Patent Publication Number: US-6988602-B2

Title: Torque transfer coupling with magnetorheological clutch actuator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 10/357,046 filed Feb. 3, 2003. 

   FIELD OF THE INVENTION 
   The present invention relates generally to power transfer systems for controlling the distribution of drive torque between the front and rear drivelines of a four-wheel drive vehicle. More particularly, the present invention is directed to a power transmission device for use in motor vehicle driveline applications and having a magnetorheological clutch actuator that is operable for controlling actuation of a multi-plate friction clutch assembly. 
   BACKGROUND OF THE INVENTION 
   In view of increased demand for four-wheel drive vehicles, a plethora of power transfer systems are currently being incorporated into vehicular driveline applications for transferring drive torque to the wheels. In many vehicles, a power transmission device is operably installed between the primary and secondary drivelines. Such power transmission devices are typically equipped with a torque transfer mechanism for selectively and/or automatically transferring drive torque from the primary driveline to the secondary driveline to establish a four-wheel drive mode of operation. For example, the torque transfer mechanism can include a dog-type lock-up clutch that can be selectively engaged for rigidly coupling the secondary driveline to the primary driveline to establish a “part-time” four-wheel drive mode. In contrast, drive torque is only delivered to the primary driveline when the lock-up clutch is released for establishing a two-wheel drive mode. 
   A modern trend in four-wheel drive motor vehicles is to equip the power transmission device with an adaptive transfer clutch in place of the lock-up clutch. The transfer clutch is operable for automatically directing drive torque to the secondary wheels, without any input or action on the part of the vehicle operator, when traction is lost at the primary wheels for establishing an “on-demand” four-wheel drive mode. Typically, the transfer clutch includes a multi-plate clutch assembly that is installed between the primary and secondary drivelines and a clutch actuator for generating a clutch engagement force that is applied to the clutch plate assembly. The clutch actuator can be a power-operated device that is actuated in response to the magnitude of an electric control signal sent from an electronic controller unit (ECU). Variable control of the control signal is typically based on changes in current operating characteristics of the vehicle (i.e., vehicle speed, interaxle speed difference, acceleration, steering angle, etc.) as detected by various sensors. Thus, such “on-demand” power transmission devices can utilize adaptive control schemes for automatically controlling torque distribution during all types of driving and road conditions. 
   Currently, a large number of on-demand transfer cases are equipped with an electrically-controlled clutch actuator that can regulate the amount of drive torque transferred to the secondary output shaft as a function of the value of the electrical control signal applied thereto. In some applications, the transfer clutch employs an electromagnetic clutch as the power-operated clutch actuator. For example, U.S. Pat. No. 5,407,024 discloses a electromagnetic coil that is incrementally activated to control movement of a ball-ramp drive assembly for applying a clutch engagement force on the multi-plate clutch assembly. Likewise, Japanese Laid-open Patent Application No. 62-18117 discloses a transfer clutch equipped with an electromagnetic actuator for directly controlling actuation of the multi-plate clutch pack assembly. 
   As an alternative, the transfer clutch can employ an electric motor and a drive assembly as the power-operated clutch actuator. For example, U.S. Pat. No. 5,323,871 discloses an on-demand transfer case having a transfer clutch equipped with an electric motor that controls rotation of a sector plate which, in turn, controls pivotal movement of a lever arm that is operable for applying the clutch engagement force to the multi-plate clutch assembly. Moreover, Japanese Laid-open Patent Application No. 63-66927 discloses a transfer clutch which uses an electric motor to rotate one cam plate of a ball-ramp operator for engaging the multi-plate clutch assembly. Finally, U.S. Pat. Nos. 4,895,236 and 5,423,235 respectively disclose a transfer case equipped with a transfer clutch having an electric motor driving a reduction gearset for controlling movement of a ball screw operator and a ball-ramp operator which, in turn, apply the clutch engagement force to the clutch pack. 
   While many on-demand clutch control systems similar to those described above are currently used in four-wheel drive vehicles, a need exists to advance the technology and address recognized system limitations. For example, the size, weight and electrical power requirements of the electromagnetic coil or the electric motors needed to provide the described clutch engagement loads may make such system cost prohibitive in some four-wheel drive vehicle applications. In an effort to address these concerns, new technologies are being considered for use in power-operated clutch actuator applications such as, for example, magnetorheological clutch actuators. Examples of such an arrangement are described in U.S. Pat. Nos. 5,915,513 and 6,412,618 wherein a magnetorheological actuator controls operation of a ball-ramp unit to engage the clutch pack. While such an arrangement may appear to advance the art, its complexity clearly illustrates the need to continue development of even further defined advancement. 
   SUMMARY OF THE INVENTION 
   Thus, its is an object of the present invention to provide a power transmission device for use in a motor vehicle having a torque transfer mechanism equipped with a magnetorheological clutch actuator that is operable to control engagement of a multi-plate clutch assembly. 
