Wedge fork clutch actuator for driveline clutches

A torque transfer mechanism is provided for controlling the magnitude of a clutch engagement force exerted on a multi-plate clutch assembly that is operably disposed between a first rotary and a second rotary member. The torque transfer mechanism includes a clutch actuator for generating and applying a clutch engagement force on the clutch assembly. The clutch actuator includes a wedge fork having a gear rack segment and a tapered tang segment and a reaction block defining a tapered edge in sliding engagement with the tapered tang segment. An electric motor drives a pinion that is meshed with the gear rack to cause bi-directional linear movement of the wedge fork which causes corresponding sliding movement of the reaction block relative to the clutch assembly.

FIELD OF THE INVENTION

The present invention relates generally to power transfer systems for controlling the distribution of drive torque between the front and rear drivelines of a four-wheel drive vehicle. More particularly, the present invention is directed to a power transmission device for use in motor vehicle driveline applications and having a 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 incorporated into vehicular driveline applications for transferring drive torque to the wheels. In many vehicles, a power transmission device is operably installed between the primary and secondary drivelines. Such power transmission devices are typically equipped with a torque transfer mechanism for selectively and/or automatically transferring drive torque from the primary driveline to the secondary driveline to establish a four-wheel drive mode of operation. For example, the torque transfer mechanism can include a dog-type lock-up clutch that can be selectively engaged for rigidly coupling the secondary driveline to the primary driveline to establish a “part-time” four-wheel drive mode. In contrast, drive torque is only delivered to the primary driveline when the lock-up clutch is released for establishing a two-wheel drive mode.

A modern trend in four-wheel drive motor vehicles is to equip the power transmission device with an adaptive transfer clutch in place of the lock-up clutch. The transfer clutch is operable for automatically directing drive torque to the secondary wheels, without any input or action on the part of the vehicle operator, when traction is lost at the primary wheels for establishing an “on-demand” four-wheel drive mode. Typically, the transfer clutch includes a multi-plate clutch assembly that is installed between the primary and secondary drivelines and a clutch actuator for generating a clutch engagement force that is applied to the clutch plate assembly. The clutch actuator can be a power-operated device that is actuated in response to electric control signals sent from an electronic controller unit (ECU). Variable control of the electric control signal is typically based on changes in current operating characteristics of the vehicle (i.e., vehicle speed, interaxle speed difference, acceleration, steering angle, etc.) as detected by various sensors. Thus, such “on-demand” power transmission devices can utilize adaptive control schemes for automatically controlling torque distribution during all types of driving and road conditions.

A large number of on-demand power transmission devices have been developed with an electrically-controlled clutch actuator that can regulate the amount of drive torque transferred to the secondary output shaft as a function of the value of the electrical control signal applied thereto. In some applications, the transfer clutch employs an electromagnetic clutch as the power-operated clutch actuator. For example, U.S. Pat. No. 5,407,024 discloses a electromagnetic coil that is incrementally activated to control movement of a ball-ramp drive assembly for applying a clutch engagement force on the multi-plate clutch assembly. Likewise, Japanese Laid-open Patent Application No. 62-18117 discloses a transfer clutch equipped with an electromagnetic actuator for directly controlling actuation of the multi-plate clutch pack assembly.

As an alternative, the transfer clutch can employ an electric motor and a drive assembly as the power-operated clutch actuator. For example, U.S. Pat. No. 5,323,871 discloses an on-demand transfer case having a transfer clutch equipped with an electric motor that controls rotation of a sector plate which, in turn, controls pivotal movement of a lever arm that is operable for applying the clutch engagement force to the multi-plate clutch assembly. Moreover, Japanese Laid-open Patent Application No. 63-66927 discloses a transfer clutch which uses an electric motor to rotate one cam plate of a ball-ramp operator for engaging the multi-plate clutch assembly. Finally, U.S. Pat. Nos. 4,895,236 and 5,423,235 respectively disclose a transfer case equipped with a transfer clutch having an electric motor driving a reduction gearset for controlling movement of a ball screw operator and a ball-ramp operator which, in turn, apply the clutch engagement force to the clutch pack.

