ボールねじの玉挙動とロストモーション（第2報）: ダブルナットあるいはオーバサイズ球予圧方式による荷重の影響

摘要

位置決めに用いられるポールねじは，バックラッシの消滅や軸方向変位量の限定あるいは剛性の増加を図る目的から，実用上の慣用手段として予圧が付与されている．ところが，予圧が付与されたボールねじであっても，ねじ軸の回転運動とナットの直線運動との間にロストモーションが生じるため，送り系の位置決め性能に影響を及ぼし，例えば，X-Yテーブルの輪郭運動軌跡に突起誤差を引き起こすことが知られている．%It is well known that preloaded ball screws with double nut produced hysteretic motion between the rotary motion of the screw shaft and the linear motion of the nut, which is called "lost motion", affects positioning accuracies of ball screws.rnPreloaded ball screws with a double nut have the characteristic that 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 balls have the velocity component perpendicular to the load line of action due to the helical raceway groove. Therefore the balls wedge into nut grooves. In the case of a ball screws with a circular-arc groove profile, "ball wedging" phenomenon is saturated when the force of resistance to ball wedging exceeds a frictional force between the ball and the raceway groove.rnIn the present study, the authors experimentally investigated the lost motion 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 under the condition of varied preload with double nut or over-sized balls have also measured.