Abstract:
A compact planetary differential gear set includes first ( 130 A) and second ( 130 B) sun gears, a first set ( 200 A) and a second set ( 200 B) of planet gears ( 220 ), and a carrier ( 160 ) with a ring gear ( 190 ). Enmeshing gear pairs ( 210 ) are formed from one planet gear from each set. The first and second planet gear sets enmesh the first and second sun gears, respectively. The ring gear does not extend into an annular region containing the planet gears thereby allowing four or more gear pairs to compactly fit into the annular region. The carrier is a weldment and substantially encloses the sun gears and the planet gears permanently. The differential requires no fasteners or post-weld machining and may have a higher capacity, lower cost, smaller size, lower part number count, and/or lower amounts of material compared with conventional differentials. The differential is suited for motor vehicle applications.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is being filed on 4 Sep. 2012, as a PCT International Patent application in the name of Eaton Corporation, a U.S. national corporation, applicant for the designation of all countries except the U.S., and, Andrew N. Edler, a citizen of the U.S., applicant for the designation of the U.S. only, and claims priority to U.S. patent application Ser. No. 61/531,611, filed 6 Sep. 2011, and U.S. patent application Ser. No. 61/673,439 filed on 19 Jul. 2012, the disclosures of which are incorporated herein by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to differential gear sets and planetary differential gear sets. Such differential gear sets are typically found in wheel driven vehicles, such as automobiles and trucks. 
       BACKGROUND 
       [0003]    Wheel driven vehicles typically are arranged with a pair of drive wheels positioned opposite each other adjacent opposite sides of the vehicle. The pair of drive wheels is typically driven by a common power source via a common drive train. The pair of drive wheels may be front wheels or rear wheels of the vehicle. When the vehicle is driven around a corner or along a curve, an outside drive wheel of the pair of drive wheels travels a longer distance than an inside drive wheel of the pair of drive wheels, which travels a shorter distance than the outside drive wheel. To accommodate the longer and the shorter distances simultaneously traveled by the opposite drive wheels, the common drive train typically includes a differential gear set. 
         [0004]    In certain all-wheel-drive vehicles, all wheels of the vehicle are drive wheels powered by a common drive train. In certain vehicles, multiple pairs of drive wheel sets (e.g., dual wheels) are positioned opposite each other adjacent opposite sides of the vehicle. In such multi-wheel drive (e.g., multi-drive axle) vehicles, a drive train typically includes a differential gear set between each pair of drive wheels or drive wheel sets (e.g., a first pair of drive wheel sets and a second pair of drive wheel sets). As the first pair of drive wheel sets may have an average travel distance different from the second pair of drive wheel sets, a differential gear set may also be positioned between the first pair of drive wheel sets and the second pair of drive wheel sets (e.g., in a transfer case of the driveline). 
         [0005]    Differential gear sets may be further used in other applications such as packaging machines, linkage arrangements, power dividers, etc. 
         [0006]    Planetary gear sets may include one or more sun gears and one or more planet gears held in position by a carrier. The planet gears typically mesh with one or more of the sun gears. Certain planetary gear sets include a ring gear that is directly coupled to the carrier. Other planetary gear sets include a ring gear that meshes with the planet gears. Certain planetary gears sets may be arranged as differential gear sets. Certain planetary gear sets may be used in multi-speed transmissions. 
         [0007]    Differential gear sets and/or planetary gear sets are often desired that are low in cost, are easily manufactured, include a low number of part numbers, are small in volume, are small in diameter, are narrow in width, are high in torque capacity, and/or are high in stiffness. The present disclosure satisfies these and other desires. 
       SUMMARY 
       [0008]    One aspect of the present disclosure relates to a compact planetary differential gear set with a higher torque capacity in a given size than conventional planetary differential gear sets. The compact planetary differential gear set may have improved torque capacity in a given width, a given diameter, a given volume, a given mass, and/or a given rotational inertia in comparison to conventional differential designs. 
         [0009]    Another aspect of the present disclosure relates to a planetary differential gear set with a lower cost for a given torque capacity than the conventional planetary differential gear sets. The lower cost may result from a low part number count, elimination of fasteners, automated assembly using fixtures, no post-weld machining, and/or a low amount of material used. 
         [0010]    Still another aspect of the present disclosure relates to a planetary differential gear set including, a first sun gear, a second sun gear, a first set of planet gears, a second set of planet gears, and a carrier. The carrier substantially encloses the first sun gear, the second sun gear, the first set of planet gears, and the second set of planet gears. The planetary differential gear set requires no fasteners to operably position the first sun gear, the second sun gear, the first set of planet gears, the second set of planet gears, and the carrier relative to each other when the planetary differential gear set is in use. 
