Abstract:
A differential assembly for a motor vehicle includes a plurality of pinion gears in meshing engagement with a plurality of side gears. At least one of the pinion gears and side gears is formed from a tubular insert and a tubular preform forged together in a forging die. A method for manufacturing a differential assembly is also provided.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to a method for manufacturing gears and, more particularly, to differential side gears, pinion gears and hypoid gears adapted for use in vehicle differentials and a method for manufacturing the gears.  
       BACKGROUND OF THE INVENTION  
       [0002]     As is well known in the art, the fabrication of typical differential side gears and pinion gears for an automotive differential assembly is complex and costly and as such, greatly adds to the cost of the differential assembly. A typical process for forming a differential gear includes forging, annealing, rough machining, carburizing, hardening and finish machining operations. Despite the almost universal use of such forming processes, several drawbacks have been noted.  
         [0003]     One such drawback relates to the initial forming of the differential gear through forging. As those skilled in the art will appreciate, the differential gear is typically blanked or rough-formed in a forging operation from a solid billet of steel. This forging operation is relatively inefficient because the shape of the “in-process” forging is substantially different from the final desired gear shape. Specifically, each of the differential gears contemplated for manufacture by the present invention include relatively large apertures extending through the center of the gear. As such, many machining operations are required after forging. In addition, a relatively large proportion of the forging material is machined off and wasted.  
         [0004]     Another drawback concerns the machining of the differential gear. The numerous machining operations that are performed on the differential gear typically account for more than 70% of the total cost of the gear. Furthermore, the protracted nature of the machining operations often results in an average cycle time that exceeds one or more days in length.  
         [0005]     Yet another drawback concerns the material from which the differential gear is formed. Typically, the steel billet from which the gear is formed is a low carbon steel having characteristics that are particularly well suited to both forging and machining. Such steels, however, generally lack the strength that is desired for a gear and as such, a time consuming and costly carburization process is typically employed to create a layer of relatively high carbon steel on the surface of the differential gear. Carburization usually entails the placement of semi-finished gears into a heated, high-carbon environment for an extended period of time to permit carbon to migrate into the gear material to a predetermined depth. The differential gear is subsequently heat treated so that the high carbon layer provides a level of strength and durability that is commensurate with the intended application.  
         [0006]     Accordingly, there remains a need in the art for an improved differential gear manufacturing method that permits increased flexibility in the design of the gears of the differential and the adaptation of lower cost processes for their manufacture.  
       SUMMARY OF THE INVENTION  
       [0007]     In one form, the present invention provides a differential assembly for a motor vehicle. The differential assembly includes a plurality of pinion gears and side gears in meshing engagement with one another. At least one of the pinion gears and side gears is formed from a tubular insert and a tubular preform forged together in a die. A method for manufacturing a differential assembly is also provided.  
         [0008]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, wherein:  
         [0010]      FIG. 1  is a schematic illustration of a motor vehicle constructed in accordance with the teachings of the present invention;  
         [0011]      FIG. 2  is a fragmentary perspective view of a portion of the motor vehicle of  FIG. 1 , illustrating the rear axle in greater detail;  
         [0012]      FIG. 3  is a section view of a portion of the rear axle illustrated in  FIG. 2 ;  
         [0013]      FIG. 4  is a perspective view of a differential side gear constructed in accordance with the teachings of the present invention;  
         [0014]      FIG. 5  is a perspective view of a tube used for constructing preforms of the present invention;  
         [0015]      FIG. 6  is a perspective view of an exemplary tubular insert used for constructing gears of the present invention;  
         [0016]      FIG. 7  is a cross-sectional side view of the tubular insert of  FIG. 6 ;  
         [0017]      FIG. 8  is a partial cross-sectional side view of a die for forming a gear from an insert and a preform, the die being in an open condition; and  
         [0018]      FIG. 9  is a partial cross-sectional side view of the die of  FIG. 6  shown in a closed position. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     With reference to  FIG. 1  of the drawings, a vehicle having a differential assembly constructed in accordance with the teachings of the present invention is generally indicated by reference numeral  10 . The vehicle  10  includes a driveline  12  drivable via a connection to a power train  14 . The power train  14  includes an engine  16  and a transmission  18 . The driveline  12  includes a drive shaft  20 , a rear axle  22  and a plurality of wheels  24 . The engine  16  is mounted in an in-line or longitudinal orientation along the axis of the vehicle  10  and its output is selectively coupled via a conventional clutch to the input of the transmission  18  to transmit rotary power (i.e., drive torque) therebetween. The input of the transmission  18  is commonly aligned with the output of the engine  16  for rotation about a rotary axis. The transmission  18  also includes an output and a gear reduction unit. The gear reduction unit is operable for coupling the transmission, input to the transmission output at a predetermined gear speed ratio. The drive shaft  20  is coupled for rotation with the output of the transmission  18 . Drive torque is transmitted through the drive shaft  20  to the rear axle  22  where it is selectively distributed in a predetermined manner to the left and right rear wheels  24   a  and  24   b,  respectively.  
