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Manufacturing of hollow transmission shafts via bulk-metal forging
ELSEVlER
Abstract More and more hollow transmission shafts are used in al! \-chicle gears as ;Lresult of the demand of Iigh~ components. DiKerent methods of bulk-metal forging are shown partially in combination with joining and machining processes. They will be discussed with reference to shaping, tolerances and economical aspects. Q 1997 Elsevier Science S.A. Kq~ords: Hollow transmission shafts; Buik-metal I‘orglng
1. Intraduction for gears have to be developed due to an increasing demand for performance with the simultaneous demand for reduction of fuel as well as noise emission and exhaust fumes. Nowadays, the Sgear-synchromesh gearbox is a standard equipment, top car models even have six gears (I]. Cars with an automatic transmission, which are expected to increase significantly in the next few years, have mainly a 4gear or 5-gear transmission. Every time a new type of transmission is introduced, the automobile industry and their suppliers are challenged from the ecological point of view to carry out the structures and/or components of the gears more cost effectively, more efficiently and more compactly, at the same time using less material. The size of the gearbox should stay the same, or even be reduced. This means that the weight of the gears will be reduced, as it already has in the last few years (21. When putting the demand of light components into practice, it has to be done without decreasing the life time and the reliability. There are in principal two ways to achieve this: the use of new materials, which combine the same properties with lower weight; or an optimized design of steel parts relative to improved mechanical For
all vehicles,
new concepts
* Corresponding author. Tel.: + 49 36925 2481 I; fax: t 49 36925 24899. 0924-0136/97/$17.00 Q 1997 Elsevier Science S.A. All rights reserved. PIISO924-0136(97)00156-8
properties. The latter is of special interest when no other material than steel can be used because of the special properties like high loads. temperatures, and high strength at sufficient ductility. These parts are above all gear shafts.
Generally, three shafts are necessary in a transmission: driveshaft (input shaft), counter shaft and output shaft (main shaft). In an automatic transmission gear, there is the intermediate shaft between the driveshaft and the output shaft. The weight of these shafts for cars is between 2 and 5 kg and for heavy duty vehicles between 8 and 20 kg. A weight reduction for the shafts mentioned and for stem pinion of rear axle transmissions, which are also part of this family, can only be achieved with hollow shafts. The principal design of these parts in general is very similar (Fig. 1). In the case of manually shifted transmissions, the driveshafts have a head on one end and a more or less structured shank on the other end. Main shafts have a flange in the middle and shanks on either side, whilst counter shafts have two or three fixed gears. The tension on these shafts concentrates on the surface area, whereas the material in the core is under less stress. Therefore, the core material is redundant. If the diameter of the bore is approximately half the outer
114
F. S&tit&r,
P. Ketttter / Jortrttnl oj‘Mttcrids
diameter, the moment of inertia or second moment of area is reduced by less than 10% compared with a solid shaft. The result is a possible reduction of material and/or weight of up to 300/, i.e. only slight changes regarding rigidness and strength occur when using a hollow shaft with the same outlines instead of a solid shaft. In principal, the following processes for manufacturing hollow shafts can be used, in case the advantages of metal forming processes should be used, e.g. little material waste, short manufacturing times, and high strength: shear spinning; forging (conventional); (wedge) cross rolling; rotary swaging (upset swaging); rotary forging; internal high pressure forming; cold extrusion (conventional); cold extrusion and friction welding; cold extrusion and deep drilling. When judging the different production methods, not only the most cost and material effective aspects, but especially criteria like reliability and reproducibility, regarding the tolerances, the shape and the nearest shape to the final workpiece geometry which can be achieved is most important. Achieving the closest tolerance is a problem when a tube slug as material has to be used. The tolerance on tubes with thick walls, especially with respect to the coaxiality between outer and inner diameter, are subject to deviations, which are very difficult to balance in the following working steps. However, since the number of revolutions on transmission shafts is very high, out of balance run-out on the diameter can only be accepted to a certain extent. As shear spinning is only used for very few parts and forging is only applied in the case of shafts for heavy duty vehicles, these processes will not be described any further.
Fig. I. Various output shafts,
Praumittg
Tdttdogy
71 (1997) 113- 118
Fig. 2. Upset waged hollow shaft (courlcsy: PtUiHhlP).