   It is a further object of the present invention to provide a magnetorheological thrust cam operator and an electromagnet for use as the clutch actuator in a torque transfer mechanism. 
   As a related object, the torque transfer mechanism of the present invention is well-suited for use in motor vehicle driveline applications to control the transfer of drive torque between a first rotary member and a second rotary member. 
   According to a preferred embodiment, the torque transfer mechanism includes a magnetorheological clutch actuator which is operable for controlling the magnitude of clutch engagement force exerted on a multi-plate clutch assembly that is operably disposed between the first rotary member and a second rotary member. The magnetorheological clutch actuator includes a first thrust cam that is fixed for rotation with the first rotary member, a second thrust cam, a chamber filled with magnetorheological fluid communicating with at least one of the thrust cams, and an electromagnet which surrounds a portion of the chamber. In operation, activation of the electromagnet creates a magnetic flux field which travels through the magnetorheological fluid for proportionally increasing its viscosity, thereby creating drag which results in an axial separation force between the thrust cams. This axial separation force results in axial movement of the first thrust cam for exerting a clutch engagement force on the clutch pack, thereby transferring drive torque from the first rotary member to the second rotary member. Upon deactivation of the electromagnet, a return spring releases the clutch pack from engagement and acts to axially move the first thrust cam to a neutral position. 
   In accordance with one preferred embodiment, the chamber is defined between the first thrust cam and the second thrust cam. Further, the first and second thrust cams have corresponding first and second cam surfaces that are arranged to normally cause common rotation of the first and second thrust cams when the electromagnet is deactivated. Upon activation of the electromagnet, a reaction force is generated between the first and second cam surfaces for causing axial movement of the first thrust cam relative to the clutch pack for engaging the clutch pack. 
   In accordance with an alternative preferred embodiment, a recess is formed in a housing within which the second thrust cam is rotatably supported. The sealed chamber is defined between the recess and the second thrust cam. The viscosity of the magnetorheological fluid is controllably varied to induce a drag force on the second thrust cam for imparting the reaction force between the first and second thrust cams. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects, features and advantages of the present invention will become apparent to those skilled in the art from analysis of the following written description, the appended claims, and accompanying drawings in which: 
       FIG. 1  illustrates the drivetrain of a four-wheel drive vehicle equipped with a power transmission device incorporating the present invention; 
       FIG. 2  is a schematic illustration of an on-demand 4WD transfer case equipped with a torque transfer mechanism having a magnetorheological clutch actuator and a multi-plate friction clutch; 
       FIG. 3  is a partial sectional view of an the transfer case showing the torque transfer mechanism arranged for selectively transferring drive torque from the primary output shaft to the secondary output shaft; 
       FIG. 4  is a partial sectional view of alternative embodiment of a torque transfer mechanism arranged for use in the transfer case of the present invention; 
       FIG. 5  is a modified version of the torque transfer mechanism shown in  FIG. 3 ; 
       FIG. 6  is a modified version of the torque transfer mechanism shown in  FIG. 4 ; 
       FIG. 7  is a schematic illustration of an alternative drivetrain for a four-wheel drive vehicle equipped with a power transmission device of the present invention; and 
       FIGS. 8 through 11  are schematic illustrations of alternative embodiments of the power transmission devices according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention is directed to a torque transfer mechanism that can be adaptively controlled for modulating the torque transferred from a first rotary member to a second rotary member. The torque transfer mechanism finds particular application in power transmission devices for use in motor vehicle drivelines such as, for example, an on-demand clutch in a transfer case or in-line torque coupling, a biasing clutch associated with a differential assembly in a transfer case or a drive axle assembly, or as a shift clutch in a multi-speed automatic transmission. Thus, while the present invention is hereinafter described in association with particular arrangements for use in specific driveline applications, it will be understood that the construction/applications shown and described are merely intended to illustrate embodiments of the present invention. 
   With particular reference to  FIG. 1  of the drawings, a drivetrain  10  for a four-wheel drive vehicle is shown. Drivetrain  10  includes a primary driveline  12 , a secondary driveline  14 , and a powertrain  16  for delivering rotary tractive power (i.e., drive torque) to the drivelines. In the particular arrangement shown, primary driveline  12  is the rear driveline while secondary driveline  14  is the front driveline. Powertrain  16  includes an engine  18 , a multi-speed transmission  20 , and a power transmission device hereinafter referred to as transfer case  22 . Rear driveline  12  includes a pair of rear wheels  24  connected at opposite ends of a rear axle assembly  26  having a rear differential  28  coupled to one end of a rear prop shaft  30 , the opposite end of which is coupled to a rear output shaft  32  of transfer case  22 . Front driveline  14  includes a pair of front wheels  34  connected at opposite ends of a front axle assembly  36  having a front differential  38  coupled to one end of a front prop shaft  40 , the opposite end of which is coupled to a front output shaft  42  of transfer case  22 . 