         [0020]      FIGS. 2 and 3  depict the rear axle  22  to include a differential assembly  30 , a left axle shaft assembly  32  and a right axle shaft assembly  34 . The differential assembly  30  includes a housing  40 , a differential unit  42  and an input shaft assembly  44 . Housing  40  supports differential unit  42  for rotation about a first axis  46  and further supports input shaft assembly  44  for rotation about a second axis  48  that is perpendicular to first axis  46 .  
         [0021]     Housing  40  is initially formed in a suitable casting process and thereafter machined as required. Housing  40  includes a wall member  50  that defines a central cavity  52  having a left axle aperture  54 , a right axle aperture  56 , and an input shaft aperture  58 .  
         [0022]     Left axle shaft assembly  32  includes a first axle tube  60  fixed to left axle aperture  54  and a first axle half-shaft  62  that is supported for rotation in first axle tube  60  about first axis  46 . Similarly, right axle shaft assembly  34  includes a second axle tube  64  that is fixed to right axle aperture  56  and which supports a second axle half-shaft  66  for rotation about first axis  46 .  
         [0023]     Differential unit  42  is disposed within central cavity  52  of housing  40  and includes a case  70 , a ring gear  72  that is fixed for rotation with case  70 , and a gearset  74  that is disposed within case  70 . Gearset  74  includes first and second side gears  82  and  86  and a pair of differential pinions  88 , which are rotatably supported on pinion shafts  90  that are mounted to case  70 . Case  70  includes a pair of trunnions  92  and  96  and a gear cavity  98 . A pair of bearing assemblies  102  and  106  are shown to support trunnions  92  and  96 , respectively, for rotation about first axis  46 . First axle half shaft  62  and second half shaft  66  extend through left and right axle apertures  54  and  56 , respectively, where they are coupled for rotation about first axis  46  with first and second side gears  82  and  86 , respectively. Case  70  is operable for supporting differential pinions  88  for rotation within gear cavity  98  about one or more axes that are perpendicular to first axis  46 . First and second side gears  82  and  86  each include a plurality of teeth  108  which meshingly engage teeth  110  that are formed on differential pinions  88 .  
         [0024]     Input shaft assembly  44  extends through input shaft aperture  58  and includes an input pinion shaft  120 , a conventional propeller shaft coupling flange  122  and a pair of conventional bearing assemblies  124  and  126 . Each of bearing assemblies  124  and  126  includes an outer race that engages housing  40  in a press-fit manner. Bearing assemblies  124  and  126  cooperate with housing  40  to support the input pinion shaft  120  for rotation on second axis  48  in input shaft aperture  58 .  
