Patent Publication Number: US-2007095628-A1

Title: Power-operated clutch actuator for torque transfer mechanisms

Description:
CROSS REFERENCE  
      This application claims the benefit of U.S. Provisional Application Ser. No. 60/731,524 filed Oct. 28, 2005, the entire disclosure of which is incorporated by reference. 
    
    
     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 and/or the left and right wheels of an axle assembly. More particularly, the present invention is directed to a power transmission device for use in motor vehicle driveline applications having a torque transfer mechanism equipped with a power-operated 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 developed for incorporation 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 which is operable 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.  
      A modern trend in four-wheel drive motor vehicles is to equip the power transmission device with a transfer clutch and an electronically-controlled traction control system. 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 may include a power-operated device that is actuated in response to electric control signals sent from an electronic controller unit (ECU). Variable control of the electric control signal is frequently based on changes in the 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.  
      A large number of on-demand power transmission devices have been developed which utilize an electrically-controlled clutch actuator for regulating the amount of drive torque transferred through the clutch assembly to the secondary driveline as a function 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 clutch actuator for directly controlling actuation of the multi-plate clutch pack assembly.  
      As an alternative, the transfer clutch may 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 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 and weight of the friction clutch components and the electrical power and actuation time requirements for the clutch actuator that are needed to provide the large clutch engagement loads may make such a system cost prohibitive in some motor vehicle applications. In an effort to address these concerns, new technologies are being considered for use in power-operated clutch actuator applications.  
     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 power-operated clutch actuator that is operable to control engagement of a multi-plate clutch assembly.  
      As a related object, the power transmission device 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 one preferred embodiment, the power transmission device is a transfer unit operable for use in a four-wheel drive motor vehicle having a powertrain and first and second drivelines. The transfer unit includes a first shaft driven by the powertrain, a second shaft adapted for connection to the second driveline and a torque transfer mechanism. The torque transfer mechanism includes a friction clutch assembly operably disposed between the first and second shafts and a clutch actuator assembly for generating and applying a clutch engagement force to the friction clutch assembly. The clutch actuator assembly includes an electric motor, a geared drive unit and a clutch apply operator. The geared drive unit includes a pinion gear having helical gear teeth meshed with helical gear teeth formed on a rotatable and axially moveable gear compound of the clutch apply operator. In operation, the electric motor drives the geared drive unit which, in turn, controls the direction and amount of rotation of a first cam member relative to a second cam member of a ballramp unit also associated with the clutch apply operator. The cam members support rollers which ride against tapered or ramped cam surfaces. The contour of the ramped cam surfaces cause the first cam member to move axially for causing corresponding translation of a thrust member. The thrust member applies the thrust force generated by the cam members as a clutch engagement force that is exerted on the friction clutch assembly. A control system including vehicle sensors and a controller are provided to control actuation of the electric motor.  
      In accordance with the present invention, the transfer unit can be configured as an in-line torque coupling for use in adaptively controlling the transfer of drive torque from the powertrain to the rear drive axle of an all-wheel drive vehicle. Pursuant to related embodiments, the transfer unit can be a transfer case for use in adaptively controlling the transfer of drive torque to the front driveline in an on-demand four-wheel drive vehicle or between the front and rear drivelines in a full-time four-wheel drive vehicle. 
    
    
     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 an all-wheel drive motor vehicle equipped with a power transmission device of the present invention;  
       FIG. 2  is a schematic illustration of the power transmission device shown in  FIG. 1  associated with a drive axle assembly;  
       FIG. 3  is a sectional view of a torque transfer mechanism associated with the power transmission device which is equipped with a friction clutch assembly and a clutch actuator assembly according to the present invention;  
       FIG. 4  is an enlarged partial view of the torque transfer mechanism taken from  FIG. 3 ;  
       FIG. 5  is a detailed view of the meshed interface between a pinion gear and a clutch apply operator gear associated with the clutch actuator assembly;  
       FIGS. 6 through 9  are schematic illustrations of alternative embodiments for the power transmission device of the present invention;  
       FIG. 10  illustrates the drivetrain of a four-wheel drive vehicle equipped with another version of the power transmission device of the present invention;  
       FIGS. 11 and 12  are schematic illustrations of transfer cases adapted for use with the drivetrain shown in  FIG. 10 ; and  
       FIG. 13  is a schematic view of a power transmission device equipped with a torque vectoring distribution mechanism 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 between a first rotary member and 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 transfer clutch installed in a transfer case or an in-line torque coupling or a biasing clutch of the type associated with a center differential in a transfer case or an intra-axle differential in a drive axle assembly. 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 arrangements 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 an all-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 front driveline while secondary driveline  14  is the rear driveline. Powertrain  16  is shown to include an engine  18  and a multi-speed transmission  20 . Front driveline  12  includes a front differential  22  driven by powertrain  16  for transmitting drive torque to a pair of front wheels  24 L and  24 R through a pair of front axleshafts  26 L and  26 R, respectively. Rear driveline  14  includes a power transfer unit  28  driven by powertrain  16  or differential  22 , a propshaft  30  driven by power transfer unit  28 , a rear axle assembly  32  and a power transmission device  34  for selectively transferring drive torque from propshaft  30  to rear axle assembly  32 . Rear axle assembly  32  is shown to include a rear differential  35 , a pair of rear wheels  36 L and  36 R and a pair of rear axleshafts  38 L and  38 R that interconnect rear differential  35  to corresponding rear wheels  36 L and  36 R.  
