Patent ID: 8291785

Claim:
A ball screw comprising: a screw shaft, in which a thread groove is formed in an outer peripheral face thereof, that includes a small-diameter portion with a diameter smaller than an outer diameter of the screw shaft formed integrally at one end of the screw shaft; a nut, through which the screw shaft passes, the nut including: a thread groove formed in an inner peripheral face thereof; and a return passage penetrating the nut along an axial length of the nut, the return passage including a plurality of ball scoops that are positioned at both ends of the return passage, and that guide the balls in the ball passage to the return passage, the ball scoops protrude into the thread groove of the screw shaft; a plurality of balls disposed in a ball passage formed by the thread groove of the screw shaft and the thread groove of the nut; and a ring-shaped stopper, immovably fitted on the small-diameter portion such that, if the nut is positioned at an end of the thread groove, then the ring-shaped stopper contacts an internal portion of the nut and prevents the nut from moving.