         [0011]    In certain embodiments, such as vehicle axle applications, the first sun gear is adapted to drive a first axle of an automobile and the second sun gear is adapted to drive a second axle of the automobile. The first sun gear may be adapted to drive a first drive shaft of a vehicle and the second sun gear may be adapted to drive a second drive shaft of the vehicle in applications such as vehicle transfer cases. The first set of planet gears may enmesh the first sun gear and the second set of planet gears may enmesh the second sun gear. The first set of planet gears and the second set of planet gears may enmesh each other. The first set of planet gears and the second set of planet gears may be positioned about the sun gears at a common radius. 
         [0012]    Yet another aspect of the present disclosure relates to a planetary differential gear set comprising a sun gear, a set of one or more planet gears, and a carrier having a welded construction that substantially encloses the sun gear and the set of planet gears such that the sun gear and the set of planet gears cannot be removed from the carrier. A method for assembling the planetary differential gear set may include: providing a first portion of the carrier; positioning the sun gear adjacent the first portion of the carrier; positioning the set of planet gears adjacent the first portion of the carrier; positioning a second portion of the carrier adjacent the first portion of the carrier; and, welding the first portion and the second portion of the carrier together. 
         [0013]    In certain embodiments, the welding of the first portion and the second portion of the carrier together in the above method may include electron beam welding. The welding of the first portion and the second portion of the carrier together may introduce only minimal distortion and/or local weld distortion such that no post-weld machining of the planetary differential gear set is required. In certain embodiments, the planetary differential gear set further includes a ring gear. The carrier may include the ring gear, a first portion welded to the ring gear, and a second portion welded to the ring gear. A method for assembling the planetary differential gear set may include: providing the first portion of the carrier; positioning the sun gear adjacent the first portion of the carrier; positioning the set of planet gears adjacent the first portion of the carrier; positioning the ring gear of the carrier adjacent the first portion of the carrier; positioning the second portion of the carrier adjacent the ring gear of the carrier; and, welding the first portion and the second portion of the carrier to the ring gear of the carrier. The welding may include electron beam welding. The sun gear may be adapted to drive a drivetrain shaft (e.g., an axle, a drive shaft, etc.) of a vehicle. 
         [0014]    Still another aspect of the present disclosure relates to a planetary differential gear set including a first sun gear, a second sun gear that is interchangeable with the first sun gear, a first set of planet gears enmeshed with the first sun gear, a second set of planet gears enmeshed with the second sun gear, and a carrier including a first piece and a second piece that are interchangeable with each other. The planet gears of the first set and the second set are interchangeable with each other. The planet gears of the first set and the second set are enmeshed with each other. The first piece forms a major portion of a first side of the carrier, and the second piece forms a major portion of a second side of the carrier. A ring gear may be welded to the first piece and the second piece of the carrier. 
         [0015]    Yet another aspect of the present disclosure relates to a planetary differential gear set including a first sun gear, a second sun gear, at least four intermeshing planet gear pairs, and a carrier. Each of the intermeshing planet gear pairs includes a first planet gear enmeshed with the first sun gear and a second planet gear enmeshed with the second sun gear. The carrier includes a ring gear piece that defines an innermost surface. The innermost surface of the ring gear piece is positioned beyond an outermost cylinder occupied by the intermeshing planet gear pairs. 
         [0016]    In certain embodiments, the innermost surface of the ring gear piece defines a radius that is spaced from the outermost cylinder occupied by the intermeshing planet gear pairs by a radial distance. The radial distance may be less than a thickness of a tooth of the planet gears. The carrier may include a first wall and a second wall that are spaced from each other. A first and a second planet gear of the intermeshing planet gear pairs may each substantially extend between the first wall and the second wall. The first and the second planet gears may each include a reduced diameter portion. The reduced diameter portion of the first planet gear may clear the second sun gear, and the reduced diameter portion of the second planet gear may clear the first sun gear. 
