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

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to power transfer systems for controlling the distribution of drive torque between front and rear wheels of a four-wheel drive vehicle and, more particularly, to a torque transfer coupling equipped with a ball-screw actuator. 
     BACKGROUND OF THE INVENTION 
     In view of increased consumer demand for four-wheel drive vehicles, a plethora of power transfer systems are currently being utilized in vehicular driveline applications for selectively directing power (i.e., drive torque) to the non-driven wheels of the vehicle. In many power transfer systems, a part-time transfer case is incorporated into the driveline and is normally operable in a two-wheel drive mode for delivering drive torque to the driven wheels. A mechanical “mode” shift mechanism can be selectively actuated by the vehicle operator for rigidly coupling the non-driven wheel to the driven wheels in order to establish a part-time four-wheel drive mode. As will be appreciated, a motor vehicle equipped with a part-time transfer case offers the vehicle operator the option of selectively shifting between the two-wheel drive mode during normal road conditions and the part-time four-wheel drive mode for operation under adverse road conditions. 
     Alternatively, it is known to use “on-demand” power transfer systems for automatically directing power to the non-driven wheels, without any input or action on the part of the vehicle operator, when traction is lost at the driven wheels. Modernly, it is known to incorporate the “on-demand” feature into a transfer case by replacing the mechanically-actuated mode shift mechanism with a clutch assembly that is interactively associated with an electronic control system and a sensor arrangement. During normal road conditions, the clutch assembly is maintained in a non-actuated condition such that the drive torque is only delivered to the driven wheels. However, when the sensors detect a low traction condition at the driven wheels, the clutch assembly is automatically actuated to deliver drive torque “on-demand” to the non-driven wheels. Moreover, the amount of drive torque transferred through the clutch assembly to the non-driven wheels can be varied as a function of specific vehicle dynamics, as detected by the sensor arrangement. 
     Conventional clutch assemblies, typically include a clutch pack operably connected between a drive member and a driven member, and a power-operated actuator for controlling engagement of the clutch pack. Specifically, torque is transferred from the drive member to the driven member by actuating the power-operated actuator for displacing an apply plate which acts on the clutch pack and increases the friction of engagement between the interleaved plates. 
     A variety of power-operated actuators have been used in the art with mixed results. Exemplary embodiments include those disclosed in U.S. Pat. No. 5,407,024 wherein a ball-ramp arrangement is used to displace the apply plate when a current is provided to an induction motor. Another example disclosed in U.S. Pat. No. 5,332,060, assigned to the assignee of the present application, which includes a linear actuator that displaces a pivoting lever arm to increase the friction forces in the clutch pack. While the above actuator devices have performed generally 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 wherein the torque transfer coupling includes a clutch pack and a ball-screw actuator. The ball-screw actuator functions to axially translates an apply plate to operatively engage the clutch pack and vary the frictional engagement. This arrangement yields numerous operational advantages over the prior art including, but not limited to, establishing a direct drive between the motor output shaft and the apply plate, concentric mounting of the actuator elements with the motor output shaft, and a simplified mechanical arrangement that reduces the number of frictional elements increasing operational efficiency and decreasing motor requirements. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the following detailed description, attached drawings and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given here below, the appended claims, and the accompanying drawings in which: 
     FIG. 1 is a schematic representation of an exemplary four-wheel drive vehicle having the power transfer system of the present invention incorporated therein; 
     FIG. 2 is a cross-sectional view of a transfer case associated with the power transfer system and which includes a clutch assembly, a drive mechanism, and an electronically-controlled ball-screw actuator; and 
     FIG. 3 is a schematic representation of the power transfer system including the clutch assembly and actuator. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In general, the present invention is directed to a power transfer system which is operably installed between the driven and non-driven wheels of a four-wheel drive vehicle. In operation, the amount of power (i.e., drive torque) transferred to the non-driven wheels is controllably regulated in accordance with various system and driver-initiated inputs for optimizing the tractive characteristics of the vehicle while concomitantly enhancing overall steering control. In addition, the power transfer system may also include a mode select mechanism for permitting a vehicle operator to select between a two-drive wheel mode, a part-time four-wheel drive mode, and an “on-demand” drive mode. 