3. Cross rolling Wedge cross rolling of solid shafts with more than one flange is a very effective method at hot forging temperatures, with production frequencies of 3- 12 s. In industry, two methods have been successful: the round yaw cross rolling according to the double roll principal, and the flat yaw cross rolling. Preliminary tests show that hollow shafts can also be cross rolled. However, we do not know any proceeding features and guidelines for the production of forged hollow shafts. Above all, it has to be settled whether tubes with thick walls can be used as material. Another element of uncertainty is the twisting of the grain flow during the wedge cross rolling process which occurs on the longitudinal axis; this can cause major inaccuracies when the teeth are manufactured in the following machining step.
4, Rotary swaging (upset swaging) Rotary swaging is a common method when reducing the diameter of rods and tubes. It is preferably used at room temperature. With this method, a11 the advan-
Fig. 3. Hollow main shaft for trucks.
art from forming a local wall thickness. the diameter the part can be reduced by cross roiling ehe workp&es wifhouf locating the part first [3]. !t is \‘ery difCxlt to manufactu:e a gear shaft economically with the general manufacturing proccsscs. it i> cery time consuming with rhc .axx settings (heating and upsetting, several reductions of t e long and sllort shanks) and the partial forming by rotary swag& Of course it would be possible to reduce the production ing machines that carry out several steps at the same time. However. it is not certain whether the same production frequencies as with cross rolling or cold forging can be achieved. Therefore. the characteristic feature of axial radial forming is its high flexibility for shaping workpieces with a similar geometry without using many tools. In the case of automation. it is more economical to make smaller quantities.
Fig. 4. Cold forged and drilled input shafts.
tages of cold forging like close tolerances, favorable grain flow, smooth surfaces and work hardening can be achieved. Generally, we differ between two methods: the infeed method and the plunching method. Two to four tool segments carry out an oscillating radial movement and form the workpiece in small singular steps. En the case of the infeed method, the workpiece is additionally moved through the oscillating dies in the axial direction. In the case of the plunching method, the tools carry out an additional radial movement which overlaps the oscillation and which is much faster than the oscillation. When using this process, the angle on the workpiece can be much steeper; it also allows reductions of the diameter between the ends of the workpiece, i.e. undercuts. When using the corresponding mandrel, bores with tight tolerances can be achieved which are cylindrical or in a cone shape. It is also possible to obtain inner profiles like splines. However, typical gear shaft geometries (Fig. 2) with a clear enlargement of the wall thickness at the flanges can only be produced in combined manufacturing steps on special machines (upset swaging). This process which is also called axial-radial-forming was developed at the University Darmstadt together with the HMP, Pforzheim. The tube is heated only at the point where
The difference between rotary forging and rotary swaging is that in the case of rotary forging the workpiece rotates and not the tools. Moreover, the shaping is carried out at forging temperatures. The tools consist mainly of two parts. Feeding is also in the radial direction. Also, in the case of rotary forging. the diameter is reduced. However, on the machines, upsetting can bc carried out also. If you have a sufficiently thick precision steel tube, the wall thickness can be doubled in some areas by upsetting with the rotary forging process. In further working steps, a further reduction at the ends of the shaft can be achieved on the same machine. It is a theoretical question whether the workpiece will be pre-forged in two or three working steps and completely machined afterwards. The tolerances with rotary forging are at some 10th millimeter. As an alternative,
Fig. 5. Hollow stem pinion.
116
ElDie Wear 6 17
0
Slug Weight: Part Weight:
4,8 kg 498 kg Solid Shaft cold extruded
498 kg 3.8 kg Hollow Shaft extruded and drilled
3.8 kg 3.8 kg Hollow Shaft cold extruded
Fig. 6. Cost comparison: solid vs. hollow shaft.
the further processing can be carried out by way of rotary swaging at room temperature. In this case, the tolerances would be optimized.
