CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of PCT International Application No. PCT/US2008/080712 filed on Oct. 22, 2008, which claims the benefit of U.S. Provisional Patent Application 60/981,587 filed on Oct. 22, 2007, which is hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     FIELD OF THE INVENTION 
     This invention generally relates to pawls and ratchet teeth of one-way clutches, and more particularly, the shape of the contact surfaces of the pawls and the ratchet teeth. 
     BACKGROUND OF THE INVENTION 
     One-way clutches are used in power transmission applications where torque is to be transmitted in one rotational direction, but not the opposite direction. This is achieved through the use of inner and outer races which contain the components of a ratchet mechanism. One of the races contains several pawl mechanisms which are forced into engagement with ratchet teeth on the opposite race. Both races rotate at the same speed when rotating in one direction. Freewheeling, or overrunning of the ratchet teeth, is permitted when rotating in the opposite direction, and torque is not transferred. 
     Referring to  FIG. 1 , a prior art one-way clutch includes an outer race  12  that houses a plurality of pawls  16  that engage ratchet teeth  18  of an inner race  14 . The one-way clutch also includes engagement members  20  that urge the pawls  16  away from the outer race  12  and into engagement with the ratchet teeth  18 . The contact surfaces  28  and  44  of the pawls  16  and the ratchet teeth  18 , respectively, are flat for conventional one-way clutches. This results in a small area carrying a significant portion of the load, leading to high stress. To lower or mitigate this high stress, some designs incorporate a large number of pawls to reduce the stress on each pawl or include relatively large pawls to limit wear due to the stress. However, such designs are relatively expensive, relatively complex, or both. Therefore, a need exists for a one-way clutch design with components designed for improved load distribution. 
     SUMMARY OF THE INVENTION 
     The present invention provides a one-way clutch having an outer race and an inner race. One of the races includes at least one pawl and an engagement member for urging the at least one pawl into engagement with ratchet teeth on the other race. At least one of the ratchet teeth and the at least one pawl include an arcuate contact surface for improved load distribution. The arcuate contact surface has a radius of curvature that is greater than a radius of curvature at corners adjacent to the arcuate contact surface. 
     In some embodiments, the contact surface of the at least one pawl has an arcuate shape for improved load distribution. In some embodiments, the contact surfaces of the ratchet teeth have an arcuate shape for improved load distribution. Further still, in some embodiments, both the contact surfaces of the ratchet teeth and the at least one pawl have arcuate shapes. 
     The foregoing and other objects and advantages of the invention will appear in the detailed description which follows. In the description, reference is made to the accompanying drawings which illustrate a preferred embodiment of the invention 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an enlarged cross-sectional view of a pawl of a prior art one-way clutch; 
         FIG. 2  is a cross-sectional view of a one-way clutch of the present invention; 
         FIG. 3  is an enlarged cross-sectional view of a pawl of the present invention with an arcuate contact surface; 
         FIG. 4  is an enlarged cross-sectional view of ratchet teeth of the inner race, illustrating a rake angle; 
         FIG. 5  is an enlarged cross-sectional view of the pawl of  FIG. 3  illustrating load distributions acting on the pawl; and 
         FIG. 6  is an enlarged cross-sectional view of a ratchet tooth of the present invention with an arcuate contact surface. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the figures, like components are designated with the same reference numerals. Additionally, like components between the prior-art and the present invention are increased by 100. 
     Referring to  FIG. 2 , a one-way clutch  110  of the present invention includes an outer race  112  and an inner race  114  that have a common axis of rotation  115 . The outer race  112  includes a plurality of pawls  116 , engagement members  120 , such as helical compression springs, Belleville springs, leaf springs, or the like, for urging the pawls  116  away from the outer race  112  and into engagement with ratchet teeth  118  on the inner race  114 , and recesses or pockets  122  for accommodating the pawls  116  and the engagement members  120 . If the outer race  112  is the driving component, clockwise rotation as viewed in  FIG. 2  causes the engagement members  120  to bias the pawls  116  into engagement with the ratchet teeth  118 . Counter-clockwise rotation of the outer race  112  causes freewheeling motion and the pawls  116  slip or ratchet over the teeth  118 . If the inner race  114  is the driving component, counter-clockwise rotation causes engagement of the pawls  116  and the ratchet teeth  118  and clockwise rotation causes freewheeling motion. 
     Referring to  FIG. 3 , each pawl  116  has a pawl support surface  126  and a pawl contact surface  128  that engage the ratchet teeth  118 , an engagement member contact surface  130  opposite the pawl support surface  126 , a pawl reaction surface  132  that defines a pivot axis  124 , a radius  134  between the pawl support surface  126  and the pawl contact surface  128 , and a radius  136  between the pawl contact surface  128  and the engagement member contact surface  130 . Each pawl  116  pivots about the pivot axis  124  which permits the pawl  116  to rotate and slip over the ratchet teeth  118  when freewheeling, or to engage one of the ratchet teeth  118  when the clutch  110  rotates in the opposite direction. 
     Each ratchet tooth  118  includes a ratchet tooth free surface  138 , a ratchet tooth support surface  140  and a ratchet tooth contact surface  144  that engage the pawls  116 , and a radius  146  between the ratchet tooth free surface  138  and the ratchet tooth contact surface  144 . The ratchet tooth support surface  140  is separated from the ratchet tooth contact surface  144  on an adjacent ratchet tooth  118  by a load separating surface  142 . The load separating surface  142  extends radially inward of the ratchet tooth support surface  140 . 