   With continued reference to the drawings, drivetrain  10  is shown to further include an electronically-controlled power transfer system for permitting a vehicle operator to select between a two-wheel high-range drive mode, a part-time four-wheel high-range drive mode, an on-demand four-wheel high-range drive mode, a neutral non-driven mode, and a part-time four-wheel low-range drive mode. In this regard, transfer case  22  is equipped with a range clutch  44  that is operable for establishing the high-range and low-range drive connections between an input shaft  46  and rear output shaft  32 , and a power-operated range actuator  48  that is operable to actuate range clutch  44 . Transfer case  22  also a transfer clutch  50  that is operable for transferring drive torque from rear output shaft  32  to front output shaft  42  for establishing the part-time and on-demand four-wheel drive modes. The power transfer system further includes a power-operated mode actuator  52  for actuating transfer clutch  50 , vehicle sensors  54  for detecting certain dynamic and operational characteristics of the motor vehicle, a mode select mechanism  56  for permitting the vehicle operator to select one of the available drive modes, and a controller  58  for controlling actuation of range actuator  48  and mode actuator  52  in response to input signals from vehicle sensors  54  and mode selector  56 . 
   Transfer case  22  is shown schematically in  FIG. 2  to include a housing  60  from which input shaft  46  is rotatably supported by a bearing assembly  62 . As is conventional, input shaft  46  is adapted for driven connection to the output shaft of transmission  20 . Rear output shaft  32  is shown rotatably supported between input shaft  46  and housing  60  via bearing assemblies  64  and  66  while front output shaft  42  is rotatably supported between transfer clutch  50  and housing  60  by a pair of laterally-spaced bearing assemblies  68  and  69 . Range clutch  44  is shown to include a planetary gearset  70  and a synchronized range shift mechanism  72 . Planetary gearset  70  includes a sun gear  74  fixed for rotation with input shaft  46 , a ring gear  76  fixed to housing  60 , and a set of planet gears  78  rotatably supported on pinion shafts  80  extending between front and rear carrier rings  82  and  84 , respectively, that are interconnected to define a carrier  86 . 
   Planetary gearset  70  functions as a two-speed reduction unit which, in conjunction with a sliding range sleeve  88  of synchronized range shift mechanism  72 , is operable to establish either of a first or second drive connection between input shaft  46  and rear output shaft  32 . To establish the first drive connection, input shaft  46  is directly coupled to rear output shaft  32  for defining a high-range drive mode in which rear output shaft  32  is driven at a first (i.e., direct) speed ratio relative to input shaft  46 . Likewise, the second drive connection is established by coupling carrier  86  to rear output shaft  32  for defining a low-range drive mode in which rear output shaft  32  is driven at a second (i.e., reduced) speed ratio relative to input shaft  46 . A neutral non-driven mode is established when rear output shaft  32  is disconnected from both input shaft  46  and carrier  86 . 
   Synchronized range shift mechanism  72  includes a first clutch plate  90  fixed for rotation with input shaft  46 , a second clutch plate  92  fixed for rotation with rear carrier ring  84 , a clutch hub  94  rotatably supported on input shaft  46  between clutch plates  90  and  92 , and a drive plate  96  fixed for rotation with rear output shaft  32 . Range sleeve  88  has a first set of internal spline teeth that are shown meshed with external spline teeth on clutch hub  94 , and a second set of internal spline teeth that are shown meshed with external spline teeth on drive plate  96 . As will be detailed, range sleeve  88  is axially moveable between three distinct positions to establish the high-range, low-range and neutral modes. Range shift mechanism  72  also includes a first synchronizer assembly  98  located between hub  94  and first clutch plate  90 , and a second synchronizer assembly  100  disposed between hub  94  and second clutch plate  92 . Synchronizers  98  and  100  work in conjunction with range sleeve  88  to permit on-the-move range shifts. 
   With range sleeve  88  located in its neutral position, as denoted by position line “N”, its first set of spline teeth are disengaged from the external clutch teeth on first clutch plate  90  and from the external clutch teeth on second clutch plate  92 . Thus, no drive torque is transferred from input shaft  46  to rear output shaft  32  when range sleeve  88  is in its neutral position. When it is desired to establish the high-range drive mode, range sleeve  88  is slid axially from its neutral position toward a high-range position, denoted by position line “H”. First synchronizer assembly  98  is operable for causing speed synchronization between input shaft  46  and rear output shaft  32  in response to sliding movement of range sleeve  88  from its neutral position toward its high-range position. Upon completion of speed synchronization, the first set of spline teeth on range sleeve  88  move into meshed engagement with the external clutch teeth on first clutch plate  90  while its second set of spline teeth are maintained in engagement with the spline teeth on drive plate  96 . Thus, movement of range sleeve  88  to its high-range position acts to couple rear output shaft  32  for common rotation with input shaft  46  and establishes the high-range drive mode connection therebetween. 
