Abstract:
A bearing configuration in which a shaft is supported by a first and second bearings, the second bearing being spaced from the first bearing. The first bearing and said second bearing are supported by a corresponding first bore and second bore in a shaft housing, a first axis being coincident with an axis of the first bore and a second axis being coincident with an axis of the second bore. The first axis and the second axis are purposefully offset from one another a sufficient distance to substantially eliminate internal clearance within the first bearing.

Description:
This application claims the benefit of earlier-filed copending U.S. provisional application No. 60/333,846, filed Nov. 28, 2001. 
    
    
     BACKGROUND 
     The present invention relates to bearings, and particularly relates to a bearing configuration for reducing noise therein. The invention is useful in gear sets for electric power steering systems. 
     Noise in gear sets is generally undesirable, and particularly undesirable in certain applications, such as in electric power steering systems where the noise is felt at the handwheel and/or heard in the passenger compartment. Prior attempts at controlling noise in such systems have focused on reducing backlash between the teeth of a pair of gears and on dampening the noise. For example of such approaches, see U.S. Pat. No. 6,164,407, issued Dec. 26, 2000 to Cheng; U.S. Pat. No. 6,269,709, issued Aug. 7, 2001 to Sangret, and the article entitled, “Electric Power Steering” by Yuji Kozaki et al., published in 1999 in the journal,  Motion  &amp;  Control,  issue 6, the latter of which is incorporated herein by reference in its entirety. 
     Although the methods used heretofore to reduce noise and the deleterious effects thereof significantly improve the performance of the gear set, they have not addressed a significant contributor to noise: the bearings. Tolerance and clearance in the roller or ball bearings supporting a shaft allows the shaft to move axially slightly, which introduces noise into the system. Traditional means for reducing bearing noise is not effective in some applications, such as in systems encountering high axial loads. In such systems, the bearings contribute noise despite axially pre-loading the bearing when a great enough axial force is exerted in the opposite direction against the pre-load. Reduction of bearing noise by radial expansion of the inner race to remove the clearance in the bearing is too expensive. The prior art has therefore not adequately addressed this source of noise. 
     SUMMARY 
     Disclosed herein is a bearing configuration including a shaft supported by a first bearing and a second bearing, the second bearing being spaced from the first bearing. The first bearing and said second bearing are supported by a corresponding first bore and second bore in a shaft housing, a first axis being coincident with an axis of the first bore and a second axis being coincident with an axis of the second bore. The first axis and the second axis are purposefully offset from one another a sufficient distance to substantially eliminate internal clearance within the first bearing. 
     The disadvantages of the prior art noted above and otherwise are overcome by a an offset intentionally created between bearing bores along the bearing axis of the shaft, thus placing inner and outer races of the bearing at a slight angle to one another. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features will be appreciated by reference to the detailed description and accompanying drawings in which: 
         FIG. 1  shows a bearing positioned with an intentional offset therein; 
         FIG. 2  shows an exemplary electric power steering system incorporating an offset-bearing; 
         FIG. 3  shows an exemplary gear set with a worm mounted on bearings; and 
         FIG. 4  shows a detail of the exemplary gear set of  FIG. 2 , showing the offset, which is exaggerated for clarity. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically represents a bearing positioned in an offset bore; the offset is exaggerated for clarity. A shaft  12  is supported within bore  16  by a bearing set  17 . Bore  16  is offset from axis  40 , which is common with an opposite bearing (not shown). Thus, bore  16  is offset from axis  40  by a distance  26 , which causes an angular displacement  42  between outer race  22  and inner bearing race  24  of theta (θ) radians, which is the same as the angular displacement between bore  16  and shaft  12 . 
     Bearing set  17  has no clearance, and therefore will not admit noise into the system. Furthermore, it handles axial loads well. When shaft  12  encounters alternating axial loads  30 , the bearing responds with a reaction force opposite the axial load. When axial load  30  is exerted to the right as seen in  FIG. 1 , a reaction force  31  is exerted through ball bearing  32 . Likewise, when the axial load  30  is exerted to the left as seen in  FIG. 1 , a reaction force  33  is applied to the right through ball bearing  34 . 
     Setting bore offset distance  26  too high will cause excessive friction in the bearings, which may cause them to wear prematurely. Setting bore offset distance  26  too low will not adequately reduce noise. Determining the optimum bore offset distance can be accomplished by simply displacing a shaft end until it stops, which occurs when the clearance is closed, and setting the bore offset by the displaced amount. If the bearing geometry is known, the displaced amount can be calculated using the following formula: 
         Δ   ⁢           ⁢   a     =     2   ⁢     m   o     ⁢     {       sin   ⁢           ⁢     α   o       +       θ   ⁢           ⁢     R   i         2   ⁢     m   o         -       1   -       (       cos   ⁢           ⁢     α   o       +       θ   ⁢           ⁢   L       4   ⁢     m   o           )     2           }           
         where:   Δa=Axial clearance (mm)   m 0 =Distance between inner and outer ring groove curvature centers (mm)   =r e +r i −D w      r e =Outer ring groove radius (mm)   r i =Inner ring groove radius (mm)   D w =Ball diameter   α 0 =Initial contact angle (deg)   θ=Angular clearance (radians)   R i =Distance between shaft center and inner-ring groove curvature center (mm)   L=Distance between left and right groove centers of inner-ring (mm)       

