Abstract:
A differential assembly including a rotatable casing, first and second axially moveable side gears disposed within the casing, at least one pinion gear disposed within the casing and intermeshed with the side gears, a cone clutch operatively coupled to the first side gear, the cone clutch being frictionally coupled to the casing in response to being exposed to a magnetic field, and a disc clutch having at least one clutch disc operatively coupled to the second side gear in response to axial movement of the second side gear.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to limited slip differentials, and more particularly to limited slip differentials having an electromagnetically actuated clutch. 
     Differentials are well known in the prior art and allow each of a pair of output shafts or axles operatively coupled to a rotating input shaft to rotate at different speeds, thereby allowing the wheel associated with each output shaft to maintain traction with the road while the vehicle is turning. Such a device essentially distributes the torque provided by the input shaft between the output shafts. 
     The completely open differential, i.e., a differential without clutches or springs which restrict relative rotation between the axles and the rotating differential casing, is not well suited to slippery conditions in which one driven wheel experiences a much lower coefficient of friction than the other driven wheel: for instance, when one wheel of a vehicle is located on a patch of ice and the other wheel is on dry pavement. Under such conditions, the wheel experiencing the lower coefficient of friction loses traction and a small amount of torque to that wheel will cause a “spin out” of that wheel. Since the maximum amount of torque which can be developed on the wheel with traction is equal to torque on the wheel without traction, i.e. the slipping wheel, the engine is unable to develop any torque and the wheel with traction is unable to rotate. A number of methods have been developed to limit wheel slippage under such conditions. 
     Prior means for limiting slippage between the axles and the differential casing use a frictional clutch mechanism, either clutch plates or a frustoconical engagement structure, operatively located between the rotating case and the axles. Certain embodiments of such prior means provide a clutch element attached to each of the side gears, and which frictionally engages a mating clutch element attached to the rotating casing or, if the clutch is of the conical variety, a complementary interior surface of the casing itself. Such embodiments may also include a bias mechanism, usually a spring, to apply an initial preload between the clutch and the differential casing. By using a frictional clutch with an initial preload, a minimum amount of torque can always be applied to a wheel having traction, e.g., a wheel located on dry pavement. The initial torque generates gear separating forces between the first pinion gears and the side gears intermeshed therewith. The gear separating forces urge the two side gears outward, away from each other, causing the clutch to lightly engage and develop additional torque at the driven wheels. Examples of such limited slip differentials which comprise cone clutches are disclosed in U.S. Pat. No. 4,612,825 (Engle), U.S. Pat. No. 5,226,861 (Engle) and U.S. Pat. No. 5,556,344 (Fox), each of which is assigned to Auburn Gear, Inc., the disclosures of which are all expressly incorporated herein by reference. 
     Certain prior art limited slip differentials provide, between the first of the two side gears and its associated clutch element, interacting camming portions having ramp surfaces or ball/ramp arrangements. In response to an initiating force, this clutch element is moved towards and into contact with the surface against which it frictionally engages, which may be a mating clutch element attached to the casing, or an interior surface of the casing itself, as the case may be, thereby axially separating the clutch element and its adjacent first side gear, the interacting camming portions slidably engaging, the rotational speed of the clutch element beginning to match that of the differential casing due to the frictional engagement. Relative rotational movement between the ramp surfaces induces further axial separation of the clutch element and the first side gear. Because the clutch element is already in abutting contact with the surface against which it frictionally engages, the first side gear is forced axially away from the clutch element by the camming portions. 
     Certain embodiments of such limited slip differentials utilize an electromagnet having an electrical coil to effect the initiating force and actuate the clutch, as disclosed in U.S. Pat. No. 5,989,147 (Forrest et al.), U.S. Pat. No. 6,019,694 (Forrest et al.), and U.S. Pat. No. 6,165,095 (Till et al.), each of which is assigned to Auburn Gear, Inc., the disclosures of which are all expressly incorporated herein by reference. Each of these references discloses that the differential casing, in which the clutches are disposed, rotates within the housing and is rotatably supported by a pair of bearings. An electromagnet, which actuates a primary cone clutch element, is mounted in fixed relationship to the axle housing and is rotatably supported by the differential casing. Alternatively, as disclosed in pending U.S. patent application Ser. No. 09/484,967, filed Jan. 18, 2000, which is assigned to Auburn Gear, Inc., the disclosure of which is expressly incorporated herein by reference, the electromagnet may be fixedly supported by the axle housing. In either case, activation of the electromagnet draws a primary cone clutch element into frictional engagement with the rotating differential housing. 
