Investigation on nonlinear behaviour of titling pad journal bearings

Due to the today's increasing demand of higher ef­ficiency for all machines, new lubricated bearings are facing extreme operating conditions and their behav­iour must be investigated. Tilting pad journal bearings are widely used in turbomachinery. The bearing stiff­ness and damping coefficients must be known for rotor dynamic analyses particularly at the design stage. The coefficients are experimentally identified with linear models by applying dynamic loads to the bearing, measuring the rotor-stator relative displacement. The motion about the static equilibrium position must be small enough to be consistent with the linearity assumption but large enough to minimize the measurement error of the displacement sensors and to replicate the vibration amplitude in real operating conditions. Several tests were performed at the University of Pisa on a test bench specifically designed for large size journal bearings operating at high peripheral speeds and static loads. Besides the classical procedures used for the identification of the dynamic coefficients, a quasi-static procedure was developed to check possible presence of nonlinearities. A slowly rotating force was applied to the floating bearing stator in addition to the static load and the relative displacement between the stator and the rotating shaft was measured. Deformed orbits instead of the elliptical ones were observed increasing the ratio between dynamic and static load, suggesting the presence of nonlinearities. Quite similar results were obtained with simple analytical models including linear and quadratic stiffness coefficients and assuming suitably tuned non-linear stiffness terms. In this work more systematic optimization methodologies are used to obtain a better fit between experimental and numerical results. The first order stiffness coefficients are also evaluated according to the nonlinear model.