     Setting Δa to zero and solving for theta (θ) will give the angle  42  between shaft  12  and bore  16 . Setting theta=tan −1 (x/D), where D=the distance between the opposite bearing and bearing center  20  and solving for x will give the offset distance  26  for zero clearance. The methods above will give a good starting point, though the optimum value may well be determined experimentally, particularly since some small amount of additional offset may be added for preloading the bearing. 
     EXAMPLE 
       FIG. 2  shows a schematic representation of an exemplary steering system  100  having electric power steering assist. Handwheel  114  is fixed to shaft  116 . Torque sensor  118  detects the torque in shaft  116  between handwheel  114  and worm gear  126 . Controller  128  receives this torque information and other information (not shown) and outputs a signal to motor  122  which is connected to worm  124 . Worm  124  engages worm gear  126  to produce an output torque against shaft  116 . Lower shaft  121  is connected with tie-rod  137  via a rack and pinion gear set  135 , thus translating rotary motion of lower shaft  121  into linear motion of tie rod  137 . Tie rod is then connected to wheel  112  in a known manner to rotate the wheel on a generally vertical axis for steering the vehicle of which this system is a part. 
     Forces acting on wheel  112  from the road will act on rack and pinion gear set  135  and will translate into torque at worm gear  126 . These rotational forces of worm gear  126  causes axial loads to bear against worm  124 . Any axial movement of worm  124  results in noise can be felt and heard by a driver. One potential source of noise in this system is the bearings supporting the worm  124 . 
       FIG. 3  shows a cross section view of the worm gear set shown in  FIG. 2 , with motor  122  removed. The specific system tested was a Fiat  188  electric power steering system. The assist mechanism housing  105  has four machined bearing bores. Two 47 mm bearing bores support the gear axis  102 . A 30 mm single row bearing  19  and bearing set  17 , which includes a 32 mm double row angular contact bearing, support shaft  12  which supports or is integral to worm  124 . Bearing  17  is press fit on the I.D. with a slip fit on the O.D. Bearing  19  has slip fit features on the I.D. and the O.D. In this way, bearing set  17  supports all axial loading of shaft  12 . 
     To eliminate internal clearance in bearing set  17  and reduce the noise in the system, bore  14 , which supports bearing set  17 , is intentionally offset from bore  15 , which supports bearing set  19 , by a certain amount.  FIG. 4  shows a detail of  FIG. 3 , along with axes  40  and  18 , which correspond to the axis of bore  14  and the axis of bore  15 , respectively. The offset shown is exaggerated for clarity. It was found that an offset of 100 microns, within a tolerance of 20 microns, significantly reduced bearing noise in this system. The offset amount may include a nominal value and a tolerance including a range of values that includes the nominal value, where the range of values does not include zero. 
     Although the Example above is directed towards electric power steering systems, the offset may be useful in other applications where the shaft is subjected to varying axial loads or where bearing noise is problematic. Thus, while the invention has been shown and described with respect to a specific embodiment, it is to be appreciated that this embodiment is exemplary only of the invention, and not limiting. As will be appreciated by one skilled in the art, these and many other variations are possible without departing from the spirit and scope of the invention. Terms such as “first” and “second” as used herein are not intended to denote an order as in importance or position, but are merely used to distinguish between like elements.