Electric power steering system

摘要

Assist force that assists an operation of a steering system of a vehicle is applied to a rack shaft (11) by transmitting driving force of a motor to the rack shaft (11) via a motor shaft (21) and a ball screw nut (40). A step (45) is formed on the outer periphery of the ball screw nut (40). An end portion (3d) of a third housing (3c) is arranged to face a bearing (51) such that the distance that the ball screw nut (40) is able to move relative to the motor shaft (21) in the direction indicated by an arrow (a2) is restricted to "C+D". In addition, the length (S) of an engaging portion (43) of the ball screw nut (40) is set longer than "C+D".