While many on-demand clutch control systems similar to those described above are currently used in four-wheel drive vehicles, a need exists to advance the technology and address recognized system limitations. For example, the size and weight of the friction clutch components and the electrical power requirements of the clutch actuator needed to provide the large clutch engagement loads may make such system cost prohibitive in some four-wheel drive vehicle applications. In an effort to address these concerns, new technologies are being considered for use in power-operated clutch actuator applications.

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 torque transfer mechanism of the present invention is well-suited for use in motor vehicle driveline applications to control the transfer of drive torque between a first rotary member and a second rotary member.

According to a preferred embodiment, a transfer case is provided for use in a four-wheel drive motor vehicle having a powertrain and first and second drivelines. The transfer case includes a first shaft driven by the powertrain and adapted for connection to the first driveline, a second shaft adapted for connection to the second driveline, and a friction clutch assembly operably disposed between the first shaft and the second shaft. The transfer case further includes a clutch actuator for generating and applying a clutch engagement force on the friction clutch assembly. The clutch actuator includes a wedge fork having a stem segment with a gear rack and a tang segment with a tapered drive surface, and a reaction block having a tapered reaction surface engaging said tapered drive surface on said tang segment and an apply surface engaging said friction clutch assembly. An electric motor drives a pinion gear that is meshed with the gear rack for causing bidirectional translational movement of the wedge fork. A controller controls actuation of the motor such that bi-directional translational movement of the wedge fork causes sliding movement of the reaction block for applying the clutch engagement force to the friction clutch assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to a torque transfer mechanism that can be adaptively controlled for modulating the torque transferred from a first rotary member to a second rotary member. The torque transfer mechanism finds particular application in power transmission devices for use in motor vehicle drivelines such as, for example, an on-demand clutch in a transfer case or in-line torque coupling, a biasing clutch associated with a differential assembly in a transfer case or a drive axle assembly, or as a shift clutch in a multi-speed automatic transmission. Thus, while the present invention is hereinafter described in association with particular arrangements for use in specific driveline applications, it will be understood that the arrangements shown and described are merely intended to illustrate embodiments of the present invention.

With particular reference toFIG. 1of the drawings, a drivetrain10for a four-wheel drive vehicle is shown. Drivetrain10includes a primary driveline12, a secondary driveline14, and a powertrain16for delivering rotary tractive power (i.e., drive torque) to the drivelines. In the particular arrangement shown, primary driveline12is the rear driveline while secondary driveline14is the front driveline. Powertrain16includes an engine18, a multi-speed transmission20, and a power transmission device hereinafter referred to as transfer case22. Rear driveline12includes a pair of rear wheels24connected at opposite ends of a rear axle assembly26having a rear differential28coupled to one end of a rear prop shaft30, the opposite end of which is coupled to a rear output shaft32of transfer case22. Front driveline14includes a pair of front wheels34connected at opposite ends of a front axle assembly36having a front differential38coupled to one end of a front prop shaft40, the opposite end of which is coupled to a front prop shaft42of transfer case22.

With continued reference to the drawings, drivetrain10is shown to further include an electronically-controlled power transfer system for permitting a vehicle operator to select between a two-wheel drive mode, a locked (“part-time”) four-wheel drive mode, and an adaptive (“on-demand”) four-wheel drive mode. In this regard, transfer case22is equipped with a transfer clutch50that can be selectively actuated for transferring drive torque from rear output shaft32to front output shaft42for establishing the part-time and on-demand four-wheel drive modes. The power transfer system further includes a power-operated mode actuator52for actuating transfer clutch50, vehicle sensors54for detecting certain dynamic and operational characteristics of the motor vehicle, a mode select mechanism56for permitting the vehicle operator to select one of the available drive modes, and a controller58for controlling actuation of mode actuator52in response to input signals from vehicle sensors54and mode selector56.