     The camming portions, described above, act between the primary cone clutch element and the first side gear to axially separate them, forcing the first side gear into abutment with a transfer block located intermediate the first and second side gears. Responsive to this force, the transfer block is moved into abutment with the second side gear, which is rotatably fixed to a secondary cone clutch element, which frictionally engages a mating interior surface of the rotating differential casing. The frictional engagement of the secondary cone clutch element and the differential casing effects further clutched engagement between the axles and the differential casing, enhancing the locking capability of the limited slip differential. Notably, the load carrying capability of the secondary cone clutch mechanism is usually significantly greater than that of the primary cone clutch mechanism, owing to a greater axial engagement force exerted thereon. Examples of prior limited slip differentials are described in more detail below, with reference to FIGS. 1 and 2. 
     FIG. 1 depicts an embodiment of prior axle assembly  10  having electrically or electromagnetically actuated limited slip differential assembly  12 . Axle assembly  10  may be a conventional axle assembly or comprise part of a transaxle assembly. Therefore, it is to be understood that the term “axle assembly” encompasses both conventional (rear wheel drive) axle assemblies as well as transaxle assemblies. Differential assembly  12  comprises electromagnet  14 , ferrous rotatable casing  16  constructed of joined first and second casing parts  16   a  and  16   b , respectively, and providing inner cavity  18 , which is defined by the interior surface of the circumferential wall portion of first casing part  16   a  and end wall portions  20 ,  22  of first and second casing parts  16   a ,  16   b , respectively. Casing part  16   a  may be a machined iron or steel casting; casing part  16   b  may also be such a casting, or a ferrous, sintered powdered metal part. Disposed within cavity  18  are side gears  24 ,  26  and pinion gears  28 ,  30 . The teeth of the side gears and pinion gears are intermeshed, as shown. Pinion gears  28 ,  30  are rotatably disposed upon cylindrical steel cross pin  32 , which extends along axis  34 . The ends of cross pin  32  are received in holes  36 ,  38  diametrically located in the circumferential wall of casing part  16   a.    
     Axles  40 ,  42  are received through hubs  44 ,  46 , respectively formed in casing end wall portions  20 ,  22 , along common axis of rotation  48 , which intersects and is perpendicular to axis  34 . Axles  40 ,  42  are respectively provided with splined portions  50 ,  52 , which are received in splines  54 ,  56  of side gears  24 ,  26 , thereby rotatably fixing the side gears to the axles. The axles are provided with circumferential grooves  58 ,  60  in which are disposed C-rings  62 ,  64 , which prevent the axles from being removed axially from their associated side gears. The terminal ends of the axles  98  and  100  may abut against the cylindrical surface of cross pin  32 , thereby restricting the axles&#39; movement toward each other along axis  48 . 
     Primary clutch element  66  is attached to side gear  24  and rotates therewith. Clutch element  66  is ferrous and of the cone clutch variety and has frustoconical surface  68  which is adjacent to, and clutchedly interfaces with, complementary surface  70  provided on the interior of casing part  16   a . Secondary clutch element  72  is also of the cone clutch variety and has frustoconical surface  74  which is adjacent to, and clutchedly interfaces with, complementary surface  76  also provided on the interior of casing part  16   a . Cone clutches  66  and  72  may be of the type described in U.S. Pat. No. 6,076,644 (Forrest et al.) or U.S. Pat. No. 6,261,202, each of which is assigned to Auburn Gear, Inc., the disclosures of which are both expressly incorporated herein by reference, or may also be of any other suitable structure. 
