Analysis of the Stiffness of Hirth Couplings in Rod-Fastened Rotors Based on Experimental Modal Parameter Identification

Hirth couplings are widely used to transport torque between discs in turbine machinery for the advantages of precise centering, reliable positioning and excellent structural stability. However, the stiffness of the rotor segment with Hirth couplings is less than that of the integrated structure because it is weaken in the discontinuous structure. For the rotor segment with Hirth couplings, there are two types of contact status depending on the relationship between the separate stress and the compression stress. The first type is that the whole zone of Hirth couplings is in contact, where all of the Hirth couplings make contribution to the stiffness of the rotor segment. The second type of contact status is that some zones of the Hirth couplings are separated, where the equivalent stiffness of the rotor segment is only determined by the zones which are in contact. Obtaining accurate stiffness of Hirth couplings is of great significance in rotor dynamic performance analysis. In this study, the flexural stiffness of Hirth couplings of a gas turbine was calculated by three dimensional (3D) nonlinear contact Finite Element Method (FEM), and based on which the natural frequencies of a rod-fastened rotor were investigated. The stiffness modifying method which modifies the elasticity modulus of the material in the connection segment was used to conduct modal analysis in order to evaluate the effect of the stiffness weakening of the Hirth couplings. One experimental rotor with Hirth couplings and a central tie rod was designed to verify the theoretical and numerical analysis results. According to the results, the stiffness coefficient is constant when the load factor γ is less than 1.0, which means the stiffness of the Hirth couplings segment is almost constant when the pre-tightening force is large enough to keep the whole zone of the Hirth couplings in contact. The stiffness coefficient constant is about 0.32 (specific for the investigated model). And the stiffness coefficient drops dramatically when the load factor γ exceeds 1.0, which indicates that the stiffness of the Hirth couplings segment decreases dramatically since the pre-tightening force is insufficient and the Hirth couplings are separated. The Hirth couplings segment stiffness obtained by experimental modal parameter identification turned out to be consistent with the calculated results by FEM. The modified stiffness of the Hirth couplings segment were applied on the dynamic performance analysis of a real gas turbine rotor and the accuracy of calculating results was improved.