Abstract:
A clutch actuator for controlling engagement of a friction clutch and having a first actuator plate rotatable about an axis, a second actuator plate adjacent to the first actuator plate, and a ballramp unit disposed between the first and second actuator plates. A piston assembly acts to induce rotation of the first actuator plate relative to the second actuator plate. Relative rotation between the first actuator plate and the second actuator plate induces linear movement of one of the first and second actuator plates along the axis to regulate engagement of the friction clutch.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. patent application Ser. No. 10/968,763 filed on Oct. 19, 2004. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to power transfer systems and, more particularly, to torque transfer mechanisms having a clutch actuator for actuating a clutch assembly in a power transfer system.  
       BACKGROUND OF THE INVENTION  
       [0003]     Power transfer systems of the type used in motor vehicles including, but not limited to, four-wheel drive transfer cases, all-wheel drive power take-off units (PTU), limited slip drive axles and torque vectoring drive modules are commonly equipped with a torque transfer mechanism. In general, the torque transfer mechanism functions to regulate the transfer of drive torque between a rotary input component and a rotary output component. More specifically, a multi-plate friction clutch is typically disposed between the rotary input and output components and its engagement is varied to regulate the amount of drive torque transferred therebetween.  
         [0004]     Engagement of the friction clutch is varied by adaptively controlling the magnitude of a clutch engagement force that is applied to the multi-plate friction clutch via a clutch actuator system. Many traditional clutch actuator systems include a power-operated drive mechanism and an operator mechanism. The operator mechanism typically converts the force or torque generated by the power-operated drive mechanism into the clutch engagement force which, in turn, can be further amplified prior to being applied to the friction clutch. Actuation of the power-operated drive mechanism is controlled based on control signals generated by a control system.  
         [0005]     Currently, a large number of the torque transfer mechanisms used in motor vehicle driveline applications are equipped with an electrically-controlled clutch actuator that can regulate the drive torque transferred as a function of the value of the electric control signal applied thereto. In some applications, an electromagnetic device is employed as the power-operated drive mechanism of the clutch actuator. For example, U.S. Pat. No. 5,407,024 discloses use of an electromagnetic coil that is incrementally activated to control movement of a ballramp operator mechanism for applying the clutch engagement force to the friction clutch. Likewise, Japanese Laid-Open Patent Application No. 62-18117 discloses an electromagnetic actuator arranged to directly control actuation of the friction clutch.  
         [0006]     As an alternative, the torque transfer mechanism can employ an electric motor as the power-operated drive mechanism of the clutch actuator. For example, U.S. Pat. No. 5,323,871 discloses a clutch actuator having an electric motor that controls angular movement of a sector cam which, in turn, controls pivoted movement of a lever arm used to apply the clutch engagement force on the friction clutch. Likewise, Japanese Laid-Open Publication No. 63-66927 discloses a clutch actuator which uses an electric motor to rotate one cam plate of a ballramp operator mechanism for engaging the friction clutch. Finally, U.S. Pat. Nos. 4,895,236 and 5,423,235, respectively, disclose a clutch actuator with an electric motor driving a reduction gearset for controlling movement of a ballscrew operator mechanism and a ballramp operator mechanism. Finally, commonly owned U.S. Pat. No. 6,595,338 discloses an electrohydraulic clutch actuator for controlling engagement of a friction clutch.  
       SUMMARY OF THE INVENTION  
       [0007]     Accordingly, the present invention is directed toward a clutch actuator that is operable to adaptively regulate engagement of a friction clutch assembly. The clutch actuator includes a power-operated drive mechanism and an operator mechanism. The operator mechanism generally includes a first actuator plate, a second actuator plate, a ballramp unit operably disposed between the first and second actuator plates, and a linear operator for controlling relative angular movement between the first and second actuator plates. Such angular movement causes the ballramp unit to move one of the first and second actuator plates axially for generating a clutch engagement force that is applied to the friction clutch assembly.  
