Patent ID: 7222553

Claim:
A ball screw apparatus comprising: a screw shaft having a first spiral screw groove on an outer circumferential face thereof; a nut having a second screw groove on an inner circumferential face in opposition to the first screw groove defined in the screw shaft and being in mesh with the screw shaft via a plurality of balls rotatably charged in a spiral ball rolling path defined between the first and second screw grooves; and a plurality of retaining pieces respectively interposed between the adjacent balls and having recess portions at both edges in ball tracking directions, wherein a contact angle θ between each of the retaining pieces and the respective balls is in the range from 30° to 37.6°, and wherein a sum total of spaces created between the balls and the retaining pieces is defined to be 30% or less of the height size of the retaining piece; wherein the sum total of spaces created between the balls and the retaining pieces is defined to be 5% or more of the height size of the retaining piece.