6. Internal high pressure forming It would also be possible to manufacture hollow shafts by internal high pressure forming. In this case the geometry of the tube could be changed by increasing the inner pressure and at the same time leading the material in the direction of the longitudinal axis of the tube. The outer diameter of the tube corresponds to the smallest diameter of the shaft. The inner geometry of the tool corresponds to the outer geometry of the hollow shaft. The material that is spread is obtained from the wall thickness and forms the length of the tube. Therefore, the tools for sealing have to follow in the longitudinal direction of the tube. As the sealing is problematic, it is necessary to have smooth slugs, so most of the time the tube ends are brushed. The enlargement of the diameter is limited by the deformation capacity of the material. If the method of internal high pressure forming is combined with upsetting in the longitudinal direction, hollow shafts with a flange in the center can be manufactured. The limits of this process depend on the geometry of the component. It is above all the relation between the diameter and the wall thickness, which allows only the manufacturing of some shapes. The smallest radii that can be produced are one and a half times the wall thickness. The tolerances of the outer outlines are between tolerance class IO and I4 and they also depend on the springback characteristics. The dimensional vari-
ations of the inner outlines are generally larger. Most of the time, they can only be defined after samples have been manufactured because of the free material flow in combination wiih the deviation of the wall thickness of the tube, radii or form and positional tolerances [4]. There is also the possibility in using the process to combine tubes conclusively with additional elements. This process, which is sometimes used for manufacturing cam shafts, is rather unlikely for the manufacture of gear shafts, because the stress and strain on those is much higher.
7. Cold extrusion Nowadays, it is more common to manufacture massive gear shafts by way of cold forging rather than hot forging, especially when the quantities are high. One reason is the little machining allowance and the closer tolerances. A further reason is that cold forged shafts do not need a final heat treatment, i.e. they can be processed further as forged. Whereas in the case of hot forging, it is necessary to heat treat the forgings, in order to convert the structure of the intermediate stage, which is built when a case hardened steel cools down. Use should be made of these advantages when hollow shafts are manufactured. However, only a few geometries of shafts can be manufactured economically only by way of cold forging due to forming limits. However, combinations with other manufacturing processes like cold forging and friction welding offer interesting possibilities. A gear shaft for a heavy duty vehicle (Fig. 3), which weighs more than 20 kg, can be replaced by a hollow
Fig.
7. Qualitative
aaluation
of difl‘crent
shaft with a weight less than 5 kg. In this case, the tclo halves of the shaft are manufactured by backward can extrusion in several forging operations. Because of the high natural strain. an intermediate annealing is necessary. After a turning operation to remove the excess material of the overflow of the extrusion process. the friction welding takes place. When comparing the costs for manuC~cturing a hollow shaft in the above mentioned way. it is more expensive than a cold forged massive shaft. The reasons are: 1. two halves of ‘; shaft have to be pressed: 2. 811 additional intermediate annealing process is necessary and therefore a following surface treatment: 3. turning and friction welding operations are necessary. All together the costs for manufacturing a shaft in such a way are much higher compared to the material savings [5]. It has to be mentioned that no problems occur when the torque is transmitted because of the lower cross-section or the friction welding. Hollow shafts with special geometries can also be manufactured economically in a combination of cold forging and deep drilling. In this case, it is important to make use of cost effective manufacturing processes. the combination of forging and machining processes, making use of the advantages which cold forging offers. If you have sheared rods, a drive shaft (Fig. 4) can first of ail be cold forged by several reduction and upsetting processes. In a further step, it can be deep drilled. If the quantities are high, this can be carried out effectively on a multiple spindle drilling machine and
production
methods l’or hollow
shafts.