         [0008]     Pursuant to a preferred construction, the ballramp unit is integrated into the first and second actuator plates to provide a compact operator mechanism. In addition, the linear operator is disposed between first and second arm segments provided on the corresponding first and second actuator plates. The linear operator may be a dual piston assembly having first and second pistons disposed in a common pressure chamber. The first piston has a first roller engaging a first cam surface formed on the first arm segment of the first actuator plate while the second piston has a second roller engaging a second cam surface formed on the second arm segment of the second actuator plate.  
         [0009]     In accordance with another feature, the operator mechanism associated with the clutch actuator of the present invention further includes an apply plate that is disposed adjacent to the second actuator plate and which is axially moveable therewith to apply the clutch engagement force to the friction clutch assembly. In yet another feature, the operator mechanism of the clutch actuator further includes a stop plate that is disposed adjacent to the first actuator plate and which inhibits axial movement of the first actuator plate.  
         [0010]     The drive mechanism associated with the clutch actuator of the present invention is operable to control the fluid pressure within the pressure chamber, thereby controlling the position of the first and second pistons and the relative angular position of the first actuator plate relative to the second actuator plate. The drive mechanism includes an electric motor, a ballscrew unit, a gearset interconnecting a rotary output of the motor to a rotary component of the ballscrew unit, and a control piston disposed in a control chamber. The control piston is fixed to an axially moveable component of the ballscrew unit while a fluid delivery system provides fluid communication between the control chamber and the pressure chamber. In operation, the location of the axially moveable ballscrew component within the control chamber controls the fluid pressure within the pressure chamber.  
         [0011]     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  
       [0012]     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:  
         [0013]      FIG. 1  illustrates an exemplary drivetrain in a four-wheel drive vehicle equipped with a power transfer system;  
         [0014]      FIG. 2  is a sectional view of a torque transfer mechanism having a friction clutch assembly and a clutch actuator according to the present invention integrated in the power transfer system;  
         [0015]      FIG. 3  is another view of the clutch actuator of the present invention;  
         [0016]      FIG. 4  illustrates an alternative version of the clutch actuator shown in  FIG. 3 ;  
         [0017]      FIG. 5  is a schematic illustration of the torque transfer mechanism of the present invention arranged to provide drive torque to an axle assembly of a motor vehicle;  
         [0018]      FIG. 6  is a schematic illustration of the torque transfer mechanism of the present invention arranged as a slip limiting and torque biasing differential in an axle assembly;  
         [0019]      FIG. 7  is a schematic illustration of a pair of torque transfer mechanisms arranged as a torque vectoring axle assembly for a motor vehicle;  
         [0020]      FIG. 8  illustrates another exemplary drivetrain equipped with a power transfer device to which the torque transfer mechanism of the present invention is applicable;  
         [0021]      FIGS. 9 through 12  are schematic illustrations of various power transfer devices adapted for use with the drivetrain of  FIG. 8 ;  
         [0022]      FIG. 13  illustrates yet another exemplary drivetrain for a four-wheel drive vehicle; and  
         [0023]      FIGS. 14 and 15  illustrate transfer cases equipped with the torque transfer mechanisms of the present invention and which are adapted for use with the drivetrain of  FIG. 13 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     The present invention is directed to a torque transfer mechanism that can be adaptively controlled for modulating the torque transferred between a first rotary member and a second rotary member. The torque transfer mechanism finds particular application in power transfer systems of the type used in motor vehicle drivelines and which include, for example, transfer cases, power take-off units, limited slip drive axles and torque vectoring drive modules. Thus, while the present invention is hereinafter described in association with one or more particular arrangements for specific driveline applications, it will be understood that the arrangements shown and described are merely intended to illustrate embodiments of the present invention.  
         [0025]     With particular reference to  FIG. 1 , a schematic layout of a vehicle drivetrain  10  is shown to include a powertrain  12 , a first or primary driveline  14  driven by powertrain  12 , and a second or secondary driveline  16 . Powertrain  12  includes an engine  18  and a multi-speed transaxle  20  arranged to normally provide motive power (i.e., drive torque) to a pair of first wheels  22  associated with primary driveline  14 . Primary driveline  14  further includes a pair of axle shafts  24  connecting wheels  22  to a front differential unit  25  associated with transaxle  20 .  
