Patent Publication Number: US-2004050643-A1

Title: Clutch actuator

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates to an actuator for a clutch and, more particularly, to a clutch actuator for transferring rotation and torque from an input shaft to an output shaft.  
       [0002] Clutches are often used in motor vehicles to selectively transfer rotational power from an input shaft to an output shaft. Clutches are typically found in automatic transmissions, differentials, and transfer cases in order to selectively transfer all or a portion of the rotational energy from an input shaft to an output shaft.  
       [0003] Commonly used clutch actuators include hydraulic actuators that use a pump driven by the engine. The pump sends the fluid (e.g., oil) to a thrust mechanism that converts the fluid pressure into a force applied to the clutch pack, causing the clutch pack to engage. Typically, the thrust mechanism is a simple piston, pressure plate, or the like. Notwithstanding the widespread acceptance of hydraulic clutch actuators, it is noted that the piston can be difficult to precisely control, is susceptible to leaks, and is expensive. Hydraulic actuators also have long response times and the pump creates a continuous load on the motor, thereby reducing fuel economy. A variation of hydraulic actuators uses an electric pump instead of a pump driven by the engine. While the electric pump eliminates the continuous load on the motor and improves fuel economy, it retains many of the other concerns of traditional hydraulic actuators.  
       [0004] Conventional clutch actuators may also include an electric motor to apply pressure to the clutch pack. Such systems commonly include a mechanism such as a ball ramp disposed between the motor output and clutch pack to transfer the rotational torque from the motor to a linear force and apply pressure to the clutch pack. One problem with conventional electric motor actuated clutch arrangements is that to provide enough pressure to engage the clutch pack, the motor must be relatively large. As the size of the actuator motor increases, so do the physical space requirements, expense, and weight. Attempts have been made in the art to provide actuator assemblies having the capacity to exert sufficient force on the clutch pack in a controllable and maintainable manner while not requiring undue space or expense. However, the need still exists for improvements in this area.  
       SUMMARY OF THE INVENTION  
       [0005] The present invention is directed to a clutch actuator assembly for a clutch pack wherein the clutch actuator assembly includes a lever rotatable about an axis, a drive mechanism that selectively rotates the lever about the axis, and a translating mechanism that receives input from the lever, converts the input to linear displacement, and is operably coupled to the clutch pack pressure plate to apply the linear displacement. The present invention is further directed to a clutch assembly and torque transferring mechanism having such a clutch actuator assembly.  
       [0006] Further scope of applicability of the present invention will become apparent from the following detailed description, claims, and drawings. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way 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. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0007] The present invention will become more fully understood from the detailed description given here below, the appended claims, and the accompanying drawings in which:  
     [0008]FIG. 1 is a side elevational view of a torque transferring unit cut away to show internal components;  
     [0009]FIG. 2 is an exploded perspective view of the torque transferring unit;  
     [0010]FIG. 3 is a front elevational view of the hollow ball screw;  
     [0011]FIG. 4 is a side elevational view of the secondary ball screw with lever arm;  
     [0012]FIG. 5 is a front elevational view of the secondary ball screw, with lever arm in hidden lines to show movement; and  
     [0013]FIG. 6 is a side elevational view of an alternative embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0014] A clutch assembly  10  constructed in accordance with the invention is illustrated in FIG. 1. In general, the clutch assembly  10  is configured to selectively transfer rotational movement and torque from an input member  12  to an output member  14 . As is conventional, the clutch assembly  10  includes a clutch pack  16  having first clutch plates  18  fixed to rotate with the input member and interleaved with second plates  20  fixed to rotate with the output member  14 . The clutch pack  16  also includes a pressure plate  38  operably coupled to the interleaved plates to impart a force that compresses the plates and transmits torque between the input and output members.  
