Abstract:
The present disclosure provides an improved, integrated electronically controlled hydraulic-based torque distribution system and related method. The torque distribution system of the present disclosure includes an electric motor that drives a pump that generates hydraulic pressure used to selectively activate a clutch pack to transfer torque to the wheels of a motor vehicle.

Description:
RELATED APPLICATION 
       [0001]    This application is a continuation of application Ser. No. 14/203,932, filed on Mar. 11, 2014, which claims the benefit of U.S. Provisional Application Ser. No. 61/779,344 filed on Mar. 13, 2013. Application Ser. No. 14/203,932 and U.S. Provisional Application Ser. No. 61/779,344 are incorporated herein by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure provides an integrated electronically controlled hydraulic torque distribution system for an automotive vehicle and a related method of torque management. 
       BACKGROUND 
       [0003]    Automotive vehicles are typically powered by an engine that drives an output shaft which powers the driven wheels. The driven wheels are typically either the two front wheels, the two back wheels, or all four wheels. 
         [0004]    To avoid instability and driveline binding and other undesirables, the drive systems are configured to allow the wheels to spin at different speeds. This feature is beneficial, for example, when the vehicle corners as it enables the wheels on the inside of the corner to spin at a slower speed than the wheels on the outside of the corner, thereby avoiding wheel slip, tire wear, and stress on the drive system. 
         [0005]    Some drive systems have integrated torque management systems that are configured to actively manage the torque delivery to each of the wheels. Improved torque management systems that are fast acting, powerful, efficient, reliable and easily serviced are desirable. 
       SUMMARY 
       [0006]    The present disclosure provides an improved, integrated electronically controlled hydraulic-based torque distribution system and related method. The torque distribution system of the present disclosure includes an electric motor that drives a pump that generates hydraulic pressure used to selectively activate a clutch pack that is configured and arranged to transfer torque to the wheels of a motor vehicle. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0007]      FIG. 1  is a schematic of a powertrain in accordance with the principles of the present disclosure; 
           [0008]      FIG. 2  is a cross-sectional view through an axle module including an example torque management system in accordance with the principles of the present disclosure; 
           [0009]      FIG. 3  is an exploded view of the torque management system of  FIG. 2 ; 
           [0010]      FIG. 4  is an elevation view of the side cover of the torque management system of  FIG. 2 ; 
           [0011]      FIG. 5  is a front, assembled, perspective view of the torque management system of  FIG. 2 ; 
           [0012]      FIG. 6  is a rear, assembled, perspective view of the torque management system of  FIG. 2 ; 
           [0013]      FIG. 7  is a front view of a differential cover of the torque management system of  FIG. 2 ; 
           [0014]      FIG. 8  is a front, perspective view of the torque management system of  FIG. 2  with a hydraulic pump and electric motor disconnected from a reservoir cover; and 
           [0015]      FIG. 9  is a front, perspective view of the torque management system of  FIG. 8  with the reservoir cover disconnected form a differential cover. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  shows an example powertrain  100  in accordance with the principles of the present disclosure. The powertrain includes a prime mover such as an engine  102  coupled to a transmission  104 . The powertrain  100  also includes a drivetrain  106  for transferring torque from the transmission  104  to a first wheel  108  (e.g., a left wheel) and a second wheel  110  (e.g., a right wheel). The drivetrain  106  includes an axle assembly  112  including an axle module  114  having an axle module housing  116  containing a differential  118  (see  FIG. 2 ). The differential  118  is coupled to a first axle shaft  120  (e.g., a left axle half-shaft) and is also coupled to a second axle shaft  122  (e.g., a right axle-shaft). The first and second axle shafts  120 ,  122  can be coaxially aligned and rotatable about an axis  124 . The first axle shaft  120  transfers torque from the differential  118  to the first wheel  108  while the second axle shaft  122  transfers torque from the differential  118  to the second wheel  110 . The differential  118  is rotatable about the axis  124  and is configured to transfer torque to the first and second axle shafts  120 ,  122  while concurrently allowing for differential rotational speeds between the first and second axle shafts  120 ,  122 . The drivetrain  106  includes a drive shaft  126  that rotates the differential  118  in about the axis  124 . The axle module housing  116  includes a first side  128  that faces toward the first wheel  108  and a second side  130  that faces toward the second wheel  110 . 
