ensured against bending the parts are joined in tapering seats (Fig. 193ft). This design sharply reduces the force necessary to compress the parts when welding. As distinct from electric arc -welding, butt resistance welding makes it possible to join parts with machined surfaces (e.g., threaded Fig. 193. Joints formed by resistance welding members). The accuratejsurfaces should be located from the plane of the joint by distance of ft >■ 4-6 mm (Fig. 193/) to prevent defor¬ mation and protect them against the sparks of molten metal. The amount of welded on metal and spark formation can be reduced and the consumption of electric energy decreased if welding is done with the use of separate projections m. In the case of spot and seam welding of thin parts (less than 2 mm thick) the diameter of the spot and the width of the weld should 3ds 6-fcr-t. s 4f' >e i \__ 1192-41; 1195-41; 1-8 ST ■ "fe 1192-41; 16-25 «(J=T3- OCT IJKT 8218/1170; 2-9 tf 5=3 ,! te 1192-41; 10-13 fS 240 Chapter 6. Riveted Joints of rivets, the type and magnitude of load, the relationship between the strength and hardness of the materials of the rivet and the parts being joined and, finally, on the method used to drive in the rivet. sd^toS) If we proceed from the functioning of a rivet in shear and base our calcula¬ tions on the condition of equal strength of the rivets (in shear and compression) and of the riveted plates (in compression, shear and rupture at the critical sections), then for the particular case of a single-row lap joint (Fig. 212a) with the same strength of the material of the rivets and the plates the following rela¬ tionships can be obtained: d — 2s; t— 2.5 d; e= 1.5 d This calculation gives exaggerated values of rivet diameter (especially when the values of s are large) and re¬ duced values of the pitch. Fig. 212. Design proportions of riveted joints Fig. 213. Determining the diameter and spacing pitch of rivets In practice, use is made of the following relationships (Fig* 2125): d = s + (4-8) (6) 3d <1 i < 6d (?) 1.5 d < e < 2 d (8) In these formulas all dimensions are in millimetres. Rivets with diameters smaller than those determined from for¬ mula (6) are difficult to forge and they may bend in the hole (Fig. 213a). Clinching of large-diameter rivets may overstress the material of the members being connected. When materials of various thickness are riveted it is necessary to take as the basis their total thickness 5 (Fig. 213b). When S — 5-60 mm the diameter of a rivet can be found from the formula 3-4 per cent) can be defor¬ med plastically in a cold state (for example, annealed steel, copper, aluminium and magnesium alloys, and annealed titanium alloys). As far as normalized and structurally improved steels are concerned plastic deformation is difficult. The methods of plastic deformation cannot be used for brittle metals (grey cast iron) and also tor steel hardened or subjected to thermo chemical treatment (carburizing, nitriding and cyaniding). The principal methods of plastic deformation are as follows: clinching, expanding, spreading, calking and punching. Thin-sheet Fig. 225. Fastening by plastic deformation methods a, d, f, ft, j—irrational designs; 0, t, e, g, i, ft—rational designs structures are also subjected to bending, outside flanging, beading and seaming. As a rule, plastic deformation should be limited to a necessary minimum (Fig. 225). The smaller the volume of deformed metal and degree of deformation, the lesser the hazard of cracks and tears and the stronger the joint. 256 Chapter 7. Fastening by Cold Plastic Deformation Methods A reduction in the amount of plastic deformation lessens the force required for deformation, makes it possible to employ harder and stronger materials for the joints and increases, with all other ■conditions being equal, the efficiency of the fastening operations. 