Transfer case22is shown inFIG. 2to include a multi-piece housing60from which rear output shaft32is rotatably supported by a pair of laterally-spaced bearing assemblies62. Rear output shaft32includes an internally-splined first end segment64adapted for connection to the output shaft of transmission20and a yoke assembly66secured to its second end segment68that is adapted for connection to rear propshaft30. Front output shaft42is likewise rotatably supported from housing60by a pair of laterally-spaced bearing assemblies70and72and includes an internally-splined end segment74that is adapted for connection to front propshaft40.

Referring primarily toFIGS. 2,3and4, transfer clutch50and mode actuator52are shown in association with transfer case22for transferring drive torque from rear output shaft32through a transfer assembly76to front output shaft42. Transfer assembly76includes a first sprocket78rotatably supported by a sleeve bushing80on rear output shaft32, a second sprocket82fixed to, or integrally formed on, front output shaft42for rotation therewith, and a power chain84encircling sprockets78and82. As will be detailed, transfer clutch50is a multi-plate friction clutch assembly86and mode actuator52is a motor-driven wedge-type clutch actuator assembly88which together define a torque transfer mechanism.

Clutch assembly86is shown to include a hub90fixed via a spline connection92to rear output shaft32, a drum94fixed via a spline connection96to first sprocket78of transfer assembly76, and a multi-plate clutch pack98operably disposed between hub90and drum94. Clutch pack98includes a set of outer clutch plates100that are splined to an annular rim segment of drum94and which are alternatively interleaved with a set of inner clutch plates102that are splined to an annular rim segment of hub90. Clutch assembly86further includes a pressure plate104having a first disk segment106journalled for sliding movement on rear output shaft32and a second disk segment108fixed via a spline connection110for rotation with drum94. Second disk segment108of pressure plate104is operably arranged to exert a compressive clutch engagement force on clutch pack98in response to axial movement of pressure plate104which, as will be detailed, is controlled by clutch actuator assembly88.

Pressure plate104is axially moveable relative to clutch pack98between a first or “released” position and a second or “locked” position. With pressure plate104in its released position, a minimum clutch engagement force is exerted on clutch pack98such that virtually no drive torque is transferred from rear output shaft32through clutch assembly86and transfer assembly76to front output shaft42so as to establish the two-wheel drive mode. In contrast, location of pressure plate104in its locked position causes a maximum clutch engagement force to be applied to clutch pack98such that front output shaft42is, in effect, coupled for common rotation with rear output shaft32so as to establish the part-time four-wheel drive mode. Therefore, accurate control of the position of pressure plate104between its released and locked positions permits adaptive regulation of the amount of drive torque transferred from rear output shaft32to front output shaft42, thereby establishing the on-demand four-wheel drive mode. A helical coil spring112coaxially surrounds a portion of rear output shaft32and acts between a retainer ring113abutting hub90and first disk segment106of pressure plate104for normally urging pressure plate toward its released position.

To provide means for moving pressure plate104between its released and locked positions, clutch actuator assembly88is generally shown to include an electric motor120and a wedge fork operator122. Electric motor120is mounted to housing60and includes a driveshaft124to which a drive pinion126is fixed. Preferably, a planetary speed reduction unit128is provided between the output of motor120and driveshaft126to increase the output torque of drive pinion126. Wedge fork operator122includes a wedge fork130and first and second reaction blocks132and134, respectively. Wedge fork130includes an elongated stem segment136, a transverse web segment138, and a pair of upstanding fork tangs140. Stem segment136has an end portion142adapted for retention in a socket144formed in housing60and an intermediate portion146having one side face surface on which a gear rack148is formed.

Gear rack148is meshed with drive pinion126such that the amount and direction of rotation of drive pinion126controls the linear movement of wedge fork130between a first or “retracted” position and a second or “extended” position. As seen, fork tangs140extend from web segment138in a common plane with stem segment136and are laterally-spaced to define a channel150. Channel150permits fork tangs140to be located for linear movement on opposite sides of rear output shaft32.

