抄録
The basic dynamic load rating of ball screws has always been calculated by an empirical formula for angular contact ball bearings. The application of the formula to ball screws, however, causes several contradictions with the characteristics of the fatigue failure which are obtained from the life tests in the previous reports. In the present report the life formula of ball screw is derived by an introducing the ball slips on the raceways, the variation of nut stroke and the state of load distribution in ball screws into the Lundberg and Palmgren's theory. Some compatibility between the life theory and the results of life tests are also discussed. The main results are as follows: (1) The surface shearing stresses which are increased by the slips between balls and raceways can be taken as a basis for an estimation of the fatigue strength of ball screws. (2) The differences between the fatigue strength of shaft and nut become clear through the considerations of number of stress cycles and length of stressed raceway which correspond to a variation of the nut stroke. (3) The theoretical explanation of the growth of initial fatigue failure at the nut raceway on the back side of ball return tube is given by the analysis of load distribution in ball screws and the introduction of their equivalent load into the equation of the basic dynamic load rating.
