Patent ID: 8863598

Claim:
An electric power steering system that includes a rack shaft provided so as to be able to reciprocate in an axial direction of the rack shaft, a hollow shaft through which the rack shaft is passed and that is rotated by a motor, and a ball screw device that converts rotation of the hollow shaft into an axial movement of the rack shaft, characterized in that: the ball screw device is formed by arranging a plurality of balls in a spiral rolling passage that is defined by a thread groove formed in an outer periphery of the rack shaft and a thread groove formed in an inner periphery of a ball screw nut that rotates together with the hollow shaft, the thread groove of the rack shaft and the thread groove of the ball screw nut facing each other; a return passage that connects two points of the rolling passage to each other to allow each of the balls that roll in the rolling passage to endlessly circulate is formed in the ball screw nut by attaching a circulation member to a fitting hole that passes through the ball screw nut in a radial direction of the ball screw nut; a protrusion is formed on an outer surface of the circulation member at a position offset from a raceway of the balls that circulate in the return passage when viewed in the radial direction of the ball screw nut; the circulation member is fixed to the ball screw nut when the protrusion contacts an inner periphery of an annular support member that is arranged on an outer periphery of the circulation member; and the protrusion extends outwardly in the radial direction from an outer surface of the circulation member.