Abstract:
A steering gear assembly includes a pinion defining a pinion axis and a rack defining a rack axis. A bearing assembly includes a roller bearing biased against the rack to generate a normal force pressing the rack into engagement with the pinion. The bearing assembly engages an outer surface of the rack at two points circumferentially spanning a large degree to provide improved support. The roller bearing is preferably a plurality of ball bearings engaging an outer race. A deformable member may be utilized that is positioned between the outer race and the rack housing. The deformable member may comprise a series of O-rings which providing a force biasing the outer race towards the pinion.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to steering gear assemblies having a rack and pinion, and more particularly relates to yoke bearings supporting engagement of the rack and pinion.  
         BACKGROUND OF THE INVENTION  
         [0002]    A steering gear assembly typically comprises a rack and pinion gear system that translates the rotational movement of the pinion into linear movement of the rack. Many modern vehicles include electric power assisted steering, and these gear assemblies utilize dual pinions. One pinion is responsive to steering wheel input, while the other pinion is powered by an electric motor to provide steering assistance or other controlled steering operations.  
           [0003]    When the pinions are subject to high torques or quick changes in torque direction, separation forces are generated between the rack and pinion gears. Accordingly, a yoke assembly is typically provided to keep the rack and pinion in engagement. The yoke assembly typically includes a slide bearing that is biased by a spring to press the rack into engagement with the pinion. Unfortunately, as higher and higher pinion torque is generated, which is especially the case with pinions driven by an electric motor, large separation forces are generated. Thus, large spring forces are required to press the yoke and yoke bearing against the rack. However, higher spring forces result in increased friction between the rack and the yoke bearing.  
           [0004]    To overcome these deficiencies, roller bearings have been proposed for use as the yoke bearing. While roller bearings allow higher spring forces with lower coefficients of friction between the rack and bearing, these designs are not without their drawbacks. For example, it has been found that the supporting force provided by current yoke roller bearings on the rack are not as robust as the support obtained by slide bearings. Accordingly, there exists a need to provide a steering gear assembly having a yoke bearing which provides robust support to the rack to provide improved engagement of the rack and pinion gears, while maintaining a low coefficient of friction between the rack and yoke bearing.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    The present invention provides a steering gear assembly including a pinion defining a pinion axis and a rack defining a rack axis. The rack operatively engages the pinion for translation along the rack axis. A bearing assembly includes a roller bearing biased against the rack to generate a normal force pressing the rack into engagement with the pinion. The bearing assembly engages an outer surface of the rack at two points circumferentially spanning greater than 140 degrees. Preferably, the the bearing assembly engages the outer surface of the rack at two points circumferentially spanning 175 to 185 degrees.  
           [0006]    According to more detailed aspects, the bearing assembly includes an outer race and the roller bearing is slidably disposed between the race and the outer surface of the rack. The race is biased towards the rack, and the roller bearing slides along the race parallel to the rack axis. Preferably, the roller bearing comprises a plurality of ball bearings. The plurality of ball bearings are arranged in rows that are circumferentially spaced about the outer surface of the rack, the rows extending along the rack axis. The rows are preferably axially spaced and circumferentially overlap. The rows extend axially a distance greater than the diameter of the pinion. The roller bearing slides relative to the outer race and within a range of motion defined by a pin attachment to the inner race, the pin moving within a slot formed in the outer race.  
           [0007]    Another embodiment of the invention provides a yoke bearing assembly for a steering gear assembly having a pinion defining a pinion axis and a rack defining a rack axis orthogonal to the pinion axis. The yoke bearing assembly includes an outer race and an inner race positioned inside the outer race. A plurality of ball bearings are positioned within a plurality of apertures formed in the inner race, each ball bearing disposed between the outer race and an outer surface of the rack. The plurality of ball bearings and the inner race translate along the outer race as the rack translates axially along the rack axis. A deformable member is positioned between the outer race and the rack housing, the deformable member providing a force biasing the outer race towards the pinion. Each ball bearing transmits the force to the rack for keeping the rack and pinion in engagement, the deformable member responsive to loads transmitted from the rack to the outer race.  