exact holes with close tolerances will be achieved. If a hollow shaft requires smaller bores at the top side of the shaft, this demand can be met by deep drilling the different diameters. The final shape of the shaft is obtained in further cold forging processes. Use will be made of the processes reducing or hollow forward extrusion. This depends on whether the wail thickness should be reduced or whether the inner geometry should be similar to the outer shape and the wail thicknrss should stay the same. An intermediate anneaiing step, or a repeated heat treating step is in most cases redundant. The inner and outer tolerances of the diameter are _+0.2 mm; the mis-match is limited to maximum 0.6 mm. The l~oiiow shaft has compared to the massive shaft 20% less weight. The wall thickness is dependent on the type. between 10 and 16mm. This combination of processes can also be applied when manufacturing step pinions (Fig. 5) in i~oliow version. The solid pinion would weigh 4.8 kg, whereas the i~oiiow version only weighs 3.8 kg. This means a weight reduction of more than 30%. As the cross-sections differ enormously, it may sometimes be necessary to upset the top of the shaft twice in the first forging process. ln order to obtain the different cross-sections and shoulders, the shaft is reduced in several forging steps after deep drilling. The wail thickness stays nearly the same and the inner geometry adapts the outer shape. This is advantagec;us. in comparison to a shaft which is drilled after the forging, in that the grain flow is optimized. Another manufacturing step would be possible in this case: starting with a tube slug or a drilled rod. the top of the shaft is upset and the shaft is
shaped in the same press cyc11:by reduction and hollow forward extrusion. In this case, the material has to be supported with a mandrel. The comparison of thr:: zests between massive shafts and hollow shafts, which) is s ;own in Fig. 6, is of course only a rough estimation! Th$ factors that influence the costs are divided into \llat&rial costs, manufacturing costs for forging, ma4zcc;.uring costs for machining and costs for the wear oitthe tools. This rough estimation shows that both hoHow shafts are approximately 20% more expensive than the cold forged massive shaft, which can be forged in one step. When judging the different possibilities for manufacturing hollow shafts, the most important factor is most probably the cost factor. The other different possibilities which have been discussed will also be taken into account. In Fig. 7 you can see some processes for manufacturing hollow gear shafts and they are judged referring to their specific feature:: The heat treatment of the hollow shafts also plays an important role with respect to the tolerances. Moreo ‘et-, the further process
steps like spline rolling of the fits for the syr&ruauus gears have to be discussed. In this context it has to be considered whether it is possible to extrude these slines in conjunction with the forming of the shaft. References [I] B. Rastinger, Die neuen Schaltgetriebe fiir die BMW-Baureihen 8, 5. 3. ATZ-Automobiltechnische Zeitschrift 93, II (1991) 698705. [2] G. Bartsch, S. Hock, P. Kiipf, Entwicklungstendenzen bei Fahrzeuggetrieben und Perspcktiven fiir ihre Bauteile. Vortrag Neuere Entwicklungen in der Massivumformung, DGM-lnformationsgesellschaft, Oberursel. 1993. [3] D. Schmoeckel, T. Ruhland, Einsatz des Axial-Radial-Umformens zur Herstellung von Getriebewellen in Leichtbauweise. Umformtechnick 27 (1993) 6. pp. 384-388. [4] A. Ebbinghaus. Prizisionswcrkstiickc in Leichtbuweise, hergestellt durch Innenhochdruckumformen. Metallumformtechnik Dli91, pp. I5 19. [5] M. Hirschvogel. Transmission Shaft Forgings+Technical and Economical Aspects of New Developments. Proceedings of the 9th International Colf Forging Congress. Solihull. UK. 1995. pp. 425-431.
Abstract More and more hollow transmission shafts are used in al! \-chicle gears as ;Lresult of the demand of Iigh~ components. DiKerent methods of bulk-metal forging are shown partially in combination with joining and machining processes. They will be discussed with reference to shaping, tolerances and economical aspects. Q 1997 Elsevier Science S.A. Kq~ords: Hollow transmission shafts; Buik-metal I‘orglng
1. Intraduction for gears have to be developed due to an increasing demand for performance with the simultaneous demand for reduction of fuel as well as noise emission and exhaust fumes. Nowadays, the Sgear-synchromesh gearbox is a standard equipment, top car models even have six gears (I]. Cars with an automatic transmission, which are expected to increase significantly in the next few years, have mainly a 4gear or 5-gear transmission. Every time a new type of transmission is introduced, the automobile industry and their suppliers are challenged from the ecological point of view to carry out the structures and/or components of the gears more cost effectively, more efficiently and more compactly, at the same time using less material. The size of the gearbox should stay the same, or even be reduced. This means that the weight of the gears will be reduced, as it already has in the last few years (21. When putting the demand of light components into practice, it has to be done without decreasing the life time and the reliability. There are in principal two ways to achieve this: the use of new materials, which combine the same properties with lower weight; or an optimized design of steel parts relative to improved mechanical For
all vehicles,
new concepts
* Corresponding author. Tel.: + 49 36925 2481 I; fax: t 49 36925 24899. 0924-0136/97/$17.00 Q 1997 Elsevier Science S.A. All rights reserved. PIISO924-0136(97)00156-8
properties. The latter is of special interest when no other material than steel can be used because of the special properties like high loads. temperatures, and high strength at sufficient ductility. These parts are above all gear shafts.
