Influence of Ball Behavior in Ball Screws on Lost Motion (1st Report)

Abstract

Linear actuators driven by ball screw mechanism lead to non-linear relations between the linear motion of the nut and the rotary motion of the screw shaft when the ball screw drive is reversed even if the ball screw is preloaded with a double nut to remove backlash between the screw shaft and the nut. The behavior of the ball screw mechanism that is referred to as “lost motion”, can contribute positioning errors of linear actuators.This paper deals with a relationship between the lost motion and the ball behavior of the X-table which is composed of a preloaded precision ball screw and a couple of linear air bearings, and ball rolling behaviors. Ball rolling behavior is measured by the gap sensor which is inserted into a hole which is bored in the perpendicular to the outer surface of the cylindrical nut and is located on the opposite side of the ball circulation tube. The results are summarized as follows : (1) “Ball wedging” and ball wedging direction can be measured by developed experimental device. (2) The ball wedging direction is reversed when the ball screw drive is reversed in two-point contact ball screws. Therefore, the differences between forward stroke and backward stroke in ball rolling speed are occurred. (3) It is considered that the lost motion is produced in two-point contact ball screws when the ball wedging direction is reversed.