Power transfer device with hydraulic clutch actuation

A power transfer system is provided and equipped with a torque transfer coupling which includes a clutch and a ball-screw actuator. The ball-screw actuator functions to axially translate an apply plate via a closed hydraulic system to operatively engage the clutch and vary the frictional engagement thereof.

FIELD OF THE INVENTION

The present invention relates generally to power transfer systems operable for controlling the distribution of drive torque between a pair of rotary shafts and, more particularly, to a torque transfer clutch assembly equipped with a hydraulic linear piston clutch 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. A power-operated actuator controls engagement of the clutch pack. Specifically, torque is transferred from the drive member to the driven member by actuating the power-operated actuator. The power-operated actuator displaces an apply plate which acts on the clutch pack and increases the frictional engagement between the interleaved plates.

A variety of power-operated actuators have been used in the art. 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, includes a linear actuator that pivots a lever arm to regulate the frictional forces applied to the clutch pack. Neither of these references incorporate a closed hydraulic system to control actuation of the associated clutch. While the above actuator devices may perform adequately for their intended purpose, a need exists for an improved actuator that is less complex, reduces the number of friction generating components which lead to inefficiencies and larger motor requirements, and an annular arrangement that provides operational simplicity and reduced space 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 equipped with a torque transfer clutch assembly having a multi-plate friction clutch pack and a hydraulic linear piston clutch actuator. The hydraulic linear piston clutch actuator includes a ball screw assembly having a threaded lead screw and a ball nut. The threaded lead screw is rotated by an electric motor through a reduction gearset for causing linear translation of the ball nut. A control piston is secured to the ball nut for linear movement in a control chamber which, in turn, is in fluid communication with apply chambers to define a closed hydraulic circuit. Multiple apply chambers are radially located about a transfer plate which is rotatably coupled to a clutch apply plate. An apply piston is retained in each apply chamber and is moveable in response to movement of the control piston for exerting a clutch engagement force on the clutch pack. This clutch actuator arrangement yields numerous operational advantages over the prior art including, but not limited to, improved response characteristics with lower hysteresis, superior torque control improved system efficiency, low cost, and weight savings.

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.

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. 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 toFIG. 1of the drawings, a drivetrain for a four-wheel drive vehicle is schematically shown interactively associated with a power transfer system10. The motor vehicle drivetrain has a pair of front wheels12and rear wheels14both drivable from a source of power, such as an engine16, through a transmission18which 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 case20operable to receive drive torque from engine16and transmission18for normally driving rear wheels14(i.e., the “driven” wheels) in a two-wheel drive mode of operation. Front wheels12and rear wheels14are shown connected at opposite ends of front and rear axle assemblies22and24, respectively. As is known, a rear differential26is interconnected between rear axle assembly24and one end of a rear drive shaft28, the opposite end of which is interconnected to a first output shaft30of transfer case20. Similarly, front axle assembly22includes a front differential32that is coupled to one end of a front drive shaft34, the opposite end of which is coupled to a second output shaft36of transfer case20. 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 wheels12.

Transfer case20is equipped with a torque transfer clutch38for selectively delivering drive torque to front wheels12(i.e., the non-driven wheels) to establish a four-wheel drive mode of operation. The operating mode of transfer clutch38is generally controlled in response to a mode signal generated by a mode selector40and which is sent to a controller42. Controller42also receives input signals from one or more vehicle sensors44that 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 shaft30to rear wheels14and transfer clutch38is maintained in a “non-actuated” condition. When the part-time four-wheel drive mode is selected, transfer clutch38is fully actuated and maintained in a “lock-up” condition such that second output shaft36is, in effect, rigidly coupled for driven rotation with first output shaft30. When the “on-demand” drive mode is selected, controller42controls the degree of actuation of transfer clutch38for varying the amount of drive torque directed to front wheels12through transfer clutch38as a function of the sensor input signals for providing improved tractive performance when needed. In addition, controller42is adapted to controllably modulate the actuated state of transfer clutch38as described in greater detail 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 clutch38during on-demand operation.

Transfer case20is shown inFIG. 2to include a housing48formed by a series of modular sections that are suitably interconnected in a conventional manner. A transmission output shaft (not shown) couples transmission18(FIG. 1) to a mainshaft50of transfer case20for supplying power thereto. In the embodiment shown, first output shaft30is connected to mainshaft50which is supported for rotation within housing48. For simplicity, the illustrated embodiment shows mainshaft50extending through the transfer case20so as to define a single-speed power transfer unit. Those skilled in the art will appreciate that a two-speed version of transfer case20could likewise be used in association with the novel active torque bias clutch system of the present invention. Examples of known planetary two-speed gearsets and range clutch arrangements are shown in commonly-owned U.S. Pat. Nos. 5,700,222, and 5,836,847.

With continued references toFIG. 2, transfer clutch38is shown for transferring drive torque from mainshaft80to front wheels12. More specifically, a drive sprocket52is fixed (i.e., splined) for rotation on a tubular extension54of a cylindrical drum56associated with transfer clutch38. In addition, extension54is rotatably supported on mainshaft50by one or more suitable bearing assemblies58. Drive sprocket52drivingly engages a chain60which is coupled to a lower driven sprocket62. Driven sprocket62is coupled to, or an integral portion of, second output shaft36of transfer case20. Second output shaft36is supported for rotation within housing48by suitable bearing assemblies64and66. As noted inFIG. 1, second output shaft36is operably connected to the motor vehicle's front wheels12via front drive shaft34.