Generally, three shafts are necessary in a transmission: driveshaft (input shaft), counter shaft and output shaft (main shaft). In an automatic transmission gear, there is the intermediate shaft between the driveshaft and the output shaft. The weight of these shafts for cars is between 2 and 5 kg and for heavy duty vehicles between 8 and 20 kg. A weight reduction for the shafts mentioned and for stem pinion of rear axle transmissions, which are also part of this family, can only be achieved with hollow shafts. The principal design of these parts in general is very similar (Fig. 1). In the case of manually shifted transmissions, the driveshafts have a head on one end and a more or less structured shank on the other end. Main shafts have a flange in the middle and shanks on either side, whilst counter shafts have two or three fixed gears. The tension on these shafts concentrates on the surface area, whereas the material in the core is under less stress. Therefore, the core material is redundant. If the diameter of the bore is approximately half the outer
114
F. S&tit&r,
P. Ketttter / Jortrttnl oj‘Mttcrids
diameter, the moment of inertia or second moment of area is reduced by less than 10% compared with a solid shaft. The result is a possible reduction of material and/or weight of up to 300/, i.e. only slight changes regarding rigidness and strength occur when using a hollow shaft with the same outlines instead of a solid shaft. In principal, the following processes for manufacturing hollow shafts can be used, in case the advantages of metal forming processes should be used, e.g. little material waste, short manufacturing times, and high strength: shear spinning; forging (conventional); (wedge) cross rolling; rotary swaging (upset swaging); rotary forging; internal high pressure forming; cold extrusion (conventional); cold extrusion and friction welding; cold extrusion and deep drilling. When judging the different production methods, not only the most cost and material effective aspects, but especially criteria like reliability and reproducibility, regarding the tolerances, the shape and the nearest shape to the final workpiece geometry which can be achieved is most important. Achieving the closest tolerance is a problem when a tube slug as material has to be used. The tolerance on tubes with thick walls, especially with respect to the coaxiality between outer and inner diameter, are subject to deviations, which are very difficult to balance in the following working steps. However, since the number of revolutions on transmission shafts is very high, out of balance run-out on the diameter can only be accepted to a certain extent. As shear spinning is only used for very few parts and forging is only applied in the case of shafts for heavy duty vehicles, these processes will not be described any further.
Fig. I. Various output shafts,
Praumittg
Tdttdogy
71 (1997) 113- 118
Fig. 2. Upset waged hollow shaft (courlcsy: PtUiHhlP).
3. Cross rolling Wedge cross rolling of solid shafts with more than one flange is a very effective method at hot forging temperatures, with production frequencies of 3- 12 s. In industry, two methods have been successful: the round yaw cross rolling according to the double roll principal, and the flat yaw cross rolling. Preliminary tests show that hollow shafts can also be cross rolled. However, we do not know any proceeding features and guidelines for the production of forged hollow shafts. Above all, it has to be settled whether tubes with thick walls can be used as material. Another element of uncertainty is the twisting of the grain flow during the wedge cross rolling process which occurs on the longitudinal axis; this can cause major inaccuracies when the teeth are manufactured in the following machining step.
4, Rotary swaging (upset swaging) Rotary swaging is a common method when reducing the diameter of rods and tubes. It is preferably used at room temperature. With this method, a11 the advan-
Fig. 3. Hollow main shaft for trucks.
art from forming a local wall thickness. the diameter the part can be reduced by cross roiling ehe workp&es wifhouf locating the part first [3]. !t is \‘ery difCxlt to manufactu:e a gear shaft economically with the general manufacturing proccsscs. it i> cery time consuming with rhc .axx settings (heating and upsetting, several reductions of t e long and sllort shanks) and the partial forming by rotary swag& Of course it would be possible to reduce the production ing machines that carry out several steps at the same time. However. it is not certain whether the same production frequencies as with cross rolling or cold forging can be achieved. Therefore. the characteristic feature of axial radial forming is its high flexibility for shaping workpieces with a similar geometry without using many tools. In the case of automation. it is more economical to make smaller quantities.
Fig. 4. Cold forged and drilled input shafts.