         [0017]    Referring to  FIG. 2 , the differential  118  includes a differential case  132  that is rotatable about the axis  124 . The differential case  132  is mounted within the axle module housing  116  by a differential mount  134 . A bearing  135  is positioned between the differential mount  134  and the differential case  132  allowing the differential case  132  to rotate about the axis  124  relative to the differential mount  134  and the axle module housing  116 . A ring gear  136  is mounted to an exterior flange  138  that extends around the periphery of the differential case  132 . The ring gear  136  engages a drive gear  140  coupled to the drive shaft  126 . In this way, torque for rotating the differential case  132  about the axis  124  can be transferred from the drive shaft  126  to the differential case  132 . It will be appreciated that the axle module housing  116  can contain lubricant for lubricating the various moving parts contained therein. 
         [0018]    The differential  118  further includes an internal torque transfer arrangement  142  for transferring torque from the differential case  132  to the first and second axle shafts  120 ,  122 . In one example, torque transfer arrangement  142  can include internal gears (e.g., side gears, pinion gears, etc.) that allow torque to be transferred from the differential case  132  to the first and second shafts  120 ,  122  while concurrently allowing the first and second axle shafts  120 ,  122  to rotate at different speeds relative to one another about the axis  124 . 
         [0019]    Referring still to  FIG. 2 , the depicted torque transfer arrangement  142  includes first and second side gears  144 ,  146  that are coaxially aligned along the axis  124 . The torque transfer arrangement  142  also includes a plurality of pinion gears  148  positioned between the first and second side gears  144 ,  146 . Each of the pinion gears  148  intermeshes with both the first and second side gears  144 ,  146 . The pinion gears  148  are depicted as being rotatably mounted on shafts  150  anchored to the differential case  132 . The torque transfer arrangement  142  further includes first and second stub-shafts  152 ,  154  (i.e., output shafts) that are coaxially aligned along the axis  124 . The first stub-shaft  152  is non-rotatably coupled (e.g., by a splined connection) to the first side gear  144  and the second stub-shaft  154  is non-rotatably coupled (e.g., by a splined connection) to the second side gear  146 . The first stub-shaft  152  is adapted to be coupled to the first axle shaft  120  and the second stub-shaft  154  is adapted to be coupled to the second axle shaft  122 . 
         [0020]    Under normal operating conditions, the differential distributes torque equally between the first and second axle shafts  120 ,  122 . Specifically, the torque is transferred from the differential case  132 , through the pinion gears  148  and the first and second side gears  144 ,  146  to the first and second stub-shafts  152 ,  154  which transfer the torque to the first and second axle shafts  120 ,  122 . The first and second side gears  144 ,  146  and the pinion gears  148  are free to rotate relative to the differential case  132  to accommodate different rotational speeds between the first and second axle shafts  120 ,  122 . This allows the wheel on the outside of a turn to rotate faster than the wheel on the inside of the turn. 
         [0021]    The configuration of the differential  118  is advantageous for allowing relative rotation between the first and second axle shafts  120 ,  122  during vehicle turning. However, this type of configuration can be problematic under certain types of driving conditions. Because equal torque is delivered to each of the first and second axle shafts  120 ,  122 , the maximum torque that can be provided to any one axle shaft  120 ,  122  is dependent upon the maximum torque that can be applied to the other of the axle shafts  120 ,  122 . This is problematic under driving conditions where one of the wheels  108 ,  110  encounters a low friction condition (e.g., ice, oil, mud, etc.) in which only a minimal amount of torque can be applied to the corresponding axle shafts  120 ,  122  before the wheel  108 ,  110  slips. In this type of situation, the amount of torque that can be applied to the axle shaft  120 ,  122  of the non-slipping wheel  108 ,  110  is limited to the amount of torque that can be applied to the axle shaft  120 ,  122  of the slipping wheel. Often, this limited amount of torque is insufficient to turn the non-slipping wheel. Thus, the vehicle is unable to move. In other applications (e.g., steer assist), it is also desirable to be able to vary the distribution of torque provided between the first and second axle shafts  120 ,  122 . 
