It is well known that preloaded ball screws with double nut produced hysteretic motion between the rotational angle of the screw shaft and linear motion of the nut, which is called “lost motion”, causes positioning error of ball screws. Preloaded ball screws with a double nut have the characteristic that the loaded balls wedge into a right angle direction to a ball rolling direction because the directions of screw shaft and nut raceways differ from the ball rolling direction. Loaded balls of preloaded ball screws with a double nut have two contact points geometrically. When the ball screw is operated under the condition of screw shaft turning and nut stationary, the wedging forces of balls are produced by differences between the screw shaft and nut raceways, and then loaded balls wedge into nut grooves. When the ball wedging motion is significantly increased, loaded balls make contact with another flank of the nut groove, that is, three points contact occur. In the present study, the authors deal with the positioning errors of the X-table which is composed of a preloaded precision ball screw and a couple of linear air bearings, and the ball wedging behaviors. The ball wedging behavior is quantified by the variation of the distance from ball surface to gap sensors which are inserted into several holes. These hole passing through the top of Gothic arch groove are bored in the perpendicular to the outer surface of the cylindrical nut.

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