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

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 K_c 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 K_c. The approach to calculating K_c in two contact states is presented. The first contact state is that the whole zone of the contact interface is in contact, K_c 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 K_cc. 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 K_sf (not include K_cc) 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 K_sf. The stiffness K_sf is equivalent to the series stiffness of the discs of the rotor segment with flexural stiffness of K_d and a spring with bending stiffness of K_cf in the location of the contact interface, so K_c is equal to the series stiffness of K_cc and K_cf in the second contact state. The results of a simplified rod-fastened rotor indicate that for a fixed pre-tightening force, K_cc decreases with bending moments in the first contact state whereas increases with bending moments in the second contact state. In addition, K_cf and K_c 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.