   Similarly, second synchronizer assembly  100  is operable for causing speed synchronization between carrier  86  and rear output shaft  32  in response to axial sliding movement of range sleeve  88  from its neutral position toward a low-range position, as denoted by position line “L”. Upon completion of speed synchronization, the first set of spline teeth on range sleeve  88  move into meshed engagement with the external clutch teeth on second clutch plate  92  while the second set of spline teeth on range sleeve  88  are maintained in engagement with the external spline teeth on drive plate  96 . Thus, with range sleeve  88  located in its low-range position, rear output shaft  32  is coupled for rotation with carrier  86  and the low-range drive mode connection is established between input shaft  46  and rear output shaft  32 . 
   To provide means for moving range sleeve  88  between its three distinct range position, range shift mechanism  72  further includes a range fork  102  coupled to range sleeve  88 . Range actuator  48  is operable to move range fork  102  for causing corresponding axial movement of range sleeve  88  between its three range positions. Range actuator  48  is preferably an electric motor arranged to move range sleeve  88  to a specific range position in response to a control signal from controller  58  that is based on the mode signal delivered to controller  58  from mode select mechanism  56 . 
   It will be appreciated that the synchronized range shift mechanism permits “on-the-move” range shifts without the need to stop the vehicle which is considered to be a desirable feature. However, other synchronized and non-synchronized versions of range clutch  44  can be used in substitution for the particular arrangement shown. Also, it is contemplated that range clutch  44  and range actuator  48  can be removed entirely from transfer case  22  such that input shaft  46  would directly drive rear output shaft  32  to define a one-speed version of the on-demand transfer case embodying the present invention. 
   Referring now primarily to  FIGS. 2 and 3 , transfer clutch  50  is shown arranged in association with front output shaft  42  in such a way that it functions to deliver drive torque from a transfer assembly  110  driven by rear output shaft  32  to front output shaft  42  for establishing the four-wheel drive modes. Transfer assembly  110  includes a first sprocket  112  fixed for rotation with rear output shaft  32 , a second sprocket  114  rotatably supported by bearings  116  on front output shaft  42 , and a power chain  118  encircling sprockets  112  and  114 . As will be detailed, transfer clutch  50  is a multi-plate clutch assembly  124  and mode actuator  52  is a magnetorheological clutch actuator  120  which together define a torque transfer mechanism. 
   Multi-plate clutch assembly  124  is shown to include an annular drum  126  fixed for rotation with second sprocket  114 , a hub  128  fixed via a splined connection  130  for rotation with front output shaft  42 , and a multi-plate clutch pack  132  operably disposed between drum  126  and hub  128 . In particular, drum  126  has a first smaller diameter cylindrical rim  126 A that is fixed (i.e., welded, splined, etc.) to sprocket  114  and a second larger diameter cylindrical rim  126 B that is interconnected to rim  126 A by a radial plate segment  126 C. Hub  128  is shown to include a first smaller diameter hub segment  128 A and a second larger diameter hub segment  128 B that are interconnected by a radial plate segment  128 C. Clutch pack  132  includes a set of outer friction plates  134  that are splined to outer rim  126 B of drum  126  and which are alternatively interleaved with a set of inner friction plates  136  that are splined to hub segment  128 B of clutch hub  128 . Clutch assembly  124  further includes a first pressure plate  138  having a plurality of circumferentially-spaced and radially-extending tangs  140  that are disposed in longitudinally-extending slots formed in hub segment  128 B prior to installation of clutch pack  132  such that a front face surface  142  of each tang  140  abuts an end surface  144  of the slots so as to define a fully retracted position of first pressure plate  138  relative to clutch pack  132 . Thus, first pressure plate  138  is coupled for common rotation with clutch hub  128  and front output shaft  42 . A second pressure plate  146  is fixed via a splined connection  147  to rim  126 B of drum  126  for rotation therewith. As seen, a plurality of circumferentially-spaced return springs  148  act between pressure plates  138  and  146 . 
   With continued reference to  FIGS. 2 and 3 , magnetorheological clutch actuator  120  is shown to generally include a thrust cam operator  150  and an electromagnetic energy source such as, for example, electromagnetic coil  152 . Thrust cam operator includes a drive ring  154  fixed via a spline connection  156  for rotation with drum  126  and a reaction ring  158  fixed to housing  60 . Recesses  154 A and  158 A are formed in corresponding portions of drive ring  154  and reaction ring  158  and together define an annular chamber  160 . Chamber  160  is filled with magnetorheological (MR) fluid  161 , preferably of a high viscosity and of a type manufactured by Lord Corporation, Erie, Pa. Drive ring  154  is supported on front output shaft  42  via a bearing assembly  162  and has a front face surface  164  in engagement with second pressure plate  146 . Seal rings  166  provide a fluid tight seal between chamber  160  and housing  60 . As seen, a first cam disk  166  is secured within recess  154 A such that it and drive ring  154  together define a first thrust cam  167 . Likewise, a second cam disk  168  is secured within recess  158 A such that it and reaction ring  158  together define a second thrust cam  169 . First cam disk  166  has a faceted face surface  166 A communicating with MR fluid  161  in chamber  160 . Similarly, second cam disk  168  has a faceted face surface  168 A communicating with MR fluid  161  in chamber  160 . The faceted face surfaces  166 A,  168 A are configured to include multiple angular and/or ramped portions of different sizes and shapes which act as cam surfaces. 
