Abstract:
A method of manufacturing a gear wheel with stub toothing for a change-speed gearbox includes drop-forging a gear wheel body and forming the stub toothing by cold die-grooving in the forging die using a forming tool with forming parts which are arranged in a fan-shaped manner. The forming ends penetrate into the tooth spaces, wherein the forming parts are guided jointly between an upper and a lower receiving plate and are moved in a direction radially inwards in such a way that material is displaced out of the region of the tooth flanks into the tooth root regions.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Inventions 
     The present invention is directed to a method and apparatus for manufacturing a gear wheel with stub toothing, wherein a gear-wheel body is drop-forged in a first forming step and the stub toothing is finish-swaged in at least one further forming step by cold die-grooving by means of a swaging tool having swaging parts disposed in a fan-like pattern, wherein the swaging ends plunge into the inter-tooth spaces. 
     2. Description of the Related Art 
     According to a known method (DE 2040413) for manufacturing a stub toothing on a gear wheel of a gearbox, it is provided that a partial toothed gear with stub toothing is manufactured separately then is welded together with the part of the gear wheel supporting the drive toothing. During the manufacture of the stub toothing of the partial toothed gear, the teeth of the stub toothing are first manufactured with tooth flanks oriented in parallel by die-pressing. This is followed by numerous forming steps by cold grooving up to finishing of the exact tooth shape of the stub toothing. This involves the formation of a roof-like shape in the region of the tooth tip as well as an undercut in the region of the tooth flanks, for which a special beveling apparatus is provided with swaging parts plunging in a fan-like pattern into the interstices of the stub toothing. The individual swaging parts are mounted pivotally and with their inwardly pivoted ends bring about grooving of the inter-tooth spaces. 
     As a consequence of the numerous machining operations, the known method leads to high manufacturing costs. During machining of the stub toothing by the pivoting swaging parts, overlaps are produced during material forming, potentially leading to malfunctions during subsequent operation. 
     SUMMARY OF THE INVENTION 
     In contrast, the object underlying the present invention is to avoid the disadvantages of the known manufacturing method in an apparatus and in a method of the type mentioned in the introduction, and in particular to enable the manufacture of a stub toothing on gear wheels that is characterized by improved accuracy of the stub toothing and ensures long-lasting use without malfunctions. An objective for the manufacturing method as such is to permit a high production rate with long service lives of the die components. 
     This object is achieved according to the invention with a method of the type mentioned in the introduction, wherein the swaging parts of the swaging tool are guided together between an upper and a lower holding plate and are moved radially inward in a direction such that precise formation of the teeth of the stub toothing is achieved by cold grooving, in which material flows from the region of the tooth flanks into the regions of the tooth roots. In the process, a stub toothing with high accuracy of shape of both the actual tooth shape and of the inter-tooth spaces is produced by material forming. 
     As a consequence of the material forming achieved with the inventive method, the hollow profile of the die becomes completely filled in the region of the tooth tips. Because of the sharp-edged structure of the lower edges of the roof-like faces achieved thereby in the region of the tooth tips—in connection with the undercut of the tooth flanks—gearing errors caused by axial separation of the shifted drive connection and known as gear jumpers are reliably avoided. 
     A particularly advantageous configuration is one in which the direction of plunging of the swaging parts of the swaging tool is selected to be oblique relative to the root space of the teeth, so that excess material after complete filling of the hollow profile of the die flows into a cavity of the die adjoining the root space. 
     In this way, it is possible to avoid the overlapping of the material that occurs as a consequence of cold forming with only superficial displacement of material and that is recognized as disadvantageous. 
     In order to guarantee precise operation of the swaging parts, it is proposed according to the invention that the gear-wheel body be pressed under a spring-loaded hydraulic force against the upper holding plate during cold grooving, while the swaging parts are pushed inward against a stop between the upper and lower holding plates. These two holding plates are axially fixed by the spring-loaded pressing action. 