tages of cold forging like close tolerances, favorable grain flow, smooth surfaces and work hardening can be achieved. Generally, we differ between two methods: the infeed method and the plunching method. Two to four tool segments carry out an oscillating radial movement and form the workpiece in small singular steps. En the case of the infeed method, the workpiece is additionally moved through the oscillating dies in the axial direction. In the case of the plunching method, the tools carry out an additional radial movement which overlaps the oscillation and which is much faster than the oscillation. When using this process, the angle on the workpiece can be much steeper; it also allows reductions of the diameter between the ends of the workpiece, i.e. undercuts. When using the corresponding mandrel, bores with tight tolerances can be achieved which are cylindrical or in a cone shape. It is also possible to obtain inner profiles like splines. However, typical gear shaft geometries (Fig. 2) with a clear enlargement of the wall thickness at the flanges can only be produced in combined manufacturing steps on special machines (upset swaging). This process which is also called axial-radial-forming was developed at the University Darmstadt together with the HMP, Pforzheim. The tube is heated only at the point where
The difference between rotary forging and rotary swaging is that in the case of rotary forging the workpiece rotates and not the tools. Moreover, the shaping is carried out at forging temperatures. The tools consist mainly of two parts. Feeding is also in the radial direction. Also, in the case of rotary forging. the diameter is reduced. However, on the machines, upsetting can bc carried out also. If you have a sufficiently thick precision steel tube, the wall thickness can be doubled in some areas by upsetting with the rotary forging process. In further working steps, a further reduction at the ends of the shaft can be achieved on the same machine. It is a theoretical question whether the workpiece will be pre-forged in two or three working steps and completely machined afterwards. The tolerances with rotary forging are at some 10th millimeter. As an alternative,
Fig. 5. Hollow stem pinion.
116
ElDie Wear 6 17
0
Slug Weight: Part Weight:
4,8 kg 498 kg Solid Shaft cold extruded
498 kg 3.8 kg Hollow Shaft extruded and drilled
3.8 kg 3.8 kg Hollow Shaft cold extruded
Fig. 6. Cost comparison: solid vs. hollow shaft.
the further processing can be carried out by way of rotary swaging at room temperature. In this case, the tolerances would be optimized.
6. Internal high pressure forming It would also be possible to manufacture hollow shafts by internal high pressure forming. In this case the geometry of the tube could be changed by increasing the inner pressure and at the same time leading the material in the direction of the longitudinal axis of the tube. The outer diameter of the tube corresponds to the smallest diameter of the shaft. The inner geometry of the tool corresponds to the outer geometry of the hollow shaft. The material that is spread is obtained from the wall thickness and forms the length of the tube. Therefore, the tools for sealing have to follow in the longitudinal direction of the tube. As the sealing is problematic, it is necessary to have smooth slugs, so most of the time the tube ends are brushed. The enlargement of the diameter is limited by the deformation capacity of the material. If the method of internal high pressure forming is combined with upsetting in the longitudinal direction, hollow shafts with a flange in the center can be manufactured. The limits of this process depend on the geometry of the component. It is above all the relation between the diameter and the wall thickness, which allows only the manufacturing of some shapes. The smallest radii that can be produced are one and a half times the wall thickness. The tolerances of the outer outlines are between tolerance class IO and I4 and they also depend on the springback characteristics. The dimensional vari-
ations of the inner outlines are generally larger. Most of the time, they can only be defined after samples have been manufactured because of the free material flow in combination wiih the deviation of the wall thickness of the tube, radii or form and positional tolerances [4]. There is also the possibility in using the process to combine tubes conclusively with additional elements. This process, which is sometimes used for manufacturing cam shafts, is rather unlikely for the manufacture of gear shafts, because the stress and strain on those is much higher.
7. Cold extrusion Nowadays, it is more common to manufacture massive gear shafts by way of cold forging rather than hot forging, especially when the quantities are high. One reason is the little machining allowance and the closer tolerances. A further reason is that cold forged shafts do not need a final heat treatment, i.e. they can be processed further as forged. Whereas in the case of hot forging, it is necessary to heat treat the forgings, in order to convert the structure of the intermediate stage, which is built when a case hardened steel cools down. Use should be made of these advantages when hollow shafts are manufactured. However, only a few geometries of shafts can be manufactured economically only by way of cold forging due to forming limits. However, combinations with other manufacturing processes like cold forging and friction welding offer interesting possibilities. A gear shaft for a heavy duty vehicle (Fig. 3), which weighs more than 20 kg, can be replaced by a hollow
Fig.
