Abstract:
A differential mechanism includes a case including first and second portions, the first portion including a first surface interrupted by recesses, the second portion including a second surface interrupted by second recesses, contacting the first surface, and secured to the first portion, a pin extending though said recesses, and a ring gear secured to said portions and overlapping said surfaces and said pin.

Description:
[0001]    This application is a divisional of pending U.S. application Ser. No. 11/827,126, filed on Jul. 10, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to a differential gear mechanism for transmitting rotary power from a power source to the wheels of a vehicle so that the wheels can rotate at mutually differential speeds and to the manufacture of the differential mechanism. 
         [0004]    2. Description of the Prior Art 
         [0005]    A differential mechanism transmits rotary power differentially to output shafts from a ring gear driven by an external power source, such as an internal combustion engine or electric motor. The differential ring gear, usually a hypoid bevel gear, is secured to the differential case, which is generally produced of cast nodular or ductile iron for high torque applications. The case is secured to the ring gear and defines a chamber containing bevel pinions driveably connected to the case by pinion shaft or spider, depending on the number of bevel pinions used, a right-side bevel gear and a left-side bevel gear in continuous meshing engagement with the bevel pinions. To support the bevel pinions, the case has equally spaced holes, equal in number to the number of bevel pinions used and positioned such that the pinion shaft or spider legs pass through the bevel pinions and through the holes in a proper spatial relationship, allowing proper meshing of the bevel pinion and bevel side gears, enabling torque transfer from the differential ring gear to the differential case to the pinion shaft to the bevel pinion gears, the bevel side gears, and right-side and left-side output shaft. 
         [0006]    Each shaft is driveably connected to a wheel of the vehicle. These shafts pass coaxially through openings in the case called hubs, which are supported circumferentially by bearings pressed into the differential carrier, supporting the entire differential assembly. Conventionally, the wall of the case is thick in the area where the pinion shaft or spider passes through the aforementioned holes, providing contact area sufficient to transmit the torsional load from the ring gear to the differential pin. It is preferable that there be no yielding of the case wall or the pinion shaft or spider in this area. 
         [0007]    The inside surface of the one-piece cast differential case must be machined to tight tolerances. To accomplish this, complicated tools are inserted axially through a hub and radially through the windows. When the case or tool is rotated, this tool must be able to compensate for the different rotational radii required at different locations in the case, and this tool, or another tool, must be able to generate pockets where the pinion gears seat and in which they rotate with a hemispherical or flat-bottomed shape to match the back of the type of pinion gears used. Depending on the tolerances of the casting, particularly concentricity about its rotational axis and uniformity in wall thickness, some cases may need to be machined on the outside after the axis of rotation has been uniquely defined by the inside machining to achieve rotational balance. 
         [0008]    Windows are cast into the one-piece case, which allows the pinion and side gears to be inserted for assembly. This is a manual operation requiring considerable dexterity. 
         [0009]    Variations in the types of differential mechanisms, which include open, limited slip, and positive locking, require variations in the components that comprise the differential assembly. These variations have heretofore required that the case take different forms in order to accommodate the various components for each of applications. 
         [0010]    There is a need in the industry, therefore, to improve the strength and stiffness of a differential assembly, to increase its torque capacity, to minimize its package space and to reduce the number of components that are unique to a particular application of the differential assembly, while minimally increasing or reducing total cost, including material and manufacturing cost. It is also desirable that weight is minimized and that NVH (noise, vibration and harshness) level be minimized. 
       SUMMARY OF THE INVENTION 
       [0011]    A differential mechanism includes a case including first and second portions, the first portion including a first surface interrupted by recesses, the second portion including a second surface interrupted by second recesses, contacting the first surface, and secured to the first portion, a pin extending though said recesses, and a ring gear secured to said portions and overlapping said surfaces and said pin. 
         [0012]    This differential assembly has lower weight and rotating inertia than a differential assembly of comparable torque capacity. A forming process is applied that allows control of the case wall along the length of the side walls and work hardens the case material, provides adequate formability to produce the component, and provides a sufficiently high elastic modulus to provide good assembly stiffness. The combination of the forming process, work-hardened material properties and component design result in excellent specific stiffness, adequate strength, manufacturability and low manufacturing cost. 
         [0013]    Cold flow forming is a process that is well suited to the production of differential cases. The flow forming process is similar to a single-point turning process in a lathe except that the single-point machining tool is replaced by a roller that causes the material that would have been removed in turning to flow radially inward or axially forward. Causing material to flow in the tangential direction is difficult. But the desired, non-uniform thicknesses are easily achieved in the longitudinal direction, where a cross section taken perpendicular to the rotational axis at any point along the length of the part is axisymmetric. 