7.1. Fastening of Bashings Figure 226 illustrates methods of fastening bushings by spreading the metal into taper seats (Fig. 226a) or into annular recesses in Fig. 226, Fastening of bushings the locating holes (Fig. 226b, c). provided on the internal surface Fig. 227. Fastening of bushings i sheet members he laps {shown by the dash line) of the bushings are intended to allow the metal to expand. The axial force of metal spread¬ ing is taken by the thrust of the end-face (Fig. 226tr, b) or the flange (Fig. 226c) of the bushing against the enveloping part. After spreading the internal surface of the bushing is finish- machined by a sizing mandrel or a cutting tool. Figure 22 (yd, e shows the ways bushings are fastened by the ex¬ panding method. Bushings are fastened in sheet members by expansion (Fig. 227n) or by spreading the protruding collar 7 under a press or with the aid of other appliances (Fig. 227b, c). 7.2. Fastening of Bars Massive cylindrical parts (columns, pillars, etc.) are fastened by expanding their end-faces (Fig. 228). The parts are usually set up with transition or interference fits. A heavy drive fit ensures the best joint, expansion serving only for additional safety. _ 7.2, Fastening^ of Bars ___ Figure 229 shows the principal modes of fastening tubular bars in massive workpieces: (1) the bar is inserted in an inverted tapered seat (Fig. 229a) and is fastened by expanding laps 1 with a cylindrical spreader (Fig. 2296); (2) the bar is locked by expanding the massive end into the cylindrical slot in the seat (Fig. 229c, d); (3) the bar is driven into a seat having a tapered insert. Bearing against shoulder 2 the bar is for- Fig. 228. Fastening of columns ced down to expand the bar mate¬ rial into the cylindrical slots of the seat (Fig. 229e, f). In an improved design the taper made integral with the bar (Fig. 229g) is connected with the latter by a thin neck. When the 258 Chapter 7. Fastening by Cold Plastic Deformation Methods 7.3. Fastening of Axles and Pins Pins made of soft material that yields lo plastic deformation are fastened by clinching and expanding their ends (Fig. 231a-c), pun¬ ching the pin ends at several points (Fig. 231d, e) and extrusion on the periphery of the pin end formed by an annular calking tool fto (v) M tx) (y) Fig. 231. Fastening of axles and pins in plates with internal teeth (Fig. 231/). Figure 231 g shows the method of fastening by expanding the end of the pin to a taper with an annular calking tool. Figure 231h-jr shows the methods of fastening when the pin is secured against rotation. Figure 231h shows the pin being locked by upsetting its square end in a square recess, and in Fig. 231 i the pin is locked by fitting in its rifled end. 7.4. Connection 0 / Cylindrical Members 259' Figure 231; shows the simplest method when the cylindrical end of the pin is calked into triangular slots made in the chamfer of the seating hole. All-over clinching may be replaced by local defor¬ mation as shown irs Fig. 231d, e. Pins made of hard materials lhal cannot be clinched are fastened in plastic metal workpieces by forcing the workpiece material into the circular groove on the pin {Fig. 231&), calking the workpiece material into the flats on the pin (Fig. 231/) and fitting with the use of rifles (Fig. 231m, n ). Figure 231 n~u illustrates the methods of fastening the pins in rigidly interconnected parts (for example, in the cheeks of forks, in shackles, etc.) when the pin is calked on both ends. In Fig. 231o the pin is secured by punching the part at several points on its periphery, in Fig. 231p-r by circular expansion and in Fig. 231s by local extrusion. Rotation of the pins is prevented by punching the metal of the part into the slots milled in the pin (Fig. 231/) or calking the mate¬ rial into flats on the pin (Fig. 231u). If the pin and the cheeks are made of hard materials that cannot be clinched fastening is done by means of flattened plugs (Fig. 231u) or rings made of plastic materials (low-carbon steel, annealed cop¬ per, etc.) which are calked into the cuts in the pin (Fig. 231m-i/). 