7. Qualitative
aaluation
of difl‘crent
shaft with a weight less than 5 kg. In this case, the tclo halves of the shaft are manufactured by backward can extrusion in several forging operations. Because of the high natural strain. an intermediate annealing is necessary. After a turning operation to remove the excess material of the overflow of the extrusion process. the friction welding takes place. When comparing the costs for manuC~cturing a hollow shaft in the above mentioned way. it is more expensive than a cold forged massive shaft. The reasons are: 1. two halves of ‘; shaft have to be pressed: 2. 811 additional intermediate annealing process is necessary and therefore a following surface treatment: 3. turning and friction welding operations are necessary. All together the costs for manufacturing a shaft in such a way are much higher compared to the material savings [5]. It has to be mentioned that no problems occur when the torque is transmitted because of the lower cross-section or the friction welding. Hollow shafts with special geometries can also be manufactured economically in a combination of cold forging and deep drilling. In this case, it is important to make use of cost effective manufacturing processes. the combination of forging and machining processes, making use of the advantages which cold forging offers. If you have sheared rods, a drive shaft (Fig. 4) can first of ail be cold forged by several reduction and upsetting processes. In a further step, it can be deep drilled. If the quantities are high, this can be carried out effectively on a multiple spindle drilling machine and
production
methods l’or hollow
shafts.
exact holes with close tolerances will be achieved. If a hollow shaft requires smaller bores at the top side of the shaft, this demand can be met by deep drilling the different diameters. The final shape of the shaft is obtained in further cold forging processes. Use will be made of the processes reducing or hollow forward extrusion. This depends on whether the wail thickness should be reduced or whether the inner geometry should be similar to the outer shape and the wail thicknrss should stay the same. An intermediate anneaiing step, or a repeated heat treating step is in most cases redundant. The inner and outer tolerances of the diameter are _+0.2 mm; the mis-match is limited to maximum 0.6 mm. The l~oiiow shaft has compared to the massive shaft 20% less weight. The wall thickness is dependent on the type. between 10 and 16mm. This combination of processes can also be applied when manufacturing step pinions (Fig. 5) in i~oliow version. The solid pinion would weigh 4.8 kg, whereas the i~oiiow version only weighs 3.8 kg. This means a weight reduction of more than 30%. As the cross-sections differ enormously, it may sometimes be necessary to upset the top of the shaft twice in the first forging process. ln order to obtain the different cross-sections and shoulders, the shaft is reduced in several forging steps after deep drilling. The wail thickness stays nearly the same and the inner geometry adapts the outer shape. This is advantagec;us. in comparison to a shaft which is drilled after the forging, in that the grain flow is optimized. Another manufacturing step would be possible in this case: starting with a tube slug or a drilled rod. the top of the shaft is upset and the shaft is
shaped in the same press cyc11:by reduction and hollow forward extrusion. In this case, the material has to be supported with a mandrel. The comparison of thr:: zests between massive shafts and hollow shafts, which) is s ;own in Fig. 6, is of course only a rough estimation! Th$ factors that influence the costs are divided into \llat&rial costs, manufacturing costs for forging, ma4zcc;.uring costs for machining and costs for the wear oitthe tools. This rough estimation shows that both hoHow shafts are approximately 20% more expensive than the cold forged massive shaft, which can be forged in one step. When judging the different possibilities for manufacturing hollow shafts, the most important factor is most probably the cost factor. The other different possibilities which have been discussed will also be taken into account. In Fig. 7 you can see some processes for manufacturing hollow gear shafts and they are judged referring to their specific feature:: The heat treatment of the hollow shafts also plays an important role with respect to the tolerances. Moreo ‘et-, the further process
steps like spline rolling of the fits for the syr&ruauus gears have to be discussed. In this context it has to be considered whether it is possible to extrude these slines in conjunction with the forming of the shaft. References [I] B. Rastinger, Die neuen Schaltgetriebe fiir die BMW-Baureihen 8, 5. 3. ATZ-Automobiltechnische Zeitschrift 93, II (1991) 698705. [2] G. Bartsch, S. Hock, P. Kiipf, Entwicklungstendenzen bei Fahrzeuggetrieben und Perspcktiven fiir ihre Bauteile. Vortrag Neuere Entwicklungen in der Massivumformung, DGM-lnformationsgesellschaft, Oberursel. 1993. [3] D. Schmoeckel, T. Ruhland, Einsatz des Axial-Radial-Umformens zur Herstellung von Getriebewellen in Leichtbauweise. Umformtechnick 27 (1993) 6. pp. 384-388. [4] A. Ebbinghaus. Prizisionswcrkstiickc in Leichtbuweise, hergestellt durch Innenhochdruckumformen. Metallumformtechnik Dli91, pp. I5 19. [5] M. Hirschvogel. Transmission Shaft Forgings+Technical and Economical Aspects of New Developments. Proceedings of the 9th International Colf Forging Congress. Solihull. UK. 1995. pp. 425-431.