         [0026]     Secondary driveline  16  includes a power take-off unit (PTU)  26  driven by the output of transaxle  20 , a propshaft  28  driven by PTU  26 , a pair of axle shafts  30  connected to a pair of second wheels  32 , a rear differential unit  34  driving axle shafts  30 , and a power transfer device  36  that is operable to selectively transfer drive torque from propshaft  28  to rear differential unit  34 . Power transfer device  36  is shown integrated into a drive axle assembly and includes a torque transfer mechanism  38 . Torque transfer mechanism  38  functions to selectively transfer drive torque from propshaft  28  to differential unit  34 . More specifically, torque transfer mechanism  38  includes an input shaft  42  driven by propshaft  28  and a pinion shaft  44  that drives differential unit  34 .  
         [0027]     Vehicle drivetrain  10  further includes a control system for regulating actuation of torque transfer mechanism  38 . The control system includes a clutch actuator  50 , vehicle sensors  52 , a mode select mechanism  54  and an electronic control unit (ECU)  56 . Vehicle sensors  52  are provided to detect specific dynamic and operational characteristics of drivetrain  10  while mode select mechanism  54  enables the vehicle operator to select one of a plurality of available drive modes. The drive modes may include a two-wheel drive mode, a locked (“part-time”) four-wheel drive mode, and an adaptive (“on-demand”) four-wheel drive mode. In this regard, torque transfer mechanism  38  can be selectively engaged for transferring drive torque from input shaft  42  to pinion shaft  44  to establish both of the part-time and on-demand four-wheel drive modes. ECU  56  controls actuation of clutch actuator  50  which, in turn, controls the drive torque transferred through torque transfer mechanism  38 .  
         [0028]     Referring now to  FIGS. 2 and 3 , a cross-section of torque transfer mechanism  38  is shown. Torque transfer mechanism  38  generally includes a friction clutch assembly  60  having a multi-plate clutch pack  62 . Clutch actuator  50  is operable to generate and apply a clutch engagement force on clutch pack  62  so as to regulate engagement and thus, the amount of drive torque transfer through clutch pack  62 . Friction clutch assembly  60  also includes a clutch hub  64  and a drum  66 . Hub  64  is adapted to be coupled for rotation with input shaft  42  while drum  66  is adapted to be coupled for rotation with pinion shaft  44 . As seen, a set of first or inner clutch plates  68  associated with clutch pack  62  are fixed for rotation with hub  64 . Likewise, a set of second clutch plates  70  are interleaved with first clutch plates  68  and are fixed for rotation with drum  66 .  
         [0029]     The degree of engagement of clutch pack  62 , and therefore the amount of drive torque transferred therethrough, is largely based on the frictional interaction of clutch plates  68  and  70 . More specifically, with friction clutch assembly  60  in a disengaged state, interleaved clutch plates  68  and  70  slip relative to one another and little or no torque is transferred through clutch pack  62 . However, when friction clutch assembly  60  is in a fully engaged state, there is no relative slip between clutch plates  68  and  70  and 100% of the drive torque is transferred from input shaft  42  to pinion shaft  44 . In a partially engaged state, the degree of relative slip between interleaved clutch plates  68  and  70  varies and a corresponding amount of drive torque is transferred through clutch pack  62 .  
         [0030]     In general, clutch actuator  50  includes an operator mechanism  72  and a power-operated drive mechanism  73 . Operator mechanism  72  is shown to include a first actuator plate  74 , a second actuator plate  76 , a stop plate  78 , an apply plate  80 , a ballramp unit  82 , and a piston assembly  84 . First and second actuator plates  74  and  76  are rotatably supported on hub  64  by a bearing assembly  86  and include corresponding arm segments  74 A and  76 A, respectively, that extend tangentially. More specifically, arms  74 A and  76 A include respective edges  87  and  89  that are generally parallel to the axis A.  