         [0022]    To address the above conditions, axle arrangements in accordance with the principles of the present disclosure can include a torque management system operable in a disengaged state and engaged state. When the torque management system is operated in a disengaged state, the differential  118  essentially functions as an open differential such that the differential  118  delivers torque equally to both the first and second wheels  108 ,  110 . As described above, the level of torque delivery is in part limited by the wheel that has lesser traction. For example, if the first wheel  108  is on dry pavement and will not slip unless 2,000 foot pounds of torque is applied thereto, and the second wheel  110  is over ice and would slip even when 40 or more foot pounds of torque is applied thereto, the torque delivered to each wheel would be 40 pounds when the torque management system is in the disengaged state. In the above described scenario, this low level of torque may be insufficient to move the vehicle. 
         [0023]    When the torque management system is in the engaged state, the differential  118  can deliver torque to the wheel with traction well in excess of the amount of torque that would cause the wheel with the least amount of traction to slip, (e.g., 40 or more pounds in the above described scenario). In particular, in one example, the rotation of the first and second wheels  108 ,  110  and their corresponding first and second axle shafts  120 ,  122  can be effectively locked together thereby causing the first and second wheels to rotate at the same rate. In one example, the torque management system can prevent relative rotation between the first stub-shaft  152  and the differential case  132  such that both axle shafts  120 ,  122  have the same rate of rotation about the axis  124  as the differential case  132 . In another example, the torque management system can control relative rotation between the first stub-shaft  152  and the differential case  132  such that the amount of torque provided to the non-slipping wheel can be controlled. This would, in the above scenario, enable the wheel on dry pavement to drive the vehicle forward. This type of functionality is particularly useful to prevent (or recover) the vehicle from being stuck in snow, mud, sand or uneven terrain where one of the drive wheels may be suspended in the air. 
         [0024]    Torque management systems in accordance with the principles of the present disclosure can also be used to provide stability to the vehicle when the vehicle is traveling on a highway at high speeds. For example, if the vehicle enters a turn and begins to over steer, the torque management system can activate to induce under steering to counter-act the impending or actual over steer. In this way, activation of the torque management system can provide a more controlled driving experience. It should be appreciated that torque control management systems in accordance with the principles of the present disclosure can have many additional alternative functions other than those specifically described above. 
         [0025]    Referring to  FIGS. 2-9 , an example torque management system  210  in accordance with the principles of the present disclosure is shown. In the depicted example, the torque management system  210  includes a differential cover  212  (i.e., a main cover) that mounts to the first side  128  of the axle module housing  116 . The cover  212  is shown including a central aperture  203  (i.e., a shaft opening) that receives the first stub-shaft  152 . A bearing  214  allows the first stub-shaft  152  to rotate about the axis  124  relative to the differential cover  212 . The first stub-shaft  152  extends outwardly from the differential cover  212  and is adapted for connection to the first axle shaft  120  coupled to the first wheel  108 . The differential cover  212  includes a first side  207  and an opposite second side  209 . When the differential cover  212  is mounted to the axle module housing  116 , the first side  207  faces toward the axle module housing  116  and the second side  209  faces away from the axle module housing  116 . 
         [0026]    Referring to  FIG. 7 , the differential cover  212  includes a cavity-defining portion  213  defining a cavity  211  that corresponds to a lower hydraulic reservoir  238 . As shown at  FIGS. 3-6 , a reservoir cover  260  mounts over the cavity defining portion  213  to enclose the hydraulic reservoir  238 . The hydraulic reservoir  238  includes at least a portion  256  that extends radially outwardly from a main body  255  of the differential cover  212 . In the depicted example, the main body  255  of the differential cover  212  includes a circumferential flange  253  having a peripheral edge  259  that defines a main outer boundary B (e.g., a footprint or outline) of the differential cover  212 . The peripheral edge  259  and the main outer boundary B surround the axis  124 . When viewed in side elevation (e.g., in an orientation along the axis  124  as shown at  FIG. 7 ), the portion  256  of the reservoir  238  is radially outside the main outer boundary B. The reservoir  238  can also include portions  258  positioned radially inside the main outer boundary B. In one example, the portion  256  can represent a majority of the total volume of the hydraulic reservoir  238 . The reservoir cover  260  has a first region  265  that radially overlaps the main body  255  and a second region  267  that projects radially outwardly from the main body  255 . The first region  265  is positioned radially inside the main outer boundary B and the second region  267  is positioned radially outside the main outer boundary B when viewed in the orientation along the axis  124 . 