   Electromagnetic coil  152  is secured to housing and is adapted to receive electric control signals from controller  58  for generating a magnetic field. In the absence of a magnetic field, first thrust cam  167  rotates relative to second thrust cam  169  in chamber  160 . However, when MR fluid  161  is exposed to a magnetic field upon activation of electromagnetic coil  152 , its magnetic particles align with the field and increase the viscosity and, therefore, the shear strength of MR fluid  161 . This increased shear strength causes the fluid to exert an axial separation force normal to the cam surfaces in the direction of relative rotation. Since second thrust cam  169  is axially restrained, the axial separation force causes first thrust cam  167  to move axially for forcibly urging second pressure plate  146  to move, in opposition to the biasing of springs  148 , into engagement with clutch pack  132 . Such engagement of clutch pack acts to transfer drive torque from rear output shaft  32  to front output shaft  42  through transfer mechanism  110 . The magnitude of the axial separation force is proportional to the cam angles and geometry of the opposing cam surfaces and the viscosity of MR fluid  161 . 
   The biasing force of springs  148  limits axial movement of first thrust cam  167  as a function of the viscosity of MR fluid  161 . For example, in its least viscous form, MR fluid  161  has no effect and first thrust cam  167  rotates relative to second thrust cam  169  within chamber  160 . In its most viscous form, MR fluid  161  has a large shear strength for inducing sufficient axial movement of first thrust cam  167  to fully engage clutch pack  132 . However, axial movement of first thrust cam  167  is limited at full engagement of clutch pack  132  and once having achieved that limit, the thrust cams function only to circulate the now highly viscous MR fluid  161  within chamber  160 . Degrees of viscosity are achievable between the least viscous and most viscous form of MR fluid  161  and vary with the intensity of the magnetic field and, thus, with the magnitude of the electric control signal sent to coil  152 . As such, the value of the clutch engagement force induced by operator  150  and applied to clutch pack  132  of clutch assembly  124  can be adaptively varied as a function of the magnitude of the electric control signal sent to coil  152  between a no torque transfer condition (two-wheel drive mode with 100% of drive torque delivered to rear output shaft  32 ) and a torque-split condition (part-time four-wheel drive mode with 50% of drive torque to front output shaft  42  and 50% to rear output shaft  32 ). Upon decease of the magnetic field strength, first thrust cam  167  is axial biased by springs  148  against second pressure plate  146 , thereby relieving engagement of clutch pack  132  and biasing first thrust cam  167  for movement to its released position. 
   In operation, when mode selector  56  indicates selection of the two-wheel high-range drive mode, range actuator  48  is signaled to move range sleeve  88  to its high-range position and transfer clutch  50  is maintained in a released condition with no electric signal sent to coil  150  of magnetorheological clutch actuator  120 , whereby all drive torque is delivered to rear output shaft  32 . If mode selector  56  thereafter indicates selection of a part-time four-wheel high-range mode, range sleeve  88  is maintained in its high-range position and a predetermined maximum electrical control signal is sent by controller  58  to coil  152  of magnetorheological clutch actuator  120  which causes axial movement of first thrust cam  167  due to the resultant change in viscosity of MR fluid  161 . Such action causes second pressure plate  146  to engage clutch pack  132  until a maximum clutch engagement force is exerted on clutch pack  132  for effectively coupling hub  128  to drum  126 . In response to such movement of second pressure plate  146 , return springs  148  are compressed and act to forcibly locate first pressure plate  138  in its fully retracted position where it acts as a reaction plate against which clutch pack  132  is compressed. 
   If a part-time four-wheel low-range drive mode is selected, the operation of transfer clutch  50  and magnetorheological clutch actuator  120  are identical to that described above for the part-time high-range drive mode. However, in this mode, range actuator  48  is signaled to locate range sleeve  88  in its low-range position to establish the low-range drive connection between input shaft  46  and rear output shaft  32 . 
