Theoretical and experimental analysis of the preload degradation of double-nut ball screws

In most applications, preload is applied to double-nut ball screws to improve the positioning accuracy and rigidity. However, the applied preload degrades owing to wear, which lowers the positioning accuracy of the ball screws, and thus affects the accuracy of computer numerical control machine tools. In this study, to obtain the dynamic contact force and sliding velocities on the ball-raceway contact, we established a load distribution model and velocity variation model, respectively. Based on these models, we propose an accurate model for predicting the preload degradation of double-nut ball screws. The degraded preload of three ball screws was measured under high axial load-low speed, medium axial load-medium speed, and low axial load-high speed, with a running stroke of 7.2 million revolutions. The experimental results are in good agreement with the theoretical values calculated by the prediction model, which confirms the validity of the prediction model. Therefore, it was concluded that the proposed preload degradation model can accurately predict the performance and service life of double-nut ball screws.