     The kinematics of the displacement of the swaging parts has a definitive influence on the accuracy of tooth formation. Advantageously they are displaced simultaneously and uniformly by means of a vertically movable actuating ring via mutually engaging conical pressure faces on swaging parts and actuating ring, so that the swaging ends of the swaging parts are always forced inward into the inter-tooth spaces under constant conditions. In this regard, close-tolerance guidance of the swaging parts between upper and lower holding plate is particularly advantageous. 
     In a method variant that is particularly suitable with respect to service life of the die on the one hand and precision of the manufactured stub toothing on the other hand, the stub toothing is produced in two grooving steps, wherein the teeth of the stub toothing are swaged with axially parallel tooth flanks in a first grooving step and are finish-grooved in a second grooving step, in which the swaging parts of the swaging tool produce an undercut of the tooth flanks followed by root rounding. 
     These two grooving steps are preceded by drop-forging of the stub toothing as the first forming step. In this process, the stub toothing can be extensively manufactured in finished condition in terms of the roof-like form of the teeth in the tip region, and so the later forming work by the subsequent grooving steps can be limited to a minimum in the interests of increasing the accuracy of shape and the service life of the dies. 
     The tooth flanks of the stub toothing, having been forged to be axially parallel at first, are advantageously given their undercut only by the grooving step using the fan-shaped swaging tool. 
     In an apparatus for implementing the inventive method, it is provided that the swaging parts of the swaging tool are individual flat pushing parts, which are disposed circumferentially in axial planes relative to the axis of the die at angular intervals corresponding to the circumferential pitch of the stub toothing. 
     According to a further proposal of the invention, a displacement of the pushing parts with high accuracy can be achieved by guiding the pushing parts between an upper and a lower holding plate of the die and displacing them radially inward in a direction from a starting position to an end position corresponding to the finished tooth shape. 
     In this regard it is advantageously provided that the gear-wheel body is mounted in such a way in the die that the stub toothing together with the tooth tips points downward. In a further configuration of the inventive apparatus, it is provided that the displacement of the pushing parts is directed obliquely relative to the axis of the die, approximately toward the root region of the stub toothing. A suitable obliqueness of the pushing direction ranges between 5° and 20°, preferably between 10° and 15° and particularly preferably is approximately 12°. For this purpose, the pushing parts are moved both radially and axially until they reach a radially inner stop position. 
     With the objective of a process workflow that is as free of vibrations as possible and largely uninfluenced by bending forces, it is further proposed according to the invention that the upper and lower holding plates be securely connected to one another and that they define guides for holding the pushing parts, and that—in a further configuration—the lower holding plate end at its radially inner side with a collar part, which points toward the gear-wheel body and the radially outer circumferential face of which acts as a stop in the end position of the pushing parts. 
     The collar part offers the additional advantage that its upper side can be used as a seat to support a ring-shaped projection of the gear-wheel body, in other words as the static part of the die. 
     In this way it is possible to create a die structure with which the objective of the present invention can be optimally achieved in view of the special cross-sectional shape of the respective gear wheel. 
     Particularly advantageously, the pushing parts can be actuated by an actuating ring encircling the swaging parts externally such that, when it is moved vertically, the pushing parts can be displaced via mutually engaging conical pressure faces on pushing parts and actuating ring into their end position in contact with the circumference of the collar part. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the invention will be explained hereinafter on the basis of the drawing, wherein 
         FIG. 1  shows an axial section through a first grooving die 
         FIG. 2  shows an axial section through a second grooving die 
         FIG. 3  shows an enlarged partial view of  FIG. 2 , and 
         FIG. 4  shows a perspective view of the gear wheel after finishing of the stub toothing 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows the structure of a first grooving die for grooving the stub toothing of a gear wheel for a gearbox. 
     In this forming step, which is followed by a second grooving step as illustrated in  FIG. 2 , a gear-wheel body  1  forged in an upstream forging die is placed in the first grooving die. In the process, the teeth of the stub toothing already formed in the forging die (not illustrated) are improved with regard to their accuracy of shape. Besides high dimensional accuracy of the radial dimensions, this improvement consists in particular of imparting a precise roof-like shape with sharply structured roof-like edges  40  to teeth  15  (see  FIG. 4 ) in the region of tooth tip  22 . The tooth flanks on both sides of each tooth  15  run parallel to one another from the tooth tip to the tooth root, and so the teeth have a constant tooth width over their entire length after the first forming step of cold grooving in the first grooving die according to  FIG. 1 . 
