POWER-OPERATED CLUTCH ACTUATOR FOR TORQUE TRANSFER MECHANISMS

FIELD OF THE INVENTION

[0001] 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 [0002] 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.

[0003] 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.

[0004] 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. Patent 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.

[0005] 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. Patent 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. Patent 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.

[0006] 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 [0007] 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.

[0008] 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.

[0009] 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.

[0010] 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

[0011] 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: [0012] FIG. 1 illustrates the drivetrain of an all-wheel drive motor vehicle equipped with a power transmission device of the present invention;

[0013] FIG. 2 is a schematic illustration of the power transmission device shown in FIG. 1 associated with a drive axle assembly;

[0014] 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;

[0015] FIG. 4 is an enlarged partial view of the torque transfer mechanism taken from FIG. 3;

[0016] 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;

[0017] FIGS. 6 through 9 are schematic illustrations of alternative embodiments for the power transmission device of the present invention;

[0018] FIG. 10 illustrates the drivetrain of a four-wheel drive vehicle equipped with another version of the power transmission device of the present invention;

[0019] FIGS. 11 and 12 are schematic illustrations of transfer cases adapted for use with the drivetrain shown in FIG. 10; and

[0020] 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 [0021] 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. [0022] 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 24L and 24R through a pair of front axleshafts 26L and 26R, 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 36L and 36R and a pair of rear axleshafts 38L and 38R that interconnect rear differential 35 to corresponding rear wheels 36L and 36R.

[0023] 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.

[0024] 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 7OL and 7OR which are fixed for rotation with corresponding axleshafts 38L and 38R. 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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. [0030] 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. [0031] 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.

[0032] 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 24L and 24R via axleshafts 26L and 26R. 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 36L and 36R. 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.

[0033] 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 24L and 24R, front differential 22 further includes a pair of side gears 210L and 210R that are connected to the front wheels via corresponding axleshafts 26L and 26R. 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 210L and 210R. A transfer

shaft 214 is provided to transfer drive torque from carrier 208 to torque coupling 192.

[0034] 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.

[0035] 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 driveline14 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.

[0036] 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 24L and 24R 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.

[0037] 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. [0038] 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.

[0039] Referring now to FIG. 10, a schematic layout of a drivetrain 10A 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. [0040] 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.

[0041] 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.

[0042] 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 38L and 38R. 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 200L is shown disposed between transfer shaft 406 and left axleshaft 38L while a second torque coupling 200R is disposed between transfer shaft 406 and right axleshaft 38R. Each of the torque couplings can be independently controlled via activation of its corresponding clutch actuator assembly 226L, 226R 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.

[0043] 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.