         [0027]    In the depicted example, the differential cover  212  includes a plurality of apertures  252  spaced-apart from one another along the peripheral edge  259  of the differential cover  212 . The apertures  252  are defined though the circumferential flange  253 . The apertures  252  are each configured to receive a fastener (e.g., a bolt) used to secure the differential cover  212  to the first side  128  of the axle module housing  116 . In this way, the differential cover  212  functions to enclose the first side  128  of the axle module housing  116  such that the differential  118  and lubricant are effectively contained and protected within the axle module housing  116 . The main outer boundary B can coincide with (i.e., conform with or match) a shape of a sealed interface between the differential cover  212  and the module housing  116 . The fasteners  252  are spaced sufficiently close to one another to ensure effective sealing between the axle module housing  116  and the differential cover  212  along the main outer boundary B. 
         [0028]    As shown at  FIG. 7 , at least one of the apertures  252  (e.g., aperture  252   a ) extends axially through the lower hydraulic reservoir  238 . For example, the cavity defining portion  213  includes a projection  215  that projects radially into hydraulic reservoir  238 . A fastener access passage  217  is defined through the projection  215  in alignment with the aperture  252   a.  As shown at  FIG. 9 , the reservoir cover  260  defines an opening  219  that aligns with the fastener access passage  217  when the reservoir cover  260  is installed on the cavity defining portion  213 . When installed, the reservoir cover  260  forms a seal against the projection  215 . The seal extends about fastener access passage  217  prevents hydraulic fluid from the reservoir  238  from entering the fastener access passage  217 . The opening  219  and the fastener access passage  217  allow a fastener to be inserted through the aperture  52   a  from the front and secured to the axle module housing  116  without needing to remove the cover  260 . Once the fastener is installed, a head of the fastener can reside in the fastener access passage  217 . The opening  219  is positioned at a central region of the reservoir cover  260 . It should be appreciated that other alternative configurations are possible. 
         [0029]    Referring back to  FIG. 3 , the torque management system  210  also includes a brushless electric motor  220  mounted adjacent to a hydraulic pump  240 . The motor  220  and/or the pump  240  can be carried with the reservoir cover  260 . In one example, the motor  220  and the pump  240  are carried with the cover  260 . In one example, the pump is mounted to the cover  260  and the electric motor  220  is mounted to the pump  220 . In the depicted embodiment, the electric motor  220  and the hydraulic pump  240  are stacked in a coaxial arrangement. In the depicted example, the pump  240  is mounted at the first region  265  of the reservoir cover  260 . The electric motor  220  functions to drive the hydraulic pump  240 . When the electric motor  220  is activated to rotate in a first direction, the hydraulic pump  220  draws hydraulic fluid from the reservoir  238  and generates hydraulic pressure used to actuate the torque management system from the disengaged state to the engaged state. In the depicted example, a drive shaft of the motor  220  is in line with the pump  220  and is generally parallel to the first and second stub-shafts  152 ,  154 . It should be appreciated that other alternative configurations are possible. 