7.4. Connection of Cylindrical Members Coaxial cylindrical parts (for instance, bars and enveloping bushings) are joined by calking or rolling the bushing around annular shoulders (Fig. 232a) or into grooves (Fig. 232&, c) in Hie bar. Fig. 232. Connection of cylindrical members If, according to its function a joint requires free rotation of one element in relation to the other the surfaces to be connected are coated with a layer of separating graphite grease before calking. In such cases the grooves should he of rectangular shape (Fig. 232(f), 17 * 260 Chapter 7, Fastening by Cold Plastic Deformation Methods 7.5. Fastening of Parts on Surfaces Small cylindrical parts such as bosses, contacts, supporting feet, etc., mounted on the surfaces of members are secured by calking in inverted cone seats (Fig. 233a-/). The same methods are employed to fasten circular elements, for example, annular seals, valve seats etc. (Fig. 233g-l). Fig. 233. Fastening of parts on surfaces Figure 234 shows some methods of fastening valve seats. Dosings a and h (Fig. 234) are used for seats made of plastic metal (bronze, austenite steel, etc.) fitted into hard and brittle (cast iron) metal housings, and designs c and d (Fig. 234) for seats manu¬ factured from hard metal and inserted into plastic metal housings (aluminium alloy). In Fig. 234c, fastening is done by calking or rolling the material of the housing around the seat (section in). In Fig. 234d, the seat is screwed into the housing and locked in position by rolling the circular groove in the hole (section «) with the following metal inflow into teeth cut in the underneath end- face of the seat. Segments, flat springs and similar parts are secured to the surface of large parts by fitting them into slots (Fig. 235a) and spreading 7.7. Fastening of Plugs 261 the material with a punch at several points. Longitudinal movement of the segment is prevented by filling semicircular cuts with metal. Fig. 235. Fastening of segments and rods A similar method is also used to secure cylindrical rod-type parts (Fig. 235 b, c). 7.6. Swaging Down of Annular Parts on Shafts The method of plastic deformation is frequently employed to calk cylindrical elements such as rings (Fig. 236a, b) and sleeves (Fig. 236c) on shafts. (a) ib) (c) Fig. 236. Swaging down of rings and sleeves on shafts Parts of this type are swaged by presses with split bushings or still better on rotary swaging machines applying the effort simul¬ taneously at several points on the periphery and smoothly increa¬ sing the force. Hand calking and all-round rolling cannot be used in this case since they spread the bushing instead of giving it the necessary compression. 7.7. Fastening of Plugs Piugs are secured in hollow shafts by expanding the shaft (Fig. 237a, b), by calking the plug periphery until the metal fills the annular groove first made in the shaft (Fig. 237c, d) and by spreading the plug rim with a tapered punch (Fig. 237c). 262 Chapter 7. Fastening by Cold Plastic Deformation Methods Figure 237/, g illustrates the methods of fastening thin-sheet plugs by expanding them into an annular slot in the hole of the shaft. Flattening of plugs is also extensively employed (Fig. 237ft). Initially, the plug has a spherical form and is spread by a flat punch (a) (h) tc) (d) (?) (f) ($) (hi Fig. 237. Fastening of plugs with the other side resting against a flat support. The plug periphery is then forced in the shaft groove. Radius R of the sphere can be found from the formula ^ 0,25d V( W 11 where d — plug diameter D = diameter of the groove for the plug On average ^ = t.03 in which case R « d. a Figure 238 shows the methods of fitting plugs into thin-walled pipes. The joint should be designed so that supports may be used to form and flatten the seam: flat ones (over surface m in Fig. 238a-c) Fig. 238. Fastening of plugs in pipes or cylindrical ones (over surface n in Fig. 238d-/). The designs that allow the admission of cylindrical supports from outside (Fig. 238e, /) are preferable to the ones where an internal support is necessary (Fig. 238 d). 7.9. Fastening of Tabes 263 7.8. Fastening of Flanges to Pipes Flanges are attached to thick-walled pipes (walls 4-6 mm thick) by rolling the pipe ends into annular grooves in the flanges (Fig. 239a, b). Fig. 239. Fastening of flanges to pipes The methods of fastening flanges to thin-walled pipes are illust rated in Fig. 239c-e. 7.9. Fastening of Tubes Figure 240a-c shows the methods of fastening tubes in sheets and plates. Thick-walled tubes with a wall thickness of 2-5 mm Fig. 240. Fastening of tubes in sheets and plates (fire and water tubes of boilers) are fastened by expanding the ends of th'e tubes by rollers. Annular grooves (Fig. 2405, c) are provided in the hole to increase the strength and tightness of the joint. Figure 24 Qd-g presents methods of fastening thin-walled tubes. Fig. 240g shows the strongest and stiffest