     Disposed between primary cone clutch element  66  and side gear  24  is annular cam plate  78 , which abuts thrust washer  82  adjacent end wall portion  22 . Ball and ramp arrangement  84 ,  86 ,  88  is comprised of a first plurality of paired spiral slots  84 ,  86  located in cam plate  78  and primary cone clutch element  66 , respectively. Slots  84 ,  86  define a helically ramping path followed by ball  88 , which may be steel, disposed in each slot pair and a first ramp angle. With electromagnet  14  de-energized, balls  88  are seated in the deepest portion of slots  84 ,  86  by Belleville spring  90 . The actuation sequence is created by the momentary difference in rotational speed between cone clutch element  66  and cam plate  78  as frustoconical surfaces  68  and  70  seat against each other. A more detailed discussion of ball/ramp camming arrangements is disclosed in U.S. Pat. No. 5,989,147. 
     In operation, a variable coil current on electromagnet  14  induces a variable amount of magnetic clamping force between casing part  16   a  and primary cone clutch element  66 , which induces a variable amount of torque to be exerted by casing part  16   a  on clutch element  66 . As electromagnet  14  is activated, axial separation of primary cone clutch element  66  and cam plate  78  is induced as cone clutch element  66  is magnetically pulled to the left against the force of Belleville spring  90  into clutched engagement with casing part  16   a  through frustoconical surfaces  68  and  70 . In response to the initial flow of magnetic flux, cone clutch element  66  is pulled by the magnetic field to the left and surfaces  68  and  70  abut, and enter frictional engagement. As cone clutch element  66  and cam plate  78  separate axially, balls  88  are caused to rotate along the ramping helical paths of slots  84 ,  86  due to the relative rotation between clutch element  66  and cam plate  78 . Cam plate  78  is urged against thrust washer  82  by the force of Belleville spring  90  and gear separation forces between pinion gears  28 ,  30  and side gear  24 . As balls  88  rotate further along the helical ramp paths, frustoconical surfaces  68 ,  70  are forced into tighter frictional engagement and cam plate  78 , still abutting thrust washer  82 , reaches the end of its rotational travel relative to cone clutch member  66 . 
     First side gear  24  moves towards the right, forcing secondary cone clutch element  72  into abutment with casing part  16   a  via transfer block  92  and second side gear  26  in the manner described above. Transfer block  92 , which may be steel, is disposed about cross pin  32  and adapted to move laterally relative thereto along axis  48  to transfer movement of first side gear  24  to second side gear  26 , thereby engaging secondary clutch element  72 . As shown, transfer block  92  is attached directly to cross pin  32 , and supports the cross pin in position within the differential casing as described in U.S. Pat. No. 6,254,505, assigned to Auburn Gear, Inc., the disclosure of which is expressly incorporated herein by reference. Alternatively, the transfer block may be loosely fitted about the cross pin, the cross pin being directly attached to the differential housing by a bolt extending through one end of the cross pin, as shown, for example, in U.S. Pat. No. 5,226,861. The shear loads associated with torque transmission are exerted on cross pin  32  near its opposite ends, particularly between the circumferential wall of casing part  16   a  and the adjacent pinion gears  28 ,  30 . 
     Transfer block  92  includes opposite bearing sides  94 ,  96  which respectively abut first and second side gears  24 ,  26 , and allows terminal ends  98 ,  100  of axles  40 ,  42 , respectively, to abut the cylindrical side surface of cross pin  32 . Transfer block  92  moves laterally relative to cross pin  32 , along axis  48 , such that rightward movement of side gear  24 , described above, is transferred to side gear  26 . Thus, during actuation of electromagnet  14 , first side gear  24  is urged rightward, as viewed in FIG. 1, into abutting contact with transfer block  92 . Transfer block  92  moves rightward, into abutting contact with second side gear  26 ; and second side gear  26  moves rightward, urging surface  74  of secondary clutch element  72  into frictional engagement with surface  76  of casing part  16   a , thereby providing additional torque transfer capacity to the differential than would otherwise be provided with single cone clutch element  66 . 
     Provided on the exterior surface of casing part  16   a , near electromagnet  14 , is flange  102 , to which ring gear  104  is attached. The teeth  136  of ring gear  104  are in meshed engagement with the teeth of pinion gear  106  which is rotatably driven by an engine (not shown), thus rotating differential casing  16  within axle housing  108 . As casing  16  rotates, the sides of holes  36 ,  38  bear against the portions of the cylindrical surface of cross pin  32  in the holes. The rotation of cross pin  32  about axis  48  causes pinion gears  28 ,  30  to revolve about axis  48 . The revolution of the pinion gears about axis  48  causes at least one of side gears  24 ,  26  to rotate about axis  48 , thus causing at least one of axles  40 ,  42  to rotate about axis  48 . Engagement of the clutches as described above arrests relative rotation between the side gears and the differential casing. 