         [0014]    When one or more of the case portions are attached to the ring gear by laser welding, or some other appropriate welding process, they result in a much stiffer design, reducing mesh point deflection and NVH. Flow forming optimizes the material thickness distribution, with respect to stress, making reduced weight possible. 
         [0015]    Because the surface finish of the mandrel is imprinted onto the interior of the case, no machining is required on the inside of the case, thereby reducing manufacturing cost. Automated assembly of components within the case is accomplished through an open end of a bell-shaped case, rather than through windows, thereby saving additional cost. 
         [0016]    The similarity in carbon equivalent between the steel differential case and a steel ring, as compared to a steel ring and cast iron case, simplifies laser welding. The elimination of bolts and holes, and frequent errors in bolt hole position, reduces weight and manufacturing error. 
         [0017]    The differential case portions are of a low carbon steel or a low carbon micro-alloyed steel, rather than nodular (also called ductile) cast iron, thereby providing a higher elastic modulus. Cold flow forming the case provides work hardening and adds to case strength and wall thickness optimization. The direct drive design allows the case wall thickness to be reduced by removing the case from the torque path. Laser welding, or some other appropriate welding process, is employed to attach one or more of the case components to the ring gear, with the internal components inserted through the open end of the flow formed case, rather than through a window in a one-piece cast design. A flange, used to bolt-on the ring gear in the baseline design, may be removed from some embodiments. Flow forming from a sheet blank and a forged blank are available to result in the desired thickness distribution. 
         [0018]    Improved stiffness and reduced mesh point deflection result from the component designs and the use of a stiffer material for the differential case. Reduced transmitted error and backlash results from the increased stiffness of the design. Net shape on the inside of the differential case with an excellent finish and dimensional accuracy result from the use of the flow forming process. Weight savings and reduced rotational moment of inertia result from the use of higher strength steel and work hardening of the differential case and possibly from the elimination of the ring gear mounting flange. The possibility of automating assembly or providing mechanically assisted assembly result from not assembling through a window. The design and manufacture process reduce manufacturing cost associated with laser welding, reduced machining, and eliminate some or all steps concerning bolting the ring gear to the flange. 
         [0019]    The differential assembly eliminates torque flow through the differential case by transmitting torque from the differential ring gear via the pinion shaft or spider directly to the bevel pinion or differential gears. This direct drive eliminates the thick walls needed in a conventional differential case to house the differential pin and spider subassembly. 
         [0020]    The differential assembly eliminates elongation of the differential pinion shaft hole in the case under repeated loading, eliminates differential pinion shaft fasteners, reduces differential case wall thickness, increases stiffness, and maximizes capacity torque in a smaller package. 
         [0021]    The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0022]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0023]      FIG. 1  is a perspective cross sectional view of a ring gear and welded differential case formed in portions, shown separated and axially spaced; 
           [0024]      FIG. 2  is perspective cross sectional view showing the case and ring gear of  FIG. 1  and components of the differential mechanism assembled in the case; 
           [0025]      FIG. 3  is a perspective cross section of a bolted differential case formed in portions, shown separated and axially spaced; 
           [0026]      FIG. 4  is partial cross sectional showing the differential case and ring gear of  FIG. 3  in their assembled positions; 
           [0027]      FIG. 5  is a perspective view of the interior of a differential case portion showing bevel pinions assembled on spider pins; 
           [0028]      FIG. 6  is a perspective view of the interior of a differential case portion showing spider pins and a retention clip; 
           [0029]      FIG. 7  is a perspective view of the exterior of a differential case portion showing the ends of the spider pins extending through the case; 
           [0030]      FIG. 8  is a front view of differential case portion showing a weld seam that secured the ring gear; 
           [0031]      FIG. 9  is a perspective view showing the components of a limited slip differential in spaced-apart relation; 
           [0032]      FIG. 10A  is a perspective view of a differential case portion containing components of an open differential; 
           [0033]      FIG. 10B  is a perspective view of a differential case portion containing components of a limit slip differential; 
           [0034]      FIG. 10C  is a perspective view of a differential case portion containing components of an electromagnetically actuated locking differential; 
           [0035]      FIG. 10D  is a perspective view of a differential case portion containing components for use with any of the assemblies of  FIGS. 10A-10C ; 