         [0017]    A variety of additional aspects will be set forth in the description that follows. These aspects can relate to individual features and to combinations of features. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the broad concepts upon which the embodiments disclosed herein are based. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a perspective view of a planetary differential gear set arrangement according to the principles of the present disclosure; 
           [0019]      FIG. 2  is an exploded perspective view of the planetary differential gear set arrangement of  FIG. 1  illustrating a pair of sun gears, a pair of planet gear sets, a pair of case halves, a ring gear, a set of pins, and a pair of bearing sets; 
           [0020]      FIG. 3  is the perspective view of  FIG. 1 , but with a wedge-shaped cut-away taken that passes through centerlines of two opposite planet gears of the pair of planet gear sets of  FIG. 2 ; 
           [0021]      FIG. 4  is the perspective view of  FIG. 1 , but with a half-cylinder cut-away taken that passes through a centerline of the sun gears of  FIG. 2  and centerlines of two planet gears of one of the planet gear sets of  FIG. 2 ; 
           [0022]      FIG. 5  is the perspective view of  FIG. 1 , but with a chord-shaped cut-away taken that passes through centerlines of a pair of opposite enmeshed planet gears of the pair of planet gear sets of  FIG. 2 ; 
           [0023]      FIG. 6  is the perspective view of  FIG. 1 , but showing only the pair of case halves and the ring gear of  FIG. 2  formed into a carrier; 
           [0024]      FIG. 7  is the perspective view of  FIG. 6 , but with a half-cylinder cut-away taken that passes through a centerline of the carrier; 
           [0025]      FIG. 8  is the perspective view of  FIG. 1 , but showing only the pair of sun gears, the pair of planet gear sets, and the set of pins of  FIG. 2 ; 
           [0026]      FIG. 9  is the perspective view of  FIG. 8 , but with a half-cylinder cut-away taken that passes through the centerline of the sun gears of  FIG. 4  and the centerlines of the two planet gears of the one of the planet gear sets of  FIG. 4 ; 
           [0027]      FIG. 10  is a perspective view illustrating the pair of opposite enmeshed planet gears of  FIG. 5  with a first of the planet gears also enmeshed with a first of the sun gears of  FIG. 2  and a second of the planet gears also enmeshed with a second of the sun gears of  FIG. 2 ; 
           [0028]      FIG. 11  is a plan view illustrating the pair of opposite enmeshed planet gears of  FIG. 5  with the first of the planet gears of  FIG. 10  also enmeshed with the first of the sun gears of  FIG. 10  and the second of the planet gears of  FIG. 10  also enmeshed with the second of the sun gears of  FIG. 10 ; 
           [0029]      FIG. 12  is a plan view of the ring gear of  FIG. 2 ; 
           [0030]      FIG. 13  is a cross-sectional elevation view of the ring gear of  FIG. 2  as called out at  FIG. 12 ; 
           [0031]      FIG. 14  is a plan view of one of the sun gears of  FIG. 2 ; 
           [0032]      FIG. 15  is a side view of the one of the sun gears of  FIG. 14 ; 
           [0033]      FIG. 16  is a plan view of one of the planet gears of  FIG. 3 ; 
           [0034]      FIG. 17  is a side view of the one of the planet gears of  FIG. 16 ; 
           [0035]      FIG. 18  is a plan view of one of the case halves of  FIG. 2  showing an outside of the case half; 
           [0036]      FIG. 19  is a cross-sectional side elevation view of the one of the case halves of  FIG. 18  as called out at  FIG. 18 ; 
           [0037]      FIG. 20  is another plan view of one of the case halves of  FIG. 2  showing an inside of the case half; 
           [0038]      FIG. 21  is a cross-sectional side elevation view of the one of the case halves of  FIG. 20  as called out at  FIG. 20 ; 
           [0039]      FIG. 22  is still another plan view of one of the case halves of  FIG. 2  showing the inside of the case half; and 
           [0040]      FIG. 23  is a cross-sectional side elevation view of the one of the case halves of  FIG. 22  as called out at  FIG. 22 . 
       
    
    
     DETAILED DESCRIPTION 
       [0041]    Reference will now be made in detail to example embodiments of the present disclosure. The accompanying drawings illustrate examples of the present disclosure. When possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
         [0042]    According to the principles of the present disclosure, a compact planetary differential gear set arrangement  100  may have improved torque capacity in a given width W D  (see  FIG. 3 ), a given diameter D D  (see  FIG. 1 ), a given volume, a given mass, and/or a given rotational inertia I X-X  (see  FIG. 1 ) in comparison to conventional differential designs. The planetary differential gear set  100  may also be built at a lower cost for a given torque capacity than the conventional planetary differential gear sets. The lower cost may result from a lower part number count, elimination of fasteners, automated assembly using fixtures, no post-weld machining, and/or a lower amount of material used. 
         [0043]    According to the principles of the present disclosure, the compact planetary differential gear set arrangement  100  includes a carrier  160  that is assembled over a first sun gear  130 A, a second sun gear  130 B, a first set  200 A of planet gears  220 A, a second set  200 B of planet gears  220 B and then welded together. By welding the carrier  160  around the gears  130 A,  130 B,  220 A,  220 B, no fasteners are required to operably position the gears  130 A,  130 B,  220 A,  220 B relative to each other when the planetary differential gear set  100  is in use. The carrier  160  substantially encloses the first sun gear  130 A, the second sun gear  130 B, the first set  200 A of planet gears  220 A, and the second set  200 B of planet gears  220 B. As the carrier  160  is a weldment, the carrier  160  permanently encloses (e.g., encases) the gears  130 A,  130 B,  220 A,  220 B, and the gears  130 A,  130 B,  220 A,  220 B are non-removable. Further details of the welding of the carrier  160  are provided hereinafter. 