     Referring to FIG. 1 of the drawings, a drivetrain for a four-wheel drive vehicle is schematically shown interactively associated with a power transfer system  10  which incorporates the novel principles of the present invention. The motor vehicle drivetrain has a pair of front wheels  12  and rear wheels  14  both drivable from a source of power, such as an engine  16 , through a transmission  18  which may be of either the manual or automatic type. In the particular embodiment shown, the drivetrain is a rear wheel drive system which incorporates a transfer case  20  operable to receive drive torque from engine  16  and transmission  18  for normally driving rear wheels  14  (i.e., the “driven” wheels) in a two-wheel drive mode of operation. Front wheels  12  and rear wheels  14  are shown connected at opposite ends of front and rear axle assemblies  22  and  24 , respectively. As is known, a rear differential  26  is interconnected between rear axle assembly  24  and one end of a rear drive shaft  28 , the opposite end of which is interconnected to a first output member  30  of transfer case  20 . Similarly, front axle assembly  22  includes a front differential  32  that is coupled to one end of a front drive shaft  34 , the opposite end of which is coupled to a section output member  36  of transfer case  20 . It is to be understood that the specific orientation of the drivetrain is merely exemplary in nature and that the drivetrain could be reversed for normally driving front wheels  12 . 
     Transfer case  20  is equipped with a 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 sensors  44  that are indicative of various operational characteristic of the vehicle. 
     When the two-wheel drive mode is selected, all drive torque is delivered from first output member  30  to rear wheels  14  and transfer clutch  38  is maintained in a “non-actuated” condition. When the part-time four-wheel drive mode is selected, transfer clutch  38  is fully actuated and maintained in a “lock-up” condition such that second output member  36  is, in effect, rigidly coupled for driven rotation with first output member  30 . When the “on-demand” drive mode is selected, controller  42  controls the degree of actuation of transfer clutch  38  for varying the amount of drive torque directed to front wheels  12  through transfer clutch  38  as a function of the sensor input signals for providing improved tractive performance when needed. In addition, controller  42  is adapted to controllably modulate the actuated condition of transfer clutch  38  to provide superior handling and steering control by minimizing the oversteer and understeer tendencies of the vehicle during a cornering maneuver. Other advantages associated with controllably modulating the actuated state of transfer clutch  38  will be detailed hereinafter. By way of example rather than limitation, the control scheme generally disclosed in U.S. Pat. No. 5,332,060 issued Jul. 26, 1994 to Sperduti et al. and assigned to the common assignee of the present invention (the disclosure of which is hereby incorporated by reference) can be used to control adaptive actuation of transfer clutch  38  during on-demand operation. 
     Transfer case  20  is shown to include a housing  56  formed by a series of modular sections that are suitably interconnected in a conventional manner. A transmission output shaft couples transmission  18  to an input shaft  60  of transfer case  20  for supplying power thereto. In the embodiment shown, first output member  30  includes an elongated mainshaft  62  which is aligned on the longitudinal axis of input shaft  60  and is supported for rotation within housing  56 . For simplicity, the illustrated embodiment shows input shaft  60  extending as a mainshaft through the transfer case to form first output member  30 . However, those skilled in the art will appreciate that a variety of intermediate sleeves or shafts splined to rotate with one another may be used to drivably couple a separate input shaft  60  for rotation with the output shaft  62 . An exemplary illustration of such a shaft and sleeve arrangement is described in detail in U.S. Pat. No. 5,332,060 which is hereby expressly incorporated into this description by reference. 
     With continued references to FIGS. 2 and 3, transfer clutch  38  is shown for transferring drive torque from input shaft  60  to front wheels  12 . More specifically, a drive sprocket  64  is fixed (i.e., splined) for rotation on a tubular extension  78  of a rotatable cylindrical drum  68  associated with transfer clutch  38 . In addition, extension  78  is rotatably supported on input shaft  60  by one or more suitable bearing assemblies  70 . Drive sprocket  64  drivingly engages a chain  72  which is coupled to a lower driven sprocket  74 . Driven sprocket  74  is coupled to, or an integral portion of, second output member  36  of transfer case  20 . Second output member  36  is shown as a front output shaft  76  which is supported for rotation within housing  56  by suitable bearing assemblies  78  and  80 . As noted, front output shaft  76  is operably connected to the motor vehicle&#39;s front wheel  12  via front drive shaft  34 . 