     When the pawl  116  engages the ratchet tooth  118 , the pawl contact surface  128  contacts the ratchet tooth contact surface  144 . These contact surfaces transmit the load between the pawl  116  and the inner race  114 . The pawl support surface  126  contacts the ratchet tooth support surface  140 . These contact surfaces hold the pawl  116  in a proper working position as shown in  FIGS. 2 and 3 . Additionally, the pawl reaction surface  132  contacts a pawl recess  148 . These contact surfaces transmit the load between the outer race  112  and the pawl  116 . 
     As briefly described above, the contact surfaces of the pawl and the ratchet tooth are both flat for conventional one-way clutches, and as a result high stress acts on these contact surfaces. Referring again to  FIG. 1 , a point on the pawl contact surface  28  near the radius  36  carries a significant portion of the load. Referring to  FIG. 3 , the pawl  116  according to the present invention includes a pawl contact surface  128  that curves as viewed in the direction of the axis of rotation  115 ; i.e. it curves as it traverses a generally radial direction, and as viewed in a radial plane that is perpendicular to the axis of rotation  115  of the clutch  110 . Preferably, the arcuate pawl contact surface  128  has a radius of curvature that is larger than the radii of curvature of the radii  134  and  136  and is semi-circular in shape. Alternatively, the arcuate pawl contact surface  128  may be parabolic, elliptical, or the like. The arcuate pawl contact surface  128  provides a small clearance from the ratchet tooth contact surface  144  where the arcuate pawl contact surface  128  meets the radii  134  and  136 . The size of the clearance depends on the dimensions of the pawl and the material used. For example, a clearance of 0.0003″ is used when the height of the arcuate pawl contact surface  128  is 0.170″. Additionally, the clearance where the arcuate pawl contact surface  128  meets the radius  134  could be decreased further. Additionally, the contact point  158  should be located near the midpoint of the arcuate pawl contact surface  128 . 
     When transmitting a load, the pawl contact surface  128  deforms, and the area around the midpoint of the arcuate pawl contact surface  128  engages the ratchet tooth contact surface  144 , which may be flat when unloaded, and carries a portion of the load. Contrary to conventional thinking, in accordance with the invention this results in more area of the pawl contact surface  128  being engaged with the ratchet tooth contact surface  144 , and the maximum stress is significantly reduced compared to prior art clutches having both flat engagement surfaces. Therefore, it is possible to transmit the same load as a conventional one-way clutch with fewer or smaller pawls, thus saving weight and cost. Additionally, decreasing the size of the radii  134  and  136  increases the size of the pawl contact surface  128 . This permits additional area of the arcuate pawl contact surface  128  to engage the ratchet tooth contact surface  144 , further reducing the maximum stress and further permitting savings in weight and cost. 
     Referring to  FIG. 4 , a rake angle  150  of the ratchet teeth  118  is the angle between a tangent line  154  tangent to the ratchet tooth contact surface  144  at a center of contact  158  between the pawl contact surface  128  and the ratchet tooth contact surface  144 , and a radial line  152  that intersects the axis of rotation  115  and the center of contact  158 . Preferably, the rake angle  150  for the present invention is 0° (±1°). Alternatively, the rake angle  150  may be increased to reduce backlash when transitioning from freewheeling motion to load transmitting motion. Also, the rake angle  150  may be decreased to reduce impact shock when freewheeling. 
     As discussed previously, the pawl  116  is loaded on three surfaces. As shown in  FIG. 5 , the pawl  116  is loaded by distributed loads  151 ,  153 , and  155  (distributions graphically illustrated in  FIG. 5 ) from the ratchet tooth contact surface  144 , the ratchet tooth support surface  140 , and the pawl reaction surface  132 , respectively. The directions of the contact forces on these surfaces when the pawl  116  is loaded ensure the pawl  116  remains in engagement with the ratchet tooth  118 . That is, as shown in  FIG. 5 , a line  156  extends through the pawl  116  between the center of contact  158  at the pawl contact surface  128  and the pivot  124 , which is the center of rotation defined by the pawl reaction surface  132 . The line  156  forms an angle  162  with a line of action  160  that intersects the center of contact  158  and is parallel to the direction of the net force of the distributed load  151 . The net force angle  162  should be less than 180° to ensure, the pawl  116  remains in engagement with the ratchet tooth  118 . If the angle is 180° or greater, the pawl  116  may remain in engagement with the ratchet tooth  118  due to friction and the spring force acting on the pawl. However, it is preferable for the pawl  116  to remain in engagement with the ratchet tooth  118  due to the contact forces on the pawl. 
     Alternatively, the ratchet teeth  118  may include an arcuate ratchet tooth contact surface  144  that curves in the direction from the root of the tooth to the free end of the tooth. In this case, the pawl  116  could have a pawl contact surface  128  that is flat or curved. Preferably, the arcuate ratchet tooth contact surface  144  is semi-circular. Alternatively, the arcuate ratchet tooth contact surface  144  may be parabolic, elliptical, or the like. As another alternative, the pawls  116  may be in pockets in the inner race  114  and the ratchet teeth  118  may be on the outer race  112 . 
     The general shape of the pawls and the ratchet teeth (besides the contact surfaces) is not limited to that shown in  FIG. 3 . For example, rectangle-like pawls and corresponding ratchet teeth are well known in the art. Arcuate contact surfaces could also be included with these components to reduce stress. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as defined within the scope of the following claims.
A one-way clutch with an outer race and an inner race is disclosed. One of the races includes at least one pawl and an engagement member for holding the at least one pawl in engagement with ratchet teeth on the other race. The contact surface of the at least one pawl or the ratchet teeth is an arcuate shape for improved load distribution. The arcuate contact surface has a radius of curvature that is greater than a radius of curvature at corners adjacent to the arcuate contact surface.