Abstract:
The present invention is directed to a power transfer system for a four-wheel drive vehicle equipped with a torque transfer coupling which includes a clutch pack and a ball-screw actuator. The ball-screw actuator functions to axially translates an apply plate to operatively engage the clutch pack and vary the frictional engagement. This arrangement yields numerous operational advantages over the prior art including, but not limited to, establishing a direct drive between the motor output shaft and the apply plate, concentric mounting of the actuator elements with the motor output shaft, and a simplified mechanical arrangement that reduces the number of frictional elements increasing operational efficiency and decreasing motor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. Ser. No. 10/263,245, filed Oct. 2, 2002 which is a continuation of U.S. Ser. No. 09/775,089, filed Feb. 1, 2001, now U.S. Pat. No. 6,484,857. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to power transfer systems for controlling the distribution of drive torque between front and rear wheels of a four-wheel drive vehicle and, more particularly, to a torque transfer coupling equipped with a ball-screw actuator. 
     BACKGROUND OF THE INVENTION 
     In view of increased consumer demand for four-wheel drive vehicles, a plethora of power transfer systems are currently being utilized in vehicular driveline applications for selectively directing power (i.e., drive torque) to the non-driven wheels of the vehicle. In many power transfer systems, a part-time transfer case is incorporated into the driveline and is normally operable in a two-wheel drive mode for delivering drive torque to the driven wheels. A mechanical “mode” shift mechanism can be selectively actuated by the vehicle operator for rigidly coupling the non-driven wheel to the driven wheels in order to establish a part-time four-wheel drive mode. As will be appreciated, a motor vehicle equipped with a part-time transfer case offers the vehicle operator the option of selectively shifting between the two-wheel drive mode during normal road conditions and the part-time four-wheel drive mode for operation under adverse road conditions. 
     Alternatively, it is known to use “on-demand” power transfer systems for automatically directing power to the non-driven wheels, without any input or action on the part of the vehicle operator, when traction is lost at the driven wheels. Modernly, it is known to incorporate the “on-demand” feature into a transfer case by replacing the mechanically-actuated mode shift mechanism with a clutch assembly that is interactively associated with an electronic control system and a sensor arrangement. During normal road conditions, the clutch assembly is maintained in a non-actuated condition such that the drive torque is only delivered to the driven wheels. However, when the sensors detect a low traction condition at the driven wheels, the clutch assembly is automatically actuated to deliver drive torque “on-demand” to the non-driven wheels. Moreover, the amount of drive torque transferred through the clutch assembly to the non-driven wheels can be varied as a function of specific vehicle dynamics, as detected by the sensor arrangement. 
     Conventional clutch assemblies, typically include a clutch pack operably connected between a drive member and a driven member, and a power-operated actuator for controlling engagement of the clutch pack. Specifically, torque is transferred from the drive member to the driven member by actuating the power-operated actuator for displacing an apply plate which acts on the clutch pack and increases the friction of engagement between the interleaved plates. 
     A variety of power-operated actuators have been used in the art with mixed results. Exemplary embodiments include those disclosed in U.S. Pat. No. 5,407,024 wherein a ball-ramp arrangement is used to displace the apply plate when a current is provided to an induction motor. Another example disclosed in U.S. Pat. No. 5,332,060, assigned to the assignee of the present application, which includes a linear actuator that is operable for pivoting a lever arm to control the magnitude of the clutch engagement force applied to the clutch pack. While the above clutch actuator devices have performed adequately for their intended purpose, a need exists for an improved actuator that is less complex and reduces the number of friction generating components which lead to inefficiencies and larger motor requirements. 