     From top to bottom, the die structure according to  FIG. 1  comprises a hold-down member  2 , which is seated on gear-wheel body  1 , which in turn is received from below in female part  3  of the lower die. An ejector  4  engages with its upper end in middle bore  5  of gear-wheel body  1 , by the fact that it is received with a circumferential rim  6  of its end face in an enlargement of middle bore  5 . 
     Hold-down member  2  and female part  3  are each fitted inside cylindrical housing parts. In this way, hold-down member  2  is guided inside a guide ring  7  and female part  3  is fixed inside a first die ring  8 , which in turn is seated inside an outer die ring  9  and is connected by means of bolts  12  with a base plate  10 . A die plate  11  mounted on base plate  10  engages in inner die ring  8 , on which female part  3  is mounted. 
     Gear-wheel body  1  has a hub part  13  encircling its middle bore  5  and ending downward with a conical part  14 . During shifting of the gear wheel, the outer cone of conical part  14  functions to adapt the speed of rotation of a clutch sleeve to that of the gear wheel by means of a synchronizing ring, the inner cone of which climbs on outer cone  41  of conical part  14 . Outer cone  41  of conical part  14  is clearly illustrated in  FIG. 4  on a gear wheel  1  with finish-swaged stub toothing  15 . 
     A toothing body  17 , on the outer circumference of which stub toothing  15  is formed as illustrated in  FIG. 4 , is connected to conical part  14  of hub part  13  in radially outward direction via a spacer channel  16 . Further outward in radial direction, a stop ring  19  (see  FIG. 4 ), which limits the axial movement of the clutch sleeve, is connected to toothing body  17  outside stub toothing  15  via a narrow spacer channel  18 . Yet further outward in radial direction, the outer toothed-gear body  21 , which is intended for the drive toothing of the gear wheel still to be produced by chip-removing processes, is connected only beyond a relatively broad spacer channel  20 . By means of the grooving die illustrated schematically in  FIG. 1 , it is possible to form the stub toothing of the forged gear-wheel body precisely, both in the region of roof-like tooth tips  22  (see  FIG. 4 ) and in the region of the tooth flanks, which albeit still have parallel tooth flanks there at first. The obliqueness thereof (shown in  FIG. 4 ), together with corresponding expansion of the inter-tooth spaces toward the respective tooth roots, will first be generated in the second grooving step by use of a swaging tool with swaging parts disposed in a fan-like arrangement, as illustrated in  FIG. 2 . By means of this swaging tool  23 , which substantially constitutes the lower die of the grooving die according to  FIG. 2 , oblique tooth flanks are generated, as illustrated in  FIG. 4  on the finished stub toothing. This undercut of teeth  15  of the stub toothing in the region of their side flanks  24  serves to prevent gear jumpers. Swaging tool  23 , as will be further described in detail hereinafter, is provided for machining the interstices of the stub toothing in such a way that material from the region of the tooth flanks will flow by appropriate material flow from the tooth flanks into the hollow profile of the die in the direction of the tooth-root region, and therefore toward spacer channel  18 . 