         [0030]    Referring to  FIG. 2 , an intake line  244  (e.g., a passage) is shown extending from the pump  240  to the lower reservoir  238 . The intake line  244  serves as a passage for hydraulic fluid to be drawn into the pump  240  from the reservoir  238 . In the depicted example of  FIG. 3 , the intake line  244  can include a tube  247  having an end positioned within the reservoir  238 . The end can include a filter screen assembly  246 . The tube  247  can connect to the reservoir cover  260  which can define an internal passage that forms a section of the intake line  244  that extends from the tube  247  to the intake of the pump  240 . When the electric motor  220  runs in the first direction, hydraulic fluid is drawn from the reservoir  238  through the intake line  244  into the pump, and is output from the pump  240  through a fluid line  241  (see  FIG. 2 ). The fluid line  241  is configured to provide the fluid pressure generated by the pump  240  to an actuator  251  that when actuated switches the torque management system  210  from the disengaged state to the engaged state. A pressure control line  243  is in fluid communication with the fluid line  241 . The pressure control line  243  includes a pressure sensor  248  for monitoring the hydraulic pressure within the fluid line  241 . The pressure control line  243  also includes a pressure regulating valve  222  that regulates the pressure in the fluid line  241  by selectively diverting flow to tank  238 . It will be appreciated that the torque management system  210  can include a controller  290  (e.g., an electronic controller, a computer, a processing unit, etc.) that interfaces with the pressure sensor  248 , the pressure regulating valve  222  and the electric motor  220 . The controller can also interface with other feedback sensors that monitor information such as the relative rotational speed between the first and second stub-shafts  152 ,  154  the torque being transferred through the sub-shafts  152 ,  154 , or other information. Based on feedback information concerning the operation of the axle assembly  112 , the electronic controller can control actuation of the actuator  251  to enhance performance of the axle assembly  112 . At least portions of the pressure sensor  248  and the pressure regulating valve  422  can be housed within discrete cavities defined by the reservoir cover  260 . 
         [0031]    In the depicted example, the step of removing the differential cover  212  from the axle module housing  116  simultaneously also removes the electric motor  220 , the pump  240 , the integral lower hydraulic reservoir  238 , the pressure sensor  248  and the pressure regulating valve  222 . In the depicted example, the electric motor  220 , the pump  240 , the pressure sensor  248 , and the pressure regulating valve  222  can also be separately removed or installed before or after the differential carrier is connected to the axle module housing  116 . The electric motor  220  and pump  240  are external to the reservoir cover  260  and can be removed and replaced without removing the differential cover  212  or the reservoir cover  260 . The pressure sensor  248  and the regulating valve  222  can be removed and replaced without removing the differential cover  212  by removing the reservoir cover  260  from the differential cover  212 . This module configuration results in a torque management system that is easy to manufacture, assembly and service. It should be appreciated that many alternative configurations are possible. 
         [0032]    In one example, the reservoir cover  260 , the pump  240 , the electric motor  220 , the clutch pack  226  and the piston  250  are carried with the cover  212  when the cover  212  is removed from the axle module housing  116 . In another example, the cover  212  can be removed from the axle module housing  116  without removing the reservoir cover  260  from the cover  212 . In another example, the reservoir cover  260 , the hydraulic pump  240  and the electric motor  220  can be removed as a unit from the cover  212  without removing the cover  212  from the axle module housing  116 . In still another example, the hydraulic pump  240  and the electric motor  220  can be removed from the reservoir cover  260  without removing the reservoir cover  260  from the cover  212  and without removing the cover  212  from the axle module housing  116 . 
         [0033]    The depicted actuator  251  of the torque management system  210  includes a clutch pack  226  having a plurality of friction disks. The clutch pack  226  is located at the first side  207  of the differential cover  212 . The friction disks include alternating first friction disks and second friction disks. The first friction disks are non-rotatably connected (e.g., coupled by a splined connection, keyed connection or other type of connection that restricts relative rotation) to a clutch basket  232  that is non-rotatably connected to the differential case  132 . The second friction disks are non-rotatably connected to a radial adapter  236  that is non-rotatably connected to the first stub-shaft  152 . In this way, the first friction disks rotate in unison with the clutch basket  232  and the differential case  132  and the second friction disks rotate in unison with the radial adapter  236  and the first stub-shaft  152 . The first and second sets of friction disks are interleaved with respect to one another. The actuator  251  further includes an annular hydraulic piston  250  positioned at the first side  207  of the differential cover  212 . The piston  250  is configured to move axially along the axis  124  based on the magnitude of hydraulic pressure applied to the piston through the fluid line  241 . The level of hydraulic pressure applied to the piston  250  controls the amount of actuation force applied to the clutch pack  226 . When the actuator  251  is fully actuated, the piston  250  applies sufficient axial force to the clutch pack  226  such that the friction disks frictionally engage one another and are prevented from rotating relative to one another. When this occurs, relative rotation is prevented between the first stub-shaft  152  and the differential case  132 . By preventing relative rotation between the first stub-shaft  152  and the differential case  132 , the side gears  144 ,  146  and the pinion gears  148  are prevented from rotating relative to the differential case such that the first and second stub-shafts  152 ,  154 , the first and second side gears  144 ,  146  and the pinion gears  148  all rotate in unison with the differential case  132  about the axis  124 . By applying an actuation pressure that is less than the full actuation pressure, the torque management system  212  can be operated to control a torque distribution between the first and second stub-shafts  152 ,  154 . 
