Study on the Stiffness Loss and the Dynamic Influence on Rotor System of the Bolted Flange Joint

Abstract

The bolted flange joint is a kind of widely used joint structure in the rotor system. Its discontinuous mechanical characteristics result from the existing of the contact surface, which will slide and deform when the spool deforms. As a consequence, the joint’s stiffness is always smaller than that of fixed configuration, which affects rotor’s stiffness distribution and the rotor’s dynamics further. The objective of this study is to investigate the mechanical characteristics of the bolted flange joint, the affecting factors and the influence on rotor’s dynamics. According to the characteristics of structure and mechanical state, using the existing equivalent axial spring-bending beam model to describe the tension and compression stiffness of bolted flange joint section, then the bending stiffness model of whole bolted flange joint is established based on that. The results show that there is a significant effect of the bolted flange joint on the local stiffness of the rotor, the loss of local bending stiffness reach a high level when the number of bolts is few. The mathematical description between stiffness loss and structure size, load and assembling condition is obtained through the analytical results. A bolted flange joint simulation model, taking the characteristics of the contact into account, is built by the nonlinear finite element method. The trends of numerical results agree with the analytical conclusion, and show the stiffness of bolted flange joint is smaller than that of the fixed configuration. The stiffness of bolted flange joint decreases a small amount with the increasing moment. When the number and the pretension force increases, the stiffness increases nonlinearly. Based on the mechanism of stiffness loss, the equivalent stiffness is used to replace the fixed configuration stiffness on the location of bolts in finite element model of high pressure rotor system. The results of dynamic analysis shows that the stiffness loss has a greater impact on bending modes than the rigid modes while the static analysis shows that the stiffness loss has a small negatively effect on clearances. The study shows that, the stiffness loss of bolted flange joint has a close relationship with the load and assembling conditions. The results show the effectiveness in controlling the mechanical and dynamic properties of the rotor with bolted flange joints by careful adjusting of structural parameter, load parameter and assembling parameter during designing.