     [0015] Clutch assembly  10  includes a clutch actuator assembly  22  that controls the position of the pressure plate  38  relative to the interleaved clutch plates. In the illustrated embodiment, the clutch actuator assembly  22  includes a translating mechanism  28 , lever  50 , and drive mechanism  60 . The drive mechanism  60  is coupled to the lever  50 , such as at lower end  52 , and is controlled in a manner known in the art to selectively rotate the lever  50  about an axis  54  (FIG. 4). The translating mechanism  28  has an input member coupled to rotate with the lever  50  and an output member operably connected to the pressure plate  38  so that rotation of the lever  50  changes the actuation force exerted by the pressure plate on the interleaved clutch plates. In the embodiment of the invention illustrated in FIGS.  1 - 5 , the translating mechanism  28  is shown as a ball screw  30 . However, those skilled in the art will appreciate that other mechanisms capable of receiving a rotational input and providing a linear output may be used with the present invention, including the ball ramp configuration shown in FIG. 6.  
     [0016] The structure and arrangement of the clutch assembly components, including the ball screw  30  and lever  50 , provide numerous advantages over conventional clutch actuator assemblies. For example, the actuator assembly provides a mechanical advantage permitting greater and controllable forces upon the clutch pack while minimizing the required size of the drive mechanism  60 , including the drive motor. Moreover, the use of a ball screw provides a controllable and maintainable pressure on the clutch pack, including active and precise control over the force exerted upon the clutch pack. The controllability of the clutch assembly reduces the response time of the actuator and control over the clamping forces. While a representative illustration of the clutch assembly component is provided below, those skilled in the art will appreciate that various modifications may be made to the illustrated embodiments without departing from the scope of the invention defined by the appended claims.  
     [0017] The ball screw  30  acts as the translating mechanism  28  in the embodiment shown in FIGS.  1 - 5  and converts rotational movement to linear movement with force magnification. The ball screw  30  is shown to include a cylindrical screw sleeve  32 , a nut  34 , and ball bearings  36 . The bearings  36  ride in opposed and facing helical threads or grooves  42  and  44  (FIG. 2) formed in the sleeve  32  and nut  34 , respectively. The grooves or threads  34  in the sleeve  42  and nut  44  define a helical path for the bearings  36 , such that rotational movement of the lever  50  and sleeve  32  creates linear movement of the nut  34 . The nut operatively engages the pressure plate  38  such that the compressive force within the clutch pack changes with linear movement of the nut  34 .  
     [0018] Use of a ball screw  30  in the translating mechanism  28  provides specific advantages in the clutch assembly. The shape and pitch of the ball screw grooves or threads  42  and  44  may be varied to tailor the mechanical advantage provided by the ball screw  30  to a particular application. The ball screw  30  provides smooth, consistent and controllable pressure to the clutch pack  20 , while the ball bearings  36  and grooves  42  and  44  effectively transfer rotational movement of the sleeve  32  to linear displacement of the nut  34  and minimize friction losses. Further, by using many small ball bearings  36 , a relatively high load may be applied to the clutch pack without deforming the ball bearings  36 . The specific configuration of the helical grooves  42  and  44  (e.g. pitch and size) may be tailored to provide the desired mechanical advantage based on the amount of pressure needed to be applied to the clutch pack as well as the configuration and capacity of other clutch actuator components, most notably the length of the lever  50  and capacity of the drive mechanism  60 .  
     [0019] Those skilled in the art will appreciate that the illustrated embodiment of the ball screw  30  and lever  50  may be varied without departing from the scope of the invention defined by the appended claims. For example, while the first ball screw nut  32  is illustrated as being radially outward of the sleeve  32 , packaging and/or operational concerns in some applications may be better addressed by positioning the nut radially inward of the sleeve  32 .  
     [0020] In the embodiment shown in FIGS.  1 - 5 , the ball screw sleeve  32  is integral with the lever arm  50 . Of course the lever arm  50  may be attached to the ball screw sleeve  32  by a variety of means such as welding, bonding, or a fastener. The length of the lever arm  50  provides the mechanical advantage for rotating the ball screw sleeve  32 , provides flexibility in the positioning of the drive mechanism  60 , and may be formed in a variety of shapes or configurations, with the length depending on the desired mechanical advantage and packaging constraints.  
     [0021] The lever arm  50  is rotated by the controlled movement of the drive mechanism  60 . As is shown in FIGS. 4 and 5, the drive mechanism  60  has an output  62  pivotally coupled to the lever  50  proximate the lever end  52 . In some instances, the lever  50  may be provided with an elongated lost motion slot  56  (FIG. 5) to facilitate smooth operation of the lever/drive mechanism connection. The slot  56  can be made in a variety of shapes, sizes, and configurations.  