     Differential casing  16  is rotatably supported within axle housing  108  by means of identical first and second bearings  110 ,  112 . Because of the proximity of ring gear flange  102  to the end of casing  16  nearest first bearing  110 , in operation, that bearing is more heavily loaded than is second bearing  112 . 
     Electromagnet  14  is rotatably supported on second differential casing portion  16   b  by third bearing  114 . Electromagnet  14  is rotatably fixed relative to axle housing  108  and disposed in close proximity to casing  16 , which rotates relative thereto. The voltage applied to electromagnet  14  to energize same and actuate primary clutch element  66  may be controlled by a control system (not shown) which is in communication with sensors (not shown) which indicate, for example, excessive relative rotation between axles  40 ,  42 , and thus the need for traction control. Housing  108  includes hole  116  fitted with rubber grommet  118  through which extend leads  120 . Through leads  120  the control system provides voltage to electromagnet  14 . As electromagnet  14  is energized, a magnetic initiating force is applied to primary cone clutch element  66  by a toroidal electromagnetic flux path (not shown) which is established about the annular electromagnet coil  126 ; the flux path flows through ferrous casing portions  16   a  and  16   b  and through clutch element  66 . Clutch element  66  is thus magnetically drawn into engagement with casing  16  during operation of electromagnet  14 . Because it is made of a magnetic material (e.g., steel) and has a solid structure, primary cone clutch element  66  is better suited for conducting the magnetic flux path therethrough than would be a clutch comprising a series of interleaved discs, which may have gaps therebetween and which would likely be formed of materials which would not so readily transmit the magnetic flux. Further, casing part  16   b  may include annular nonmagnetic portion  122  to help direct the toroidal magnetic flux path through primary cone clutch element  66 , as described in U.S. Pat. No. 6,019,694 (Forrest et al.), assigned to Auburn Gear, Inc., the disclosure of which is expressly incorporated herein by reference. 
     FIG. 2 depicts a second embodiment of a prior axle assembly which is identical in structure and operation to the above-described axle assembly  10  except as follows: Axle assembly  10 ′ comprises electromagnet  14 ′ which is fixed to the axle housing, rather than being rotatably supported by a bearing  114  disposed about casing part  16   b . Bearing  110 ′ is disposed in cup  124  which extends inwardly of the axle housing to engage and support electromagnet  14 ′ in the manner described in pending U.S. patent application Ser. No. 09/484,967, filed Jan. 18, 2000. Notably, bearing  110 ′ is somewhat smaller than bearing  110  (and identical bearing  112 ) and, as noted above, would be more heavily loaded during operation than larger bearing  112  due to the proximity of the ring gear. 
     Although cone clutches of the type disclosed above are better suited than disc-type clutches as primary clutch elements in electromagnetically-actuated limited slip differentials, for the reasons set forth above, their load carrying capability is limited, for a give axial engagement force, by the angle of the included angle formed by the cone clutch engagement surfaces. Typically, these angles range from 9° to 12.5°. The smaller this angle, the greater the torque capacity of the cone clutch. The smaller this angle, however, the harsher the clutch engagement, and the smaller the tendency for the clutch to release. Clutches having multiple interleaved discs, or “clutch packs,” are well known in the art and generally have greater torque capacity than a cone clutch of approximately equal package size. Moreover, the required tolerances associated with manufacturing cone clutches tend to be somewhat smaller than with disc clutches. 
     Further still, compared to the axial movement needed to engage disc clutches, a greater distance is needed when using cone clutches because a portion of the movement is absorbed by the casing as it is being radially stretched. Therefore, relatively more movement between the pinion and side gears is needed to accommodate proper movement of the cone clutch, and optimal gear mesh clearances therebetween, which are on the order of ±0.010 inch, may be compromised. An electromagnetically-actuated limited slip differential assembly which provides the respective benefits of cone clutches and clutch packs is highly desirable. 