      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 locked (“part-time”) four-wheel drive mode and an adaptive (“on-demand”) four-wheel drive mode. In this regard, power transmission device  34  is equipped with a transfer clutch  50  that can be selectively actuated for transferring drive torque from propshaft  30  to rear axle assembly  32  for establishing the part-time and on-demand four-wheel drive modes. The power transfer system further includes a power-operated clutch actuator  52  for actuating transfer clutch  50 , vehicle sensors  54  for detecting certain dynamic and operational characteristics of motor vehicle  10 , 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 clutch actuator  52  in response to input signals from vehicle sensors  54  and mode selector  56 .  
      Power transmission device, hereinafter referred to as torque coupling  34 , is shown schematically in  FIG. 2  to be operably disposed between propshaft  30  and a pinion shaft  60 . As seen, pinion shaft  60  includes a pinion gear  62  that is meshed with a hypoid ring gear  64  fixed to a differential case  66  of rear differential  35 . Differential  35  is conventional in that pinions  68  driven by case  66  are arranged to drive side gears  70 L and  70 R which are fixed for rotation with corresponding axleshafts  38 L and  38 R. Torque coupling  34  is shown to generally include transfer clutch  50  and clutch actuator  52  arranged to control the transfer of drive torque from propshaft  30  to pinion shaft  60  and which together define the torque transfer mechanism of the present invention.  
      Referring primarily to  FIGS. 3 through 5 , the components and function of torque coupling  34  will be disclosed in detail. As seen, torque coupling  34  generally includes a housing  72 , an input shaft  74  rotatably supported in housing  72  via a bearing assembly  76 , transfer clutch  50  and clutch actuator  52 . A yoke  78  is fixed to a first end of input shaft  74  to permit connection with propshaft  30 . Transfer clutch  50  includes a drum  80  fixed for rotation with input shaft  74 , a hub  82  fixed for rotation with pinion shaft  60 , and a multi-plate clutch pack  84  comprised of alternating outer and inner clutch plates that are fixed (i.e., splined) to corresponding ones of drum  80  and hub  82 . As shown, a bearing assembly  86  rotatably supports a second end of input shaft  74  on pinion shaft  60 , which, in turn, is rotatably supported in housing  72  via a pair of laterally-spaced bearing assemblies  88 .  
      Clutch actuator  52  is generally shown to include an electric motor  90 , a geared drive unit  92  and a clutch apply operator  94 . Electric motor  90  is secured to housing  72  and includes a rotary output shaft  96 . Geared drive unit  92  includes a pinion gear  100  driven by motor output shaft  96  that is in meshed engagement with a transfer gear  101 . More specifically, pinion gear  100  includes helical gear teeth  102  that mesh with corresponding helical gear teeth  104  of transfer gear  101 . As such, geared drive unit  92  is defined by the meshed helical gearset comprised of pinion gear  100  and transfer gear  101 .  