         [0034]    The differential cover  212  is configured to accommodate the piston  250  and the clutch pack  226 . For example, the side  207  of the differential cover  212  that faces toward the axle module housing  116  can include a cavity  271  for receiving and housing the clutch pack  226  and the adapter  236 . The differential cover  212  also defines a piston chamber  273  for receiving the piston  250 . 
         [0035]    When the torque management system  212  is activated, hydraulic pressure from the pump  240  causes the piston  250  to impart an axial force on a thrust bearing  228  that compresses the clutch pack  226  between the thrust bearing  228  and the clutch basket  232 . A thrust bearing  230  and bearing race  231  are also provided between the rear side of the clutch basket  232  and the differential mount  134 . In the depicted example, the clutch basket  232  rotates with the differential casing  118 . The compression of the clutch pack  226  can be sufficient to cause the radial adapter  236  to rotate with the clutch pack  226  which causes the first stub-shaft  152  to rotate with the differential casing  132 . The relative rotational speed between the radial adapter  236  and the differential casing  132  can be controlled by the selected and/or modulated activation of the clutch pack  226 . When the clutch pack  226  is fully compressed, the first stub-shaft  152  rotates in unison with the second stub-shaft  154  and the differential casing  132 . It will be appreciated that other alternative configurations are also possible. 
         [0036]    In the depicted example, the clutch pack  226  can be configured to be progressively activated/ engaged based on modulation of the hydraulic pressure that acts on the piston  250 . If only a small increase in torque is desirable at a given one of the wheels  108 ,  110 , the pressure applied to the piston  250  will be relatively low and the clutch pack will be partially engaged causing some lower level of additional torque to be delivered to the torque deficient wheel. This hydraulic pressure level would be less than the level of hydraulic pressure sufficient to fully lock the differential (i.e., cause the differential to act as a mechanically locked differential) where both wheels  108 ,  110  are rotated at the same speed regardless of traction. Accordingly, when the differential is locked, enough torque can be transferred to slip one tire on dry pavement at the maximum axle capacity rating. It should be appreciated that many alternative configurations are possible. For example, torque management systems which are not capable of locking the differential are also included within the scope of the present disclosure. In one example, the clutch pack  226  controls (i.e., stops, limits, prevents, regulates, etc.) relative rotation between the differential case  132  and the shaft  152  when actuated. 
         [0037]    In the depicted example, the level of pressure applied to the piston  250  is monitored via the pressure sensor  248  and is electronically controlled/modulated in part by controlling the pressure regulating valve  222 . In the depicted example, a multiple wired electrical connector  254  is located adjacent the exterior of the electric motor  220 . Control signals (e.g., control instructions to the motor  220 , control instructions to the pressure regulating valve  222 , etc.) and feedback signals (e.g., the hydraulic pressure applied to the piston  250 , the temperature of the hydraulic fluid or other various components of the system, etc.) are transmitted to the system controller by a wire having an electrical connector that mates with the electrical connector  254 . In the depicted example, the control unit can interface with memory to reference a look-up table that correlates the hydraulic pressure applied to the piston with the torque load applied to the wheels. The hydraulic pressure corresponding to a particular torque request can be dependent on the wear on the system (clutch wear), the temperature of the system, and other factors which can be accounted for by the control system. Accordingly, the system of the depicted embodiment determines the appropriate hydraulic pressure based on the desired torque load. It should be appreciated that other alternative configurations are possible. In the depicted example, the friction disks of the clutch pack  226  are positioned outside of the differential casing  132 . This arrangement allows for the friction disks to be relatively large since they do not need to fit within the differential casing  132 . In the depicted example, each of the friction disks is generally circular and has an outer diameter that is greater than an overall cross-sectional diameter of the differential casing  132 . In one example, the friction disks can have outer diameters that are less than 30 centimeters, and the outer diameter of the differential casing  132  is also less than 30 centimeters. It will be appreciated that many alternative configurations are possible. 