     A further issue associated with electromagnetically-actuated limited slip differentials is that the electromagnet tends to magnetize ferrous components within the axle housing, particularly those in close proximity to the electromagnet. This can be of particular concern where relatively moving, interengaging components such as bearings or gears of the differential or axle assembly become magnetized and attract metal shavings or other ferrous debris, or where the shavings and debris are themselves magnetized and become attached to these interengaging components. The collection of such contamination on these components can substantially accelerate their wear and lead to premature failure. 
     One known approach to addressing this issue is to provide a magnetic drain plug in the axle housing, which may attract and retain some of the debris. However, the debris may be equally attracted to other magnetized components within the axle housing, rather than to only the drain plug. Another approach to addressing this issue is described in 
     U.S. Pat. No. 6,165,095, which discloses an apparatus and method for demagnetizing the components initially magnetized by the electromagnet. While effective, this means for demagnetization involves providing additional controls for directing current through the electromagnet(s). It is desirable to provide a simple and effective means for reducing the likelihood or severity of magnetization of at least some of the relatively moving, interengaging components within the axle housing. 
     Further, one way to reduce the cost and improve the reliability of an axle assembly is to reduce the number of components parts, or at least the number of complex, high precision parts. For example, reducing the number of ball or roller bearings may reduce the cost of material, the cost of assembly labor, and the number of moving parts, thereby improving durability and reliability. Reduction in the number of parts, however, may compromise the ability of the remaining parts to perform satisfactorily. For example, reducing the number of bearings may increase the load to be borne by the remaining bearings, which may adversely affect the durability of those remaining bearings. The reduction of costs without compromising performance is an ongoing and important goal in virtually every commercial endeavor, and means for accomplishing that goal are therefore highly desirable. 
     SUMMARY OF THE INVENTION 
     The present invention provides a differential assembly including a rotatable casing, first and second axially moveable side gears disposed within the casing, at least one pinion gear disposed within the casing and intermeshed with the side gears, a cone clutch operatively coupled to the first side gear, the cone clutch being frictionally coupled to the casing in response to being exposed to a magnetic field, and at least one clutch disc operatively coupled to the second side gear in response to axial movement of the second side gear. 
     The present invention also provides a differential assembly including a rotatable casing having opposite ends, a differential gear mechanism and a magnetically-activated clutch disposed within the casing, relative rotation of at least a portion of the gear mechanism being selectively frictionally engaged with the casing by the clutch, an electromagnet being disposed proximal to one of the casing ends, and a ring gear attached to the casing at a location proximal to the other of the casing ends. 
     The present invention also provides a differential assembly including a rotatable casing, a differential gear mechanism and a magnetically-activated clutch disposed within the casing, relative rotation of at least a portion of the gear mechanism being selectively frictionally engaged with the casing by the clutch, an electromagnet disposed proximal to the casing, the casing and the electromagnet having relative rotation therebetween, and a self lubricating bearing disposed between the electromagnet and the casing, the electromagnet being supported relative to the casing by the bearing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of the embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a sectional side view of a first embodiment of a prior art electrically actuated limited slip axle assembly having its clutch-activating electromagnet rotatably supported on the differential casing by a separate bearing; 
     FIG. 2 is a sectional side view of a second embodiment of a prior art electrically actuated limited slip axle assembly having its clutch-activating electromagnet rotatably supported by an extended bearing cup of a bearing which supports the differential casing within the axle housing; 
     FIG. 3 is a sectional side view of a first embodiment of an electrically actuated limited slip axle assembly according to the present invention having its clutch-activating electromagnet rotatably supported on the differential casing by a separate bearing; 
     FIG. 4 is a sectional side view of a second embodiment of an electrically actuated limited slip axle assembly according to the present invention having its clutch-activating electromagnet rotatably supported by an extended bearing cup of a bearing which supports the differential casing within the axle housing; 
     FIG. 5 is an enlarged, fragmentary view of the axle assembly of FIG. 3; 
     FIG. 6 is an enlarged, fragmentary view of an axle assembly according to a third embodiment of the present invention having its electromagnet supported by a self-lubricating bearing; 
     FIG. 7 is an enlarged, fragmentary view of an axle assembly according to a fourth embodiment of the present invention having its electromagnet supported by an alternative self-lubricating bearing; 
     FIG. 8 is an enlarged, fragmentary view of an axle assembly according to a fifth embodiment of the present invention having its electromagnet supported by another alternative self-lubricating bearing; 
     FIG. 9A is a plan view of a first embodiment of a ball spacer used in the axle assemblies of FIGS. 3 and 4; and 
     FIG. 9B is an oblique view of a second embodiment of a ball spacer used in the axle assemblies of FIGS.  3  and  4 . 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. Although the drawings represent embodiments of the present invention, the drawings are not necessarily to scale and certain features may be exaggerated in order to better illustrate and explain the present invention. The exemplification set out herein illustrates embodiments of the invention in several forms, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiments disclosed below are not intended to be exhaustive or limit the invention to the precise forms disclosed in the following detailed description. Rather, the embodiments are chosen and described so that others skilled in the art may utilize their teachings. 
