Abstract:
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.

Description:
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. 
    
    
     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 the power transfer system of the present invention incorporated therein; 
     FIG. 2 is a sectional view of a transfer case associated with the power transfer system and which includes a clutch assembly and an electronically-controlled linear piston hydraulic clutch actuator; 
     FIG. 3 is a sectional view of the linear piston power unit associated with the transfer case shown in FIG. 2; 
     FIG. 3A is a sectional view of the linear piston power unit which shows an alternate embodiment of the invention incorporating more than one apply piston; 
     FIG. 4 is a sectional view of an axial arrangement of the linear piston power unit shown in FIG. 3A; and 
     FIG. 5 is an alternate arrangement of the four-wheel drive vehicle shown in FIG.  1 . 
    
    
     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 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 . 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 shaft  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 second output shaft  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 torque transfer clutch  38  for selectively delivering drive torque to front wheels  12  (i.e., the non-driven wheels) to establish 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 one or more vehicle 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 shaft  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 shaft  36  is, in effect, rigidly coupled for driven rotation with first output shaft  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 state of transfer clutch  38  as 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 clutch  38  during on-demand operation. 
     Transfer case  20  is shown in FIG. 2 to include a housing  48  formed by a series of modular sections that are suitably interconnected in a conventional manner. A transmission output shaft (not shown) couples transmission  18  (FIG. 1) to a mainshaft  50  of transfer case  20  for supplying power thereto. In the embodiment shown, first output shaft  30  is connected to mainshaft  50  which is supported for rotation within housing  48 . For simplicity, the illustrated embodiment shows mainshaft  50  extending through the transfer case  20  so as to define a single-speed power transfer unit. Those skilled in the art will appreciate that a two-speed version of transfer case  20  could 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 to FIG. 2, transfer clutch  38  is shown for transferring drive torque from mainshaft  50  to front wheels  12 . More specifically, a drive sprocket  52  is fixed (i.e., splined) for rotation on a tubular extension  54  of a cylindrical drum  56  associated with transfer clutch  38 . In addition, extension  54  is rotatably supported on mainshaft  50  by one or more suitable bearing assemblies  58 . Drive sprocket  52  drivingly engages a chain  60  which is coupled to a lower driven sprocket  62 . Driven sprocket  62  is coupled to, or an integral portion of, second output shaft  36  of transfer case  20 . Second output shaft  36  is supported for rotation within housing  48  by suitable bearing assemblies  64  and  66 . As noted in FIG. 1, second output shaft  36  is operably connected to the motor vehicle&#39;s front wheels  12  via front drive shaft  34 . 
     Transfer clutch  38  is a multi-plate clutch assembly that is arranged to concentrically surround a portion of mainshaft  50 . As noted, cylindrical drum  56  is fixedly secured to drive sprocket  52   50  as to drive, or be driven by, front output shaft  36  of transfer case  20 . In a preferred form, transfer clutch  38  also includes a clutch hub  68  that is concentrically surrounded by drum  56  and which is fixed (i.e., splined) to mainshaft  50  for rotation therewith. Thus, clutch hub  68  and drum  56  are capable of rotating relative to one another and form an internal chamber therebetween. Disposed within the internal chamber is a clutch pack  70  comprised of two sets of alternatively interleaved friction clutch plates  72  that are operable for transferring torque from mainshaft  50  through clutch hub  68  to drum  56  and, ultimately, to front output shaft  36  in 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 hub  68  while the second set of clutch plates, referred to as outer clutch plates, are mounted (i.e., splined) for rotation with drum  56 . In addition, a reaction plate  74  is mounted on or integral with one end of clutch hub  68 . A pressure apply plate  76  is rotatable with drum  56  and yet is axially moveable with respect to the interleaved friction clutch plates of clutch pack  70 . Thus, apply plate  76  acts as a pressure plate for compressing the interleaved clutch plates  72   50  as to cause drive torque to be transferred through transfer clutch  38  as a function of the clutch engagement force exerted on apply plate  76  which is generated by a power-operated clutch actuator  78 . 
