Abstract:
A method of transitioning between modes of differentiation in a differential includes measuring a vehicle parameter, determining a mode of differentiation, selectively providing an electrical signal to an electromagnet based on the determination, generating a magnetic field, moving the electromagnet and selectively operating the differential in one of an open mode of differentiation and a locked mode of differentiation in response to movement of the electromagnet.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of U.S. patent application Ser. No. 10/674,024, filed on Sep. 29, 2003. The disclosure of the above application is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention generally relates to differentials for motor vehicles and, more particularly, to a locking differential employing an electromagnet to control operation of the differential.  
         [0003]     As is known, many motor vehicles are equipped with driveline systems including differentials which function to drivingly interconnect an input shaft and a pair of output shafts. The differential functions to transmit drive torque to the output shafts while permitting speed differentiation between the output shafts.  
         [0004]     Conventional differentials, such as a parallel-axis helical differential, include a pair of side gears fixed for rotation with the output shafts and two or more sets of meshed pinion gears mounted within a differential case. However, the conventional differential mechanism has a deficiency when a vehicle is operated on a slippery surface. When one wheel of the vehicle is on a surface having a low coefficient of friction, most or all of the torque will be delivered to the slipping wheel. As a result, the vehicle often becomes immobilized. To overcome this problem, it is known to provide a mechanical differential where an additional mechanism limits or selectively prevents differentiation of the speed between the output shafts. Typically, the mechanical device to provide the limited-slip or non-slip function is a friction clutch. The friction clutch is a passive device which limits the differential speed between the output shafts only after a certain differential speed has been met. Additionally, such mechanical devices may not be selectively disengaged during operation of anti-lock braking systems or vehicle traction control systems. For example, four-wheel anti-lock braking systems attempt to measure and control the rotational speed of each wheel independently. If a mechanical type limited slip differential is present, independent control of the speed of each wheel coupled to a differential is no longer possible. Accordingly, it would be desirable to provide an improved differential which may be actively controlled in conjunction with other control systems present on the vehicle.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention relates to a differential system including a case, a pair of pinion gears, a pair of side gears and an electrically operable coupling including an electromagnet. The coupling selectively drivingly interconnects one of the side gears and the case. In one instance, the present invention includes an axially moveable actuator having an electromagnet. The electromagnet may be selectively actuated to move a ring into engagement with one of the side gears of the differential. In this manner, the differential may function as an “open” differential when the ring is disconnected from the side gear or as a “locked” differential when the ring engages the side gear thereby fixing the side gear to the case.  
         [0006]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0008]      FIG. 1  is a schematic view of an exemplary motor vehicle drivetrain including a differential assembly of the present invention;  
         [0009]      FIG. 2  is an end view of a first embodiment differential assembly of the present invention;  
         [0010]      FIG. 3  is a cross-sectional side view of the differential of the present invention;  
         [0011]      FIG. 4  is an end view of a second embodiment differential assembly of the present invention;  
         [0012]      FIG. 5  is a cross-sectional side view of the second embodiment of the present invention; and  
         [0013]      FIG. 6  is a cross-sectional end view of the second embodiment differential assembly.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]     The present invention is directed to an improved differential for a drivetrain of a motor vehicle. The differential of the present invention includes an actuator operable to place the differential in an “open” or “locked” condition. It should be appreciated that the differential of the present invention may be utilized with a wide variety of driveline components and is not intended to be specifically limited to the particular application described herein. In addition, the actuator of the differential of the present invention may be used in conjunction with many types of differentials such as a bevel gear design which are of a completely open or limited-slip variety.  
         [0015]     With reference to  FIG. 1 , a drivetrain  10  for an exemplary motor vehicle is shown to include an engine  12 , a transmission  14 , having an output shaft  16  and a propeller shaft  18  connecting output shaft  16  to a pinion shaft  20  of a rear axle assembly  22 . Rear axle assembly  22  includes an axle housing  24 , a differential assembly  26  supported in axle housing  24  and a pair of axle shafts  28  and  30  respectively interconnected to left and right and rear wheels  32  and  34 . Pinion shaft  20  has a pinion gear  36  fixed thereto which drives a ring gear  38  that is fixed to a differential case  40  of differential assembly  26 . A gearset  41  supported within differential case  40  transfers rotary power from differential case  40  to axle shafts  28  and  30 , and facilitates relative rotation (i.e., differentiation) therebetween. Thus, rotary power from engine  12  is transmitted to axle shafts  28  and  30  for driving rear wheels  32  and  34  via transmission  14 , propeller shaft  18 , pinion shaft  20 , differential case  40  and gearset  41 . While differential assembly  26  is depicted in a rear-wheel drive application, the present invention is contemplated for use in differential assemblies installed in trailing axles, transaxles for use in front-wheel drive vehicles, transfer cases for use in four-wheel drive vehicles and/or any other known vehicular driveline application.  