           [0008]    According to more detailed aspects, the deformable member is preferably an elastomeric ring, and most preferably a series of axially spaced elastomeric rings. Alternatively, the deformable member may comprise a sheet of elastomeric material. The inner race has a semi-circular cross-sectional shape, and the plurality of ball bearings are spaced circumferentially about the outer surface of the rack an angle greater than 140 degrees.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention. In the drawings:  
         [0010]    [0010]FIG. 1 is a perspective view of a steering gear assembly constructed in accordance with the teachings of the present invention;  
         [0011]    [0011]FIG. 2 is a perspective view similar to FIG. 1, having the rack housing removed;  
         [0012]    [0012]FIG. 3 is a perspective view similar to FIG. 2, but taken from the rear;  
         [0013]    [0013]FIG. 4 is a cross-sectional view of an alternate embodiment of the steering gear assembly constructed in accordance with the teachings of the present invention; and  
         [0014]    [0014]FIG. 5 is a cross-sectional view taken above the 5-5 in FIG. 4. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    Turning now to the figures, FIG. 1 is a perspective view of a steering gear assembly  20  constructed in accordance with the teachings of the present invention. The steering gear assembly  20  generally includes a pinion  22  operatively connected to a rack  24 . The pinion  22  generally includes a pinion housing  23  substantially enclosing a pinion gear  26  (FIGS. 2 and 3). The pinion gear  26  includes teeth  27  (FIG. 3) which is driving engagement with the rack  24 . The rack  24  generally includes a housing  25  which substantially encloses a rack gear  28 . The rack gear  28  is linearly translated by virtue of its engagement with the pinion gear  26 , as is well understood in the art. A bearing assembly  30  engages the rack  24  and biases the same into the pinion  22  to provide reliable engagement of the rack  24  and pinion  22 .  
         [0016]    Uniquely, the bearing assembly  30  of the present invention provides improved support to the rack  24 . Not only does the bearing assembly  30  provide increased support along the rack axis  32  (FIG. 3), but also provides increased support relative to the pinion axis  34  (FIG. 3). In addition to typical separation forces between the pinion  22  and rack  24 , it has also been found that various forces are transmitted through the rack gear  28  itself, a portion of which can be directed along the pinion axis  34 . That is, the rack gear  28  may be subject to loads which force it to slide along the pinion axis  34 , vertically in FIGS. 1-3.  
         [0017]    Accordingly, the present invention provides additional support to the rack  24  in the direction parallel to the pinion axis  34 . As best seen in FIGS. 2 and 3, the bearing assembly  30  includes an inner race  40  positioned within an outer race  36 . The inner race  40  includes a plurality of apertures  42  sized to receive a plurality of ball bearings  44 . Notably, the inner race  40  has a C-shaped cross-section. Stated another way, the inner race  40  has a semi-circular cross-section. Preferably, the inner race  40  extends circumferentially around the rack gear  28  at least 140 degrees, and preferably 170 to 190 degrees, and most preferably about 180 degrees. By virtue of this structure, the bearing assembly  30  provides support to the rack gear  28  in a direction both perpendicular to the pinion axis  34 , as well as parallel to the pinion axis  34 .  
         [0018]    The ball bearings  44  have a diameter greater than the thickness of the inner race  40 . Accordingly, the ball bearings  44  engage an outer surface of the rack gear  28 . Similarly, the ball bearings  44  also engage an inner surface of the outer race  36 . Thus, as the pinion gear  26  translates the rack gear  28  linearly along the rack axis  32 , the inner race  40  and the ball bearings  44  slide along both the outer surface of the rack gear  28  as well as the inner surface of the outer race  36 . In short, the inner race  40  and the ball bearings  44  roll axially along the rack axis  32  as the rack gear  28  moves linearly along the same axis.  
         [0019]    The axial movement of the inner race  40  and the ball bearings  44  are constrained by way of a pin  46  formed on the inner race  40 . The outer race  36  includes a slot  37  which extends parallel to the rack axis  32 . The slot  37  is sized to receive the pin  46 , and the ends of the slot  37  define the range of movement allowed to the pin  46 , and hence the inner race  40 . Furthermore, the pin  46 , by extending through slot  37 , prevents the rotation of the inner race  40  about the rack gear  28  and the rack axis  32 . It is important to keep the inner race  40  in the position shown in the figures in order to ensure proper support to the rack gear  28  and engagement with the pinion gear  26 .  
