Patent ID: 9499194
Filing Date: 2016-11-22
CPC Classification: B62D,Y10T

Claim Text:
1. An electric power steering system comprising: a worm that has a first end portion and a second end portion; a worm wheel that engages with the worm at an engagement region; a speed reducer that includes the worm and the worm wheel and that reduces a speed of rotation of an electric motor; a housing that houses the speed reducer; a first bearing that includes an outer ring fixed to the housing and an inner ring loosely fitted to the first end portion of the worm, the first bearing supporting the first end portion of the worm so that the first end portion is rotatable and movable in an axial direction; a second bearing that includes an outer ring and an inner ring, the inner ring of the second bearing being press-fitted to the second end portion of the worm, the second bearing being supported by the housing so that the second bearing is able to be displaced in a direction in which an inter-center distance between the worm and the worm wheel increases and in a direction in which the inter-center distance decreases, and the second bearing supporting the second end portion of the worm so that the second portion is rotatable and movable in an axial direction; a first elastic member and a second elastic member that are disposed on respective sides of the inner ring of the first bearing in the axial direction, and that elastically support the worm in the axial direction with respect to the inner ring of the first bearing; and an urging member that urges an outer circumference of the outer ring of the second bearing in the direction in which the inter-center distance decreases, further comprising: a joint that includes a first rotation element having a boss fixed to the first end portion of the worm and a flange extending from the boss, and a second rotation element fixed to a rotary shaft of the electric motor, the joint connecting the worm and the rotary shaft so as to transmit a torque; a first annular backing plate that is interposed between a first end face of the inner ring of the first bearing and the first elastic member; and a second annular backing plate that is interposed between a second end face of the inner ring of the first bearing and the second elastic member, wherein the first elastic member has an annular shape and the first elastic member includes: (i) a first end face fixed to the first backing plate, (ii) a second end face frictionally engaging with an annular stepped portion formed on an outer circumference of the worm, (iii) an outer circumference, and (iv) an inner circumference surrounding the outer circumference of the worm, and wherein the second elastic member has an annular shape, and the second elastic member includes: (i) a first end face fixed to the second backing plate, (ii) a second end face frictionally engaging with the flange of the first rotation element, (iii) an outer circumference, and (iv) an inner circumference surrounding an outer circumference of the boss of the first rotation element; wherein a sum of first frictional resistance in the axial direction between the inner ring of the first bearing and the worm and second frictional resistance in the axial direction between the urging member and the outer circumference of the outer ring of the second bearing is less than an engagement frictional resistance in the axial direction of the worm, and wherein when an urging load of the urging member is defined as F, a coefficient of friction in the axial direction between the urging member and the outer ring of the second bearing is defined as μe, a coefficient of friction in the axial direction between the inner ring of the first bearing and the worm is defined as μt, a distance between a bearing center of the first bearing and the engagement region in the axial direction of the worm is defined as L