         [0044]    According to the principles of the present disclosure, the planetary differential gear set arrangement  100  includes a low number of part numbers. In particular, the first sun gear  130 A and the second sun gear  130 B may be interchangeable with each other (i.e., have the same part number). The first sun gear  130 A and the second sun gear  130 B may collectively be known as sun gear  130  (see  FIGS. 14 and 15 ). In addition, the planet gears  220 A and the planet gears  220 B may be interchangeable with each other (see  FIGS. 8 and 9 ). The planet gear  220 A and the planet gear  220 B may collectively be known as planet gear  220  (see  FIGS. 16 and 17 ). In addition, the carrier  160  may include a first piece  170 A and a second piece  170 B that are interchangeable with each other (see  FIGS. 1-4  and  7 ) and also include a ring gear  190 . The first piece  170 A and the second piece  170 B may collectively be known as carrier piece  170  (see  FIGS. 18-23 ). Furthermore, the planet gears  220  may be rotatably mounted on pins  260  that are all interchangeable with each other, and the differential arrangement  100  may be rotatably mounted on a pair of identical bearings  270 . Thus, as illustrated, the planetary differential gear set arrangement  100  need only include a first part, sun gear  130 ; a second part, carrier piece  170 ; a third part, ring gear  190 ; a fourth part, planet gear  220 ; a fifth part, pin  260 ; and a sixth part, bearing  270 . Further details of part commonality are provided hereinafter. 
         [0045]    According to the principles of the present disclosure, the compact planetary differential gear set arrangement  100  includes a compact radial arrangement. In particular, as illustrated at  FIG. 4 , the sun gears  130 ,  130 A,  130 B operate within a radial region R S  from a centerline C L  of the differential gear set  100 . The planet gears  220 ,  220 A,  220 B operate within an annular region A. And, the ring gear  190  operates within an annular region A R . As depicted, the annular region A P  overlaps the radial region R S  by an amount sufficient to allow meshing of the sun gears  130  with the planet gears  220 . As depicted, a clearance C P  exists between the annular region A P  and the annular region A R . The clearance C P  can be quite small and thereby contribute to radial compactness of the planetary differential gear set arrangement  100 . In the depicted embodiment, the clearance C P  may be less than a thickness t T  of a tooth T of any one and/or all of the gears  130 ,  190 ,  220  (see  FIG. 11 ). 
         [0046]    Radial and/or axial compactness in proportion to torque capacity of the planetary differential gear set arrangement  100  may be accomplished by selecting appropriate gear proportions of the gears  130 ,  190 ,  220 . In certain embodiments, the gears  130 ,  190 ,  220  may be straight spur gears. As depicted, the gears  130 ,  220  are straight spur gears that may have a higher torque capacity than other types of gears (e.g., helical gears). Noise that may be generated by using straight spur gears for the gears  130 ,  220  may be acceptable given that the gears  130 ,  220  typically have low or no relative movement in typical operation of a vehicle when the vehicle is normally operated. By using straight spur gears for the gears  130 ,  220 , low or no axial thrust may be generated by the gears  130 ,  220 . In certain embodiments using straight spur gears for the gears  130 ,  220 , no thrust washers and/or thrust bearings are needed to carry thrust loads of the gears  130 ,  220 . Elimination of thrust bearing and/or thrust washers for the gears  130 ,  220  may increase the axial compactness of the planetary differential gear set arrangement  100 . In other embodiments, helical gears and/or other gears may be used for the gears  130 ,  190 ,  220 . In other embodiments, thrust washers and/or thrust bearings may be used and may carry thrust loads of the gears  130 ,  220 . 
         [0047]    In certain embodiments, as depicted, choosing a helical gear as the ring gear  190  may be desired. A pinion gear that meshes with the ring gear  190  may have a high rotational velocity when the vehicle is normally operated. The high velocity may generate significant undesired noise if straight spur gears were used as the pinion gear and the ring gear  190 . The pair of the bearings  270  may carry thrust loads generated by the pinion gear and the ring gear  190 . The pair of the bearings  270  may further carry separating loads generated by the pinion gear and the ring gear  190 . 
         [0048]    Choosing an appropriate number of gear teeth T of the gears  130 ,  190 ,  220  may increase the radial and/or axial compactness in proportion to torque capacity and thereby contribute to radial compactness of the planetary differential gear set arrangement  100 . In the depicted embodiment, the sun gears  130  include 24 gear teeth T, the ring gear  190  includes 56 gear teeth T, and the planet gears  220  include 10 gear teeth T. Choosing an appropriate ratio of pitch diameters of the gears  130 ,  220  may increase the radial and/or axial compactness in proportion to torque capacity and thereby contribute to radial compactness of the planetary differential gear set arrangement  100 . In the depicted embodiment, the ratio of the pitch diameter of the sun gears  130  to the pitch diameter of the planet gears  220  is 12:5. 
         [0049]    The welding of the carrier  160  may increase the radial and/or axial compactness in proportion to torque capacity and thereby contribute to compactness of the planetary differential gear set arrangement  100 . In particular, as depicted at  FIG. 7 , a weld W joins each of the first piece  170 A and the second piece  170 B to the ring gear  190 . In certain embodiments, a weld W may join the first piece  170 A and the second piece  170 B to each other. In the illustrated embodiment, the welds W are made by electron beam welding. Electron beam welding results in the welds W occupying only a thin radial region of the planetary differential gear set  100 . In addition, electron beam welding results in: a small heat affected zone; substantially no distortion, low distortion, and/or distortion that is locally limited; no detempering of the teeth T of the gears  130 ,  220 , and especially the teeth T of the ring gear  190 ; no post-weld machining of the planetary differential gear set  100 ; and allowing a relatively thin gear base  196  of the ring gear  190  (see  FIGS. 7 ,  12 , and  13 ). Welding may further distribute stresses within the carrier  160 , including the ring gear  190  and the pins  260 , more uniformly than conventional methods (e.g., fasteners). This may allow less material to be used and/or further contribute to compactness of the planetary differential gear set arrangement  100 . This may further increase stiffness of the planetary differential gear set arrangement  100 . 
