Method and apparatus for manufacturing a shift gearwheel and shift gearwheel manufactured by same

The subject matter of the invention is a method of manufacturing a shift gearwheel (1) with coaxially disposed and axially projecting shift teeth (5), which have an axial undercut. To said end, a shift gearwheel blank (1) having preformed shift teeth (5) and a forming tool (16) having corresponding tooth recesses (18) are brought into engagement and set in gyratory motion relative to one another. Further described are a device for effecting the method and a shift gearwheel (1), which may be manufactured by the method. The latter is notable for the fact that the flanks (23, 24) of the shift teeth (5) upon completion of the manufacturing method are concave so as to produce an undercut.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to a method of manufacturing a shift gearwheel with
 coaxially disposed and axially projecting shift teeth, which have an axial
 undercut.
 2. Description of the Related Art
 Shift gearwheels of the type indicated above are used, for example, in
 synchromesh transmissions for motor vehicles. For general appreciation, a
 cutout of a synchronizing unit belonging to prior art is first described
 with reference to FIGS. 1 and 2a-c.
 FIG. 1 shows an exploded view of a loose wheel 1 with clutch body 7, a
 synchronizing ring 2, a sliding sleeve 3 and a synchronizer body 4.
 The loose wheel 1 is provided with an external gearing 20. Integrally
 formed on its end face is a clutch body 7. The clutch body 7 comprises a
 coaxial ring of shift teeth 5, which--starting from a coaxial groove 6
 forming an undercut--extend in axial direction to the left and project
 beyond the end face of the loose wheel 1. The shift teeth 5 have an axial
 undercut, such that the width of said shift teeth 5--starting from the
 tooth tip--diminishes towards the base of the groove 6. The shift teeth 5
 at their tip end have a roof slope, which terminates at an annular surface
 14 extending at right angles to the axis. Raised up from said annular
 surface 14 is a coaxial friction cone 8.
 The synchronizing ring 2 at its outside is provided with a ring of shift
 teeth 9, which extend in axial direction between two annular surfaces 10
 and 11 extending at right angles to the axis. The inner surface of the
 synchronizing ring 2 matches the friction cone 8 on the clutch body 7.
 The sliding sleeve 3 is provided with axially extending shift teeth 12 and
 arranged so as to be axially displaceable but non-rotatable on the
 synchronizer body 4. To said end, the synchronizer body 4 in turn has
 axially extending teeth 13, which mesh with the inner shift teeth 12 of
 the sliding sleeve 3. The shift teeth 12 of the sliding sleeve likewise
 have an undercut close to their two axial ends.
 FIG. 2 shows the positions of the shift teeth of the transmission elements
 shown in FIG. 1 in three different shift positions, and indeed as a
 developed sectional view.
 FIG. 2a shows the neutral position of the synchronizing shift device. Here,
 the sliding sleeve 3 is in centre position. The loose wheel 1 with its
 clutch body and a further loose wheel (not shown) is freely rotatable
 relative to the synchronizer body. The shift teeth 12 of the sliding
 sleeve 3, the shift teeth 9 of the synchronizing ring 2 and the shift
 teeth of the clutch body have clearance relative to one another.
 FIG. 2b shows the transmission elements in blocking position. The sliding
 sleeve 3 and the synchronizing ring 2 have been displaced to the right.
 The synchronizing ring 2 therefore comes into frictional engagement with
 the friction cone 8 of the clutch body 7 and is rotated so that the shift
 teeth 12 of the sliding sleeve and the shift teeth 9 of the synchronizing
 ring 2 lie with their roof slopes against one another.
 FIG. 2c shows the "gear engaged" position. Here, the sliding sleeve has
 been slid so far to the right that the shift teeth 12 of the sliding
 sleeve 3 engage fully through the shift teeth 9 of the synchronizing ring
 2 and moreover project into the shift teeth 5 of the clutch body 7. It may
 be seen that the undercuts of the shift teeth 12 of the sliding sleeve 3
 upon transmission of a torque and the undercuts of the shift teeth 5 of
 the clutch body 7 engage one behind the other, thereby preventing an axial
 separation of the transmission elements. This is the purpose of the
 undercuts.
 A plurality of methods of manufacturing shift gearwheels with undercut
 shift teeth are already known.
 Thus, the prior publication DE 20 40 413 A describes a known method whereby
 tapered gearwheels with undercut teeth may be manufactured in that teeth
 with parallel extending tooth flanks are first manufactured by
 rough-pressing and then finish-edged so as to form a roof shape.