         [0016]      FIGS. 2 and 3  depict differential assembly  26  including differential case  40  and gearset  41 . Gearset  41  includes a pair of pinion gears  42  rotatably supported on a cross shaft  44 . First and second side gears  45  and  46  are drivingly interconnected to pinion gears  42  and axle shafts  28  and  30 . Differential assembly  26  also includes an actuator assembly  50  operable to selectively couple side gear  45  to differential case  40 , thereby placing differential assembly  26  in a fully locked condition  
         [0017]     A cap  48  is coupled to differential case  40  to define a pocket  49  for receipt of actuator assembly  50 . Actuator assembly  50  includes a solenoid assembly  52 , an actuating ring  54 , a draw plate  56 , and a retainer  58 . Cap  48  includes a flange  60  coupled to a flange  62  of case  40 . Flange  60  of cap  48  includes a recess  64  sized to receive solenoid assembly  52  during actuation. Cap  48  includes a pair of stepped bores  66  and  68  which define pocket  49 . Specifically, first bore  66  includes an annular surface  70  while second bore  68  includes an annular surface  72 . First bore  66  includes an end face  74  radially inwardly extending from annular surface  70 . An aperture  76  extends through the cap  48  and is in communication with second bore  68  where aperture  76  and second bore  68  are sized to receive the portion of the axle shaft.  
         [0018]     Actuating ring  54  includes a generally hollow cylindrical body  78  having an annular recess  80  formed at one end. Side gear  45  includes a similarly sized annular recess  82  formed in an outboard face  84 . A compression spring  85  is positioned between actuating ring  54  and side gear  45  within annular recesses  80  and  82 . A plurality of axially extending dogs  86  protrude from an end face  88  of actuating ring  54 . A corresponding plurality of dogs  90  axially extend from face  84  of side gear  45 . Actuating ring  54  is moveable from a disengaged position as shown in  FIG. 3  to an engaged position (not shown). In the disengaged position, dogs  86  of actuating ring  54  are released from engagement with dogs  90  of side gear  45 . In contrast, when actuating ring  54  is moved to its engaged position, dogs  86  engage dogs  90  to rotatably fix side gear  45  to differential case  40 .  
         [0019]     Solenoid assembly  52  includes a metallic cup  94  and a coil of wire  96 . The wire is positioned within cup  94  and secured thereto by an epoxy  98 . Cup  94  includes an inner annular wall  100 , an outer annular wall  102  and an end wall  104  interconnecting annular walls  100  and  102 . Retainer  58  is a substantially disc-shaped member having an outer edge  106  mounted to end wall  104  of cup  94 . Retainer  58  is spaced apart from end wall  104  to define a slot  108 . Draw plate  56  is positioned within slot  108  and coupled to actuating ring  54  via a plurality of fasteners  110 . A washer  112  is positioned between cap  48  and actuating ring  54 . Preferably, washer  112  is constructed from a non-ferromagnetic material so as to reduce any tendency for actuating ring  54  to move toward metallic cap  48  instead of differential case  40  during energization of solenoid assembly  52 . A bearing  114  supports cup  94  on an outer journal  116  of cap  48 .  
         [0020]     Coil  96  is coupled to a controller  118  ( FIG. 1 ) which operates to selectively energize and de-energize coil  96 . During coil energization, a magnetic field is generated by current passing through coil  96 . The magnet field causes actuator assembly  50  to be drawn toward cap  48 . As solenoid assembly  52  enters recess  64 , dogs  86  of actuating ring  54  engage dogs  90  of side gear  45 . Once the dogs are engaged, actuating ring  54  is in its engaged position and differential assembly  26  is in a fully locked condition. One skilled in the art will appreciate that the axially moveable electromagnet of the present invention provides a simplified design having a reduced number of components. Additionally, the present invention utilizes the entire differential case as the armature for the electromagnet. This allows a more efficient use of the available magnetic force. These features allow a designer to reduce the size of the electromagnet because the armature more efficiently utilizes the electromotive force supplied by the electromagnet. Such a compact design allows for minor modification of previously used components and packaging with a standard sized axle housing.  
         [0021]     To place differential assembly  26  in the open, unlocked condition, current is discontinued to coil  96 . The magnetic field ceases to exist once current to coil  96  is stopped. At this time, compression in spring  85  causes actuator assembly  50  to axially translate and disengage dogs  86  from dogs  90 . Accordingly, side gear  45  is no longer drivingly coupled to differential case  40 , thereby placing differential assembly  26  in the open condition. It should also be appreciated that actuation and deactuation times are very short due to the small number of moving components involved. Specifically, no relative ramping or actuation of other components is required to cause engagement or disengagement of dogs  86  and dogs  90 .  