   When the mode signal indicates selection of the on-demand four-wheel high-range drive mode, range actuator  48  moves or maintains range sleeve  88  in its high-range position and magnetorheological clutch actuator  120  is placed in a ready or “stand-by” condition. In particular, the amount of drive torque sent to front output shaft  42  through transfer clutch  50  with clutch actuator in its stand-by condition can be zero or a slight amount (i.e., in the range of 2–10%) as required for the specific vehicular application. This minimum stand-by torque transfer is generated by controller  58  sending a control signal to coil  152  having a predetermined minimum value. Thereafter, controller  58  determines when and how much drive torque needs to be transferred to front output shaft  42  based on tractive conditions and/or vehicle operating characteristics, as detected by vehicle sensors  54 . For example,  FIG. 2  illustrates a first speed sensor  180  which sends a signal to controller  58  indicative of the rotary speed of rear output shaft  32  while a second speed sensor  182  sends a signal indicative of the rotary speed of front output shaft  42 . Controller  58  can vary the value of the electric control signal sent to coil  152  between the predetermined minimum value and the predetermined maximum value based on defined relationships or detected characteristics such as, for example, the speed difference (ΔRPM) between shafts  32  and  42 , vehicle acceleration, a braking condition and the steering angle. 
   Referring now to  FIG. 4 , an alternative version of a torque transfer mechanism for use in transfer case  22  is shown. In particular, this torque transfer mechanism includes multi-plate clutch assembly  124  which is now associated with a different magnetorheological clutch actuator  190 . For purposes of clarity and brevity, similar components are identified with common reference numerals throughout the drawings. Magnetorheological clutch actuator  190  is shown to generally include a brake operator  192  and an electromagnetic coil  152 . Brake operator  192  includes a first thrust cam  194  fixed for rotation with second pressure plate  146  and having a surface defining a series of first cams  196  that are engaged or interdigitated with a series of second cams  198  formed on a surface of a second thrust cam  200 . Second thrust cam  200  has an annular brake rotor  202  extending coaxially from its disk segment  204  and which is disposed for rotation in an annular cavity  206  formed in housing  60 . A bearing assembly  208  rotatably supports rotor  202  from housing  60 . Rotor  202  is formed with a circumferential recess in its outer surface that defines a fluid chamber  210  with an inner wall surface of housing  60  within cavity  206 . A pair of laterally-spaced seal rings  212  are provided to seal chamber  210  which is filled with MR fluid  161 . Coil  152  is secured in housing  60  and is located in proximity to chamber  210 . As previously detailed, activation of coil  152  results in a magnetic field being established in MR fluid  151 . 
   The profile of cams  196  and  198  are such that second thrust cam  200  rotates with first thrust cam  194  when MR fluid  161  has a low viscosity. Once an operating condition is detected that warrants actuation of the torque transfer mechanism, controller  58  sends an appropriate control signal to coil  152 . This results in an increase in the shear strength of MR fluid  161  acting between housing  60  and rotor  202  of second thrust cam  200  which, in turn, exerts a reaction or brake torque on rotor  202 . This braking of rotor  202  relative to housing  60  causes second thrust cam  200  to rotate relative to first thrust cam  194 . As such, an axial separation force is applied to first thrust cam  194  which is proportional to the cam angle between cams  196  and  198  and the magnitude of the reaction torque exerted on rotor  202 . This axial force then acts to cause second pressure plate  146  to exert a corresponding clutch engagement force on clutch pack  132 , in opposition to the biasing force exerted thereon by return springs  148 . 
   As previously disclosed, controller  58  cam vary the value of the electric control signal sent to electromagnetic coil  152  between predetermined minimum and maximum values based on defined relationships (ΔRPM), vehicle operating characteristics and/or the mode signal from mode selector  56  so as to establish any one of the available drive modes. 
   While both of the torque transfer mechanisms have been shown arranged on front output shaft  42 , it is evident that they could easily be installed on rear output shaft  32  for selectively transferring drive torque to a transfer assembly coupled to drive front output shaft  42 . Furthermore, the present invention can be used as a torque transfer coupling in an all-wheel drive (AWD) vehicle to selectively and/or automatically transfer drive torque on-demand from the primary (i.e., front) driveline to the secondary (i.e., rear) driveline. Likewise, in full-time transfer cases equipped with an interaxle differential, transfer clutch  50  could be used to limit slip and bias torque across the differential. 
   Referring now to  FIG. 5 , a torque transfer mechanism, hereinafter referred to as torque coupling  220 , is shown to include a multi-plate clutch assembly  222  operably installed between an input member  224  and an output member  226 , and a magnetorheological clutch actuator  228 . Clutch assembly  222  includes a set of inner clutch plates  230  fixed via a spline connection  232  for rotation with input member  224 , a clutch drum  234  fixed to output member  226 , and a set of outer clutch plates  236  fixed via a spline connection  238  to clutch drum  234 . As seen, outer clutch plates  236  are alternatively interleaved with inner clutch plates  230  to define a clutch pack. Drum  234  has a radial plate segment  240  which functions as a reaction plate against which the interleaved clutch plates can be frictionally engaged. A bearing assembly  242  is shown supporting drum  234  for rotation relative to input member  224 . 
   Clutch actuator  228  is similar to clutch actuator  120  shown in  FIG. 3  and includes a thrust cam operator  150  and electromagnetic coil  152 . However, in this arrangement, drive ring  154  is fixed via a splined connection  244  to input member  224 . Again, common reference numerals are used identified components of clutch actuator  228  that are similar to corresponding components of clutch actuator  120 . 
