Research on the axial stiffness softening and hardening characteristics of machine tool spindle system

Bearing stiffness exhibits non-linear characteristic when the bearing is subjected to a certain external load, which will inevitably cause changes in the spindle stiffness, since bearing stiffness has a decisive impact on the spindle stiffness. In this paper, the axial stiffness softening and hardening characteristics of machine tool spindle were studied. A novel spindle axial load automatic applying device and the spindle stiffness test bench were proposed. Meantime, an axial stiffness model for the experimental spindle was established based on the bearing load–displacement model and mechanical analysis of spindle. Then, the axial stiffness of fixed position preload spindle under different preload was measured experimentally and the evolution mechanism was analyzed. The results show that when bearing preload reaches a certain relatively large threshold, a “sag” shape occurs in the axial stiffness curve, indicating the “stiffness hardening” characteristic of the spindle. On the other hand, for a small preload, no “sag” shape occurs in spindle stiffness curve, indicating the “stiffness softening” characteristic of the spindle. This phenomenon is of great significance for the acquisition of the excellent spindle stiffness properties.