     SUMMARY OF THE INVENTION 
     In view of the above, the present invention is directed to a power transfer system for a four-wheel drive vehicle having a torque transfer coupling equipped with a clutch pack and a ball-screw actuator. The ball-screw actuator functions to axially translates an apply plate for operatively engaging the clutch pack and varying the frictional engagement. This arrangement yields numerous operational advantages over the prior art including, but not limited to, establishing a direct drive between the motor output shaft and the apply plate, concentric mounting of the actuator elements with the motor output shaft, and a simplified mechanical arrangement that reduces the number of frictional elements increasing operational efficiency and decreasing motor requirements. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the following detailed description, attached drawings and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given here below, the appended claims, and the accompanying drawings in which: 
     FIG. 1 is a schematic representation of an exemplary four-wheel drive vehicle having a power transfer system according to the present invention; 
     FIG. 2 is a cross-sectional view of a transfer case associated with the power transfer system and which includes a multi-plate clutch assembly and an electronically-controlled ball-screw actuator; 
     FIG. 3 is a schematic representation of the transfer case shown in FIG. 2; 
     FIGS. 4 and 5 are schematic illustrations of transfer cases according to alternative embodiments of the present invention; 
     FIG. 6 is a schematic representation of an alternative four-wheel drive vehicle having the power transfer system of the present invention incorporated therein; 
     FIGS. 7 and 8 are schematic illustrations of on-demand power transfer arrangement associated with the vehicle shown in FIG. 7; and 
     FIG. 9 is a schematic illustration of a full-time power transfer arrangement for the vehicle shown in FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In general, the present invention is directed to a power transfer system which is operably installed between the driven and non-driven wheels of a four-wheel drive vehicle. In operation, the amount of power (i.e., drive torque) transferred to the non-driven wheels is controllably regulated in accordance with various system and driver-initiated inputs for optimizing the tractive characteristics of the vehicle while concomitantly enhancing overall steering control. In addition, the power transfer system may also include a mode select mechanism for permitting a vehicle operator to select between a two-drive wheel mode, a part-time four-wheel drive mode, and an “on-demand” drive mode. 
     Referring to FIG. 1 of the drawings, a drivetrain for a four-wheel drive vehicle is schematically shown interactively associated with a power transfer system  10  which incorporates the novel principles of the present invention. The motor vehicle drivetrain has a pair of front wheels  12  and rear wheels  14  both drivable from a source of power, such as an engine  16 , through a transmission  18  which may be of either the manual or automatic type. In the particular embodiment shown, the drivetrain is a rear wheel drive system which incorporates a transfer case  20  operable to receive drive torque from engine  16  and transmission  18  for normally driving rear wheels  14  (i.e., the “driven” wheels) in a two-wheel drive mode of operation. Front wheels  12  and rear wheels  14  are shown connected at opposite ends of front and rear axle assemblies  22  and  24 , respectively. As is known, a rear differential  26  is interconnected between rear axle assembly  24  and one end of a rear drive shaft  28 , the opposite end of which is interconnected to a first output member  30  of transfer case  20 . Similarly, front axle assembly  22  includes a front differential  32  that is coupled to one end of a front drive shaft  34 , the opposite end of which is coupled to a section output member  36  of transfer case  20 . It is to be understood that the specific orientation of the drivetrain is merely exemplary in nature and that the drivetrain could be reversed for normally driving front wheels  12 . 
     Transfer case  20  is equipped with a transfer clutch  38  for selectively delivering drive torque to front wheels  12  (i.e., the non-driven wheels) for establishing a four-wheel drive mode of operation. The operating mode of transfer clutch  38  is generally controlled in response to a mode signal generated by a mode selector  40  and which is sent to a controller  42 . Controller  42  also receives input signals from sensors  44  that are indicative of various operational characteristic of the vehicle. 
     When the two-wheel drive mode is selected, all drive torque is delivered from first output member  30  to rear wheels  14  and transfer clutch  38  is maintained in a “non-actuated” condition. When the part-time four-wheel drive mode is selected, transfer clutch  38  is fully actuated and maintained in a “lock-up” condition such that second output member  36  is, in effect, rigidly coupled for driven rotation with first output member  30 . When the “on-demand” drive mode is selected, controller  42  controls the degree of actuation of transfer clutch  38  for varying the amount of drive torque directed to front wheels  12  through transfer clutch  38  as a function of the sensor input signals for providing improved tractive performance when needed. In addition, controller  42  is adapted to controllably modulate the actuated condition of transfer clutch  38  to provide superior handling and steering control by minimizing the oversteer and understeer tendencies of the vehicle during a cornering maneuver. Other advantages associated with controllably modulating the actuated state of transfer clutch  38  will be detailed hereinafter. By way of example rather than limitation, the control scheme generally disclosed in U.S. Pat. No. 5,332,060 issued Jul. 26, 1994 to Sperduti et al. and assigned to the common assignee of the present invention (the disclosure of which is hereby incorporated by reference) can be used to control adaptive actuation of transfer clutch  38  during on-demand operation. 