           [0036]      FIG. 11  is a side cross section of a machined forging that integrates the ring gear and common differential case portion; 
           [0037]      FIG. 12  shows schematically a workpiece being formed by flow forming; 
           [0038]      FIGS. 13A ,  13 B and  13 C illustrate process steps for form forming a first axial side of a differential case portion; 
           [0039]      FIGS. 14A ,  14 B and  14 C illustrate process steps for form forming a second axial side of the differential case portion; 
           [0040]      FIG. 15  is a cross section taken at a diametric plane through a forged preform having a wall thickness that varies before flow forming; 
           [0041]      FIGS. 16A ,  16 B and  16 C are cross sections taken at a diametric plane through a workpiece illustrating successive process steps for flow forming a differential case portion; and 
           [0042]      FIGS. 17A ,  17 B and  17 C are cross sections taken at a diametric plane through the workpiece illustrating later, successive process steps for flow forming a differential case portion. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0043]    Referring now to  FIGS. 1 and 2 , a differential assembly  10  includes a differential case  12  formed in portions  13  and  14  that are secured mutually, the case  12  being supported for rotation about an axis  15 , which extends laterally toward the left and right wheels of a driven wheel set of an automotive vehicle. The case  12  is driveably connected to a driveshaft  16 , which rotates about an axis  17  and is driven from a transmission output (not shown). Axis  17  can be either perpendicular or parallel to axis  15 . A bevel pinion  18 , secured to driveshaft  16 , is engaged with a ring gear  20 , which is secured to case portion  13  and drives the differential assembly  10  in rotation about axis  15 . 
         [0044]    The case portions  13 ,  14  enclose a cavity containing bevel pinions  22 - 25 , each formed with bevel gear teeth and spaced in equal angular increments about axis  15 . Each pinion  22 - 25  is secured to case  12  by a spider  26 , which includes a differential pin  28 , fitted into two holes in case  12 , and two pins  30 ,  32 , directed normal to pin  28 . Each of pins  30 ,  32  is fitted into a hole in case  12  and into a spherical depression in pin  28 . Pin  28  passes through hole  34  in bevel pinion  22  and a similar hole in pinion  24 . Pinions  22 ,  24  are located at mutually diametric opposite sides on case  12  and near axially opposite ends of pin  28 . Pin  32  passes through a hole  36  through bevel pinion  25 , and pin  30  passes a similar hole through bevel pinion  23 . As case  12  rotates about axis  15 , pinions revolve about axis  15  and rotate about the axes of their respective spider pin  28 ,  30 , and  32 . 
         [0045]    Two side gears  38 ,  40 , each formed with bevel gear teeth in meshing engagement with the teeth of the pinions  22 - 25 , are located in the cavity of casing  12 . Side gear  38  includes an axial surface  42 , extending away from the spider  26  and formed on its inner surface with a spline  44 , by which it is engaged with an axle shaft (not shown) connected to the right-hand side wheel of the vehicle. The right-hand side axle shaft extends laterally through an opening  46  formed in case portion  14 . Similarly, at the left side of the differential assembly  10 , side gear  40  includes an axial surface  48 , extending away from the spider  26  and formed on its inner surface with a spline  50 , by which it is engaged with an axle shaft (not shown) connected to the left-hand side wheel of the vehicle. The left-hand side axle shaft extends laterally through an opening  52  formed in case portion  13 . 
         [0046]    The case  14  and gear  20  are secured mutually by a weld  54 , preferably a laser weld, which extends radially toward axis  15  at a plane where the case  14  and gear  20  are in mutual contact. Weld  54  also extends circumferentially about axis  15 . After case portion  14  and gear  20  are interconnected by weld  54 , case portion  13  and ring gear  20  are secured mutually by a series of bolts  56 , each bolt fitted into a hole  58  on a circle of bolt holes formed in a radial flange  60  on case portion  13 . 
         [0047]      FIG. 8  is a front view of differential case portion  14  showing the circular laser weld seam  54  welded in the axial direction from the side shown there, by which ring gear  20  is secured to the case portion  14 . The intersection between the ring gear bore and the case outside diameters in nominally a line-to-line fit, with either a slight interference or a slight slip fit being acceptable. The interface between ring gear  20  and case portion  14  is welded, preferably by laser welding, in the axial direction. 
         [0048]    Case portion  13  includes an axial protrusion  80 , which contacts an annular rim  82  on case portion  14 , when the parts are assembled. The axial protrusion  80  is formed with a series of arcuate recesses  84 , spaced angularly about axis  15 . Each recess  84  partially surrounds a respective spider pin  28 ,  30 ,  32  (as shown in  FIG. 6 ) and is aligned with a spider pin hole portion  86  formed in case portion  14 . Each recess  84  and hole portion  86  complete a circular or slotted hole  88 , which contains a spider pin  28 ,  30 ,  32  when the case is assembled. 
