Patent ID: 7201076

Claim:
A ball screw, comprising: a shaft including a spiral-shaped shaft groove having a semicircular-shaped section; a nut including a spiral-shaped nut ball groove corresponding to the shaft groove and having a semicircular-shaped section; a large number of balls rotatably fitted between the nut ball groove and the shaft groove so as to be held by and between the two grooves; and, a circulation member defining a ball circulation passage for allowing the balls to circulate endlessly therealong, wherein a portion of the nut ball groove is formed larger in the effective diameter than are the remaining portions of the nut ball groove; and wherein a length of the larger-diameter portion along the spiral-shaped nut ball groove is a length equivalent to that of two balls or more and is less than a length corresponding to a half winding of the balls along the spiral-shaped nut ball groove; wherein the larger-diameter portion of the nut ball groove is disposed in a plurality of locations of the nut; wherein a number of load areas is set as n, and a number of the larger-diameter portions of the nut ball groove is set as n- 1 ; and wherein the n- 1 larger-diameter portions are arranged in a balanced manner in a circumferential direction of the shaft.