 A method of manufacturing a synchronizing component provided with an
 abridged gearing with undercut teeth for shift transmissions is further
 known (DE 34 27 156 C2), in which rough-forging is used first to
 manufacture a semifinished product, the abridged gearing of which
 comprises teeth having an overmeasure exceeding the height of the finished
 tooth tip. Then, by means of a plurality of sizing impacts the cold
 semifinished product is worked in such a way that first the tooth tips are
 rough-edged, during which the teeth are supported at their radially outer
 sides against the forging die. At the same time, by means of the
 rough-edging or a further sizing impact a strain-hardening is produced in
 each case in the tooth region of the teeth. Then the tooth tips are
 finish-edged so that they have a roof shape and the tooth flanks have the
 inclination corresponding to their undercut.
 Finally, a method of machining clutch gearwheels for motor vehicle
 transmissions is additionally known (DE 34 38 454 A1), whereby in a first
 working step a blank of a round rod is formed by cold- or hot-forging from
 a material which is deformable under pressure. In a second working step,
 the blank is pressed by means of a forging die. In said manner, the cross
 section having the defined final dimensions and at the same time a
 taper-face projection are obtained. In a third working step the taper-face
 projection is converted into a tooth shape. In a fourth working step the
 tooth shape is pressed into the forging die from an end face of the
 opposite end of the taper-face projection so that the tooth shape is
 provided with an undercut.
 An object of the invention is to indicate a non-cutting method of
 manufacturing a shift gearwheel having axially undercut shift teeth, which
 is particularly easy to effect and suitable for mass production.
 According to one aspect of the invention, there is provided a novel method
 of manufacturing a shift gearwheel with coaxially disposed and axially
 projecting shift teeth and which have an axial undercut. This method
 includes a first step of bringing into mutual engagement, a shift
 gearwheel blank having coaxially disposed and axially projecting preformed
 shift teeth, and a forming tool having corresponding axial tooth recesses,
 such that the preformed gearweel blank shift teeth become engages in the
 forming tool tooth recesses with a clearance; a second step of setting the
 shift gearwheel blank and the forming tool in gyratory motion relative to
 one another in such a way that the contours of the tooth recesses move in
 a pressing and material-shaping manner over the flanks of the performed
 gear wheel shift teeth so as to form an undercut; and a third step of
 separating the thus machined shift gearwheel blank and the forming tool
 from one another.
 The method according to the invention is particularly notable for the fact
 that it may be effected continuously throughout without interruption of
 the individual teeth and leads to reproducible products. The method is
 simple as well as time- and cost-saving.
 Advantageous refinements of the method according to this aspect of the
 invention are described hereinafter.
 The invention in another aspect relates to a device, with which the
 previously described method may be effected. According to a further
 refinement, the shift gearwheel blank is maintained stationary and the
 forming tool is made to simultaneously execute a rotation about an axis of
 nutation, a nutating movement about a vertical axis of rotation and a
 lifting movement in the axial direction.
 According to a still further refinement, the shift gearwheel is caused to
 execute a lifting movement in the axial direction while the forming tool
 simultaneously executes a nutating movement.
 In yet another refinement, the invention relates to a shift gearwheel
 manufactured by the above described method.
 An additional refinement involves successively increasing the angle of
 nutation between the shift gearwheel blank and the forming tool from
 0.degree. up to a maximum angle of nutation, maintaining the angle at the
 maximum angle of nutation and then reducing the angle back down to
 0.degree..
 Yet another refinement involves carrying out the gyratory motion first in
 one direction of nutating movement and then repeating the gyratory motion
 in the opposite direction of nutating movement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 3 shows a receiving part in the form of a bottom die 21 for receiving
 a shift gearwheel blank and/or the finished shift gearwheel 1 after
 implementation of the method. For the shift gearwheel 1 in FIGS. 3 to 11
 the same reference characters have been used as for the loose wheel 1 with
 clutch body 7 in FIG. 1. The axis of the shift gearwheel 1 is denoted by
 15 and is referred to hereinafter as "axis of rotation".
