Patent ID: 7568550

Claim:
A power assist unit for providing steering assist power to a wheel steering mechanism, the steering mechanism including a steering shaft, said power assist unit comprising: a motor including an output shaft, said motor providing rotational motion; and a worm gear mechanism for reducing the rotation power and outputting the rotation power as said steering assist power; said worm gear mechanism further comprising: a housing connecting to said motor; a worm disposed within said housing, said worm including first and second shaft ends, said first shaft end connecting to the output shaft of said motor, said worm including a gear section; a worm wheel engaged with said worm gear section, said worm wheel being connected to the steering shaft; first and second bearings disposed in said housing, said first bearing supporting said first shaft end of said worm and said second bearing supporting said second shaft end of said worm; and said first bearing being a deep-groove ball bearing, said bearing including an inner ring, an outer ring, and a bearing ball, said inner ring and said outer ring each including a racetrack groove, wherein: said inner ring racetrack groove includes a curvature radius of between 52.5% and 75% of the diameter of said ball; or said outer ring racetrack groove includes a curvature radius of between 53.5% and 85% of the diameter of said ball; and wherein: the raceway groove of the inner ring of said first bearing includes a composite curve surface, said composite curve defining opposing shoulder regions and a bottom region, each region including a curvature radius; and said curvature radius of said shoulder regions being smaller than said curvature radius of said bottom region.