         [0049]      FIGS. 3 and 4  illustrate an alternative embodiment of a differential assembly  62 , in which two differential case portions  64 ,  66  are secured mutually and to a ring gear  67  by a series of bolts, each bolt being fitted into a hole  68  located on a circle of bolt holes formed in a radial flange  70  on case portion  64  and a hole  72  located on a circle of bolt holes formed in a radial flange  74  and aligned with a hole  68  on case portion  66 . Flanges  70 ,  74  are in mutual contact, and the ring gear contacts the face  76  of flange  74 . 
         [0050]      FIG. 4  shows bearings  78 ,  79  fitted over the outer surfaces of case portions  64 ,  66  and axially spaced mutually along axis  15  for supporting the case  62 , and ring gear  67  in rotation about axis  15 . 
         [0051]    Case portion  64  includes an annular, axial-extending protrusion  80 , which contacts an annular rim  82  on case portion  66 , when the parts are assembled, as shown in  FIG. 4 . The axial protrusion  80  is formed with a series of arcuate recesses  84 , spaced angularly about axis  15 . Each recess  84  partially surrounds its respective spider pin  28 ,  30 ,  32  and is aligned with a spider pin hole portion  86  formed in case portion  66 . Each recess  84  and hole portion  86  complete a circular hole  88 , which contains a spider pin  28 ,  30 ,  32  when the case is assembled as shown in  FIG. 4 . 
         [0052]      FIG. 5  illustrates the pinions  22 - 25  supported on a case portion  14 ,  66  for rotation about the spider pins  28 ,  30 ,  32 .  FIG. 6  shows parallel slots  90  formed at diametrically opposite sides of spider pin  30 , and parallel slots  92  formed at diametrically opposite sides of spider pin  32 . A clip  94  includes legs  96 ,  97 , each of which engages a slot  90  on spider pin  30 , and legs  98 ,  99 , each of which is similar respectively to a leg  96 ,  97  and engages a slot  92  on spider pin  32 . Upon assembly, clip  94  retains spider pins  30 ,  32  in a correct position in their respective holes  88 , such that a protrusion  100  at the inner end of pin  30  is retained in and supported on a hole  101  formed in spider pin  28 , and a protrusion  102  at the inner end of pin  32  is retained in and supported on hole  103 , which is aligned with hole  101 . 
         [0053]      FIG. 7  shows the ends  104 ,  106  of spider pin  28  extending through holes  88 , the end  108  of spider pin  30  extending through a hole  88 , and the end  110  of spider pin  32  extending through a hole  88 . During assembly of case  66 , ring gear  67  is located radially outboard of the spider pin ends  104 ,  106 ,  108 ,  110  and is bolted to flanges  70 ,  74 . 
         [0054]    In a second embodiment, the limited slip differential (LSD)  118  shown in  FIGS. 9 and 10 , angularly spaced radial tabs  120  on the clutch plates  122  are aligned with and located in angularly spaced grooves  124  formed in the LSD case  126 , thereby preventing rotation of the clutch plates. Alternatively, many shallower tabs  120  and grooves  124  or splines could be used for this purpose. If the depth of the locking feature is much less than the case wall thickness, then the grooves or splines can be machined into a mandrel, and easily flow formed into the inside diameter of the LSD case  126 . Friction discs  128 , interleaved between successive clutch plates  122 , are secured to the side gear  48 . When the friction discs  128  and clutch plates  122  are forced axially into mutual frictional contact by side gear separating forces, a drive connection between side gear  48  and LSD case  126  is either fully closed or partially closed depending on the degree of the slip that occurs between the side gear and case. 
         [0055]    A bridge piece  130 , formed with the annular, axial-extending protrusion  80 , includes a flange  132 , which is secured by bolts  56  to the flange  134  of case  126  and to the ring gear  20 . An alternate construction eliminates flange  132  and presses the bridge piece  130  into case  14 ,  66 . 
         [0056]      FIG. 10A  shows the first embodiment, the open differential  10  of  FIGS. 1-4 , whose case portion  13  includes the annular, axial-extending protrusion  80 . 
         [0057]      FIG. 10  B shows the second embodiment, the limited slip differential (LSD)  118  shown in  FIG. 9 . 
