Contact Stiffness Calculation and Effects on Rotordynamic of Rod Fastened Rotor

Abstract

Rod fastened rotor is widely used as one kind of structures of gas turbine rotors. Due to its discontinuity, the stiffness of the combined rotor is different from integral rotor. In order to obtain the dynamic of rod fastened rotor, the stiffness of contact needs to be calculated. There are two main methods to calculate the stiffness of contact. One is focus on the microscopic spots in contact based on the fact that maching surface is full of asperities. Hertz contact theory and Greenwood model are used to calculate the contact stiffness under both moment and preload. The other method is finite element method and assumes the contact surfaces are absolutely smooth which is from macroscopic aspect. Contact stiffness is calculated using the two methods and their influencing factors are discussed. Also the contact stiffness from the two methods is used to build a finite element model of a rod fastened rotor and their influences on dynamics of combined rotor are investigated and compared. Results show that the effect of preload of rods on contact stiffness is due to the roughness of contact surfaces. When no separation appears in contact, the bending stiffness is independent of bending moment in both methods. However, it shows a contrary variation when separation appears. Although there is big difference of bending stiffness from both methods, it shows little influence on the rotordynamic of the rod-fastened rotor.