Patent ID: 8230963
Filing Date: 2012-07-31
Classification: B62D

Abstract:
1. An electric power steering system, comprising: a rack shaft that is able to reciprocate in an axial direction; a motor shaft through which the rack shaft passes; and a ball screw device that converts rotation of the motor shaft into axial movement of the rack shaft, wherein the ball screw device is formed by arranging a plurality of balls within a helical ball-rolling path that is formed by aligning a thread groove formed in an inner periphery of a ball screw nut and a thread groove formed in an outer periphery of the rack shaft with each other, wherein the ball screw nut is inserted into a motor housing that has openings at respective axial ends, one of the openings being blocked by a side housing fitted to the motor housing, through the remaining opening that is not blocked, and the ball screw nut is screwed to the motor shaft, which is rotatably supported in the motor housing, in the axial direction and secured to an axial end portion of the motor shaft, and wherein a hollow threaded shaft that is screwed to a threaded portion formed in the axial end portion of the motor shaft is formed in an axial end portion of the ball screw nut, a flange member that is larger in diameter than the ball screw nut and that has an engaged portion that restricts rotation of the flange member when being engaged with a restricting member is fitted to the motor shaft, and simultaneous rotation between the motor shaft and the ball screw nut in the same direction when the ball screw nut is screwed to the motor shaft is restricted by restricting the rotation of the flange member.