         [0058]      FIG. 10  C shows an electronic locking differential  135  including a case portion  136 , an electromagnetic coil of wire  138  secure to case portion  136 , a side gear  140  formed with axial facing dog clutch teeth  142 , and an actuating ring  144  secured to case portion  136 . Ring  144  moves axially into engagement with the clutch teeth  142  when coil is energized by electric current, thereby securing side gear  140  to case portion  136 . Ring  144  moves axially out of engagement with the clutch teeth  142  when the electromagnet  138  is deenergized, thereby allowing side gear  140  to rotate differentially. The case portion  136  is formed with a flange  146 , which is secured by bolts  56  to the ring gear  20 . 
         [0059]      FIG. 10  D shows the common case portion  14  attached to the ring gear by welding. If serviceability is not an issue, the three interchangeable case portions, case  13  of the open differential  10 , case  126  of the limited slip differential  118  and case  136  of the locking differential, can also be secured to ring gear  20  by welding. 
         [0060]      FIG. 11  shows a cross sectional view of a fourth embodiment  150 , in which the ring gear  20 , and the common case portion  14  are consolidated into a single precision forging  152 . The second case portion used with the consolidated forging  152  can be case portion  13  of the open differential  10 , case portion  126  of the limited slip differential  118  and case portion  136  of the locking differential, each of which is secured to the bowl shaped region of the ring gear  20 . The forging  152  requires machining on radial inner surfaces. 
         [0061]      FIG. 12  shows schematically a tubular workpiece  160  of metal, such as steel, being formed by a flow forming process. A preformed tube  162 , fitted over a mandrel  164 , is held at one end  166  between an end face  168  of the mandrel and a tail stock  170 , which is forced axially into contact with the workpiece to hold the workpiece in position on the mandrel. A roller  172  driven in rotation about axis  174  moves axially along mandrel  164  and radially with respect to the axis  176  of the mandrel. The material of workpiece  160  flows axially along the mandrel as the roller moves along the mandrel, thereby forming the thickness and contour of the outer surfaces of the workpiece  160 . The inner contour and wall thickness of workpiece  160  varies along the length of the workpiece also in response to changes in the outer contour of the mandrel  164 . 
         [0062]    A steel workpiece  160  is significantly strengthened by work hardening that occurs due to the flow forming process. For example, a workpiece preform  162  of AISI 1006 steel with fine, equiaxed grains and a very low inclusion level, can be cold worked from an initial hardness of 115 HB, to a hardness of 225 HB. 
         [0063]      FIGS. 13A and 13B  show a disc preform  162 , a cylindrical mandrel  164 , a plate  180  supporting the preform, and a cylindrical roller  172  being forced radially toward axis  176 , contacting the perform and forming a hollow cylindrical workpiece hub  182  located at a first axial side of the disc  162 .  FIG. 13C  illustrates a second roller  184  being used to wrap the hub  182  tighter onto mandrel  164 , thereby improving the surface finish, dimensional accuracy and repeatability of hub  182 . 
         [0064]      FIGS. 14A and 14B  show a second mandrel  186  and a third roller tool  188  being used to split disc  162  and to form the inner and outer contour and wall thickness of a portion  190  of the workpiece as the roller is forced radially toward axis  176 .  FIG. 14C  illustrates roller  188  being used to wrap the portion  190  tighter onto mandrel  186 , thereby improving the surface finish, dimensional accuracy and repeatability of hub  180 . 
         [0065]    The process steps of  FIGS. 13A ,  13 B,  14 A and  14 B produce lower contact stress at the workpiece surface during the process than true flow forming, because of the increased contact area between the tool rollers and the workpiece, thus reducing accuracy and finish of the workpiece surfaces. The process steps of  FIGS. 13C and 14C  show true flow forming being used to produce the desired accuracy and finish of the workpiece. 
         [0066]      FIG. 15  shows a forged preform  190  having a wall thickness that varies before flow forming. Preform  190  will be formed by subsequent flow forming into a differential case portion  13 ,  14 ,  126 ,  136 , thereby allowing flow forming to be more effective and requiring less forming time. 
         [0067]      FIG. 16A-16C  show successive steps for flow forming the forged preform  190  of  FIG. 15  by moving various tool rollers  192  axially along a mandrel  194  and radially with respect to axis  196 . A full anneal or some other heat treating process to increase cold formability may be required after the step shown in  FIG. 16C . 
         [0068]      FIG. 17A-17C  show later, successive steps for flow forming the workpiece  198  of  FIG. 16C  by moving various tool rollers  200  axially along another mandrel  202  and radially with respect to axis  196  to achieve the desired inside tolerances and inside surface finish of a differential case portion  13 ,  14 ,  126 ,  136 . 
         [0069]    In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.