     Power-operated clutch actuator  78  includes a linear piston hydraulic power unit  80  and an apply piston  82  interconnected via a closed hydraulic circuit (see FIG.  3 ). Apply piston  82  is shown in FIG. 2 to be an annular component retained in an apply chamber  84  connected to housing  48 . An inlet passage  86  communicates with apply chamber  84  and receives hydraulic fluid through one or more supply passages  88 . An alternative embodiment of the apply piston arrangement, as shown in FIGS. 3A and 4, includes multiple apply pistons  82  retained within apply chambers  84 . 
     Linear piston hydraulic power unit  80  is fixed to housing  48  and is shown in FIG. 3 to generally include a ball-screw assembly  90  operably coupled to an electric motor  92  via a reduction gearset  94 . Ball-screw assembly  90  is retained in a cylindrical housing  96  that is integral to or connected to control cylinder  98 . Ball-screw assembly  90  includes a lead screw  100  having threads  102 , a ball nut  104  having threads  106 , and rollers  108  retained between the threads  102  and  106 . Lead screw  100  is supported for rotation in housing  96  by a bearing assembly  110 . Rotation of lead screw  100  in a first rotary direction causes linear translation of ball nut  104  in a first axial direction while rotation of lead screw  100  in the opposite second rotary direction causes linear translation of ball nut  104  in a second axial direction. Reduction gearset  94  is shown to include a first gear  112  that is fixed for rotation with lead screw  100 . A second gear  114  that is fixed for rotation with a rotor shaft  116  of electric motor  92  is meshed with first gear  112 . Thus, rotation of rotor shaft  116  upon actuation of electric motor  92  controls the resulting direction and magnitude of linear movement of ball nut  104 . 
     Control piston  118  is shown fixed to ball nut  104  for linear bi-directional movement therewith. Control piston  118  is a closed-ended cylindrical member concentrically mounted over the end of lead screw  100  and which is sealed relative to a control chamber  120  formed in control cylinder  98  via a seal ring  122 . Supply passage(s)  88  are in fluid communication with control chamber  120  via a corresponding number of control ports  124 . FIGS. 3A and 4 show supply passages  88  interconnecting to a common control port  124 . 
     Referring again to FIG. 2, apply piston(s)  82  acts on a transfer plate  126  journalled on mainshaft  50  which, in turn, transfers the clutch engagement force to apply plate  76  through a thrust bearing assembly  128 . Transfer plate  126  is an annular component adapted to radially accommodate multiple apply pistons  82 . A return spring  130  acts between clutch hub  68  and apply plate  76  toward a released position. 
     Referring to FIGS. 1 to  5  collectively, controller  42  determines the operational mode based on the current mode signal delivered thereto via mode selector  40 . If the two-wheel drive mode is selected, controller  42  sends an electric control signal to electric motor  92  causing rotation of rotor shaft  116  in a direction which, in turn, causes linear retraction (i.e., toward the electric motor  92  in FIG. 4) of control piston  118  in control chamber  120  to a first position. Since hydraulic fluid is virtually incompressible, the fluid displaced by such movement of control piston  118  causes corresponding retraction of apply piston  82  (i.e., away from apply plate  76  in FIG.  2 ). Concurrently, return spring  130  forcibly urges apply plate  76  to its released position such that no drive torque is transferred through clutch pack  70  to second output shaft  36 . 
     When the part-time four-wheel drive mode is selected, controller  42  sends an electric signal to motor  92  causing rotation of rotor shaft  116  in a direction causing linear extension (i.e., away from the electric motor  92  in FIG. 4) of control piston  118  in control cylinder  98  to a second position. The fluid displaced by such movement of control piston  118  to its second position causes corresponding expansion of apply piston  82  (i.e., toward apply plate  76  in FIG. 2) for exerting a predetermined maximum clutch engagement force on clutch pack  70 , thereby rigidly coupling clutch drum  56  for rotation with clutch hub  68 . 
     When the on-demand drive mode is selected, the amount of drive torque transferred through clutch pack  70  is 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. Controller  42  calculates a desired clutch engagement force and generates the same by controlling the position of control piston  118  between its first and second positions. 
     FIG. 3A depicts linear piston hydraulic power unit  80  where electric motor  92  is axially aligned with ball-screw assembly  90 . This axial arrangement eliminates reduction gearset  94  to 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 piston  118  or apply piston(s)  82 . 
     In view of the above arrangement, rotor shaft  116  acts 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.