         [0031]     First and second actuator plates  74  and  76  also include first and second ballramp groove sets  90  and  92 , respectively. Balls  94  are disposed between first and second actuator plates  74  and  76  and ride within ballramp groove sets  90  and  92 . As best seen from  FIG. 3 , each set has three equally spaced grooves aligned circumferentially relative to the “A” axis. Thus, ballramp unit  82  is shown to be integrated into actuator plates  74  and  76  so as to provide a compact arrangement. Stop plate  78  is supported on hub  64  and is inhibited from axial movement by a lock ring  96 . More specifically, stop plate  78  is disposed between lock ring  96  and first actuator plate  74  and is separated from first actuator plate  74  by a thrust bearing assembly  98 . Apply plate  80  is disposed between clutch pack  62  and second actuator plate  76  and is separated from second actuator plate  76  by another thrust bearing assembly  100 . Apply plate  80  is adapted to move axially to regulate engagement of clutch pack  62 , as is explained in further detail below.  
         [0032]     Piston assembly  84  is actuated by drive mechanism  73  to control relative rotation between first and second actuator plates  74  and  76 . More specifically, piston assembly  84  includes a first piston  104  and a second piston  106  that are disposed for sliding movement within a pressure chamber  108  formed in a cylinder housing  110 . As seen, first and second pistons  104  and  106  have first and second rollers  112  and  114 , respectively, attached thereto. First and second rollers  112  and  114  engage corresponding first and second cam surfaces  116  and  118  formed on first and second arms  74 A and  76 A, respectively. First and second rollers  112  and  114  are induced to ride against first and second cam surfaces  116  and  118  in response to movement of pistons  104  and  106  caused by actuation of drive mechanism  73 . Specifically, rolling movement of first and second rollers  112  and  114  against first and second cam surfaces  116  and  118  results in relative rotation between first and second actuator plates  74  and  76 . Pistons  104  and  106  are shown in  FIG. 3  in a first or “retracted” position within pressure chamber  108  such that first and second actuator plates  74  and  76  are located in a corresponding first angular position relative to each other. A return spring  120  is provided for normally biasing first and second actuator plates  74  and  76  toward this first angular position. With the actuator plates located in their first angular position, ballramp unit  82  functions to axially locate second actuator plate  76  in a corresponding first or “released” position whereat apply plate  80  is released from engagement with clutch pack  62 . In this position, a minimum clutch engagement force is applied to clutch pack  62  such that little or no drive torque is transmitted from input shaft  42  to pinion shaft  44 .  
         [0033]     As will be detailed, drive mechanism  73  is operable to cause pistons  104  and  106  to move toward a second or “expanded” position within pressure chamber  108  such that actuator plates  74  and  76  are caused by engagement with rollers  112  and  114  to circumferentially index to a second angular position. Such rotary indexing of actuator plates  74  and  76  causes ballramp unit  82  to axially displace second actuator plate  76  from its released position toward a second or “locked” position whereat apply plate  80  is fully engaged with clutch pack  62 . With second actuator plate  76  in its locked position, a maximum clutch engagement force is applied to clutch pack  62  such that pinion shaft  44  is, in effect, coupled for common rotation with input shaft  42 .  
         [0034]     Drive mechanism  73  is shown in  FIG. 3  to include a piston housing  122 , a ballscrew and piston assembly  124 , a gearset  126 , and an electric motor  128 . Electric motor  128  rotatably drives gearset  126  which, in turn, rotatably drives a leadscrew  130  associated with piston assembly  124 . Such rotation of leadscrew  130  results in axial movement of a nut  131  mounted thereon which, in turn, causes corresponding axial movement of a piston plunger  132  within a fluid control chamber  134  formed in housing  122 . Control chamber  134  is in fluid communication with pressure chamber  108  via a closed hydraulic control system. Specifically, as piston plunger  132  translates along an axis “B”, it regulates the volume of fluid in control chamber  134 . As the volume of control chamber  134  decreases, fluid is supplied through a conduit  136  to pressure chamber  108  in piston assembly  84 , thereby causing pistons  104  and  106  to move in concert toward their expanded position. In contrast, as the volume of control chamber  134  increases, the fluid flows back through conduit  136  from piston chamber  108  to relieve the pressure exerted by first and second rollers  112  and  114  against first and second cam surfaces  116  and  118 .  