      Clutch apply operator  94  is best shown in  FIG. 4  to include a first cam plate  130  non-rotatably fixed via a lug or spline connection  132  to housing  72 , a second cam plate  134  that is supported for rotations about pinion shaft  60 , and balls  138 . Second cam plate  134  has transfer gear  101  fixed thereto or integrally formed thereon such that second cam plate  134  functions as a rotatable and axially moveable thrust generating component. A ball  138  is disposed in each of a plurality of aligned cam grooves  140  and  142  formed in corresponding facing surfaces of first and second cam plates  130  and  134 , respectively. Preferably, three equally-spaced sets of such facing cam grooves  140  and  142  are formed in cam plates  130  and  134 , respectively. Grooves  140  and  142  are formed to define cam surfaces that are ramped, tapered or otherwise contoured in a circumferential direction. Balls  138  roll against cam surfaces  140  and  142  such that rotation of second cam plate  134  with transfer gear  101  causes axial movement of second cam plate  134  relative to first cam plate  130 . In addition, a thrust bearing assembly  144  is disposed between second cam plate  130  and an actuator plate  146  of clutch pack  84 . As seen, a return spring  148  is disposed between hub  82  and actuator plate  146 . As an alternative to the arrangement shown, one of cam surfaces  140  and  142  can be non-tapered such that the ramping profile is configured entirely within the other of the cam plates. Also, balls  138  are shown be spherical but are contemplated to permit use of cylindrical rollers disposed in correspondingly shaped cam grooves.  
      Second cam plate  134  is axially moveable relative to clutch pack  84  between a first or “released” position and a second or “locked” position. With second cam plate  134  in its released position, a minimum clutch engagement force is exerted on clutch pack  84  such that virtually no drive torque is transferred from input shaft  74  through clutch pack  84  to pinion shaft  60 . In this manner, a two-wheel drive mode is established. In contrast, location of second cam plate  134  in its locked position causes a maximum clutch engagement force to be applied to clutch pack  84  such that pinion shaft  60  is, in effect, coupled for common rotation with input shaft  74 . In this manner, the part-time four-wheel drive mode is established. Therefore, accurate bi-directional control of the axial position of second cam plate  134  between its released and locked positions permits adaptive regulation of the amount of drive torque transferred from input shaft  74  to pinion shaft  60 , thereby establishing the on-demand four-wheel drive mode. Return spring  148  is operable to bias second cam plate  134  toward its released position.  
      The tapered contour of cam surfaces  140  and  142  is selected to control the range of axial travel of second cam plate  134  relative to clutch pack  84  from its released position to its locked position in response to pinion gear  100  being driven by electric motor  90  in a first rotary direction. Such rotation of pinion gear  100  in a first direction induces rotation of transfer gear  101 . Due to the meshed helical tooth profiles, such rotation of pinion gear  100  results in axial translation of transfer gear  101  relative to pinion gear  100  such that second cam plate  134  axially moves toward its locked position. In addition, the resulting relative rotation between first cam plate  130  and second cam plate  134  causes balls  138  to ride against contoured cam surfaces  140  and  142 . However, since first cam plate  130  is restrained against axial movement, this relative rotation causes axial movement of second cam plate  134  toward its locked position for increasing the clutch engagement force exerted on clutch pack  84 . Thus, the combination of the helical gearset and the ballramp unit work cooperatively to control movement of second cam plate  134  and amplify the clutch engagement force generated and applied by actuator plate  146  on clutch pack  84 .  
      In operation, when mode selector  56  indicates selection of the two-wheel drive mode, controller  58  signals electric motor  90  to rotate motor shaft  96  in the second direction for causing second cam plate  134  to move axially until it is located in its released position, thereby fully releasing engagement of clutch pack  84 . If mode selector  56  thereafter indicates selection of the part-time four-wheel drive mode, electric motor  90  is signaled by controller  58  to rotate driveshaft  96  in the first direction for inducing linear translation of second cam plate  134  until it is located in its locked position. As noted, such movement of second cam plate  134  to its locked position acts to fully engage clutch pack  84 , thereby coupling pinion shaft  60  to input shaft  74 .  
      When mode selector  56  indicates selection of the on-demand four-wheel drive mode, controller  58  energizes motor  90  to rotate motor shaft  96  until second cam plate  134  is located in a ready or “stand-by” position. This position may be its released position or, in the alternative, an intermediate position. In either case, a predetermined minimum amount of drive torque is delivered to pinion shaft  60  through clutch pack  84  in this stand-by condition. Thereafter, controller  58  determines when and how much drive torque needs to be transferred to pinion shaft  60  based on current tractive conditions and/or operating characteristics of the motor vehicle, as detected by sensors  54 . As will be appreciated, any control schemes known in the art can be used with the present invention for adaptively controlling actuation of transfer clutch  50  in a driveline application. The arrangement described for clutch actuator  52  is an improvement over the prior art in that the torque amplification provided by geared drive unit  92  permits use of a small low-power electric motor and yet provides extremely quick response and precise control. Other advantages are realized in the reduced number of components and packaging flexibility.  