 Disposed above the bottom die 21 is a forming tool 16 having at its
 underside a circular hollow 17, at the edge of which are situated tooth
 recesses 18 which are open both axially towards the end face of the
 forming tool 16 and in a radially inward direction. The forming tool may
 be set by a drive (not shown) in gyratory motion relative to the bottom
 die 21, wherein the angle of nutation .tau. is adjustable. The axis of
 nutation is denoted by 19. When the angle of nutation .tau. is greater
 than 0.degree., the axis of rotation 15 and the axis of nutation 19
 intersect at the point of intersection 14. During implementation of the
 gyratory motion the axis of nutation 19 rotates about the axis of rotation
 15. Thus, the forming tool 16 is in each case at one position in intensive
 forming contact with the relevant shift teeth 5 of the shift gearwheel 1,
 while at the axially opposite point a--depending on the angle of nutation
 .tau.--partial or full separation between the forming tool 16 and the
 corresponding shift teeth 5 of the shift gearwheel 1 occurs.
 Upon completion of the method and removal of the forming tool 16, the
 finished shift gearwheel 1 may be pushed out of the bottom die 21 by means
 of an ejector 22.
 FIGS. 4-9 show, on the left, a section A--A through a shift tooth 5 of the
 shift gearwheel 1 in FIG. 3 from the viewing direction indicated by the
 arrow R in FIG. 3. This means that, in each case, the rear shift tooth 5
 is viewed. Next to this, on the right, FIGS. 4-10 each show a circle which
 reveals the position x of the axis of nutation 19 above the point of
 intersection between the axis of nutation 19 and the axis 15 of the shift
 gearwheel 1. From the position x of the axis of nutation 19 it is possible
 to infer the angle of nutation .tau..
 FIG. 4 shows the state when the forging die is closed. This means that the
 forming tool 16 is placed, with the angle of nutation .tau.=0.degree.,
 onto the shift gearwheel 1 initially provided in the form of a blank. The
 shift gearwheel blank 1 is provided with preformed shift teeth 5. The
 latter have parallel flanks 23 and 24. At their axial end they are
 provided with a rounded portion 27. Instead of the latter, the preformed
 shift teeth 5 may however already have a roof shape. The forming tool 16
 is provided with corresponding tooth recesses 18, which in turn have a
 wedge-shaped base in order to form a roof shape on the preformed shift
 teeth 5, if they do not already have one. The boundary edges 25 and 26 of
 the tooth recesses 18 are--as will be described below--used to form the
 undercuts. As may be seen from the circle situated next to the sectional
 view, the axis of rotation 15 and the axis of nutation 19 here extend
 coaxially without clearance (.tau.=0.degree.).
 A machining sequence of the method of manufacturing a shift gearwheel is
 described below by way of example and with reference to FIGS. 5-9.
 FIG. 5 shows a first manufacturing stage of the machining sequence, during
 which a maximum angle of nutation .tau. is adjusted. This is apparent from
 the position x of the axis of nutation 19 on the circle situated next to
 the sectional view. Here, the axis of nutation 19 is situated above the
 axis 15. This means that, as yet, no circular movement of the forming tool
 16 about the axis of rotation 15 has occurred. During said first
 manufacturing stage a slight roof shape is formed on the preformed shift
 tooth 5; the shift tooth 5 is moreover slightly widened.
 FIG. 6 shows the next stage of the machining sequence, during which once
 more the maximum angle of nutation .tau. is adjusted. Here, the axis of
 nutation 19 is situated to the right of the axis 15. This means that the
 axis of nutation 19 and, with it, the forming tool 16 have executed a
 circular movement through 90.degree. compared to FIG. 5. It is evident
 from the sectional view that the boundary edge 26 of the tooth recess 18
 is forming an undercut on the preformed shift tooth 5. The tooth flank 24
 is now no longer straight but concave.
 FIG. 7 shows the axis of nutation 19 after it has executed a circular
 movement of 180.degree. about the axis of rotation 15 compared to FIG. 5.
 The forming tool 16 at the point of the presently illustrated shift tooth
 5 has lifted relative to the shift gearwheel blank 1 without a separation
 of shift tooth 5 and tooth recess 18 having been effected.
 In FIG. 8, the axis of nutation 19 has executed a circular movement of
 270.degree.. Here, the forming tool 16 is lowered again in the direction
 of the partially still preformed shift tooth 5 and is forming on the left
 flank 23 of the latter a concave undercut. The tooth flank 23 also is
 therefore no longer straight but now likewise concave.
 FIG. 9 shows the finished shift tooth with undercut, wherein the angle of
 nutation .tau. is reset to 0.degree.. The axis of rotation 15 and the axis
 of nutation 19 again extend parallel to one another and without clearance.