   With reference to  FIG. 6 , an alternative torque transfer mechanism, hereinafter referred to as torque coupling  250 , is shown to include a multi-plate clutch assembly  252  operably installed between a first rotary member  254  and a second rotary member  256 . Clutch assembly  252  includes a clutch drum  258  fixed for rotation with first rotary member  254 , a hub  260  associated with second rotary member  256  and a clutch pack  262 . Clutch pack  262  includes inner clutch plates  264  splined to hub  260  and which are interleaved with outer clutch plates  266  splined to drum  258 . Torque coupling  250  also includes a magnetorheological clutch actuator  270  that is similar to clutch actuator  190  shown in  FIG. 4 . Clutch actuator  270  includes brake operator  192  and electromagnetic coil  152 . Operation of MR clutch actuator  270  is substantially similar to that of clutch actuator  190  in that activation of coil  152  brakes rotation of rotor  202  for causing axial movement of first thrust cam  194  for exerting a clutch engagement force on clutch pack  262 . It is contemplated that torque couplings  220  and  250  could be readily used in various driveline applications including, without limitation, as the on-demand transfer clutch or the full-time bias clutch in 4WD transfer units, as an in-line coupling or power take-off unit, or as a limited slip coupling in drive axles and AWD systems. 
   To illustrate an alternative power transmission device to which the present invention is applicable,  FIG. 7  schematically depicts a front-wheel based four-wheel drivetrain layout  10 ′ for a motor vehicle. In particular, engine  18  drives a multi-speed transmission  20 ′ having an integrated front differential unit  38 ′ for driving front wheels  34  via axle shafts  33 . A transfer unit  35  is also driven by transmission  20 ′ for delivering drive torque to the input member of an in-line torque transfer coupling  300  via a drive shaft  30 ′. In particular, the input member of transfer coupling  300  is coupled to drive shaft  30 ′ while its output member is coupled to a drive component of rear differential  28 . Accordingly, when sensors indicate the occurrence of a front wheel slip condition, controller  58  adaptively controls actuation of torque coupling  300  such that drive torque is delivered “on-demand” to rear wheels  24 . It is contemplated that torque transfer coupling  300  would include a multi-plate transfer clutch and a magnetorheological clutch actuator that are generally similar in structure and function to that of any of the devices previously described herein. While shown in association with rear differential  28 , it is contemplated that torque coupling  300  could also be operably located for transferring drive torque from transfer unit  35  to drive shaft  30 ′. 
   Referring now to  FIG. 8 , torque coupling  300  is schematically illustrated in association with an on-demand four-wheel drive system based on a front-wheel drive vehicle similar to that shown in  FIG. 7 . In particular, an output shaft  302  of transaxle  20 ′ is shown to drive an output gear  304  which, in turn, drives an input gear  306  fixed to a carrier  308  associated with front differential unit  38 ′. To provide drive torque to front wheels  34 , front differential unit  38 ′ includes a pair of side gears  310  that are connected to front wheels  34  via axleshafts  33 . Differential unit  38 ′ also includes pinions  312  that are rotatably supported on pinion shafts fixed to carrier  308  and which are meshed with side gears  310 . A transfer shaft  314  is provided to transfer drive torque from carrier  308  to a clutch hub  316  associated with a multi-pate clutch assembly  318 . Clutch assembly  318  further includes a drum  320  and a clutch pack  322  having interleaved clutch plates operably connected between hub  316  and drum  320 . 
   Transfer unit  35  is a right-angled drive mechanism including a ring gear  324  fixed for rotation with drum  320  of clutch assembly  318  which is meshed with a pinion gear  326  fixed for rotation with drive shaft  30 ′. As seen, a magnetorheological clutch actuator  328  is schematically illustrated for controlling actuation of clutch assembly  318 . According to the present invention, magnetorheological actuator  328  can be similar to any one of the various magnetorheological clutch actuators previously described in that an electromagnetic coil is supplied with electric current for changing the viscosity of a magnetorheological fluid which, in turn, functions to control translational movement of a thrust cam for engaging clutch pack  322 . In operation, drive torque is transferred from the primary (i.e., front) driveline to the secondary (i.e., rear) driveline in accordance with the particular mode selected by the vehicle operator via mode selector  56 . For example, if the on-demand 4WD mode is selected, controller  58  modulates actuation of magnetorheological clutch actuator  328  in response to the vehicle operating conditions detected by sensors  54  by varying the value of the electric control signal sent to the electromagnetic coil. In this manner, the level of clutch engagement and the amount of drive torque that is transferred through clutch pack  322  to the rear driveline through transfer unit  35  and drive shaft  30 ′ is adaptively controlled. Selection of a locked or part-time 4WD mode results in full engagement of clutch assembly  318  for rigidly coupling the front driveline to the rear driveline. In some applications, the mode selector  56  may be eliminated such that only the on-demand 4WD mode is available so as to continuously provide adaptive traction control without input from the vehicle operator. 
