Research On Stiffness Of Bolted Joints Considering Joint Surfaces Contact

Large number of joint surfaces in mechanical structure destroy the continuity of the mechanical structure and make the system shows complex mechanical properties, which greatly affects the static and dynamic performance of the machine. Bolted joint is one of the main form for fixed connection between the structural components of machine tools. Accurate calculation of the connection stiffness of bolted joint is the foundation of the stiffness analysis and forecasting of machine tools. There are a large number of joint surfaces in bolted joints, which bring great nonlinear characteristics and a large number of stiffness loss. Existing stiffness calculation model for bolted joints only considers the stiffness of the joints'parts and structure itself, but the contact stiffness of joint surfaces in bolted joints is often neglected, which restricts the accuracy of the connection stiffness calculation results of bolted joint. Therefore, it has great theoretical and practical significance to establish a bolted joints stiffness model which considers the contact stiffness of joint interfaces and apply it to engineering practice. The work in this paper is as follows:(1) A multi-scale prediction method of normal contact stiffness of joint surfaces is put forward based on finite element analysis. The actual topography characteristics are obtained by using wavelet decomposition and power spectrum density analysis technique, on the basis of the actual joint surfaces topography data that measured by the instruments. The finite element contact models of two rough surfaces are directly established with the ANSYS APDL command. The finite element elastic-plastic contact models of microscopic and macroscopic scale are both analyzed. Normal contact stiffness of the joint surfaces, which contains complete topography information of the actual surfaces, is obtained by combining the two scale analysis. The effect of related factors on the normal contact stiffness is analyzed.(2) The contact stiffness experiments for grinding joint surfaces of Q235 are designed, and validate the effectiveness of the proposed contact stiffness prediction method.(3) The stiffiiess calculation formulas of the parts and the joint surfaces in bolted joints are derived respectively, and a stiffness model for a single bolted joint is set up. The results show that after introducing into the contact stiffness of the joint surfaces, the bolted joint stiffness is no longer a constant but changes nonlinearly with loads; under the recommended pre-tightening force, the bolted joint stiffness decreases about 8%-25% than that only considers the stiffness of the parts; the larger pre-tightening force is, the smaller stiffness loss of bolted joint at the joint surfaces is.(4) A stiffness analytical model for the bolts group of the bed-column joint in a horizontal machining center is established. The performance of z-direction stiffness of the machining center is analyzed. According to the actual condition, the structure parameters and the bolts'location of the bolted joint are optimized respectively. A good optimization result of the stiffness is obtained.