design. In this design the tube is secured by a thick bushing 1 with a tapered projection m expanded when the tube is installed. Figure 241 shows methods of fastening oil-feed tubes in shafts, drawn (Fig. 241a-/) and turned (Fig. 24ig-i). Fig, 241. fastening of oil-feed tubes in shafts 7.10. Fastening by Means of Lugs This mode of connection is used for thin-sheet structures. The lugs (Fig. 242o-c) on one of the parts to be connected are introduced into the slits in the adjacent part and bent over. Another method is flanging the lugs perpendi¬ cular to their plane (Fig. 242d). This method is used when the design does not allow a bending force to be applied. The joint can be tightened with a certain interference if a cham¬ fered cut is used (Fig. 242c). The strength of such joints is not high. In some cases these methods are employed in power structures. Figure 243 shows a unit for faste¬ ning blades to the shells of an annular return-circuit rig of an axial air compressor. The large number of attachment points makes this design sufficiently strong and rigid. 7.11. Various Connections 265 7.11. Various Connections Figure 244 illustrates the methods of connecting sheets and plates by fluting them with punch 1 in thin sheet (Fig. 244a) or in thick plate (Fig. 244b), the closing head being formed by die 2 installed on the side opposite to the motion of the punch. Fig. 244. Fastening of lining sheets The fastening of lining sheets to massive members by rivets (Fig. 244c) or bolts (Fig. 244d) can be reinforced by plastic defor¬ mation of the lining at the joint. Figure 245a, b illustrates a method of spreading the shaft mate¬ rial into tapered recesses in disks when mounting light gears and Fig, 245. Fastening of disk-shaped Fig. 246. Units connected by plas- parts on shafts tic deformation methods other disk-shaped parts on stepped shafts. Figure 246 shows; fastening of a hub on a shaft by means of a flattened washer (Fig. 246a), faste¬ ning of a disk by calking the metal into an annular recess in a shaft (Fig, 246fe), fastening of a sleeve on a shaft by calking the metal into recesses (Fig. 246c) or into an annular groove (Fig. 246d) in a shaft. 266 Chapter 7. Fastening by Cold Plastic Deformation Methods 7.12. Seaming Seaming is used to connect sheet workpieces of various thick¬ nesses ranging from several tenths of a millimetre (tin) to ,1-2 mm. Fig. , 247. Seaming of pipes fm) (a) (o) (p) Fig. 248. Seam joints Figure 247 shows seam joints used to connect thin-walled pipes and shells. 7.12. Seaming 267 The most common joint is the one in which the edges are first flanged (Fig. 248a) to form a lock (Fig. 2486) after which the lock is bent over and flattened to make a four-layer seam (Fig. 248c). Fig. 249. Seaming of covers Figure 248d-/ presents a seam overlapped by a strip of sheet mate¬ rial, Fig. 248 g-k —a strengthened six-layer seam (combination seam) and Fig. 2481-p—a seven-layer seam. The seam joints shown in Fig. 248 are employed to con¬ nect flat sheets and form longitudinal seams of cylindrical shells. Figure 249 shows methods of seaming bottoms and covers to cylindrical shells. Figure 249a, b shows designs employed to connect comparatively thick materials (0.5-2 mm). Tin products are connected by three-layer (Fig. 249c-/), four-layer (Fig. 249m-o), five-layer (Fig. 249p-r) or seven-layer (Fig. 249s-w) 268 Chapter 7. Fastening by Cold Plastic Deformation Methods seams. The seam is flattened during the last operation by pressing it against a centre mandrel placed in the bottom recess. The most popular types of seaming are shown in Fig. 249p-r. Figure 250 illustrates mechanized seaming of such joints on multiple-posi¬ tion rotor machines. Seaming is done in chucks consisting of a centre mandrel 1 and rollers 2 and 3 performing planetary motion around the workpiece. Ordinarily, use is made of two rollers arranged dianietrally on the periphery. The initial stage of the operation is shown in Fig. 250a. The cover is delive¬ red for seaming with its edges already bent. The edges of the shell are also flanged in advance. At first, rollers 2 used for the first operation are brought to the workpiece (Fig. 2506) to form the seam, and then rollers.? for the second operation (Fig. 250c) to flatten and compact the seam. The rollers for