         [0035]     Accordingly, rotation of leadscrew  130  in a first rotary direction results in axial movement of piston plunger  132  in a first direction (right in  FIG. 3 ), thereby causing pistons  104  and  106  to be forcibly moved toward their expanded position for angularly indexing first and second actuator plates  74  and  76  toward their second angular position in opposition to the biasing force exerted thereon by return spring  120 . In contrast, rotation of leadscrew  130  in a second rotary direction results in axial movement of piston plunger  132  in a second direction (left in  FIG. 3 ), thereby permitting the biasing force of return spring  120  to forcibly rotate actuator plates  74  and  76  toward their first angular position which, in turn, causes pistons  104  and  106  to move back toward their retracted position. A pressure sensor  140  is responsive to the pressure within conduit  136  and generates a signal that is sent to ECU  56 . Preferably, ECU  56  is functional to correlate line pressure readings from pressure sensor  140  to the torque output of friction clutch assembly  60 .  
         [0036]     In its neutral, clutch actuator  50  imparts no clutch engagement force on clutch pack  62  such that first and second clutch plates  68  and  70  are permitted to slip relative to one another. As first and second actuator plates  74  and  76  are caused to rotate relative to one another, balls  94  ride within ballramp grooves  90  and  92  to axially move second actuator plate  76 . Since stop plate  78  inhibits axial movement of first actuator plate  74 , as balls  94  ride up ballramp grooves  90  and  92 , second actuator plate  76  is separated from first actuator plate  74  and moves linearly to impart the clutch engagement force on apply plate  80  through thrust bearing assembly  100 . Apply plate  80 , in turn, imparts this linear clutch engagement force on clutch pack  62 , thereby regulating engagement of clutch pack  62 .  
         [0037]     With second actuator plate  76  in its released position, virtually no drive torque is transferred from input shaft  42  to pinion shaft  44  through friction clutch  60 , thereby effectively establishing the two-wheel drive mode. In contrast, axial movement of second actuator plate  76  to its locked position causes a maximum amount of drive torque to be transferred through friction clutch  60  to pinion shaft  44  for, in effect, coupling pinion shaft  44  for common rotation with rear prop shaft  28 , thereby establishing the part-time four-wheel drive mode. Accordingly, controlling the position of second actuator plate  76  between its released and locked positions permits variable control of the amount of drive torque transferred from rear prop shaft  28  to pinion shaft  44 , thereby establishing the on-demand four-wheel drive mode. Thus, the control signal supplied to electric motor  128  controls the angular position of actuator plates  74  and  76  for controlling axial movement of apply plate  80  relative to clutch pack  62 .  
         [0038]     ECU  56  sends electrical control signals to electric motor  128  for accurately controlling the position of control piston  132  within control chamber  134  by utilizing a predefined control strategy that is based on the mode signal from mode selector  54  and the sensor input signals from vehicle sensors  52 . In operation, if the two-wheel drive mode is selected, motor  156  drives leadscrew  130  in its second direction for moving control piston  132  so as to reduce the fluid pressure within pressure chamber  108 . As such, return spring  120  forcibly biases actuator plates  74  and  76  toward their first angular position until second actuator plate  76  is axially moved to its released position. In contrast, upon selection of the part-time four-wheel drive mode, motor  128  drives leadscrew  130  in its first rotary direction for increasing the fluid pressure in pressure chamber  108  until pistons  104  and  106  are located in their expanded position. As noted, such movement causes actuation plates  74  and  76  to rotate to their second angular position such that second actuator plate  76  is axially moved to its locked position for fully engaging friction clutch  60 .  