         [0050]    Using no fasteners may increase the radial and/or axial compactness in proportion to torque capacity and thereby contribute to radial compactness of the planetary differential gear set arrangement  100 . In particular, fasteners (e.g., rivets, threaded fasteners, etc.) and their associated holes, bosses, flanges, etc. typically occupy both radial and axial space. 
         [0051]    As depicted, positioning centerlines C LP  of the first set  200 A of planet gears  220 A and the second set  200 B of planet gears  220 B at a common radius R P  about the centerline C L  of the differential gear set  100  and the sun gears  130 A,  130 B may increase the radial and/or axial compactness in proportion to torque capacity and thereby contribute to radial compactness of the planetary differential gear set arrangement  100  (see  FIGS. 4 and 20 ). By positioning the centerlines C LP  of all of the planet gears  220  at the common radius R P , the common radius R P  can be a minimum radius consistent with the required torque capacity of the planetary differential gear set arrangement  100 . In certain prior art planetary differential gear sets, a centerline radius of a first planetary gear set is different from a centerline radius of a second planetary gear set and thereby results in radial space being consumed to accommodated the larger of the centerline radii. Notwithstanding, in certain other embodiments of the planetary differential gear set arrangement  100 , the centerlines C LP  of the first set  200 A of planet gears  220 A and the second set  200 B of planet gears  220 B may be positioned at different radii. 
         [0052]    According to the principles of the present disclosure, the planetary differential gear set arrangement  100  may be configured to include at least four intermeshing planet gear pairs  210   1-4  (see  FIGS. 5 ,  8 ,  10 , and  11 ). Such configuration may be made possible, at least in part, from the compacting features disclosed herein. Certain prior art planetary differential gear sets include only three planet gear pairs due, at least in part, to non-efficient used of space. By including more than three planet gear pairs, additional torque capacity is gained for the planetary differential gear set arrangement  100 . 
         [0053]    In particular detail, each of the intermeshing planet gear pairs  210  includes one of the first planet gears  220 A enmeshed with the first sun gear  130 A and one of the second planet gears  220 B enmeshed with the second sun gear  130 B. As depicted, the first planet gear  220 A meshes with the first sun gear  130 A along an axial zone Z A  generally corresponding to a width of the teeth T of the first sun gear  130 A, and the second planet gear  220 B meshes with the second sun gear  130 B along an axial zone Z B  generally corresponding to a width of the teeth T of the second sun gear  130 B. Each of the intermeshing planet gear pairs  210  intermeshes within an axial zone Z C  generally corresponding to an axial space between the teeth T of the first sun gear  130 A and the teeth T of the second sun gear  130 B. In particular, the first planet gear  220 A of the intermeshing planet gear pair  210  meshes with the second planet gear  220 B of the same intermeshing planet gear pair  210 . A tooth pitch, pitch circle, tooth form, etc. of the first planet gear  220 A may remain the same along a width of its teeth T and across the axial zones Z A  and Z C . Likewise, a tooth pitch, pitch circle, tooth form, etc. of the second planet gear  220 B may remain the same along a width of its teeth T and across the axial zones Z B  and Z C . 
         [0054]    Turning now to  FIGS. 12 and 13 , the ring gear  190  will be described in detail. As depicted, the ring gear  190  extends between a first side  192 A and a second side  192 B. The first and second sides  192 A and  192 B are generally parallel to each other and perpendicular to the centerline C L  of the differential gear set  100  and the sun gears  130 A,  130 B and the centerlines C LP  of the planet gears  220 . As depicted, the teeth T of the ring gear  190  generally extend between the first and second sides  192 A and  192 B and are helical teeth. The teeth T of the ring gear  190  also extend from an outer perimeter  194  of the ring gear  190  to the gear base  196 . The gear base  196  radially extends between the teeth T of the ring gear  190  and an innermost surface  198  of the ring gear  190 . As depicted, the innermost surface  198  is positioned beyond an outermost cylinder that is occupied by the planet gears  220  (i.e., an outer cylinder of the annular region A P ). 