Transfer clutch38is a multi-plate clutch assembly that is arranged to concentrically surround a portion of mainshaft50. As noted, cylindrical drum56is fixedly secured to drive sprocket52so as to drive, or be driven by, front output shaft36of transfer case20. In a preferred form, transfer clutch38also includes a clutch hub68that is concentrically surrounded by drum56and which is fixed (i.e., splined) to mainshaft50for rotation therewith. Thus, clutch hub68and drum56are capable of rotating relative to one another and form an internal chamber therebetween. Disposed within the internal chamber is a clutch pack70comprised of two sets of alternatively interleaved friction clutch plates72that are operable for transferring torque from mainshaft50through clutch hub68to drum56and, ultimately, to front output shaft36in response to a clutch engagement force applied thereto. One set of clutch plates, referred to as inner clutch plates, are mounted (i.e., splined) for rotation with clutch hub68while the second set of clutch plates, referred to as outer clutch plates, are mounted (i.e., splined) for rotation with drum56. In addition, a reaction plate74is mounted on or integral with one end of clutch hub68. A pressure apply plate76is rotatable with drum56and yet is axially movable with respect to the interleaved friction clutch plates of clutch pack70. Thus, apply plate76acts as a pressure plate for compressing the interleaved clutch plates72so as to cause drive torque to be transferred through transfer clutch38as a function of the clutch engagement force exerted on apply plate76which is generated by a power-operated clutch actuator78.

Power-operated clutch actuator78includes a linear piston hydraulic power unit80and an apply piston82interconnected via a closed hydraulic circuit (see FIG.3). Apply piston82is shown inFIG. 2to be an annular component retained in an apply chamber84connected to housing48. An inlet passage86communicates with apply chamber84and receives hydraulic fluid through one or more supply passages88. An alternative embodiment of the apply piston arrangement, as shown inFIGS. 3A and 4, includes multiple apply pistons82retained within apply chambers84.

Linear piston hydraulic power unit80is fixed to housing48and is shown inFIG. 3to generally include a ball-screw assembly90operably coupled to an electric motor92via a reduction gearset94. Ball-screw assembly90is retained in a cylindrical housing96that is integral to or connected to control cylinder98. Ball-screw assembly90includes a lead screw100having thread102, a ball nut104having threads106, and rollers108retained between the threads102and106. Lead screw100is supported for rotation in housing96by a bearing assembly110. Rotation of lead screw100in a first rotary direction causes linear translation of ball nut104in a first axial direction while rotation of lead screw100in the opposite second rotary direction causes linear translation of ball nut104in a second axial direction. Reduction gearset94is shown to include a first gear112that is fixed for rotation with lead screw100. A second gear114that is fixed for rotation with a rotor shaft116of electric motor92is meshed with first gear112. Thus, rotation of rotor shaft116upon actuation of electric motor92controls the resulting direction and magnitude of linear movement of ball nut104.

Control piston118is shown fixed to ball nut104for linear bi-directional movement therewith. Control piston118is a closed-ended cylindrical member concentrically mounted over the end of lead screw100and which is sealed relative to a control chamber120formed in control cylinder98via a seal ring122. Supply passage(s)88are in fluid communication with control chamber120via a corresponding number of control ports124.FIGS. 3A and 4show supply passages88interconnecting to a common control port124.

Referring again toFIG. 2, apply piston(s)82acts on a transfer plate126journalled on mainshaft50which, in turn, transfers the clutch engagement force to apply plate76through a thrust bearing assembly128. Transfer plate126is an annular component adapted to radially accommodate multiple apply pistons82. A return spring130acts between clutch hub68and apply plate76toward a released position.

Referring toFIGS. 1to5collectively, controller42determines the operational mode based on the current mode signal delivered thereto via mode selector40. If the two-wheel drive mode is selected, controller42sends an electric control signal to electric motor92causing rotation of rotor shaft116in a direction which, in turn, causes linear retraction (i.e., toward the electric motor92inFIG. 4) of control piston118in control chamber120to a first position. Since hydraulic fluid is virtually incompressible, the fluid displaced by such movement of control piston118causes corresponding retraction of apply piston82(i.e., away from apply plate76in FIG.2). Concurrently, return spring130forcibly urges apply plate76to its released position such that no drive torque is transferred through clutch pack70to second output shaft36.

When the part-time four-wheel drive mode is selected, controller42sends an electric signal to motor92causing rotation of rotor shaft116in a direction causing linear extension (i.e., away from the electric motor92inFIG. 4) of control piston118in control cylinder98to a second position. The fluid displaced by such movement of control piston118to its second position causes corresponding expansion of apply piston82(i.e., toward apply plate76inFIG. 2) for exerting a predetermined maximum clutch engagement force on clutch pack70, thereby rigidly coupling clutch drum56for rotation with clutch hub68.

When the on-demand drive mode is selected, the amount of drive torque transferred through clutch pack76is adaptively controlled as a function of various vehicle conditions which may include, without limitation, interaxle speed difference, vehicle speed, throttle position, brake status, steering angle, etc. Controller42calculates a desired clutch engagement force and generates the same by controlling the position of control piston118between its first and second positions.

FIG. 3Adepicts linear piston hydraulic power unit80where electric motor92is axially aligned with ball-screw assembly90. This axial arrangement eliminates reduction gearset94to further reduce the friction loss associated with known clutch actuation assemblies. Any required mechanical advantage can be accomplished through a change in diameter of either control piston118or apply piston(s)82.

In view of the above arrangement, rotor shaft116acts as the input to the ball-screw yielding a mechanically simple system that eliminates more complex mechanical designs generally used in the art that include a plurality of gears and/or linkages. As each of the mechanical components of the actuator contain friction elements, the elimination of some 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 clutch actuation electric motors and more accurate clutch torque estimation. 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.

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.