First reaction block132is shown to include an annular hub segment152mounted on rear output shaft32via a bushing154and a plate segment156having a front face surface158and a rear face surface160. Rearward axial movement of first reaction block132relative to rear output shaft32is restrained via a snap ring162and a thrust bearing assembly164that is located between rear face surface160and snap ring162. Front face surface158is shown to be orthonganal to the rotary axis of rear output shaft32and in contact with a rear face surface166of each fork tang140on wedge fork130. Second reaction block134includes an annular hub segment168that is mounted on rear output shaft32via a bushing170and a plate segment172having a front face surface174and a rear face surface176. A thrust bearing assembly178is located between front face surface174of second reaction block134and first disk segment106of pressure plate104. Front face surface174is shown to be orthonganal to the rotary axis of rear output shaft32. In contrast, rear face surface176is best shown inFIG. 3to be angled or tapered and is in contact with a similarly tapered front face surface180of each fork tang140on wedge fork130. In operation, linear translation of wedge fork130causes tapered face surface180on fork tangs140to act against tapered face surface176of second reaction block134. This action results in sliding movement of second reaction block134which, in turn, causes corresponding axial movement of pressure plate104.

Wedge fork130is shown inFIGS. 2 and 3in its retracted position which corresponds to pressure plate104being located in its released position. When electric motor120is thereafter energized, driveshaft126is rotated in a first direction such that the torque on drive pinion126is converted into an axial force on gear rack148. This axial force causes wedge fork130to move from its retracted position in a first (i.e., upward inFIG. 2) linear direction toward its extended position. Such linear movement of wedge fork130causes the angular relationship between face surface180on fork tangs140and face surface176of second reaction block134to generate a longitudinal force coaxial to the rotary axis of rear output shaft32. This longitudinal force is a function of the tangent of the fork tang/second reaction block taper angle and is of a magnitude several times greater than the axial force actually applied to wedge fork130. This longitudinal force is transmitted by second reaction block134through thrust bearing assembly178to apply plate104and causes movement of apply plate104, in opposition to the biasing force exerted by return spring112, for exerting a corresponding clutch engagement force on clutch pack98.

In operation, when mode selector56indicates selection of the two-wheel drive mode, controller58signals electric motor120to rotate drive pinion126in the second direction for moving wedge fork130in a second (i.e., downward inFIG. 2) linear direction until it is located in its retracted position. Such action permits return spring112to forcibly urge pressure plate104to move to its released position. If mode selector56thereafter indicates selection of the part-time four-wheel drive mode, electric motor120is signaled by controller58to rotate drive pinion126in the first direction for linearly translating wedge fork130in the first direction until it is located in its extended position. Such movement of wedge fork130to its extended position acts to cause corresponding movement of pressure plate104to its locked position, thereby coupling front output shaft42to rear output shaft32through clutch assembly86and transfer assembly76.

When mode selector56indicates selection of the on-demand four-wheel drive mode, controller58energizes motor120to rotate drive pinion126until wedge fork130is located in a ready or “stand-by” position. This position may be its retracted position or, in the alternative, an intermediate position. In either case, a predetermined minimum amount of drive torque is delivered to front output shaft42through clutch assembly86in this stand-by condition. Thereafter, controller58determines when and how much drive torque needs to be transferred to front output shaft42based on current tractive conditions and/or operating characteristics of the motor vehicle, as detected by sensors54. Many control schemes are known in the art for adaptively controlling actuation of a transfer clutch in a driveline application. In this regard, commonly owned U.S. Pat. No. 5,323,871 discloses a non-limiting example of a clutch control scheme and the various sensors used therewith, the entire disclosure of which is incorporated by reference.

