Re-Evaluation of Basic Dynamic Load Rating and Life Formula for a Ball Screw

摘要

The existing method of calculation for the basic dynamic load rating for a ball screw mentioned in the ISO standard is based on the method used for the angular contact ball bearing, and thus it does not align with the one used for the linear bearing. The unit for the rating life; i.e., the unit running life (URL) is defined in terms of 10{sup}6 rev. As a result of this, the effect of the linear stroke of the nut or screw shaft, the lead, the number of starts, and the number of circulating ball circuits and their turns are not clearly reflected in the load rating formula. In this paper, the basic dynamic load rating and the life formula for the ball screw based on the method used for the linear bearing; i.e., in terms of a unit URL = 100 km is proposed and some representative calculation results for a ball screw are discussed. The results are summarized as follows: (1) Basic dynamic load rating C{sub}a for standard stroke corresponding to the nut raceway length is defined along with a stroke factor to generalize the relationship for any stroke of the ball screw operation. (2) The C{sub}a value for the ball screw is proportioned according to the number of ball circuits and their turns to the power of 0.7 as well as the number of starts. Any increase in the number of starts especially affects an increase in the helical raceway length of the nut and the number of balls, which results in an exceptional increase in the C{sub}a value.