     Referring primarily to FIGS. 2 and 3, transfer case  20  includes a housing  56  formed by a series of modular sections that are suitably interconnected in a conventional manner. A transmission output shaft couples transmission  18  to an input shaft  60  of transfer case  20  for supplying power thereto. In the embodiment shown, first output member  30  includes an elongated mainshaft  62  which is aligned on the longitudinal axis of input shaft  60  and is supported for rotation within housing  56 . For simplicity, the illustrated embodiment shows input shaft  60  extending as a mainshaft through transfer case  20  to form first output member  30 . However, those skilled in the art will appreciate that a variety of intermediate sleeves or shafts splined to rotate with one another may be used to drivably couple a separate input shaft  60  for rotation with output shaft  62 . An exemplary illustration of such a shaft and sleeve arrangement is described in detail in U.S. Pat. No. 5,332,060 which is hereby expressly incorporated into this description by reference. 
     With continued references to FIGS. 2 and 3, transfer clutch  38  is shown for transferring drive torque from input shaft  60  to front wheels  12 . More specifically, a drive sprocket  64  is fixed (i.e., splined) for rotation on a tubular extension  78  of a rotatable cylindrical drum  68  associated with transfer clutch  38 . In addition, extension  78  is rotatably supported on input shaft  60  by one or more suitable bearing assemblies  70 . Drive sprocket  64  drivingly engages a chain  72  which is coupled to a lower driven sprocket  74 . Driven sprocket  74  is coupled to, or an integral portion of, second output member  36  of transfer case  20 . Second output member  36  is shown as a front output shaft  76  which is supported for rotation within housing  56  by suitable bearing assemblies  78  and  80 . As noted, front output shaft  76  is operably connected to the motor vehicle&#39;s front wheel  12  via front drive shaft  34 . 
     Transfer clutch  38  includes a multi-plate clutch assembly  45  and a power-operated actuator  46  that are arranged to concentrically surround a portion of input shaft  60 . As noted, cylindrical drum  68  is fixedly secured to drive sprocket  64  so as to drive, or be driven by, front output shaft  76  of transfer case  20 . In a preferred form, clutch assembly  45  includes a clutch hub  82  that is concentrically surrounded by drum  68  and which is fixed (i.e., splined) to input shaft  60  for rotation therewith. Thus, clutch hub  82  and drum  68  are capable of rotating relative to one another and form an internal chamber therebetween. Disposed within the internal chamber are two sets of alternatively interleaved friction clutch plates that are operable for transferring torque from input shaft  60  through clutch hub  82  to drum  68  and, ultimately, to front output shaft  76  in response to a clutch “engagement” force applied thereto. One set of clutch plates, referred to as inner clutch plate  84 , are mounted (i.e., splined) for rotation with clutch hub  82  while the second set of clutch plates, referred to as outer clutch plates  86 , are mounted (i.e., splined) for rotation with drum  68 . In addition, a reaction plate  88  is mounted on or integral with one end of clutch hub  82 . In addition, an apply plate  90  is rotatable with clutch hub  68  and yet is axially movable with respect to interleaved friction clutch plates  84  and  86 . Thus, apply plate  90  acts as a pressure plate for compressing the interleaved clutch so as to cause drive torque to be transferred through clutch assembly  45  as a function of the clutch engagement force exerted on apply plate  90  by power-operated actuator  46 . 
     Power-operated actuator  46  is a ball-screw actuator  92  which operatively engages apply plate  90  and is controlled by controller  42  to selectively control the amount of torque transferred through clutch assembly  45 . Ball-screw actuator  92  provides a concentrically-mounted clutch actuator that reduces the mechanical and frictional components when compared to prior art actuators while further directly transferring rotational movement of the motor output to axial movement of apply plate  90  to provide more precise and repeatable and easily controlled reaction plate movement. 