     Transfer clutch  38  is a multi-plate clutch assembly that is arranged to concentrically surround a portion of input shaft  60 . As noted, cylindrical drum  68  is fixedly secured to drive sprocket  64  so as to drive, or be driven by, front output shaft  76  of transfer case  20 . In a preferred form, transfer clutch  38  also includes a clutch hub  82  that is concentrically surrounded by drum  68  and which is fixed (i.e., splined) to input shaft  60  for rotation therewith. Thus, clutch hub  82  and drum  68  are capable of rotating relative to one another and form an internal chamber therebetween. Disposed within the internal chamber are two sets of alternatively interleaved friction clutch plates that are operable for transferring torque from input shaft  60  through clutch hub  82  to drum  68  and, ultimately, to front output shaft  76  in response to a clutch “engagement” force applied thereto. One set of clutch plates, referred to as inner clutch plate  84 , are mounted (i.e., splined) for rotation with clutch hub  82  while the second set of clutch plates, referred to as outer clutch plates  86 , are mounted (i.e., splined) for rotation with drum  68 . In addition, a reaction plate  88  is mounted on or integral with one end of clutch hub  82 . In addition, an apply plate  90  is rotatable with clutch hub  68  and yet is axially movable with respect to interleaved friction clutch plates  84  and  86 . Thus, apply plate  90  acts as a pressure plate for compressing the interleaved clutch so as to cause drive torque to be transferred through transfer clutch  38  as a function of the clutch engagement force exerted on apply plate  90  by a power-operated actuator  46 . 
     Power-operated actuator  46  is a ball-screw actuator  92  which operatively engages apply plate  90  and is controlled by controller  42  to selectively control the amount of torque transferred through transfer clutch  38 . Ball-screw actuator  92  provides a concentrically mounted actuator that reduces the mechanical and frictional components when compared to prior art actuators while further directly transferring rotational movement of the motor output to axial movement of apply plate  90  to provide more precise and repeatable and easily controlled reaction plate movement. 
     In general, ball-screw actuator  92  includes an electric motor  100  having a fixed stator  102  and a rotary output shaft  104 , a screw  106 , and a plurality of circumferentially spaced balls  108 . Electric motor  100 , screw  106 , and balls  108  are each concentrically mounted with one another and about output shaft  62 . Screw  106  is mounted within housing  56  for axial, non-rotational movement relative thereto. A thrust bearing  110  is disposed between the application face surfaces of screw  106  and apply plate  90  to permit rotation of apply plate  90  relative to screw  106 . A spring  112  is located between hub  82  and apply plate  90  for normally exerting a return biasing a force on apply plate  90 . 
     In operation, controller  42  selectively delivers current to electric motor  100  which causes motor  100  to rotate its output shaft  104 . Motor output shaft  104  has helical grooves formed on its outer circumferential surface while screw  106  has helical grooves formed on its inner circumferential surface. Balls  108  are disposed within the grooves causing axial displacement of lead screw  106  toward or away from apply plate  90  in response to direction of rotation of motor output shaft  104 . In turn, axial displacement of lead screw  106  causes corresponding movement of apply plate  90  and the desired increase or decrease in the frictional engagement between interleaved clutch plates  84  and  86 . Thus, control of the direction and amount of rotation of shaft  104  controls the magnitude of the clutch engagement force exerted on clutch assembly  38 . 
     In view of the above arrangement, electric motor output shaft  104  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 these components and the more simple design provided by the present invention reduces the overall friction and therefore increases the efficiency of the assembly. Increased efficiency is translated into more economical motors and more accurate clutch torque estimation. The novel annular packaging of the motor and ball-screw actuator permits the outer diameter of motor  100  to be grounded to housing  56 . Those skilled in the art will appreciate that a variety of electric motors may be used including a dc brush, dc brushless, and stepper motors. 
     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.