     Between a pressure plate  25  of the upper die and an outer bracing ring  26  of the lower die, the grooving die according to  FIG. 2  comprises a hold-down member  27 , which is pressed down by pressure plate  25  by means of elastomeric compression springs  28 . Hold-down member  27  is configured in such a way with its lower swaging face that it engages over the entire surface with the shape of gear-wheel body  1  and in this way holds gear-wheel body  1  together with its outer toothed-gear body  21  in contact with an upper holding plate  29 . Upper holding plate  29  is connected securely with a lower holding plate  30 , in which guides to permit a radial displacement movement of pushing parts  31  are machined. In their radially outer region, pushing parts  31  have pressure faces  33 , which project conically upward toward the axis of the die and which cooperate with correspondingly shaped pressure faces  32  of an actuating ring  35  connected to pressure plate  25  via bolts  34 . Corresponding to this, pushing parts  31  are pushed inward in radial direction when actuating ring  35  travels downward together with pressure plate  25  until hold-down member  27  reaches its lower end position by means of springs  28 . At their inner end, pushing parts  31  each have a swaging end  36 , which in the inner position of pushing parts  31  brings about swaging of the inter-tooth spaces, by plunging into them and causing undercutting of the stub toothing in the flank region of the teeth. Each pushing part  31  is used for formation of only one inter-tooth space, which is bounded by the two flanks of adjacent teeth. This plunging by pushing parts  31  is achieved by guiding them precisely between upper holding plate  29  and lower holding plate  30 , thus ensuring that an exact tooth shape will be created. Because of the high accuracy of shape of the stub toothing generated in the preceding grooving step according to  FIG. 1 , it is possible to limit the load of the swaging tool in the second grooving step in favor of a long service life thereof. The exact inner end position of the pushing parts is assured by a stop, which is formed on lower holding plate  30 . For this purpose, lower holding plate  30  ends at its radially inner side with a collar part  37 , which points toward gear-wheel body  1  and the radially outer circumferential face 38 of which acts as a stop in the inner end position of pushing parts  31 . 
     It is particularly advantageous to guide pushing parts  31  in such a way that they are angled obliquely relative to the axis of the die during their shape-imparting radial movement toward it, so that the material flow in the region of the inter-tooth spaces takes place upward toward the tooth-root region, where the excess material from the tooth flanks can flow into the tooth-root region and possibly into an adjacent unoccupied cavity of the die thereabove, located underneath spacer ring  16  ( FIG. 3 ). 
     The obliqueness of the pushing direction of pushing parts  31  advantageously ranges between 5° and 20°, or is approximately 12° in the example of the grooving die illustrated in  FIG. 2 . 
     Besides its function as the inner end stop for pushing parts  31 , collar  37  of lower holding plate  30  together with its upper end face functions as the support for toothing body  17  of gear-wheel body  1 , so that disturbing vibrations in the engaging region of swaging ends  36  of pushing parts  31  are avoided. 
     The schematic detail of  FIG. 2  shown in  FIG. 3  illustrates a pushing part  31  in two positions of its inner edge, namely in dashed representation in the retracted starting position PA and as solid lines in its shape-imparting end position PE. This limits the inwardly directed pushing movement along arrow P 2 , in which swaging end  36  of pushing part  31  plunges to the maximum into an inter-tooth space and in doing so creates the final shape of the space between two adjacent teeth. Above swaging end  36 , namely between its upper contour and the adjacent spacer channel  16 , a narrow cavity, into which excess material formed during shaping of the inter-tooth space can flow without leading to material overlaps, is present in the die. This favorable displacement direction of the material is achieved by the fact that the pushing part is guided such that it can move obliquely upward toward the axis of the die, so that material displacement necessarily takes place in the explained direction into the tooth-root region and possibly also into spacer channel  16 . 
       FIG. 3  also illustrates the axial extent of pushing part  31 , namely up to lower edge  39 , which rests in a slot in lower holding plate  30  and is mounted displaceably within this slot. The end position PE of the pushing part shown in  FIG. 3  corresponds to its illustration in  FIG. 2 , right side. The retracted position PA shown as dashed lines in  FIG. 3  corresponds to the illustration of pushing part  31  according to  FIG. 2 , left side. 
     The fixation of upper holding plate  29  on lower holding plate  30  is not illustrated in more detail in the drawing. Upper holding plate  29  is designed as a continuous annular plate, which with its underside limits the upward guidance of pushing parts  31  in such a way that pushing parts  31  are guided on all sides. 
     After the die has been opened, and therefore when the pushing parts are retracted to their starting position PA and pressure plate  25 , on the underside of which actuating ring  35  is fixed, has been raised, the gear wheel together with finish-swaged stub toothing can be removed upward from the lower die, as illustrated in  FIG. 4 . At that time tooth tips  22  of the stub toothing point downward. Thereafter the drive toothing is machined into toothed-gear body  21  in a chip-removing process. It is self-evident that the outer circumference of toothed-gear body  21  together with the middle bore of the gear wheel and conical part  14  will have been machined centrally beforehand.