         [0025]     As shown in  FIG. 4 , first side gear  82  includes a plurality of teeth  108  circumferentially surrounding an aperture  130 . Teeth  108  extend from a front face  132  to a back face  134 . A cylindrical sleeve  136  extends from back face  134 . Cylindrical sleeve  136  includes an outer surface  138  and an end face  140 . An internal spline  142  is formed within aperture  130  and extends through first side gear  82  from front face  132  to end face  140 . Internal spline  142  is useful for drivingly interconnecting first side gear  82  and first axle half-shaft  62 .  
         [0026]     With reference to  FIGS. 5-9 , first side gear  82  and its method of manufacture will be described. However, it should be appreciated that many types of gears including second side gear  86 , differential pinions  88  and the gear of input shaft  44  may be constructed incorporating the teachings of the present invention. For clarity, only first side gear  82  will be described in detail with the understanding that any one of the gears within differential unit  42  may be constructed as described hereinafter.  
         [0027]     First side gear  82  includes is constructed from a preform  144  and an insert  146 . Preform  144  is shaped as a section of substantially cylindrical tubing cut to length. Preform  144  includes an outer cylindrical surface  148  and an inner cylindrical surface  150  defining an aperture  152 . Aperture  152  extends from a first end face  154  to a second end face  156 .  
         [0028]     Preform  144  may be constructed from a billet, welded tubing or seamless tubing. Welded tubing is manufacturing by rolling a substantially flat sheet in the shape of a cylinder. The edges of the flat sheet are joined to one another by a weld. The weld extends substantially to a longitudinal axis of the tube. Seamless tubing may be prepared by a variety of techniques including hot extrusion. Seamless tubing may be fabricated using low carbon steel or other alloys. In one embodiment, steel tubing having a sufficiently high carbon content to be hardened by an induction heat treat method is used.  
         [0029]     Insert  146  is a substantially cylindrical member including a hollow body  158  having a first end face  160  and a second end face  162 . An aperture  164  axially extends through hollow body  158  from first end face  160  to second end face  162 . Internal spline  142  is formed within aperture  164  and extends substantially the entire length of the aperture. A flange  170  radially outwardly extends from hollow body  158 . Flange  170  is axially positioned between first end face  160  and second end face  162  to define a first outer cylindrical surface  172  and a second outer cylindrical surface  174 . First outer cylindrical surface  172  is positioned between flange  170  and first end face  160 . Second outer cylindrical surface  174  is positioned between flange  170  and second end face  162 . Insert  146  may be constructed from a powder metal material or a metal tube which has been swaged and/or rolled to produce the geometrical features previously described. A powder metal insert would most likely be sintered prior to insertion within the forging die of the present invention.  
         [0030]     Flange  170  includes an upper annular surface  176  and a lower annular surface  178  interconnected by a third outer cylindrical surface  180 . A plurality of recesses or scallops  182  are formed in third outer cylindrical surface  180 . Scallops  182  are circumferentially spaced apart from one another such that the outer cylindrical surface  180  is interrupted about its entire circumference to form a plurality of protrusions  184 . One skilled in the art will appreciate that the shape of scallops  182  shown in the figures is merely exemplary and that other interrupted geometrical forms such as gear teeth, axial splines or the like may be formed in flange  170  without departing from the scope of the present invention. Protrusions  184  are mechanically interconnected with preform  144  after the forging process has been completed. A radial interlock is formed between insert  146  and preform  144  as material from insert  146  enters scallops  182 . The mechanical interconnection between insert  146  and preform  144  allows torsional energy to be transferred between the components without relative slipping between the preform and the insert.  
         [0031]      FIGS. 8 and 9  depict a tool  186  used to forge first side gear  82 . In the process, insert  146  is heated and placed on a lower die  188 . A pin  190  extends through aperture  164  to properly position insert  146  on lower die  188 . Pin  190  includes a shaped outer surface  191  which corresponds to the shape of aperture  164 . In the embodiment shown, internal spline  142  is formed on the inner surface of aperture  164 . The contours of pin  190  and aperture  164  are matched to minimize any deformation of insert  146  during forging.  