         [0039]     When mode selector  54  indicates selection of the on-demand four-wheel drive mode, ECU  56  energizes motor  128  for initially rotating leadscrew  130  until second actuator plate  76  is located in an intermediate or “ready” position. Accordingly, a predetermined minimum amount of drive torque is delivered to pinion shaft  44  through friction clutch  60  in this adapt-ready condition. Thereafter, ECU  56  determines when and how much drive torque needs to be transferred to pinion shaft  44  based on the current tractive conditions and/or operating characteristics of the motor vehicle, as detected by sensors  52 . Sensors  52  detect such parameters as, for example, the rotary speed of the shafts, the vehicle speed and/or acceleration, the transmission gear, the on/off status of the brakes, the steering angle, the road conditions, etc. Such sensor signals are used by ECU  56  to determine a desired output torque value utilizing a control scheme that is incorporated into ECU  56 . This desired torque value is used to actively control actuation of electric motor.  
         [0040]     In addition to adaptive torque control, the present invention permits release of friction clutch  60  in the event of an ABS braking condition or during the occurrence of an over-temperature condition. Furthermore, while the control scheme was described based on an on-demand strategy, it is contemplated that a differential or “mimic” control strategy could likewise be used. Specifically, the torque distribution between prop shaft  28  and pinion shaft  44  can be controlled to maintain a predetermined rear/front ratio (i.e., 70:30, 50:50, etc.) so as to simulate the inter-axle torque splitting feature typically provided by a mechanical center differential unit. Regardless of the control strategy used, accurate control of clutch actuator  50  will result in the desired torque transfer characteristics across friction clutch  60 . Furthermore, it should be understood that mode select mechanism  54  could also be arranged to permit selection of only two different drive modes, namely the on-demand 4WD mode and the part-time 4WD mode. Alternatively, mode select mechanism  54  could be eliminated such that the on-demand 4WD mode is always operating in a manner that is transparent to the vehicle operator.  
         [0041]     Referring to  FIG. 4 , clutch actuator  50  is now shown to include a modified operator mechanism  72 ′ wherein first actuator plate  74  is held against angular movement such that only second actuator plate  76  is rotated relative to first actuator plate  74 . In this regard, anti-rotation members  150  and  152  are located on opposite sides of arm segment  74 A so as to prevent bidirectional rotation of first actuator plate  74 . In addition, grooves  90  on first actuator plate  74  have been removed to permit balls  94  to ride on a planar face cam surface on first actuator plate  74 . Also, piston assembly  84 ′ now only includes piston  106 ′ which is still retained for sliding movement within pressure chamber  108  such that roller  114  rides against cam surface  118  on arm segment  76 A of second actuator plate  76 . As before, drive mechanism  73  functions to control the position of piston  106 ′ so as to control the rotated position of second actuator plate  76  relative to first actuator plate  74 . In particular, piston  106 ′ is moveable between retracted and expanded positions to cause corresponding angular movement of second actuator plate between its first and second angular positions. When second actuator plate  76  is in its first angular position, ballramp unit  82 ′ causes second actuator plate  76  to also be axially located in its released position. In contrast, rotation of second actuator plate  76  to its second angular position causes ballramp unit  82 ′ to axially move second actuation plate  76  to its locked position.  
         [0042]     It is contemplated that alternative drive mechanisms can be used in place of the closed-circuit hydraulic system disclosed. For example, a motor-driven leadscrew could be implemented to drive one or both of first and second pistons  104  and  106  of operator mechanism  72  between their retracted and expanded positions. Likewise, it is to be understood that the particular drivetrain application shown is merely exemplary of but one application to which the clutch actuator of the present invention is well suited.  