         [0055]    The gear base  196  may serve as a structural support for the teeth T of the ring gear  190 , as a locating and stopping feature for the first piece  170 A and the second piece  170 B, as a joining piece for the first piece  170 A and the second piece  170 B, and as a weld pad that isolates the teeth T of the ring gear  190  from weld distortion and a heat affected zone of the welds W. As depicted, the gear base  196  includes a first cylindrical surface  250 A and a second cylindrical surface  250 B. The first cylindrical surface  250 A may be a first high precision cylindrical surface, and the second cylindrical surface  250 B may be a second high precision cylindrical surface. The high precision cylindrical surfaces  250 A,  250 B may be capable of holding a press fit. The cylindrical surfaces  250 A,  250 B may be concentric with each other and/or with the innermost surface  198  of the ring gear  190 . As depicted, the gear base  196  includes a first shoulder stop  252 A and a second shoulder stop  252 B. The shoulder stops  252 A,  252 B extend radially inwardly from the cylindrical surfaces  250 A,  250 B, respectively, to the innermost surface  198  of the ring gear  190 . The shoulder stops  252 A,  252 B may be separated from each other by a recessed area  254 . The recessed area  254  may include a third cylindrical surface  254  with generally the same diameter as the cylindrical surfaces  250 A,  250 B. The cylindrical surfaces  250 A and/or  250 B may be concentric with the third cylindrical surface  254 . The shoulder stops  252 A,  252 B may be positioned symmetrically about the ring gear  190  along the centerline C L  of the differential gear set  100 , the sun gears  130 A,  130 B, and the ring gear  190 . The ring gear  190  may include a groove  256 . The groove  256  may be positioned on the second side  192 B and may serve as an indicator of the second side  192 B and may therefore serve as an orientation indicator of the planetary differential gear set arrangement  100 . 
         [0056]    Turning now to  FIGS. 14 and 15 , the sun gear  130  will be described in detail. As depicted, the sun gear  130  extends between a first side  132  and a second side  134 . The first and second sides  132  and  134  are generally parallel to each other and perpendicular to the centerline C L  of the sun gear  130  and the centerlines C LP  of the planet gears  220 . As depicted, the teeth T of the sun gear  130  generally extend between the first side  132  and a medial plane  136 . The teeth T of the sun gear  130  generally radially extend between a gear base  138  and an outer perimeter  140  of the sun gear  130 . A reduced diameter portion  142  extends between the medial plane  136  and the second side  134 . The reduced diameter portion  142  of each of the sun gears  130 A,  130 B forms about half of the axial zone Z C  and allows the teeth T of the planet gears  220  to extend within a cylinder defined by the outer perimeter  140  without meshing with the teeth T of the sun gear  130 . The gear base  138  radially extends between the teeth T of the sun gear  130  and an innermost surface  144  of the sun gear  130 . As depicted, the innermost surface  144  is included on a female spline  146  that is adapted to rotationally couple to a male spline of a drivetrain shaft (e.g., an axle, a drive shaft, etc.) of a vehicle. 
         [0057]    Turning now to  FIGS. 16 and 17 , the planet gear  220  will be described in detail. As depicted, the planet gear  220  extends between a first side  222  and a second side  224 . The first and second sides  222  and  224  are generally parallel to each other and perpendicular to the centerline C L  of the sun gear  130  and the centerlines C LP  of the planet gears  220 . As depicted, the teeth T of the planet gear  220  generally extend between the first side  222  and a medial plane  226 . The teeth T of the planet gear  220  generally radially extend between a gear base  228  and an outer perimeter  230  of the planet gear  220 . A reduced diameter portion  232  extends between the medial plane  226  and the second side  224 . The reduced diameter portion  232  of the planet gears  220  extends over the axial zones Z A  or Z B  and allows the teeth T of the sun gear  130 A or  130 B, respectively, to extend within a cylinder defined by the outer perimeter  230  without meshing with the teeth T of the planet gear  220 A or  220 B, respectively. The gear base  228  radially extends between the teeth T of the planet gear  220  and an innermost surface  234  of the planet gear  220 . As depicted, the innermost surface  234  includes a bearing surface that is adapted to rotatably mount to the pins  260 . The reduced diameter portion  232  of the first planet gear  220 A may clear the second sun gear  130 B, and the reduced diameter portion  232  of the second planet gear  220 B may clear the first sun gear  130 A. 
         [0058]    Turning now to  FIGS. 18-23 , the carrier piece  170  will be described in detail. As depicted, the carrier piece  170  extends between a first side  172  and a second side  174 . The first and second sides  172  and  174  are generally parallel to each other and perpendicular to the centerline C L  of the sun gear  130  and the carrier piece  170  and the centerlines C LP  of the planet gears  220 . As depicted, the carrier piece  170  includes a first cylindrical surface  240  and a second cylindrical surface  180 . The first cylindrical surface  240  may be a first high precision cylindrical surface, and the second cylindrical surface  180  may be a second high precision cylindrical surface. The high precision cylindrical surfaces  180 ,  240  may be capable of holding a press fit. The cylindrical surfaces  180 ,  240  may be concentric with each other. As depicted, the carrier piece  170  includes a first stop  242  (e.g., a shoulder stop) and a second stop  182  (e.g., a shoulder stop). The stop  242  extends radially inwardly from the cylindrical surface  240 . The stop  182  extends radially outwardly from the cylindrical surface  180 . The stops  182 ,  242  may be spaced from each other by a distance D C  (see  FIG. 23 ). The distance D C  may be a high precision distance. The shoulder stops  182 ,  242  of the first piece  170 A may be positioned symmetrically with respect to the shoulder stops  182 ,  242  of the second piece  170 B about the ring gear  190  along the centerline C L  of the differential gear set  100 , the sun gears  130 A,  130 B, and the ring gear  190 . The shoulder stops  182 ,  242  of the first piece  170 A may be positioned symmetrically with respect to the shoulder stops  182 ,  242  of the second piece  170 B about the planetary differential gear set arrangement  100  along the centerline C L . 
