Experimental and theoretical investigations of the dynamic properties for tilting-pad journal bearings

Abstract

Rotors of high-speed machinery are usually supported in tilting-pad journal bearings (TPJB), which offer inherent stability resulting from theoretically zero cross coupling dynamic coefficients. The rotordynamic analysis is usually based on linear bearing stiffness and damping coefficients calculated after ignoring pad mass and assuming synchronous shaft vibration (synchronously reduced coefficients). Measurement of the bearing dynamic properties has been a challenge for many years. This is particularly true for TPJB, which demonstrate a certain dependence on frequency of excitation. The paper presents theoretical and experimental evaluation techniques for stiffness and damping coefficients of TPJB. Some results are also shown. The theoretical results have been obtained from a computer model, which accounts for thermal and elastic effects, including those of pivot flexibility. Experimentally, the dynamic coefficients have been evaluated from the measured excitation force, acceleration and bearing/shaft displacement using a power spectral density technique. The experimental investigations have been supplemented by uncertainty considerations for the frequency domain method. These considerations have been based on analysis of propagation of elemental errors associated with the measurements of displacement and acceleration. The results illustrate the effects of pivot flexibility on the bearing stiffness and damping properties and their variations with frequency.