Nonlinear Effects in the Dynamic Characterization of Tilting Pad Journal Bearings

The experimental identification of the dynamic coefficients of bearings is very important for predicting the behavior of rotors. Usually linear coefficients are considered, assuming small perturbations around the static equilibrium position. In the literature, investigations on nonlinear effects in plain bearings are reported, especially from a numerical point of view. Few similar studies are found on tilting pad journal bearings (TPJB). The present paper reports some peculiar aspects observed during the experimental procedure for the identification of TPJB linear dynamic coefficients. The tests were performed on an experimental apparatus realized for large size journal bearings operating at high peripheral speeds and static loads. A quasi-static procedure for the determination of the stiffness coefficients was developed, as a quick check of the results obtained by the usually adopted dynamic excitation. It consists in applying a slowly rotating force to the floating stator and measuring the relative displacement of the stator from the rotating shaft. Different static and dynamic load levels were applied. Deformed orbits were observed for increasing ratios of dynamic to static load suggesting the presence of nonlinearities. Similar results were obtained by numerical simulation on a simple bearing model assuming nonlinear stiffness terms