     [0022] While a variety of drive mechanisms  60  may be used to move the lever arm  50 , the drive mechanism  60  is shown to include a secondary ball screw  70  driven by a drive device  72 , such as an electric motor. The use of a secondary ball screw  70  enhances the mechanical advantage provided by and controllability of the clutch actuator assembly  22 . The secondary ball screw  70  may be formed in a variety of sizes, shapes, and configurations without departing from the spirit of the invention and is shown in FIG. 5 to include a helically threaded or grooved screw shaft  74  and a second ball nut  76  which moves linearly along the shaft as the shaft  74  rotates. A pin  78  on the second ball nut  76  is disposed in the slot  56  on the lever arm  50 . The pin  78  slides in the slot  56  as the secondary ball screw  70  moves the lever arm  50 , allowing for the radial movement of the lever arm  50  relative to the shaft  74 . A radial bearing  68  supports the screw shaft to permit the secondary ball screw  70  to freely rotate while maintaining its position and preventing warping, bending, or unwanted movement. Of course, the second ball nut  76  or secondary ball screw  70  may be fastened directly to the lever as shown in FIG. 4.  
     [0023] As is noted throughout this detailed description, various alternatives to the illustrated embodiments may be used without departing from the scope of the invention. For example, FIG. 6 illustrates the clutch actuator assembly  22  wherein a ball ramp  90  is disposed between the lever arm  50  and the pressure plate  38 . While a ball ramp  90  does not provide the actively controllable and maintainable forces to the extent of a ball screw, the alternative illustrates that various known components may be substituted in the present invention. The ball ramp  90  acts as a translating mechanism and includes an inner plate  92  operably coupled to the pressure plate  38 , an outer plate  94  coupled to rotate with the lever arm  50 , and ball ramp bearings  96 . The ball ramp bearings  96  ride in ramps (not shown) in a conventional manner such that rotational movement of the lever  50  and outer plate  94  creates linear movement of the inner plate  92 . The specific shape and angle of the ramps may be varied to tailor the mechanical advantage provided by the ball ramp  90  to a particular application. The mechanical advantage of the ball ramp  90 , lever  50 , and secondary ball screw  90  again allow a small pressure input by the rotational pressure device  72  to be multiplied to a larger pressure applied to the clutch pack  16 .  
     [0024] In operation, the torque transferred by the clutch pack  20  is controlled by the linear displacement of the pressure plate  38  which is in turn dictated by the helix angle of the ball screw grooves  42  and  44  and the rotational displacement of the lever  50 . In short, the rotational pressure device  72  turns the secondary ball screw shaft  74 , which causes the second ball nut  76  to move along the secondary ball screw shaft  74 . The pin  78  within the slot  56  on the lever  50  rotationally displaces the lever arm  50  about axis  54 . The ball screw sleeve  32  rotates with the lever  50 , causing linear displacement of the ball nut  34  and pressure plate  38 . The pressure plate  38  in turn applies pressure to the clutch pack  20 , which engages the torque transfer unit causing the rotational input shaft  22  to turn in the same direction as the rotational output shaft  24 .  
     [0025] The use of a ball screw  30  attached to a lever arm  50  provides a mechanical advantage that allows the use of a less robust drive mechanism  60 . The size of the drive mechanism  60  depends on a number of factors including (1) the amount of torque being transmitted across the clutch pack, (2) the number of friction surfaces on the clutch pack, (3) the inside and outside diameter of the clutch plates, (4) the stroke to actuate the clutch pack, (5) the ball screw helix, and (6) the ball screw efficiency.  
     [0026] Those skilled in the art will appreciate that the appropriate size and configuration of the actuator  22  may be determined in a variety of ways. By way of example rather than limitation, determination of an appropriate size actuator may include determining the desired clutch pack clamp force, verifying an acceptable plate interface pressure, calculating the appropriate ball screw input torque, factoring the mechanical advantage of the lever arm, and accounting for the mechanical advantage contribution of the drive mechanism, such as the illustrated secondary ball screw.  
     [0027] The remaining paragraphs of this specification set forth a representative calculation for determining the appropriate size and configuration of the actuator. Those skilled in the art will appreciate that the example is provided for illustration only. In this example, the amount of torque to be transferred across the clutch pack is 32040 in-lbf. The exemplary clutch pack  16  has an outside diameter (D) of 2.800 inches, an inside diameter (d) of 2.00 inches, and sixty frictional surfaces (N). Equation (1) is used to determine the pressure to be applied to the clutch pack  16 .  
             P   :=     3   ·   T   ·     [       (       D   2     -     d   2       )       N   ·   μ   ·     (       D   3     -     d   3       )         ]     ·     sin        (   α   )                 (   1   )                       
 