         [0043]      FIG. 5  is provided to show incorporation of friction clutch  60  and clutch actuator  50  associated with torque transfer mechanism  38  in power transfer device  36 . As seen, pinion shaft  44  drives a pinion  160  that is meshed with a ring gear  162  fixed to a carrier  164  of differential unit  34 . Carrier  164  rotatably supports and drives a pair of pinion gears  166  that each mesh with a pair of side gears  168 . Each side gear  168  is fixed for rotation with a corresponding one of axleshafts  30 . The arrangement shown for the drive axle assembly of  FIG. 5  is operable to provide on-demand four-wheel drive by adaptively controlling the transfer of drive torque from the primary driveline to the secondary driveline. In contrast, a drive axle assembly  170  is shown in  FIG. 6  wherein a torque transfer mechanism, hereinafter referred to as torque coupling  38 A, is now operably installed between differential case  164  and one of axleshafts  30  to provide an adaptive “side-to-side” torque biasing and slip limiting feature. Torque coupling  38 A is schematically shown to again include friction clutch  60  and clutch actuator  50 , the construction and function of which are understood to be similar to the detailed description previously provided herein for each sub-assembly. As see, drum  66  is shown to be driven by carrier  164  while hub  64  is driven by one of axleshafts  30 .  
         [0044]     Referring now to  FIG. 7 , the power transfer device is shown as having a pair of torque couplings  38 L and  38 R that are operably installed between propshaft  28  or pinion shaft  44  and axleshafts  30 . The driven shaft drives a right-angled gearset including pinion  160  and ring gear  162  which, in turn, drives a transfer shaft  174 . First torque coupling  38 L is shown disposed between transfer shaft  174  and the left one of axleshafts  30 L while second torque coupling  38 R is disposed between transfer shaft  174  and the right axleshaft  30 R. Each torque coupling includes a corresponding friction clutch  60 L and  60 R and clutch actuator  50 L and  50 R. Accordingly, independent torque transfer and slip control is provided between the driven shaft and each of rear wheels  32 L and  32 R pursuant to this arrangement.  
         [0045]     To illustrate additional alternative power transfer systems to which the present invention is applicable,  FIG. 8  schematically depicts a front-wheel based four-wheel drive drivetrain layout  10 ′ for a motor vehicle. In particular, engine  18  drives multi-speed transaxle  20  which has front differential unit  25  for driving front wheels  22  via first axleshafts  24 . As before, PTU  26  is driven by transaxle  20 . However, in this arrangement, a power transfer device  176  functions to transfer drive torque to propshaft  28 . Power transfer device  176  includes a torque coupling  180  having an output member coupled to propshaft  28  which, in turn, drives rear wheels  32  via rear axleshafts  34 . The rear axle assembly can be a traditional driven axle with a differential or, in the alternative, be similar to the drive axle arrangements described in regard to FIGS.  6  or  7 . Accordingly, in response to detection of certain vehicle characteristics by sensors  52  (i.e., the occurrence of a front wheel slip condition), the power transfer system causes torque coupling  180  to deliver drive torque “on-demand” to rear wheels  32 . It is contemplated that torque coupling  180  would be generally similar in structure and function to that of torque transfer coupling  38  previously described herein  
         [0046]     Referring now to  FIG. 9 , torque coupling  180  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. 8 . In particular, an output shaft  182  of transaxle  20  is shown to drive an output gear  184  which, in turn, drives an input gear  186  that is fixed to a carrier  188  associated with front differential unit  25 . To provide drive torque to front wheels  22 , front differential unit  25  includes a pair of side gears  190  that are connected to front wheels  22  via axleshafts  24 . Differential unit  25  also includes pinions  192  that are rotatably supported on pinion shafts fixed to carrier  188  and which are meshed with side gears  190 . A transfer shaft  194  is provided for transferring drive torque from carrier  188  to a clutch hub  64  associated with friction clutch  60 . PTU  26  is a right-angled drive mechanism including a ring gear  196  fixed for rotation with drum  66  of friction clutch  60  and which is meshed with a pinion gear  198  fixed for rotation with propshaft  28 . According to the present invention, the components schematically shown for torque transfer coupling  180  are understood to be similar to those previously described. In particular, clutch actuator  50  includes a power-operated drive mechanism  73  that controls operation of an operator mechanism  72  or  72 ′ to adaptively control the clutch engagement force applied to clutch pack  62 . As such, drive torque is adaptively transferred on-demand from the primary (i.e., front) driveline to the secondary (i.e., rear) driveline.  