Referring now toFIG. 6, a transfer case22A is shown with a torque transfer mechanism having a clutch actuator assembly88A incorporating a modified wedge fork operator122A. For purposes of clarity and brevity, common reference numerals are used to identify those components of transfer case22A shown inFIG. 6which are similar to those components described previously in association with transfer case22, with the exception that primed reference numerals indicate slightly modified components. In general, wedge fork operator122A differs from wedge fork operator122in that rollers are retained in guide slots formed between the reaction blocks and the wedge fork. In particular, each fork tang140′ of wedge fork130′ has at lease one elongated guide slot190formed in its rear face surface166′ at and at least one guide slot192formed in its front face surface180′. Guide slots190in fork tangs140′ are aligned with guide slots194formed in front face surface158′ of first reaction block132′. Similarly, guide slots192in fork tangs140′ are aligned with guide slots196formed in rear face surface176′ of second reaction block134′. Rollers198are disposed in the aligned sets of guide slots and are provided to reduce friction generated during movement of wedge fork130′ relative to reaction blocks132′ and134′. Preferably, aligned sets of guide slots190and194are oriented to be orthonganal to the rotary axis of rear output shaft32while the aligned sets of guide slots192and196combine to establish a cam pathway that is angled relative to the rotary axis. The angular orientation of the cam pathway is selected to provide the desired force multiplication and travel characteristics required for the particular application. It will be appreciated that the torque transfer mechanism shown inFIG. 6is controlled and operates similar to that described previously for the torque transfer mechanism shown inFIG. 2 through 4.

Referring now toFIGS. 7 and 8, a transfer case22B is shown equipped with another alternative embodiment of a torque transfer mechanism that is generally similar to that shown inFIG. 6with the exception that a pilot clutch220has now been operably installed between clutch assembly86and clutch actuator assembly88A. Pilot clutch220generally includes a second friction clutch assembly222, a ball ramp operator224, and a second pressure plate226. Second friction clutch assembly222includes a hub228, an extended rim segment230on drum94′, and a clutch pack232having at least one outer plate234that is splined to rim segment230of drum94′ and which is interleaved with a set of inner clutch plates236that are splined to hub228. Clutch assembly222also includes a reaction plate240splined to rim segment230of drum94′ and which is axially located thereon via a snap ring242. Second pressure plate226is shown to include an apply plate segment244also splined to rim segment230of drum94′ and a plate segment246which is supported for sliding movement on rear output shaft32.

In operation, actuation of clutch actuator assembly88′ causes second reaction block134to move second pressure plate226for engaging second friction clutch assembly222which, in turn, functions to actuate ball ramp operator224. Ball ramp operator224includes a first cam ring250to which hub228of second friction clutch assembly222is fixed, and a second cam ring252fixed via a spline connection264to rear output shaft32. Ball ramp operator224further includes load transferring rollers, such as balls254, that are retained in a plurality of aligned sets of cam tracks256and258respectively formed in first cam ring250and second cam ring252. Cam tracks256and258have a varying or ramped groove depth such that relative rotation between first cam ring250and second cam ring252causes axial movement of second cam ring252. Axial movement of second cam ring252results in corresponding movement of first pressure plate104for controlling engagement of first friction clutch assembly86. As seen, a thrust bearing assembly260is disposed between second cam ring252and pressure plate104while another thrust bearing assembly262is located between first cam ring250and drum segment246of second pressure plate226. Preferably, cam tracks256and258represent oblique sections of a helical torus. However, balls254and cam tracks256and258may be replaced with alternative components that cause axial displacement of second cam ring250and second cam ring252. In any arrangement, the load transferring components must not be self-locking or self-engaging so as to permit adaptive actuation of the clutch assembly.

Ball ramp operator224is provided to multiply the apply force exerted by wedge fork operator122A for increasing the clutch engagement force ultimately exerted by pressure plate104on clutch pack98. Spring112functions to apply a biasing force on pressure plate104and second cam ring252of ball ramp operator224which will release second friction clutch assembly222when wedge fork130′ is moved to its retracted position. Variable control of the location of wedge fork130′ generates a variable apply force that is exerted by second reaction block134′ on second pressure plate226which, in turn, controls engagement of clutch pack232and the cam thrust force outputted from ball ramp operator224to pressure plate104. The use of pilot clutch220for amplifying the clutch engagement force applied to primary friction clutch assembly86allows the size and number of clutch plates to be reduced and further permits finer control over clutch engagement which results in smoother operation.