      To illustrate an alternative power transmission device to which the present invention is applicable,  FIG. 6  schematically depicts a front-wheel based four-wheel drivetrain layout  10 ′ for a motor vehicle. In particular, engine  18  drives multi-speed transmission  20  having an integrated front differential unit  22  for driving front wheels  24 L and  24 R via axleshafts  26 L and  26 R. A power transfer unit  190  is also driven by powertrain  16  for delivering drive torque to the input member of a torque transfer coupling  192  that is operable for selectively transferring drive torque to propshaft  30 . Accordingly, when sensors indicate the occurrence of a front wheel slip condition, controller  58  adaptively controls actuation of torque coupling  192  such that drive torque is delivered “on-demand” to rear driveline  14  for driving rear wheels  36 L and  36 R. It is contemplated that torque transfer coupling  192  would include a multi-plate transfer clutch  194  and a clutch actuator  196  that are generally similar in structure and function to multi-plate transfer clutch  50  and clutch actuator  52  previously described herein.  
      Referring to  FIG. 7 , power transfer unit  190  is schematically illustrated in association with an on-demand all-wheel drive system based on a front-wheel drive vehicle similar to that shown in  FIG. 6 . In particular, an output shaft  202  of transmission  20  is shown to drive an output gear  204  which, in turn, drives an input gear  206  fixed to a carrier  208  associated with front differential unit  22 . To provide drive torque to front wheels  24 L and  24 R, front differential  22  further includes a pair of side gears  210 L and  210 R that are connected to the front wheels via corresponding axleshafts  26 L and  26 R. Differential unit  22  also includes pinions  212  that are rotatably supported on pinion shafts fixed to carrier  208  and which are meshed with both side gears  210 L and  210 R. A transfer shaft  214  is provided to transfer drive torque from carrier  208  to torque coupling  192 .  
      Power transfer unit  190  includes a right-angled drive mechanism having a ring gear  220  fixed for rotation with a drum  222  of transfer clutch  194  and which is meshed with a pinion gear  224  fixed for rotation with propshaft  30 . As seen, a clutch hub  216  of transfer clutch  194  is driven by transfer shaft  214  while a multi-plate clutch pack  228  is disposed between hub  216  and drum  222 . Clutch actuator  196  is operable for controlling engagement of transfer clutch  194 . Clutch actuator  196  is intended to be similar to motor-driven clutch actuator  52  previously described in that an electric motor is supplied with electric current by controller  58  for controlling relative rotation of a geared drive unit which, in turn, controls translational movement of a cam plate operator for controlling engagement of clutch pack  228 .  
      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 four-wheel drive mode is selected, controller  58  modulates actuation of clutch actuator  196  in response to the vehicle operating conditions detected by sensors  54  by varying the value of the electric control signal sent to the motor. In this manner, the level of clutch engagement and the amount of drive torque that is transferred through clutch pack  228  to rear driveline  14  through power transfer unit  190  is adaptively controlled. Selection of the part-time four-wheel drive mode results in full engagement of transfer clutch  194  for rigidly coupling the front driveline to the rear driveline. In some applications, mode selector  56  may be eliminated such that only the on-demand four-wheel drive mode is available so as to continuously provide adaptive traction control without input from the vehicle operator.  
       FIG. 8  illustrates a modified version of  FIG. 7  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 driveline  14  while selectively transmitting drive torque to front wheels  24 L and  24 R through torque coupling  192 . In this arrangement, drive torque is transmitted directly from transmission output shaft  202  to transfer unit  190  via a drive shaft  230  interconnecting input gear  206  to ring gear  220 . To provide drive torque to the front wheels, torque coupling  192  is shown operably disposed between drive shaft  230  and transfer shaft  214 . In particular, transfer clutch  194  is arranged such that drum  222  is driven with ring gear  220  by drive shaft  230 . As such, actuation of clutch actuator  196  functions to transfer torque from drum  222  through clutch pack  228  to hub  216  which, in turn, drives carrier  208  of front differential unit  22  via transfer shaft  214 . Again, the vehicle could be equipped with mode selector  56  to permit selection by the vehicle operator of either the adaptively controlled on-demand four-wheel drive mode or the locked part-time four-wheel drive mode. In vehicles without mode selector  56 , the on-demand four-wheel drive mode is the only drive mode available and provides continuous adaptive traction control without input from the vehicle operator.  