         [0020]    As just discussed, the inner race  40  is positioned between the rack gear  28  and the outer race  36 . The plurality of ball bearings  44  engage both the outer surface of the rack gear  28  and the inner surface of the outer race  36 . To ensure reliable engagement of the rack and pinion gears  28 ,  26 , the outer race  36  is biased toward the pinion  22 . Uniquely, the present invention accomplishes this objective by utilizing a plurality of O-rings  38  attached to the outer periphery of the outer race  36 . The O-rings  38  also engage the rack housing  25  and are compressed between the housing  25  and the outer race  36 . Since the O-rings  38  are constructed of an elastomeric material, the O-rings  38  can be deformed to provide a force against the outer race  36 . Preferably, this force is primarily directed toward the pinion  22 , and is normal to the pinion axis  34 . However, it will be recognized that other radial components of this force are also present, including forces along the pinion axis  34 .  
         [0021]    This engagement force against the outer race  36  is transmitted through the ball bearings  44  to the rack gear  28 , pressing it into engagement with the pinion gear  26 . Due to vibrations or other manufacturing tolerances, the rack gear  28  is allowed some movement away from the pinion  22  which will result in a compression of the O-rings  38 . However, due to the deformable and elastomeric nature of the O-rings  38 , the rack gear  28  will continue to be pressed into engagement with the pinion gear  26 .  
         [0022]    It will be recognized that other biasing means can be provided in place of or in addition to the O-rings  38 . For example, a large elastomeric sheet can be provided between the outer race  36  and the rack housing  25 . Likewise, other deformable members, such as a plurality of springs could also be employed. One such alternate embodiment has been depicted in FIGS. 4 and 5.  
         [0023]    In this embodiment, like parts have been given like reference numerals. As in the prior embodiment, a pinion  22  and rack  24  are operatively engaged such that rotation of a pinion gear  26  drives the rack gear  28  linearly along the rack axis  32 . An inner race  40  houses a plurality of ball bearings  44 . The ball bearings  44  engage an inner surface of the outer race  36  as well as the outer surface of the rack gear  28 . Here, however, the O-rings  38  have been replaced with a traditional biasing assembly, namely a plug  50  engaging the rack housing  25  to position a spring  52  for biasing a yoke  54  into engagement with the outer race  36 . In turn, the outer race  36  presses the ball bearings  44  into the rack gear  28 , and hence into engagement with the pinion gear  26 .  
         [0024]    It can be seen clearly in FIG. 4 that the inner race  40  and ball bearings  44  provide support to the rack gear  28 , not only in a direction orthogonal or normal to the pinion axis  34 , but also provides support in a direction generally parallel to and along the pinion axis  34 . That is, the inner race  40  and ball bearings  44  extend circumferentially around the outer surface of the rack gear  28   a  large angle, preferably about 180°. In this way, the present invention prevents the rack gear  28  from being jostled axially along the pinion axis  34  by forces such as those induced by tie-rod loads.  
         [0025]    Turning to the cross-sectional view of FIG. 5, it can be seen that the inner race  40  and the ball bearings  44 , through which the normal biasing force of the spring  52  is transmitted to the rack gear  28 , spans a significant distance along the rack axis  32 . In this way, the bearing assembly  30  also provides robust support along a significant portion of the rack  24 . Preferably, the inner race  40  and ball bearings  44  extend a distance along the rack axis  32  that is greater than a diameter of the pinion gear  26 . Most preferably, the inner race  40  and ball bearings  44  extend an axial distance that is greater than one-third the range of motion provided to the inner race  40 . In this way, at least some portion of the inner race  40  and ball bearings  44  are positioned below the yoke spring  52  and orthogonal to both the rack and pinion axes  32 ,  34 . Stated another way, the bearing assembly  30  is structured that at least a portion of the inner race  40  and ball bearings  44  are always located between the pinion gear  26  and the rack gear  28 .  
         [0026]    Preferably, the plurality of ball bearings  44  are arranged in rows that are circumferentially space about the outer surface of the rack gear  28 . The rows of ball bearings  44  extend along the rack axis  32 . As best seen in FIGS. 2 and 3, the rows of ball bearings  44  are axially spaced and circumferentially overlap to provide a compact force transmitting area. Stated another way, each row of ball bearings  44  zigzags circumferentially as the row extends along the rack axis  32 .  
         [0027]    The foregoing description of various embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Numerous modifications or variations are possible in light of the above teachings. The embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.