Patent ID: 8567553

Claim:
An electric power steering system comprising: a worm shaft that has a first end portion and a second end portion; a worm wheel that is in mesh with the worm shaft and that is coupled to a steering mechanism; a first bearing that rotatably supports the first end portion such that the worm shaft is allowed to oscillate along a central axis of the worm shaft about the first end portion, and that includes an inner ring having a raceway groove, an outer ring having a raceway groove and rolling elements interposed between the raceway grooves, wherein the outer ring has a pair of end surfaces that are aligned in an axial direction of the worm shaft; a second bearing that rotatably supports the second end portion; a first elastic member that elastically urges the second bearing in such a direction that an axis-to-axis distance between the worm shaft and the worm wheel reduces; a joint that couples the first end portion to an output shaft of an electric motor such that torque is transmittable; a second elastic member that is interposed between the joint and the first end portion, that elastically urges the worm shaft toward the second end portion, and that elastically presses the rolling elements via the worm shaft and the inner ring toward the outer ring; a housing that accommodates the worm shaft and the worm wheel and that has a facing surface facing one of the pair of end surfaces of the first bearing outer ring and a facing surface facing the other one of the pair of end surfaces of the first bearing outer ring; a third elastic member that is arranged between the one of the pair of end surfaces and the corresponding facing surface; and a fourth elastic member that is arranged between the other one of the pair of end surfaces and the corresponding facing surface, wherein a dynamic spring constant of the second elastic member when the second elastic member vibrates at a high frequency is higher than a dynamic spring constant of the third elastic member when the third elastic member vibrates at a high frequency and a dynamic spring constant of the fourth elastic member when the fourth elastic member vibrates at a high frequency.