     In general, ball-screw actuator  92  includes an electric motor  100  having a fixed stator  102  and a rotary output shaft  104 , a screw  106 , and a plurality of circumferentially spaced balls  108 . Electric motor  100 , screw  106 , and balls  108  are each concentrically mounted with one another and about output shaft  62 . Screw  106  is mounted within housing  56  for axial, non-rotational movement relative thereto. A thrust bearing  110  is disposed between the application face surfaces of screw  106  and apply plate  90  to permit rotation of apply plate  90  relative to screw  106 . A spring  112  is located between hub  82  and apply plate  90  for normally exerting a return biasing a force on apply plate  90 . 
     In operation, controller  42  selectively delivers current to electric motor  100  which causes motor  100  to rotate its output shaft  104 . Motor output shaft  104  has helical grooves formed on its outer circumferential surface while screw  106  has helical grooves formed on its inner circumferential surface. Balls  108  are disposed within the grooves causing axial displacement of lead screw  106  toward or away from apply plate  90  in response to direction of rotation of motor output shaft  104 . In turn, axial displacement of lead screw  106  causes corresponding movement of apply plate  90  and the desired increase or decrease in the frictional engagement between interleaved clutch plates  84  and  86 . Thus, control of the direction and amount of rotation of shaft  104  controls the magnitude of the clutch engagement force exerted on clutch assembly  45 . 
     In view of the above arrangement, electric motor output shaft  104  acts as the input to the ball-screw operator which yields a mechanically simple system that eliminates more complex mechanical designs previously used in the art including gears and/or linkages. As each of the mechanical components of the actuator contain friction elements, the elimination of these components and the more simple design provided by the present invention reduces the overall friction and therefore increases the efficiency of the assembly. Increased efficiency is translated into more economical motors and more accurate clutch torque estimation. The novel annular packaging of motor  100  and ball-screw actuator  92  permits the outer diameter of motor  100  to be grounded on housing  56 . Those skilled in the art will appreciate that a variety of electric motors may be used including a dc brush, dc brushless, and stepper motors. 
     In addition to the on-demand versions of the power transfer system previously shown, the present invention is likewise adapted for use in alternative four-wheel drive vehicles. Referring to FIG. 4, a full-time 4WD version of a transfer case  20 A is shown to now include an interaxle differential  120 . Differential  120  is operable to transfer drive torque from input shaft  60  to both front and rear output shafts  76  and  62 , respectively, while permitting speed differentiation therebetween. In addition, transfer clutch  38  is operably arranged between the outputs of interaxle differential  120  to bias the torque distribution therebetween. Preferably, mode selector  40  permits selection of an adaptive full-time four-wheel drive mode and a locked four-wheel drive mode. When the adaptive full-time four-wheel drive mode is selected, controller  42  controls the degree of actuation of transfer clutch  38  for varying the torque bias and limiting interaxle speed differentiation (i.e., slip) as a function of the sensor input signals. In contrast, when the locked four-wheel drive mode is selected, transfer clutch  38  is fully actuated and maintained in its lock-up condition such that interaxle differential  120  is locked and the output shafts are, in effect, rigidly coupled together. 
     Interaxle differential  120  is shown to include a carrier unit  122  from which meshed pairs of long pinions  124  and short pinions  126  are rotatably supported. Long pinions  124  are also meshed with a first sun gear  128  that is fixed for rotation with rear output shaft  62 . Short pinions  126  are shown to be meshed with a second sun gear  130  that is fixed for rotation with drive sprocket  64 . Power-operated actuator  46  is schematically shown in FIG. 4 to include ball screw operator  92  and electric motor  100  which are operably arranged on shaft  62  similar to that shown in FIGS. 2 and 3. 
     Referring now to FIG. 5, transfer case  20 A of FIG. 4 is now shown to be further equipped with a two-speed planetary gear assembly  132  and a synchronized range shift system  134 . Planetary gear assembly  132  includes a ring gear  136 , a sun gear  138  fixed for rotation with input shaft  60 , and a set of planet gears  140  which are each rotatably supported from a planet carrier  142  and meshed with sun gear  138  and ring gear  136 . Planet carrier  142  is shown to include a front carrier ring  144  interconnected to a rear carrier ring  146 . Rear carrier ring  146  of planet carrier  142  is fixed for rotation with pinion carrier  122  of interaxle differential  120  via a transfer shaft  148 . 