 During practical implementation of the method, the angle of nutation in the
 course of a plurality of nutation sequences is successively increased from
 0.degree. up to the maximum final angle. Then a plurality of nutation
 sequences are effected while maintaining the final angle. Finally, the
 angle of nutation in the course of effecting a plurality of further
 nutation sequences is successively reduced back down to 0.degree..
 To guarantee that the undercut is formed as symmetrically as possible on
 the shift teeth 5, it is possible in practice for the same operation to be
 repeated once more, wherein the circular movement is effected in the
 opposite direction.
 The maximum angle of nutation depends upon the nutating machines used.
 Currently, with the latter a maximum angle of nutation of 2.degree. is
 adjustable. In the case of the present method, the desired undercut angle
 on the shift teeth corresponds to the maximum angle of nutation at which
 the device for effecting the method operates.
 When greater undercut angles are required, either the nutating machine has
 to be suitably re-designed or the flanks 30 and 31 of the tooth recesses
 18 in the forming tool 16 are in turn provided with a suitable undercut.
 In said case, the maximum angle of nutation and the undercut angle of the
 flanks 30, 31 of the tooth recesses 18 sum up to the achievable undercut
 angle on the shift teeth 5. Thus, for example, the tooth recesses 18 may
 be provided with an undercut angle of 2.3.degree. in order, together with
 the maximum angle of nutation of 2.degree., to realize the undercut angle
 of 4.3.degree. on the shift teeth 5.
 FIG. 10 shows a tooth recess 18 with undercut. Otherwise, FIG. 10
 corresponds to FIG. 9.
 FIG. 11 shows an enlarged partial section through a part of a finished
 shift gearwheel 1. Here, it is particularly evident that the shift teeth 5
 are partially countersunk in the groove 6, with the result that the shift
 gearwheel 1 as a whole may be of a relatively flat construction. With the
 conventional methods, it is impossible to go below a specific ratio of
 groove width a to groove depth b. With the method according to the
 invention, said ratio is more advantageous in view of the desired aim to
 make the shift gearwheels of the presently discussed type as flat as
 possible.
 Finally, it should be mentioned that particularly FIGS. 3-10 show the
 essential features such as angle of nutation and undercut in a highly
 exaggerated manner. If illustrated true to scale, said features would be
 barely discernible.
 It should also be mentioned that the data regarding the maximum angle of
 nutation and the number of nutating revolutions are merely by way of
 example and may be altered without thereby affecting the essential
 substance of the invention.
 It is moreover additionally pointed out that the method described above,
 the bottom die 21 and the forming tool 16 may equally be used in various
 types of nutating press. The most common nutating presses may be
 classified, e.g. according to their tool movements, into three types.
 In the nutating press of type 1, the bottom die 21 with the shift gearwheel
 blank 1 inserted therein executes a driven rotational movement about the
 axis of rotation 15. The top forming tool 16 rotates in synchronism with
 the rotational movement of the bottom die 21 about the axis of nutation 19
 inclined by the angle of nutation .tau.. The translatory movement, which
 corresponds to the lifting movement during the forming operation, is
 likewise initiated hydraulically by the top forming tool 16. If the
 rotational movement of the forming tool 16 is not driven, i.e. if the
 forming tool 16 co-rotates with the shift gearwheel 1 freely about the
 axis of nutation 19, then said nutating press is described as type 1A. If,
 on the other hand, the rotational movement of the forming tool 16 is
 additionally driven, said nutating press is described as type 1B.
 The nutating press of type 2 has a fixed bottom die 21 which is incapable
 of executing either translatory or rotational movements. The shift
 gearwheel 1 is stationary throughout the machining operation. Instead, the
 top forming tool 16 executes a total of three movements: the rotation
 about the axis of nutation 19, the nutating movement about the vertical
 axis of rotation 15 and the hydraulically driven lifting movement in axial
 direction.
 Nutating presses of type 3, for which the method was also described above
 with reference to the drawings, are however the most common type of
 nutating press. Here, the hydraulically driven lifting movement is
 effected by a cylinder in the bottom part of the machine. The nutating
 movement is initiated by the top forming tool 16, while bottom die 21 and
 shift gearwheel 1 remain stationary.
 Irrespective of the type of nutating press used, all that matters about the
 according to the invention is that the forming tool 16 executes a gyratory
 movement relative to the shift gearwheel and/or shift gearwheel blank 1.
 How the individual components of motion of the nutating movement are
 allocated to the various tool parts of the nutating press used in each
 case is, in said case, fundamentally irrelevant.