Relaxation of static load bolt and its influence on early accuracy decline of guideway

Abstract

The straightness error of guideway connected with machine bed can exceed the allowable value due to the pretension relaxation in bolted joints. It is inevitable that the relaxation caused by creep displacement of contact surfaces at ambient temperature. The spring and damping elements are introduced to equivalent the contact surfaces. A mathematical model is established for characterizing the relaxation. The effects of equivalent parameters, surface roughness, and initial tightening torque on pretension relaxation are ulteriorly analyzed. It is found that the relaxation degree of pre-tightening force increases with increase of elastic element stiffness, surface finish, and initial tightening torque. The straightness of guideway subjected to different bolt preloads is studied based on the finite element method (FEM). Finally, experiments are conducted to verify the proposed model and FEM. The results show that the model error is 6.67%, and the straightness error in horizontal/vertical direction is 7.79%/7.28%.