     FIGS. 3 and 4 respectively depict first and second embodiments of electrically or electromagnetically-actuated limited slip differentials according to the present invention. Axle assembly  210  (FIG. 3) is structurally and operationally similar to prior art axle assembly  10  (FIG. 1) except as described hereinbelow. Axle assembly  210 ′ (FIG. 4) is structurally and operationally similar to prior art axle assembly  10 ′ (FIG. 2) except as described hereinbelow. Identical parts between all of these axle assemblies are identically numbered. 
     Referring to FIG. 3, axle assembly  210  includes differential assembly  212  rotatable comprising casing  216 . Casing  216  includes first, second and third parts  216   a ,  216   b  and  216   c , respectively. At least casing parts  216   a  and  216   b  are ferrous, and may be machined iron or steel castings. Casing part  216   b  may be a sintered powdered metal part having nonmagnetic annular portion  218  to facilitate the proper magnetic flux path as described above. 
     Rotatably supported on casing  216  is electromagnet  220 , which is rotatably fixed relative to axle housing  108 . As described above, current is supplied to electromagnet  220  via leads  120 . 
     Disposed within casing  216  and proximal to casing part  216   b  is primary clutch element  222  which is ferrous and of the cone clutch variety. In the manner described above, frustoconical surface  224  of cone clutch  222  is magnetically drawn into frictional engagement with complementary interior surface  226  of differential casing part  216   a  to initiate clutching and slows the relative rotation between casing  216  and cone clutch  222 . 
     A ball/ramp arrangement comprising spiral slots  230  provided in planar portion  232  of primary cone clutch  222 , spiral slots  234  provided in first side gear  236 , and balls  88 , act to axially force first side gear  236 , which is rotatably coupled to axle  42  via interfitted splined portions  52  and  56  therein, leftward as viewed in FIG.  3 . Ball spacer  238 , also shown in FIGS. 9A and 9B, is provided between the interfacing axial surfaces of planar cone clutch element portion  232  and first side gear  236 . Ball spacer  238  is flat and annular, and provided with a plurality of circumferentially distributed identically-sized holes  238  within which balls  88  are disposed; the diameter of holes  238  is slightly larger than the diameter of balls  88  to facilitate free movement of the balls through the holes. Spacer  238  maintains proper positioning of balls  88  as the interfacing axial surfaces of planar cone clutch element portion  232  and first side gear  236  separate, and ensures that all the balls transmit and equal force between all paired surfaces of slots  230  and  234 . Should a ball  88  tend to lead or lag the revolution of the other balls  88  about axis  48 , it will contact a side of its spacer hole  240  and be urged thereby back into its proper circumferential position. Proper positional relationship between the balls  88  is thus maintained at all times. Spacer  238  may be flat, stamped sheet steel part. Alternatively, the ball spacer may be formed as a steel Belleville spring as shown in FIG.  9 B. Ball spacer  238 ′ is provided with circumferentially distributed holes  240  like ball spacer  238  to maintain proper relative ball positions, but provides the additional function of facilitating the axial separation of primary cone clutch element  222  and first side gear  236  by urging them axially apart and more quickly effecting locking of the differential. 