         [0022]     Electronic controller  118  controls the operation of actuator assembly  50 . Electronic controller  118  is in receipt of data collected by a first speed sensor  120  and a second speed sensor  122 . First speed sensor  120  provides data corresponding to the rotational speed of axle shaft  28 . Similarly, second speed sensor  122  measures the rotational speed of axle shaft  30  and outputs a signal to controller  118  indicative thereof. Depending on the data collected at any number of vehicle sensors such as a gear position sensor  124 , a vehicle speed sensor  126 , a transfer case range position sensor or a brake sensor  128 , controller  118  will determine if an electrical signal is sent to coil  96 . Controller  118  compares the measured or calculated parameters to predetermined values and outputs an electrical signal to place differential assembly  26  in the locked position only when specific conditions are met. As such, controller  118  assures that an “open” condition is maintained when events such as anti-lock braking occur. Limiting axle differentiation during anti-lock braking would possibly counteract the anti-lock braking system. Other such situations may be programmed within controller  118 .  
         [0023]      FIGS. 4-6  depict an alternate embodiment differential assembly  200 . Differential assembly  200  is substantially similar to differential assembly  26  except that differential assembly  200  relates to a parallel axis helical differential. Accordingly, like elements will retain the reference numerals previously introduced.  
         [0024]     Differential assembly  200  includes a planetary gearset  202  which is operable for transferring drive torque from differential case  40  to axle shafts  28  and  30  in a manner facilitating speed differential and torque biasing therebetween. Gearset  202  includes a pair of helical side gears  204  and  206  having internal splines that are adapted to mesh with external splines on corresponding end segments of axle shafts  28  and  30 . In addition, side gears  204  and  206  respectively include hubs  208  and  210  which are seated in corresponding annular sockets  212  and  214 . Side gear  204  also includes a plurality of axially extending dogs  215 . Gearset  202  further includes a spacer block  216  for maintaining side gears  204  and  206  and axle shafts  28  and  30  in axially spaced relation to each other. Once installed, spacer block  216  is free to rotate with respect to either axle shaft  28  and  30  and differential case  40 .  
         [0025]     Planetary gearset  202  also includes a first set of helical pinions  218  journally supported in first gear pockets  220  formed in differential case  40 . A set of second helical pinions  222  are journally supported in second gear pockets  224  formed in differential case  40 . While not limited thereto, differential  200  is shown to include three first pinions  218  and three second pinions  222  arranged in meshed pairs, referred to as meshed pinion sets. Gear pockets  220  and  224  are longitudinally extending, elongated, partially cylindrical bores and are formed in paired overlapping sets such that they communicate with an interior volume of differential case  40 .  
         [0026]     First pinions  218  are shown to include a long, larger diameter gear segment  230  and a short, smaller diameter stub shaft segment  232 . When installed in first gear pockets  220 , first pinions  218  are arranged such that the teeth of gear segments  230  are meshed with the teeth of side gear  204  while their outer diameter tooth end surfaces are journally supported by the bearing wall surface of pockets  220 .  
         [0027]     Likewise, second pinions  222  are shown to include a long, larger diameter gear segment  234  and a short, smaller diameter stub shaft  236 . When installed in second gear pockets  224 , second pinions are arranged such that the teeth of gear segments  234  are meshed with the teeth of side gear  206  while their outer diameter tooth end surfaces are journally supported by the bearing wall surface of second gear pockets  224 . Since pinions  218  and  222  are arranged in meshed sets, gear segment  230  of one of first pinions  218  also meshes with gear segment  234  and the corresponding one of second pinions  222 . Preferably, gear segments  230  and  234  are of an axial length to effectively maintain meshed engagement substantially along their entire length.  
         [0028]     A set of support members  238  support stub shaft sections  232  on each of pinions  218  against the bearing wall surface of its corresponding first gear pocket  220  and against the outer diameter tooth end surfaces of side gear  206  and gear segment  234  of its meshed second pinion. Support members  238  similarly support stub shaft segment  236  of second pinions  222 . A more complete description of parallel-axis gear differentials is found in U.S. Pat. No. 6,013,004 to Gage et al. which is hereby incorporated by reference.  
         [0029]     As previously described, actuator assembly  50  is positioned within a pocket  49  defined by cap  48  and differential case  40 . Actuator assembly  50  is selectively energizable to cause dogs  86  of actuating ring  54  to engage dogs  215  of side gear  204 . Differential assembly  200  functions substantially similarly to differential assembly  26  in that it is placed in a locked mode when dogs  86  engage dogs  215 . The differential assembly can be placed in an open mode by discontinuing current supply to coil  96 . Compression spring  85  axially displaces actuator assembly  50  to cause dogs  86  to disengage from dogs  215 .  
         [0030]     While a rear drive axle assembly has been described in detail, it should be appreciated that the differential system of the present invention is not limited to such an application. Specifically, the differential system of the present invention may be used in transaxles for front-wheel drive vehicles, transfer cases for use in four-drive vehicles and/or a number of other vehicular driveline applications.  
         [0031]     Furthermore, the foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations may be made therein without department from the spirit and scope of the invention as defined in the following claims.