Ball screw mechanism and electric power steering device using the same

A ball screw mechanism comprising:

• a ball screw shaft with a ball rolling groove formed spirally on an outer circumferential surface thereof;
• a ball nut with a ball rolling groove formed spirally on an inner circumferential surface thereof;
• a plurality of rolling balls circulatably arranged between the respective ball rolling groove of the ball screw shaft and the ball nut, wherein the balls are able to roll in the direction of a length of the ball rolling groove;
• and
• a retainer provided between the outer circumference of the ball screw shaft and the inner circumference of the ball nut and having a plurality of retainer grooves arranged in the circumferential direction for retaining the rolling balls;
• wherein each of the retainer grooves of the retainer takes a form whereby the width, in the circumferential direction, of a radially inner portion of a respective retainer groove is smaller than the diameter of the rolling balls and the width, in the circumferential direction, of a radially outer portion of the respective retainer groove is greater than the diameter of the rolling balls, whereby the rolling balls are able to move radially outward but are restrained from moving radially inward, and
• wherein each of the retainer grooves is inclined relative to the axis of the balls screw shaft to extend at right angles to the length of the ball rolling groove, over a distance at least equal to the spacing between the turns of the spiral ball rolling groove, from the groove centers thereof, at portions of the ball rolling groove at the same circumferential angular position as the respective retainer grooves.