To illustrate an alternative power transmission device to which the present invention is applicable,FIG. 9schematically depicts a front-wheel based four-wheel drivetrain layout10′ for a motor vehicle. In particular, engine18drives a multi-speed transmission20′ having an integrated front differential unit38′ for driving front wheels34via axle shafts33. A transfer unit35is also driven by transmission20′ for delivering drive torque to the input member of an in-line torque transfer coupling270via a drive shaft30′. In particular, the input member of transfer coupling270is coupled to drive shaft30′ while its output member is coupled to a drive component of rear differential28which, in turn, drives rear wheels24via axleshafts25. Accordingly, when sensors indicate the occurrence of a front wheel slip condition, controller58adaptively controls actuation of torque coupling270such that drive torque is delivered “on-demand” to rear wheels24. It is contemplated that torque transfer coupling270would include a multi-plate clutch assembly and a clutch actuator that is generally similar in structure and function to that of any of the devices previously described herein. Furthermore, while shown in association with rear differential28, it is contemplated that torque coupling270could also be operably located at the front of the motor vehicle for transferring drive torque from transfer unit35to drive shaft30′.

Referring toFIG. 10, torque coupling270is schematically illustrated operably disposed between drive shaft30′ and rear differential28. Rear differential28includes a pair of side gears272that are connected to rear wheels24via rear axle shafts25. Differential28also includes pinions274that are rotatably supported on pinion shafts fixed to a carrier276and which mesh with side gears272. A right-angled drive mechanism is associated with differential28and includes a ring gear278that is fixed for rotation with carrier276and which is meshed with a pinion gear280that is fixed for rotation with a pinion shaft282.

Torque coupling270includes a mutli-plate clutch assembly284operably disposed between driveshaft30′ and pinion shaft282and which includes a hub286fixed to driveshaft30′, a drum288fixed to pinion shaft282, and a clutch pack290. Torque coupling270also includes a clutch actuator292for controlling engagement of clutch assembly284and thus the amount of drive torque transferred from drive shaft30′ to differential28. Accordingly, to the present invention, clutch actuator292is contemplated to be similar to motor-driven wedge-type clutch actuators88or88′ in that an electric motor controls translation of a wedge fork operator which, in turn, controls engagement of clutch pack290.

Torque coupling270permits operation in any of the drive modes previously disclosed. For example, if the on-demand 4WD mode is selected, controller58regulates activation of clutch actuator292in response to operating conditions detected by sensors54by varying the electric control signal sent to the electric motor. Selection of the part-time 4WD mode results in complete engagement of clutch pack290such that pinion shaft282is rigidly coupled to driveshaft30′. Finally, in the two-wheel drive mode, clutch pack290is released such that pinion shaft282is free to rotate relative to driveshaft30′.

Referring now toFIG. 11, a torque coupling300is schematically illustrated in association with an on-demand four-wheel drive system based on a front-wheel drive vehicle similar to that shown in FIG.9. In particular, an output shaft302of transaxle20′ is shown to drive an output gear304which, in turn, drives an input gear306fixed to a carrier308associated with front differential unit38′. To provide drive torque to front wheels34, front differential unit38′ includes a pair of side gears310that are connected to front wheels34via axleshafts33. Differential unit38′ also includes pinions312that are rotatably supported on pinion shafts fixed to carrier308and which are meshed with side gears310. A transfer shaft314is provided to transfer drive torque from carrier308to a clutch hub316associated with a multi-pate clutch assembly318. Clutch assembly318further includes a drum320and a clutch pack322having interleaved clutch plates operably connected between hub316and drum320.

