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

The rod-fastened rotor (RFR) 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 K-c in the RFR is the key concern for accurate rotor dynamic performance analysis. Each contact interface was modeled as a bending spring with a stiffness of K-c and a hinge in this study. The contact states of the contact interfaces, which depend on the pretightening 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 Greenwood and Williamson (GW) 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 relatively large); the equivalent flexural stiffness of the rotor segment K-sf (not including 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 RFR indicate that, for a fixed pretightening 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 decrease abruptly with the increase of bending moments in the second contact state when the rotor is subjected to a relatively large pretightening force. Finally, the multipoint exciting method was used to measure the modal parameters of the experimental RFR. It is found that the experimental modal frequencies decrease as the pretightening force decreases. [DOI: 10.1115/1.4007026]