         [0059]    As depicted, the carrier piece  170  may be stamped, spun, and/or forged from a single piece of raw material. The raw material may be a plate, a billet, a tube, etc. The stamping, spinning, and/or forging may work harden the carrier piece  170 . In other embodiments, the carrier piece  170  may be a casting, a machined piece, etc. In certain embodiments, a portion or all of the carrier piece  170  may be stress and/or strain relieved (e.g., by heating). In certain embodiments, a portion or all of the carrier piece  170  may be shot-peened. As depicted, the carrier piece  170  generally defines a wall  178  with a wall thickness t W . The wall thickness t W  may vary or the wall thickness t W  may remain substantially constant. The wall  178  may form at least a portion of the cylindrical surfaces  180 ,  240  and the stops  182 ,  242 . Additionally, the wall  178  may form at least a portion of a radial portion  176  and/or a lateral portion  186  of the carrier piece  170 . The radial portion  176  of the first piece  170 A may form a first side  162 A, and the radial portion  176  of the second piece  170 B may form a second side  162 B of a hub (see  FIG. 1 ). At least a portion of the cylindrical surface  180  may form a snout  184  upon which the bearing  270  may be mounted (see  FIG. 21 ). The bearing  270  may bottom against the stop  182  and thereby be located by the stop  182 . By welding the carrier piece  170  to the ring gear  190  and forming the lateral portion  186  on the carrier piece  170 , the planet gears  220  may be separated (i.e., spaced) from the ring gear  190  by as little as the wall thickness t W . 
         [0060]    The carrier  160  may include a first wall  164 A and a second wall  164 B formed by insides of the first side  162 A and the second side  162 B, respectively. The walls  164 A,  164 B are spaced from each other. The planet gears  220  may each substantially extend between the first wall  164 A and the second wall  164 B. 
         [0061]    As depicted at  FIGS. 18-23 , the radial portion  176  include a series of pin holes  266 , a series of holes  268 A, a series of holes  268 B, and a series of holes  268 C. As depicted, the holes  268 C extend beyond the radial portion  176  and into the lateral portion  186 . As depicted, the holes  266 ,  268 A, and  268 B are round or substantially round holes. In other embodiments, the holes  266 ,  268 A, and  268 B may have other shapes. As depicted, the hole  268 C includes a round portion and a slot-shaped portion that extends to the lateral portion  186  (see also  FIG. 6 ). In other embodiments, the holes  268 C may have other shapes. In the depicted embodiment, the pin holes  266  mount the pins  260 . In particular, the pin holes  266  of the first piece  170 A mount first end portions  262 A of the pins  260 , and the pin holes  266  of the second piece  170 B mount second end portions  262 B of the pins  260  (see  FIG. 2 ). The holes  268 A,  268 B, and  268 C may reduce rotational inertia, may reduce mass, may tailor stiffness of the carrier piece  170 , and/or may improve lubrication and oil flow. The tailored stiffness of the carrier piece  170  may accommodate press fitting the first cylindrical surface  240  into the cylindrical surfaces  250 A or  250 B. The tailored stiffness of the carrier piece  170  may accommodate a lower tolerance of the first cylindrical surface  240  and/or the cylindrical surfaces  250 A,  250 B. 
         [0062]    In the depicted embodiment, the lateral portion  186  undulates and forms pockets  188  centered on the pin holes  266 . The pockets  188  may each house at least a portion of one of the planet gears  220 . By undulating, the lateral portion  186  may stiffen the carrier piece  170  and thereby stiffen the planetary differential gear set arrangement  100 . By undulating, the lateral portion  186  may reduce rotational inertia. 
         [0063]    In the depicted embodiment, the differential gear set  100 , including the pinion gear, is governed by the equation 
         [0000]        K ×( V   1   +V   2 )/2= V   3  
 
         [0000]    where K is a gear ratio of the pinion and ring gear set, V 1  is a rotational velocity of the first sun gear  130 A, V 2  is a rotational velocity of the second sun gear  130 B, and V 3  is a rotational velocity of the pinion gear that drives the ring gear  190  of the carrier  160 . 
         [0064]    In the depicted embodiment, the differential gear set  100 , excluding the pinion gear, is governed by the equation 
         [0000]      ( V 1 +V 2)/2= V   3    
         [0000]    where V 1  is the rotational velocity of the first sun gear  130 A, V 2  is the rotational velocity of the second sun gear  130 B, and V 3  is a rotational velocity of the carrier  160 . 