     Planetary gear assembly  132  functions as a two-speed gear reduction unit which, in cooperation with a range clutch  150  of synchronized range shift mechanism  134 , is operable to establish a first or high-range drive connection between input shaft  60  and carrier  142  by coupling ring gear  136  for rotation with a first clutch plate  152  that is fixed to transfer shaft  148  for rotation with planet carrier  142 . When the first drive connection is established, ring gear  136  is effectively coupled for rotation with planet carrier  142 . Thus, the first drive connection establishes a high-range drive mode in which planet carrier  142  is driven by input shaft  60  at a first (i.e., direct) speed ratio. Likewise, a second or low-range drive connection is established between input shaft  60  and planet carrier  142  by coupling ring gear  136  to a second clutch plate  154  that is fixed to housing  56 . When the second drive connection is established, ring gear  136  is braked against rotation and planet carrier  142  is driven by input shaft  60 . Thus, the second drive connection establishes a low-range mode in which carrier  142  is driven at a second (i.e., reduced) speed ratio with respect to input shaft  60 . A neutral mode is established when range clutch  150  uncouples ring gear  136  from both first clutch plate  152  and second clutch plate  154  such that carrier  142  is not driven by input shaft  60 . As will be detailed, synchronized range shift mechanism  134  is operable for permitting transfer case  20 A to be shifted “on-the-move” between its high-range and low-range drive modes. 
     With continued reference of FIG. 5, synchronized range shift mechanism  134  is shown to include range clutch  150 , a first synchronizer assembly  156  that is disposed between range clutch  150  and first clutch plate  152 , and a second synchronizer assembly  158  that is disposed between range clutch  150  and second clutch plate  154 . Range clutch  150  includes a range sleeve  160  having a set of internal longitudinal splines  162  maintained in constant mesh with external longitudinal splines  164  formed on an outer surface of ring gear  136 . Thus, range sleeve  160  is mounted for rotation with ring gear  136  and is further supported for bidirectional sliding movement relative thereto. With range sleeve  160  in a neutral position (denoted by position line “N”) its spline teeth  162  are disengaged for mesh engagement with clutch teeth  166  on first clutch plate  152  and clutch teeth  168  on second clutch plate  154 . First synchronizer assembly  156  is operable for causing speed synchronization between input shaft  60  and planet carrier  142  in response to movement of range sleeve  160  from its N position toward a high-range position (denoted by position line “H”). Once the speed synchronization process is completed, spline teeth  162  on range sleeve  160  are permitted to move through first synchronizer  156  and into meshed engagement with clutch teeth  166  on first clutch plate  152 . 
     With range sleeve in its H position, it couples ring gear  136  to first clutch plate  152  such that planet carrier  142  is coupled to rotate at the same speed as input shaft  60  for establishing the first drive connection therebetween. Second synchronizer  158  is operable to cause speed synchronization between ring gear  136  and housing  56  in response to movement of range sleeve  160  from its N position toward a low-range position (denoted by position line “L”). Once speed synchronization is complete, spline teeth  162  on range sleeve  160  move through second synchronizer  158  and into meshed engagement with clutch teeth  168  on second clutch plate  154 . With range sleeve  160  positioned in its L position, ring gear  136  is coupled to housing  56  such that planet carrier  142  is driven at a reduced speed ratio relative to input shaft  60 , thereby establishing the second drive connection and the low-range drive mode. 
     To provide means for moving range sleeve  160  between its three range positions, transfer case  20 A includes a shift system  170  which is shown schematically to include a power-operated actuator  172  which receives control signals from controller  42 . Actuator  172  is operable for controlling movement of a shift fork  174  which, in turn, moves range sleeve  136  between its three distinct range positions. 
     To illustrate an alternative power transmission device to which the present invention is applicable, FIG. 6 schematically depicts a front-wheel based four-wheel drive layout. In particular, engine  16  drives a multi-speed transmission  18 ′ having an integrated front differential unit  32 ′ for driving front wheels  12  via axle shafts  13 . A transfer unit  200  is also driven by transmission  18 ′ for delivering drive torque to the input member of an in-line torque transfer coupling  202  via a drive shaft  28 ′. In particular, the input member of transfer coupling  202  is coupled to drive shaft  28 ′ while its output member is coupled to a drive component of rear differential  26 . Accordingly, when sensors  44  indicate the occurrence of a front wheel slip condition, controller  42  adaptively controls actuation of torque coupling  202  such that drive torque is delivered “on-demand” to rear wheels  14 . It is contemplated that torque transfer coupling  202  would include a multi-plate clutch assembly  45  and a ball screw actuator  92  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  26 , it is contemplated that torque coupling  202  could be operably located for transferring drive torque from transfer unit  200  to drive shaft  28 ′. 
