INFLUENCE OF FLUID FILM NONLINEARITY ON THE EXPERIMENTAL-DETERMINATION OF DYNAMIC STIFFNESS AND DAMPING COEFFICIENTS FOR 3-LOBE JOURNAL BEARINGS

Recently, several papers have been published reporting measured journa l bearing characteristics including the linear dynamic force coefficie nts. In some instances, analyses were performed to give confidence lev els for the data based on uncertainties in the measured data. However, the effect of fluid film nonlinearity on the measured data used to ca lculate the dynamic coefficients has not previously been reported. Thi s paper presents results of a nonlinear simulation to assess these eff ects for one particular bearing test rig previously documented. The no nlinearity arises from the finite size orbital motion imposed on the j ournal or housing during the measurement process to obtain response da ta used to extract the dynamic coefficients from a linear model of the journal bearing dynamic system. The orbit size may be large enough to introduce nonlinearity in the fluid film forces which may in turn aff ect the resulting calculation of the linearized force coefficients. Th e nonlinear simulation includes a time-transient solution of the Reyno lds equation for a three-lobe bearing and replicates the testing proce dure used in an experimental journal bearing test rig previously repor ted upon. Comparisons of orbital motion and dynamic force coefficients to data obtained from that rig are re-ported herein. The fluid film n onlinearity was found to produce an uncertainty in the coefficients of up to 20 percent compared to the linearized coefficients obtained fro m a small perturbation solution of the Reynolds equation which are com monly used in rotating machinery design and analysis. ii further objec tive of the simulation is to evaluate the effect of different external forcing functions to assist guiding the test procedure In the actual test rig to reduce the influence of fluid film nonlinear ty on the tes t results.