     As first side gear  236  is moved leftward, as viewed in FIG. 3, it is brought into abutment with bearing side  96  of transfer block  92 , which moves laterally relative to cross pin  32  as described above. Opposite transfer block bearing side  94  abuts second side gear  242 , which is rotationally fixed to axle  40  via interfitted splined portions  50  and  54  therein. Leftward movement of second side gear  242  urges a plurality of interleaved discs  244 ,  246 , which comprise secondary clutch  248 , into mutual frictional engagement. Discs  244  are rotatably fixed to side gear  242 , and discs  246  are rotatably fixed to casing  216 ; hence, their frictional engagement tends to slow their relative rotation, and lock the axle  40  into rotation with casing  216 . Because axles  40  and  42  are connected through side gears  234 ,  242  and pinion gears  28 ,  30 , once one axle is clutchedly engaged to casing  216 , both axles are so engaged. 
     In marked distinction from the differentials shown in FIGS. 1 and 2, differential  212  provides ring gear mounting flange  250  at the axial end of casing  216  which is opposite that at which electromagnet  220  is located, thereby substantially decreasing the likelihood that ring gear  104  will become magnetized, and thus minimizing the possibility that magnetic shavings or other debris which may be in cavity  18  will come between the intermeshed teeth of ring gear  104  and pinion  106 . As described above, the toroidal flux path about the annular electromagnet coil is directed through the adjacent portions of the ferrous casing parts, and the primary cone clutch. By greatly separating ring gear  104  from this flux path in accordance with the present invention, gear wear, and the durability of axle assembly  210  is improved vis-a-vis prior art electromagnetically-actuated limited slip axle assemblies which more proximally locate the ring gear and electromagnet. 
     FIG. 4 depicts a second embodiment of an axle assembly according to the present invention which is identical in structure and operation to above-described axle assembly  210  except as follows: Axle assembly  210  comprises electromagnet  220 ′ which is fixed to the axle housing  108 , rather than being rotatably supported about casing part  216   b ′. Bearing  110 ′ is disposed in cup  124  which extends inwardly of the axle housing to engage and support electromagnet  220 ′. Notably, bearing  110 ′ is somewhat smaller than bearing  112 , or bearing  110  of FIG.  3 . By moving the electromagnet to the axial end of casing  216 ′ opposite that at which ring gear  104  is located, however, larger bearing  112 , located near the ring gear  104 , is more heavily loaded during operation. 
     With reference now to FIGS. 5-8, there are shown various bearing means for axially and radially supporting electromagnet  220  in axle assembly  210 . FIG. 5, which is an enlarged fragmentary view of FIG. 3, shows electromagnet  220  (which comprises coil  252 ) is separated from casing  216  by flat annular roller thrust bearing  254 , and by annular bearing  256  molded of a self-lubricating, SP polyimide resin such as, for example, Vespel®, manufactured by DuPont. Bearing  256  has an L-shaped partial cross section providing integral cylindrical portion  258  and flat annular portion  260 . Snap ring  262  disposed in annular groove  264  provided in hub  266  of casing part  216   b  retains electromagnet  220  to casing  216 . Notably, line  268  indicates the toroidal magnetic flux path of electromagnet coil  252 . 
     FIG. 6 shows an alternative to the electromagnet mounting scheme of FIG. 5 which eliminates roller thrust bearing  254 , and replaces bearing  256  with bearing  270 . Bearing  270 , which may also be molded of Vespel®, has a U-shaped partial cross section providing integral annular flat portions  272  and  274  located on opposite sides of central cylindrical portion  276 . Effectively, the function of roller thrust bearing  254  (FIG. 5) is performed by bearing portion  272 . 
     FIG. 7 shows a further alternative to the electromagnet mounting scheme of FIG. 5 which eliminates roller thrust bearing  254 , and replaces it with flat, annular Vespel® bearing  278 . Further, bearing  256  (FIG. 5) is replaced with individual Vespel® bearings  280 ,  282  which are respectively substituted for portions  258  and  260  of bearing  256 . Flat annular bearing  278  of FIG. 7, and annular L-shaped bearing  256  of FIG. 5, are both used in the variant shown in FIG.  8 . 
     While this invention has been described as having exemplary designs, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.