Transfer unit35is a right-angled drive mechanism including a ring gear324fixed for rotation with drum320of clutch assembly318which is meshed with a pinion gear326fixed for rotation with drive shaft30′. As seen, a clutch actuator assembly328is schematically illustrated for controlling actuation of clutch assembly318. According to the present invention, clutch actuator assembly328is similar to either the motor-driven wedge-type clutch actuators88,88′ previously described in that an electric motor is supplied with electric current for controlling translational movement of a wedge fork operator which, in turn, controls engagement of clutch pack322. 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 selector56. For example, if the on-demand 4WD mode is selected, controller58modulates actuation of clutch actuator328in response to the vehicle operating conditions detected by sensors54by 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 pack322to the rear driveline through transfer unit35and drive shaft30′ is adaptively controlled. Selection of a locked or part-time 4WD mode results in full engagement of clutch assembly318for rigidly coupling the front driveline to the rear driveline. In some applications, the mode selector56may 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. 12illustrates a modified version ofFIG. 11wherein 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 wheels24while selectively transmitting drive torque to front wheels34through a torque coupling300A. In this arrangement, drive torque is transmitted directly from transmission output shaft302to transfer unit35via a drive shaft330interconnecting input gear306to ring gear324. To provide drive torque to front wheels34, torque coupling300A is shown operably disposed between drive shaft330and transfer shaft314. In particular, clutch assembly318is arranged such that drum320is driven with ring gear324by drive shaft330. As such, actuation of clutch actuator328functions to transfer torque from drum320through clutch pack322to hub316which, in turn, drives carrier308of front differential unit38′ via transfer shaft314. Again, the vehicle could be equipped with mode selector56to permit selection by the vehicle operator of either the adaptively controlled on-demand 4WD mode or the locked part-time 4WD mode. In vehicles without mode selector56, the on-demand 4WD 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. 13schematically illustrates a full-time four-wheel drive system which is generally similar to the on-demand four-wheel drive system shown inFIG. 12with the exception that an interaxle differential unit340is now operably installed between carrier308of front differential unit38′ and transfer shaft314. In particular, output gear306is fixed for rotation with a carrier342of interaxle differential340from which pinion gears344are rotatably supported. A first side gear346is meshed with pinion gears344and is fixed for rotation with drive shaft330so as to be drivingly interconnected to the rear driveline through transfer unit35. Likewise, a second side gear348is meshed with pinion gears344and is fixed for rotation with carrier308of front differential unit38′ so as to be drivingly interconnected to the front driveline. A torque transfer mechanism300B is now shown to be operably disposed between side gears346and348. Torque transfer mechanism300B is similar to torque transfer mechanism300A except that it is operably arranged between the driven outputs of interaxle differential340for providing a torque biasing and slip limiting function. Torque transfer mechanism300B is shown to include multi-plate clutch assembly318and clutch actuator328. Clutch assembly318is operably arranged between transfer shaft314and driveshaft330. In operation, when sensor54detects a vehicle operating condition, such as excessive interaxle slip, controller58adaptively controls activation of the electric motor associated with clutch actuator328for controlling engagement of clutch assembly318and thus the torque biasing between the front and rear driveline.

Referring now toFIG. 14, a full-time 4WD system is shown to include a transfer case22C equipped with an interaxle differential350between an input shaft351and output shafts32′ and42′. Differential350includes an input defined as a planet carrier352, a first output defined as a first sun gear354, a second output defined as a second sun gear356, and a gearset for permitting speed differentiation between first and second sun gears354and356. The gearset includes meshed pairs of first planet gears358and second pinions360which are rotatably supported by carrier352. First planet gears358are shown to mesh with first sun gear354while second planet gears350are meshed with second sun gear356. First sun gear354is fixed for rotation with rear output shaft32′ so as to transmit drive torque to rear driveline12. To transmit drive torque to front driveline14, second sun gear356is coupled to transfer assembly76which includes first sprocket78rotatably supported on rear output shaft32′, a second sprocket82fixed to front output shaft42′, and a power chain84.

Transfer case22C further includes a biasing clutch50′ having a multi-plate clutch assembly86and a mode actuator52′ having a clutch actuator assembly88. Clutch assembly86includes drum94fixed for rotation with first sprocket78, hub90fixed for rotation with rear output shaft32′, and multi-plate clutch pack98operably disposed therebetween.

Referring now toFIG. 15, a drive axle assembly400is schematically shown to include a pair of torque couplings operably installed between a driven pinion shaft282and rear axle shafts25. Pinion shaft282drives a right-angle gearset including pinion280and ring gear278which, in turn, drives a transfer shaft402. A first torque coupling270A is shown disposed between transfer shaft402and one of axle shaft25while a second torque coupling270B is disposed between transfer shaft402and the other of axle shafts25. Each of the torque couplings can be independently controlled via activation of its corresponding clutch actuator292A,292B to adaptively control side-to-side torque delivery. In a preferred application, axle assembly400can 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.