         [0047]     Referring to  FIG. 10 , a modified version of the power transfer device shown in  FIG. 9  is now shown to include a second torque coupling  180 A that is arranged to provide a limited slip feature in association with primary differential  25 . As before, adaptive control of torque coupling  180  provides on-demand transfer of drive torque from the primary driveline to the secondary driveline. In addition, adaptive control of second torque coupling  180  provides adaptive torque biasing (side-to-side) between axleshafts  24  of primary driveline  14 . As seen, components of torque coupling  180 A that are common to those of torque coupling  180  are identified with an “A” suffix.  
         [0048]      FIG. 11  illustrates another modified version of  FIG. 9  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  32  while selectively transmitting drive torque to front wheels  22  through torque coupling  180 . In this arrangement, drive torque is transmitted directly from transmission output shaft  182  to power transfer unit  26  via a drive shaft  200  which interconnects input gear  186  to ring gear  196 . To provide drive torque to front wheels  22 , torque coupling  180  is shown operably disposed between drive shaft  200  and transfer shaft  194 . In particular, friction clutch  60  is arranged such that drum  66  is driven with ring gear  196  by drive shaft  200 . As such, clutch actuator  50  functions to transfer drive torque from drum  66  through clutch pack  62  to hub  64  which, in turn, drives carrier  188  of differential unit  25  via transfer shaft  194 .  
         [0049]     In addition to the on-demand four-wheel drive systems shown previously, the power transmission technology of the present invention can likewise be used in full-time four-wheel drive systems to adaptively bias the torque distribution transmitted by a center or “interaxle” differential unit to the front and rear drivelines. For example,  FIG. 12  schematically illustrates a full-time four-wheel drive system which is generally similar to the on-demand four-wheel drive system shown in  FIG. 11  with the exception that an interaxle differential unit  210  is now operably installed between carrier  188  of front differential unit  25  and transfer shaft  194 . In particular, output gear  186  is fixed for rotation with a carrier  212  of interaxle differential  210  from which pinion gears  214  are rotatably supported. A first side gear  216  is meshed with pinion gears  214  and is fixed for rotation with drive shaft  200  so as to be drivingly interconnected to the rear driveline through power transfer unit  26 . Likewise, a second side gear  218  is meshed with pinion gears  214  and is fixed for rotation with carrier  188  of front differential unit  25  so as to be drivingly interconnected to the front driveline. Torque coupling  180  is now shown to be operably disposed between side gears  216  and  218 . Torque coupling  180  is operably arranged between the driven outputs of interaxle differential  210  for providing an adaptive torque biasing and slip limiting function between the front and rear drivelines.  
         [0050]     Referring now to  FIG. 13 , a drivetrain layout for a four-wheel drive vehicle is shown to include a power transfer device, hereinafter referred to as a transfer case  240 , arranged to transfer drive torque from engine  18  and transmission  20  to rear propshaft  28  and a front propshaft  242  that is arranged to drive front wheels  22  in via front differential  25  and axleshafts  24 . Transfer case  240  is shown to include a rear output shaft  244  coupled to rear propshaft  28  and a front output shaft  246  coupled to front propshaft  242 . From  FIG. 14 , transfer case  240  is further shown to include an input shaft  248  driven by transmission  20 , a transfer unit  250  driving front output shaft  246 , and a differential  252  interconnecting input shaft  248  to transfer unit  250  and rear output shaft  244 . Transfer unit  250  includes a first sprocket  254  rotatably supported on rear output shaft  244 , a second sprocket  256  fixed to front output shaft  246  and a power chain  258  therebetween. Differential  252  includes an input  260  driven by input shaft  248 , a front output  262  driving first sprocket  254 , a second output  264  driving rear output shaft  244 , and a speed differentiating gearset therebetween. As seen, torque coupling  180  is operably disposed between transfer unit  250  and rear output shaft  244  to control adaptive torque biasing therebetween.  FIG. 15  illustrates a modified version of transfer case  240  wherein differential  252  is removed such that input shaft  248  is directly coupled to rear output shaft  244  with friction clutch  60  arranged to permit on-demand transfer of drive torque from rear output shaft  244  to front output shaft  246 .  
         [0051]     Various 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.