         [0038]    In the depicted example, the relatively large diameters of the friction disks improve the longevity of the disks as well as improves the overall performance of the system. As compared to smaller friction disks that are fit within the differential casing, the larger friction disks of the depicted example can provide the same level of torque delivery with less axial force (e.g., less hydraulic pressure is needed or a smaller piston could be used). The disclosed configuration having relatively large friction disks is less noisy, has fewer and smaller vibrations, and generates less heat than systems with smaller friction disks that are positioned within the differential casing. The disclosed configuration is generally more efficient as fewer friction disks can be used to generate the same amount of torque. Due to mechanical factors (e.g., binding of years of friction disks), increasing the number of friction disks generally decreases the efficiency of the clutch pack. It should be appreciated that many alternative configurations are possible. For example, in alternative examples of the torque management system, the system could include friction disks housed within the differential housing. 
         [0039]    In the depicted example, the friction disks of a clutch pack  226  are positioned adjacent to the differential cover  212 . The clutch pack can be accessed by removing the differential cover  212 , removing the piston  250 , removing the thrust bearing  228 . This modular configuration results in a torque management system that is easy to manufacture, assemble and service. However, as discussed above, it will be appreciated that many alternative configurations are possible. 
         [0040]    As described above, in the depicted example, the axial force needed to activate the clutch pack  226  from complete disengagement to full engagement is relatively small. In the depicted example, the system does not rely on an accumulator to provide reserve hydraulic pressure. Instead, the system only uses the electric motor  220  to generate hydraulic pressure via the hydraulic pump  240 , as needed. The motor  220  can also be run in reverse, thereby causing the hydraulic pump to run in reverse and quickly decrease the hydraulic pressure acting on the piston  250 . However, as discussed above, it should be appreciated that many alternative configurations are possible including, for example, examples that include accumulators. 
         [0041]    In the depicted example, when the system is operating normally, the clutch pack  226  is not engaged and therefore results in very little friction loss. The electric motor  220  can be run slowly and/or periodically to maintain a target hydraulic pressure. The default target hydraulic pressure can be modified based on driving conditions through either user input (e.g., moving a dial, switch or other user interface), sensed conditions, or both. When the torque management system is directed to engage the clutch pack  226 , the motor  220  can be run at maximum speed or near maximum speed, thereby causing the hydraulic pump  240  to quickly draw hydraulic fluid from the reservoir  238  through the intake line  244  thereby generating reserve hydraulic pressure on the upstream side of the regulating valve  222 . The regulating valve  222  can be directed to supply the precise level of hydraulic pressure needed to generate the desired amount of axial force on the clutch pack  226  thereby providing the desired level of torque at a given one of the wheels  108 ,  110 . In the depicted example, the disclosed physical arrangement and configuration of the components enables the use of a relatively small electric motor (e.g., 200 to 300 watt) and relatively low hydraulic pressure (e.g., 200 to 300 psi). For example, as discussed above, the use of large friction disks located outside the differential case  132  can enable the system to fully “lock” the wheels  108 ,  110  without reliance on an accumulator, large electric motor, and/or high hydraulic pressures. It should be appreciated that other alternative configurations are also possible. 
         [0042]    The above specification, examples and drawings included herewith disclose examples of how inventive aspects of the disclosure may be practiced. It will be appreciated that changes may be made and the specifics of the disclosed examples without departing from the spirit and scope of the broad inventive aspects of the disclosure.