     [0028] where the friction coefficient (μ) between clutch plates is 0.10 and the angle (α) on the cone clutch is 90°. Inserting the above values into the equation (1) yields:  
             P   :=     3   ·     (     32040                   in   ·   lbf       )     ·     [           (     2.800                 in     )     2     -       (     2.00                 in     )     2           (   60   )     ·     (   0.10   )     ·     (         (     2.800                 in     )     3     -       (     2.00                 in     )     3       )         ]     ·     sin        (   90   )                 (   2   )                       
 
     [0029] Therefore, the pressure of the clamp load needed to transfer the required torque across the clutch pack is 4,409 lbf.  
     [0030] The plate interface pressure is commonly calculated during clutch pack design in order to ensure that the calculated clutch pack pressure does not cause excessive wear. Equation (3) may be used to determine plate interface pressure in the representative actuator.  
             Pressure   :=     P     [     π   ·       (       D   2     -     d   2       )     4       ]               (   3   )                       
 
     [0031] This calculation for the representative actuator yields a plate interface pressure of 1462 lbf/in 2 . For the sake of this illustration, it is assumed that a plate interface pressure less than 1,500 psi is acceptable.  
     [0032] The ball screw input torque (Γ) required to create the calculated clutch pack clamp load is determined based on:  
             Γ   :=     β   ·     P     2   ·   π   ·   ɛ                 (   4   )                       
 
     [0033] where, in this exemplary illustration, the ball screw lead (β) is 0.3937 in. and the ball screw efficiency (ε) is 0.90. The above calculation gives a required input torque of 307 in-lbf. This input torque is the direct rotational torque needed to rotate the ball screw 30.  
     [0034] Of course, the lever arm also provides a mechanical advantage. To determine the load (P2) needed to be applied to the lever arm  50 , the input torque (Γ) is divided by the radius or length of the lever arm:  
             P2   :=     Γ   Radius             (   5   )                       
 
     [0035] If the radius of the lever arm is 3.0 inches, the load applied to the lever arm  50  would need to be just over 102 in-lbf.  
     [0036] In the illustrated embodiment, a secondary ball screw  70  is used to apply pressure to the lever arm  50 . The secondary ball screw  70  also provides a helix advantage and, as noted above, the amount of input torque to the ball screw may be determined from:  
             MotorTorque   :=     β2   ·     P2     2   ·   π   ·   ɛ                 (   6   )                       
 
     [0037] where β2 is the secondary ball screw lead, (ε) is the efficiency of the secondary ball screw, and P2 is the amount of load needed to be applied to the lever arm. In the illustrated example, the lead on the secondary ball screw is 0.118 inches and the efficiency is 0.90. Inserting these values into the equation (6) yields a ball screw input torque from the rotational pressure device of approximately 2.135 in-lbf. This example illustrates that the use of two ball screws  30  and  70  as well as a lever  50  allows the use of a much smaller drive device  72  to provide the input torque to engage the clutch pack  16 . The advantage of using two ball screws  30  and  70  and a lever arm  50  is significant in that the input torque of 2.135 in-lbf is magnified to exert a clamping load of approximately 4,409 lbf.  
     [0038] 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.