      In addition to the on-demand 4WD systems shown previously, the power transmission 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. 9  schematically illustrates a full-time four-wheel drive system which is generally similar to the on-demand four-wheel drive system shown in  FIG. 7  with the exception that power transfer unit  190  now includes an interaxle differential unit  240  that is operably installed between carrier  208  of front differential unit  22  and transfer shaft  214 . In particular, output gear  206  is fixed for rotation with a carrier  242  of interaxle differential  240  from which pinion gears  244  are rotatably supported. A first side gear  246  is meshed with pinion gears  244  and is fixed for rotation with drive shaft  230  so as to be drivingly interconnected to rear driveline  14  through gearset  220  and  224 . Likewise, a second side gear  248  is meshed with pinion gears  244  and is fixed for rotation with carrier  208  of front differential unit  22  so as to be drivingly interconnected to the front driveline.  
      Torque transfer mechanism  192  is shown to be operably disposed between side gears  246  and  248 . As such, torque transfer mechanism  192  is operably arranged between the driven outputs of interaxle differential  240  for providing a torque biasing and slip limiting function. Torque transfer mechanism  192  is shown to again include multi-plate transfer clutch  194  and clutch actuator  196 . Transfer clutch  194  is operably arranged between transfer shaft  214  and driveshaft  230 . In operation, when sensor  54  detects a vehicle operating condition, such as excessive interaxle slip, controller  58  adaptively controls activation of the electric motor associated with clutch actuator assembly  196  for controlling engagement of clutch assembly  194  and thus the torque biasing between the front and rear drivelines.  
      Referring now to  FIG. 10 , a schematic layout of a drivetrain  10 A for a four-wheel drive vehicle having powertrain  16  delivering drive torque to a power transfer unit, hereinafter referred to as transfer case  290 . Transfer case  290  includes a rear output shaft  302 , a front output shaft  304  and a torque coupling  292  therebetween. Torque coupling  292  generally includes a multi-plate transfer clutch  294  and a power-operated clutch actuator  296 . As seen, a rear propshaft  306  couples rear output shaft  302  to rear differential  34  while a front propshaft  308  couples front output shaft  304  to front differential  22 . Power-operated clutch actuator  296  is again schematically shown to provide adaptive control over engagement of transfer clutch  294  incorporated into transfer case  290 .  
      Referring now to  FIG. 11 , a full-time 4WD system is shown to include transfer case  290  equipped with an interaxle differential  310  between an input shaft  312  and output shafts  302  and  304 . Differential  310  includes an input defined as a planet carrier  314 , a first output defined as a first sun gear  316 , a second output defined as a second sun gear  318 , and a gearset for permitting speed differentiation between first and second sun gears  316  and  318 . The gearset includes meshed pairs of first planet gears  320  and second planet gears  322  which are rotatably supported by carrier  314 . First planet gears  320  are shown to mesh with first sun gear  316  while second planet gears  322  are meshed with second sun gear  318 . First sun gear  316  is fixed for rotation with rear output shaft  302  so as to transmit drive torque to the rear driveline. To transmit drive torque to the front driveline, second sun gear  318  is coupled to a transfer assembly  324  which includes a first sprocket  326  rotatably supported on rear output shaft  302 , a second sprocket  328  fixed to front output shaft  304 , and a power chain  330 .  
      As noted, transfer case  290  includes transfer clutch  294  and clutch actuator  296 . Transfer clutch  294  has a drum  332  fixed to sprocket  326  for rotation with front output shaft  304 , a hub  334  fixed for rotation with rear output shaft  302  and a multi-plate clutch pack  336  therebetween. Again, clutch actuator  296  is schematically shown but intended to be substantially similar in structure and function to that disclosed in association with clutch actuator  52  shown in  FIGS. 3 and 4 .  FIG. 12  is merely a modified version of transfer case  290  which is constructed without center differential  310  to provide an on-demand four-wheel drive system.  
      Referring now to  FIG. 13 , a drive axle assembly  400  is schematically shown to include a pair of torque couplings operably installed between driven propshaft  30  and rear axleshafts  38 L and  38 R. Propshaft  30  drives a right-angle gearset including pinion  402  and ring gear  404  which, in turn, drives a transfer shaft  406 . A first torque coupling  200 L is shown disposed between transfer shaft  406  and left axleshaft  38 L while a second torque coupling  200 R is disposed between transfer shaft  406  and right axleshaft  38 R. Each of the torque couplings can be independently controlled via activation of its corresponding clutch actuator assembly  226 L,  226 R to adaptively control side-to-side torque delivery. In a preferred application, axle assembly  400  can be used in association with the secondary driveline in four-wheel drive motor vehicles.  
      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.