Effects of Bending Moments and Pre-Tightening Forces on the Flexural Stiffness of Contact Interfaces in Rod-Fastened Rotors

Abstract

The rod-fastened rotor is comprised of a series of discs clamped together by a central tie rod or several tie rods on the pitch circle diameter. The equivalent flexural stiffness of contact interfaces in the rod-fastened rotor is the key concern for accurate rotor dynamic performance analysis. Each contact interface was modeled as a bending spring with stiffness of Kc and a hinge in this study. The contact states of the contact interfaces which depend on the pre-tightening forces and bending moments (static) have effects on Kc. The approach to calculating Kc in two contact states is presented. The first contact state is that the whole zone of the contact interface is in contact, Kc is determined by the contact layer which consists of asperities of the contact surfaces. Hertz contact theory and the GW (Greenwood and Williamson) statistical model are used to calculate the equivalent flexural stiffness of the contact layer Kcc. The second contact state is that some zones of the contact interface are separated (when the bending moment is relative large), the equivalent flexural stiffness of the rotor segment Ksf (not include Kcc) decreases as the material in the separated zone has no contribution to the bending load carrying capacity of the rotor. The strain energy which is calculated by the finite element method (FEM) is used to determine Ksf. The stiffness Ksf is equivalent to the series stiffness of the discs of the rotor segment with flexural stiffness of Kd and a spring with bending stiffness of Kcf in the location of the contact interface, so Kc is equal to the series stiffness of Kcc and Kcf in the second contact state. The results of a simplified rod-fastened rotor indicate that for a fixed pre-tightening force, Kcc decreases with bending moments in the first contact state whereas increases with bending moments in the second contact state. In addition, Kcf and Kc decreases abruptly with the increase of bending moments in the second contact state when the rotor is subjected to a relatively large pre-tightening force. Finally, the multipoint exciting method was used to measure the modal parameters of the experimental rod-fastened rotor. It is found that the experimental modal frequencies decrease as the pre-tightening force decreases.