the first and second operations'are usually mounted in a stag¬ gered order in one chuck. As the rotor revolves the rollers for the first operation and then the rollers for the second operation are automatically brought into use. Multiple-chuck seaming machines operating on this principle can finish up to 500 pieces per minute. Index A Assembly, 9-54 axial, 11-21 facilitating, 48-54 locations of, 29-30 radial, 11-21 selective, 10 successive, 22-25 tools, access of, 34-36 wrong, prevention of, 30-33 food proof, 33 c Castings, open, 67-69 separation into parts, 78, 80 shape, simplification of, 78 strength of, 61-63 variations in dimensions, 102-108 wall thickness of, 61-63 Casting methods, 60-61 cavityless (full-form), 61 centrifugal, 61 chill, 60 pressure die, 61 sand mould, 60 semi-permanent mould, 61 shell mould, 60 Centre holes, 103-165 Chamfering of form surfaces, 150-151 Chapiets, 75 Cluster gear assembly patterns, 18-21 Cod, 67 Composite structures, 119-121 Cutting tools, approach of, 132-130 overtravel of, 127-132 Conjugation of walls, 87-89 Connection of cylindrical members, 259 Contact between teeth, 38 Contour milling, 148-150 Controlled cooling, 86 Cores, 60, 69-78 band, 72 fastening of, 73-76 installation of, 71, 73 prints, 73-78 holes for, 76-78 unification of, 72-73 Core moulds, 60 Cutting tools, 153-163 elimination of, deformations caused by, 155-157 unilateral pressure on, 153- 155 reduction of the range of, 161-163 shockless operation of, 158-159 D Design rules, 87-101, 199-209, 243-245 Design tapers, 80 Dimensioning, 109-111 Disassembly, facilitating, 48-54 independent, 21-22 Dismantling of flanges, 28 E Elimination of massive elements, 89- 91 Escape of gases, 71-72 F Fastening, of axes and pins, 258-259 270 Index of bars, 256-258 of bushings, 256 of flanges to pipes, 263 by means of lugs, 264 of parts on surfaces, 260-261 of plugs, 261-262 of tubes, 263-264 Fillet welds, 184 concave convex (reinforced) dimensions of straight (normal) Finish of machines, 57-59 Flanges, 94 G Gear drives, 37-47 bevel, 41-46 spur, 37-41 spur-and-bevel, 46-47 H Holes, 95 I Interlocking devices, 56-57 K Kinematic accuracy, 38 L Locations, axial, 101-102 casting (rough), 101 rough surface, 101 Machining, of bosses in housings, 152- 153 cutting down amount of, 114-117 elimination of superfluously accu¬ rate, 121-122 of frictional end surfaces, 153 of holes, 159-161 increasing the efficiency of, 167- 171 joint, of assembled parts, 146-147 of parts of different hardness, 157-158 multiple, 171-173 consecutive, 171 parallel, 171 parallel-consecutive, 171 in a single setting, 144-146 of sunk surfaces, 151 through-pass, 123-127 Measurement datum surfaces, 165-167 Moulding, 63-78 drafts, 80-81 mechanical, 60 Mould parting, 66-67 N N on-recoverable punch, 250 P Press forging and forming, 117-119 Prevention of blowholes, 93 Protection against damage, 54-56 R Reduction of shrinkage stresses, 91-92 Ribs, 95 Rigging devices, 36-37 Rimming, 94 Riveted joints, 229-254 strengthening of, 245-246 types of, 234-236 Riveting, cold, 231-233 hot, 229-231 of thin sheets, 253-254 Rivets, blind, 249-253 calculation of, 230-231 heading allowances, 242 installation of, 244-245 materials, 233-234 set, 249, 251-253 shapes of, 245 solid, 246-247 special, 253 tubular, 247-248 thin-walled, 249 types of, 237-239 varieties of, 232 Rule of shadows, 64 s Seaming, 266-268 Separation of surfaces, of different accuracy and finish, 136-141 rough and machined, 141-143 Shrinkage, 82-83 free, 83 linear, 82 restricted, S3 Index 271 rules of, 83 volume, 82 Socket wrenches, 34-35 Solidification, 85-87 - directional, 87 simultaneous, 85-86 Smoothness of run, 38 Stresses, internal, 83-85 u Undercuts, 184 elimination of, 63-66 V Various connections, 265 w Wall thickness, 100-101 Warping, 175, 183 Welded frames, 221-224 Welded joints, drawings of, 196-199 increasing the strength of, 199, 209-213 as shown on drawings, 180 truss, 225-228 types of, 184-193 butt, 184, 186, 189-190 corner, 184, 186, 192 lap, 184-185, 191 slotted (plug) welds, 185 transfusion, 185 . tee, 184, 186, 193 Welding of pipes, 215-216 Welding-on, of bars, 219-220 of Dushings, 217-219 of flanges, 216-217 Withdrawal facilities, 25-28 for flanges, 28 in standard machine elements, 26-27 for tightly fitted hubs, 25, 27