Electric power steering apparatus

An electric power steering apparatus that uses rotational force of a motor to assist motion of a rack shaft, which moves linearly in accordance with a steering operation, the apparatus comprising:

• a power conversion mechanism that converts rotation of the motor to linear motion of the rack shaft;
• and
• a housing that accommodates the rack shaft and the power conversion mechanism, wherein:
• the power conversion mechanism includes a ball screw mechanism and a transmission mechanism,
• the ball screw mechanism includes a ball nut that is threaded to the rack shaft via a plurality of balls and is rotationally supported on an inner circumferential surface of the housing via a bearing,
• the transmission mechanism includes (i) a driving wheel that rotates as a drive of the motor, (ii) a driven wheel that is formed into a cylindrical shape, has an outer circumferential surface and an inner circumferential surface on which a female thread portion is formed, and is fitted about an outer circumferential surface of the ball nut to transmit rotation of the driving wheel to the ball nut, and (iii) a rotation transmitting member that is located on the outer circumferential surface of the driven wheel and that is capable of transmitting the rotation of the driving wheel to the driven wheel,
• the ball nut includes a first end in which is formed a male thread portion and that is inserted in and directly coupled to the driven wheel by fastening the male thread portion to the female thread portion of the driven wheel, and a second end where a flange is formed on the outer circumferential surface of the ball nut, an inner race of the bearing being pressed by the flange toward the driven wheel in the axial direction of the ball nut so as to be clamped between the driven wheel and the flange, and
• in a direction perpendicular to the axial direction of the ball nut, the driven wheel fills the entire space between the female thread portion of the driven wheel and the portion of the rotation transmitting member located on the outer circumferential surface of the driven wheel.