A new two-stage degradation model for the preload of ball screws considering geometric errors

Owing to the geometric errors, preload of ball screws usually exhibits a two-stage degrading characteristic during operation, which is taken as the running-in stage (the first stage) and steady wear stage (the second stage), respectively. While existing researches were mainly focused on the steady wear stage, no references can be found in modelling the preload degradation during the running-in stage. Therefore, we proposed a two-stage model to compute the wear volume and preload degradation of ball screws. In the first stage, a new model considering macroscopic, geometric errors (the pitch errors, profile errors, ball radius errors, and waviness) was constructed to compute the wear volume. In the second stage, fractal theory was utilized to depict the wear volume from the microscopic perspective, in which both the adhesive wear and abrasive wear were considered. The predicted preload of the proposed model coincides well with the experimental results. Simulation results show that when the axial load increases from 5000 N to 7000 N, the first stage can be shortened as much as 50%. When the rotational speed increases from 600 rpm to 1000 rpm, the degradation rate of the first stage increases by 178%, and the first stage decreases by more than 50%. When the peak value of geometric errors increases from 1 μm to 1.8 μm, the first stage can be extended as much as 416%.