     FIG. 9  illustrates a modified version of  FIG. 8  wherein an on-demand four-wheel drive system is shown based on a rear-wheel drive motor vehicle that is arranged to normally deliver drive torque to rear wheels  24  while selectively transmitting drive torque to front wheels  34  through a torque coupling  300 A. In this arrangement, drive torque is transmitted directly from transmission output shaft  302  to transfer unit  35  via a drive shaft  330  interconnecting input gear  306  to ring gear  324 . To provide drive torque to front wheels  34 , torque coupling  300 A is now shown operably disposed between drive shaft  330  and transfer shaft  314 . In particular, clutch assembly  318  is arranged such that drum  320  is driven with ring gear  324  by drive shaft  330 . As such, actuation of magnetorheological clutch actuator  328  functions to transfer torque from drum  320  through clutch pack  322  to hub  316  which, in turn, drives carrier  308  of front differential unit  38 ′ via transfer shaft  314 . Again, the vehicle could be equipped with mode selector  56  to permit selection by the vehicle operator of either the adaptively controlled on-demand 4WD mode or the locked part-time 4WD mode. In vehicles without mode selector  56 , the on-demand 4WD mode is the only mode available and which provides continuous adaptive traction control with input from the vehicle operator. 
   In addition to the on-demand 4WD systems shown previously, the power transmission (magnetorheological clutch actuator and clutch assembly) technology of the present invention can likewise be used in full-time 4WD systems to adaptively bias the torque distribution transmitted by a center or “interaxle” differential unit to the front and rear drivelines. For example,  FIG. 10  schematically illustrates a full-time four-wheel drive system which is generally similar to the on-demand four-wheel drive system shown in  FIG. 9  with the exception that an interaxle differential unit  340  is now operably installed between carrier  308  of front differential unit  38 ′ and transfer shaft  314 . In particular, output gear  306  is fixed for rotation with a carrier  342  of interaxle differential  340  from which pinion gears  344  are rotatably supported. A first side gear  346  is meshed with pinion gears  344  and is fixed for rotation with drive shaft  330  so as to be drivingly interconnected to the rear driveline through transfer unit  35 . Likewise, a second side gear  348  is meshed with pinion gears  344  and is fixed for rotation with carrier  308  of front differential unit  38 ′ so as to be drivingly interconnected to the front driveline. Torque transfer mechanism  300 B is shown operably installed between side gears  346  and  348 . In operation, when sensor  54  detects a vehicle operating condition, such as excessive interaxle slip, controller  58  adaptively controls activation of the electromagnetic coil associated with magnetorheological clutch actuator  328  for controlling engagement of clutch assembly  318 , thereby adaptively controlling the torque biasing between the front and rear drivelines. 
   Referring now to  FIG. 11 , a full-time 4WD system is shown to include a transfer case  22 ′ equipped with an interaxle differential  350  between an input shaft  46 ′ and output shafts  32 ′ and  42 ′. Differential  350  includes an input defined as a planet carrier  352 , a first output defined as a first sun gear  354 , a second output defined as a second sun gear  356 , and a gearset for permitting speed differentiation between first and second sun gears  354  and  356 . The gearset includes meshed pairs of first planet gears  358  and second pinions  360  which are rotatably supported by carrier  352 . First planet gears  358  are shown to mesh with first sun gear  354  while second planet gears  350  are meshed with second sun gear  356 . First sun gear  354  is fixed for rotation with rear output shaft  32 ′ so as to transmit drive torque to rear driveline  12 . To transmit drive torque to front driveline  14 , second sun gear  356  is coupled to a transfer assembly  110 ′ which includes a first sprocket  112 ′ rotatably supported on rear output shaft  32 ′, a second sprocket  114 ′ fixed to front output shaft  42 ′, and a power chain  118 ′. 
   Transfer case  22 ′ further includes a biasing clutch  50 ′ having a multi-plate clutch assembly  124 ′ and a mode actuator  52 ′ having a magnetorheological clutch actuator  120 ′. Clutch assembly  124 ′ includes a drum  126 ′ fixed for rotation with first sprocket  112 ′, a hub  128 ′ fixed for rotation with rear output shaft  32 ′, and a multi-plate clutch pack  132 ′ operably disposed therebetween. Magnetorheological clutch actuator  120 ′ includes an electromagnetic coil that can be energized for controlling the viscosity of the magnetorheological fluid for controlling movement of a screw cam relative to clutch pack  132 ′. 
   A number of preferred embodiments have been disclosed to provide those skilled in the art an understanding of the best mode currently contemplated for the operation and construction of the present invention. The invention being thus described, it will be obvious that various modifications can be made without departing from the true spirit and scope of the invention, and all such modifications as would be considered by those skilled in the art are intended to be included within the scope of the following claims.