     Referring now to FIG. 7, torque coupling  202  is schematically illustrated in association with an on-demand four-wheel drive system based on a frontwheel drive vehicle similar to that shown in FIG.  6 . In particular, an output shaft  204  of transaxle  18 ′ is shown to drive an output gear  206  which, in turn, drives an input gear  207  fixed to a carrier  208  associated with front differential unit  32 ′. To provide drive torque to front wheels  12 , front differential unit  32 ′ includes a pair of side gears  210  that are connected to front wheels  14  via axleshafts  13 . Differential unit  32 ′ also includes pinions  212  that are rotatably supported on pinion shafts fixed to carrier  208  and which are meshed with side gears  210 . A transfer shaft  214  is provided to transfer drive torque from carrier  208  to a clutch hub  82  associated with a multi-pate clutch assembly  45 . Clutch assembly  45  includes drum  68  and a clutch pack having interleaved clutch plates operably connected between hub  82  and drum  68 . 
     Transfer unit  200  is a right-angled drive mechanism including a ring gear  224  fixed for rotation with drum  68  of clutch assembly  38  which is meshed with a pinion gear  226  fixed for rotation with drive shaft  28 ′. As seen, ball screw clutch actuator  46  is schematically illustrated for controlling actuation of clutch assembly  28 . According to the present invention, actuator  46  is similar to, and includes, ball screw operator  92  and motor  100 . 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  40 . For example, if the on-demand 4WD mode is selected, controller  42  modulates actuation of clutch actuator  46  in response to the vehicle operating conditions detected by sensors  44  by varying the value of the electric control signal sent to motor  100 . In this manner, the level of clutch engagement and the amount of drive torque that is transferred through the clutch pack to the rear driveline through transfer unit  200  and drive shaft  28 ′ is adaptively controlled. Selection of the locked or part-time 4WD mode results in full engagement of clutch assembly  45  for rigidly coupling the front driveline to the rear driveline. In some applications, mode selector  40  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. 8 illustrates a modified version of FIG. 7 wherein an on-demand four-wheel drive system based on a rear-wheel drive motor vehicle that is arranged to normally deliver drive torque to rear wheels  14  while selectively transmitting drive torque to front wheels  12  through torque coupling  202 . In this arrangement, drive torque is transmitted directly from transmission output shaft  204  to transfer unit  200  via a drive shaft  230  interconnecting input gear  207  to ring gear  224 . To provide drive torque to front wheels  12 , torque coupling  202  is now shown operably disposed between drive shaft  230  and transfer shaft  214 . In particular, clutch assembly  45  is arranged such that drum  68  is driven with ring gear  224  by drive shaft  230 . As such, actuation of torque coupling  202  functions to transfer torque from drum  68  through the clutch pack to hub  82  which, in turn, drives carrier  208  of front differential unit  32 ′ via transfer shaft  214 . 
     In addition to the on-demand 4WD systems shown previously, the power transmission (ball screw 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. 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. 8 with the exception that an interaxle differential unit  240  is now operably installed between carrier  208  of front differential unit  32 ′ and transfer shaft  214 . In particular, output gear  207  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 the rear driveline through transfer unit  200 . Likewise, a second side gear  248  is meshed with pinion gears  248  and is fixed for rotation with carrier  208  of front differential unit  32 ′ so as to be drivingly interconnected to the front driveline. In operation, when sensor  44  detects a vehicle operating condition, such as excessive interaxle slip, controller  42  adaptively controls activation of motor  100  associated with ball screw actuator  46  for controlling engagement of clutch assembly  38  and thus the torque biasing between the front and rear driveline. 
     The foregoing discussion discloses and describes an exemplary embodiment of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the true spirit and fair scope of the invention as defined by the following claims.