         [0065]    In other embodiments, the differential gear set  100  may be governed by the equation 
         [0000]      ( n   1   ×V   1   +n   2   ×V   2 )=( n   1   +n   2 )× V   3  
 
         [0000]    where n 1  and n 2  are gear ratios of the differential gear set  100 , V 1  is a rotational velocity of a first input/output member  130 A, V 2  is a rotational velocity of a second input/output member  130 B, and V 3  is a rotational velocity of a third input/output member (e.g., a pinion). 
         [0066]    A method for assembling the planetary differential gear set  100  may include one or more of the steps below. The steps need not necessarily be performed in the order in which they appear. All of the steps need not necessarily be performed. Additional steps may be added. 1) Provide the first piece  170 A of the carrier  160 . 2) Position the first sun gear  130 A adjacent the first piece  170 A. In particular, position the first side  132  of the first sun gear  130 A adjacent the first wall  164 A of the first piece  170 A with the centerline C L  of the first sun gear  130 A aligned with the centerline C L  of the first carrier piece  170 A. 3) Position the first set  200 A of the planet gears  220 A adjacent the first piece  170 A. In particular, position the first side  222  of the planet gears  220 A adjacent the first wall  164 A of the first piece  170 A with the centerline C LP  of each of the planet gears  220 A aligned with a corresponding center of one of the pin holes  266 . 4) Position the ring gear  190  adjacent the first piece  170 A. In particular, position the first cylindrical surface  250 A around the first cylindrical surface  240  and move and/or press the ring gear  190  and the first piece  170 A together until the first stop  242  abuts the first shoulder stop  252 A. 5) Position the second sun gear  130 B adjacent the first sun gear  130 A. In particular, position the second sides  134  of the sun gears  130 A and  130 B adjacent each other with their centerlines C L  aligned. 6) Position the second set  200 B of the planet gears  220 B adjacent the first set  200 A. In particular, position the second side  224  of the planet gears  220 B adjacent the first wall  164 A of the first piece  170 A with the centerline C LP  of each of the planet gears  220 B aligned with a corresponding center of one of the pin holes  266 . 7) Position the second piece  170 B of the carrier  160  adjacent the ring gear  190 . In particular, position the second cylindrical surface  250 B around the second cylindrical surface  240  and move and/or press the ring gear  190  and the second piece  170 B together until the first stop  242  abuts the second shoulder stop  252 B while the centerline C LP  of each of the planet gears  220  is aligned with a corresponding center of one of the pin holes  266  of the second piece  170 B. 8) Secure and/or position some or all of the above parts with a fixture. 9) Insert and/or press the pins  260  into and/or through the holes  266 ,  234  with a bearing surface  264  (see  FIG. 2 ) of each of the pins  260  engaging a corresponding one of the bearing surfaces  234  of the planet gears  220 . 10) Form the weld W between the first piece  170 A and the ring gear  190 . 11) Form the weld W between the second piece  170 B and the ring gear  190 . 12) Weld the first piece  170 A and the second piece  170 B of the carrier  160  together. 13) Form a weld W P  between the first piece  170 A and the pins  260 . 14) Form a weld W P  between the second piece  170 B and the pins  260 . And/or, 15) remove the fixture. 
         [0067]    As mentioned above, the above steps do not necessarily need to be preformed in the order listed. Some or all of the steps may be performed substantially simultaneously. Some of the above steps may be omitted. Other steps may be added. The welding may include electron beam welding. 
         [0068]    In one example, the width W D  (i.e., hub span) can be shown to be about 54% of a width of a typical differential mechanism with the same torque capacity. The overall diameter D D  can be about the same as the typical differential mechanism, but weight of the planetary differential gear set arrangement  100  can be about 88% of the typical differential mechanism, including the ring gear  190 . In certain embodiments of the planetary differential gear set arrangement  100 , torque bias can be one-to-one and torque capacity can be the same or greater than the typical differential mechanism. In certain embodiments, the planetary differential gear set arrangement  100  may be a compact open differential. In certain embodiments, the planetary differential gear set arrangement  100  may have near zero bias. In certain embodiments, the planetary differential gear set arrangement  100  may be configured as a limited slip, a viscous coupled, and/or a locking differential and include corresponding components. In the depicted embodiment, the bearings  270  are roller bearings. 
         [0069]    In certain embodiments, such as vehicle axle and/or transfer case applications, the first sun gear  130 A is adapted to drive a first drivetrain shaft (e.g., an axle, a drive shaft, etc.) of a vehicle, and the second sun gear  130 B is adapted to drive a second drivetrain shaft (e.g., an axle, a drive shaft, etc.) of the vehicle. 
         [0070]    Various modifications and alterations of this disclosure will become apparent to those skilled in the art without departing from the scope and spirit of this disclosure, and it should be understood that the scope of this disclosure is not to be unduly limited to the illustrative embodiments set forth herein.