         [0032]     Lower die  188  includes a pocket  192  for receipt of a portion of hollow body  158 . First end face  160  is supported by a land  194  of pocket  192 . Pocket  192  is sized such that lower annular surface  178  of insert  146  is spaced apart from a forging face  196  of lower die  188 . As will be described in greater detail hereinafter, the gap formed between forging face  196  and lower annular surface  178  is filled with material from preform  144  during the forging process.  
         [0033]     Preform  144  is also heated and placed on lower die  188 . Preform  144  is positioned on lower die  188  such that at least a portion of insert  146  extends into aperture  152  of preform  144 .  
         [0034]     An upper die  198  is moveable from a first position shown in  FIG. 8  to a second position shown in  FIG. 9 . The upper die position in  FIG. 8  corresponds to an open die position. The position of upper die  198  in  FIG. 9  corresponds to a closed die position where upper die  198  and lower die  188  define a closed cavity  200  therebetween. During the closing movement of upper die  198 , preform  144  is contacted and deformed to fill cavity  200 . Upon closure, side gear  82  is formed to include a first portion  202  corresponding to substantially deformed preform  144  and a second portion  204  including substantially undeformed insert  146 . One skilled in the art will appreciate that the die configuration depicted is a trapped die design. In a trapped die, nearly all of the material of preform  144  is restrained within cavity  200  during the forging process. On the contrary, an open-ended die concept includes a pathway for superfluous material to exit during the forging process. The trapped die design advantageously provides a gear having little to no flash thereby minimizing the need for subsequent machine operations.  
         [0035]     Upper die  198  compresses preform  144  to form teeth  108  as a set of net-shaped teeth. Net-shaped teeth refers to the condition of teeth  108  as being completely formed and not requiring subsequent machine operations to properly form the geometry of the teeth. In this manner, the time and cost required to form a completed first side gear  82  is minimized. It should also be appreciated that the use of tubular preform  144  in conjunction with tool  186  substantially minimizes the quantity of scrap material generating during gear manufacture. As previously mentioned, standard rough gears exiting the forging process will require expensive and time-consuming processes such as boring and broaching to produce a splined aperture such as aperture  164  of insert  146 . Material previously present in the gear aperture would be wasted.  
         [0036]     During the forging process, axial and radial forces are of such a magnitude to cause material from preform  144  to envelop the geometrical features of insert  146 . Specifically, material flows between forging face  196  and lower annular surface  178  of insert  146  to form a lower lip  206 . Additionally, material flows between an end forging face  208  of upper die  198  and upper annular surface  176  of insert  146  to form an upper lip  210 . Upon completion of the forging step, flange  170  is enveloped by preform  144 . Because lower lip  206  engages lower annular surface  178  and upper lip  210  engages upper annular surface  176 , insert  146  is axially restrained from movement relative to preform  144 .  
         [0037]     Depending on the mechanical interlock desired, an additional operation may be performed to couple insert  146  to first portion  202 . Optionally, a brazing powder or solder is applied to the outer surfaces of insert  146  which are to be encapsulated by deformed preform  144 . After the brazing powder or solder has been applied to insert  146 , a heated preform  144  is positioned over insert  146  within lower die  188 . The heated preform causes the brazing powder to melt. As such, an additional bond is created by brazing or soldering. During forging, preform  144  is substantially deformed during the closure of upper die  198  and lower die  188  to cause material from preform  144  to frictionally engage the brazing powder or solder as well as the outer surface of insert  146  to create a brazed or soldered joint therebetween.  
         [0038]     It should be appreciated that only a portion of insert  146  is encapsulated within first portion  202  and that the amount of insert  146  extending from first portion  202  is determined by the die design. Accordingly, a variety of final gear designs are contemplated which include one or more portions of the insert protruding from the substantially deformed first portion. Furthermore, it is within the scope of the present invention to form the insert and/or preform using other materials or processes than previously described.  
         [0039]     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations may be made therein without department from the spirit and scope of the invention as defined in the following claims.