A test rig for the identification of rotordynamic coefficients of fluid film bearings

Abstract

A test rig and measurement procedure to identify the rotordynamic coefficients of generic fluid film bearing elements are described. The test apparatus allows the exchange of test articles without modifications to the basic design of the machine. The maximum operating speed of the rig is 9,000 rpm, with static loads to 4,450 N (1,000 lb) and available fluid supply pressures to 689 kPa (100 psi). Impact load excitations in two orthogonal directions bring the test bearing into dynamic motion, and a data acquisition and computer system record the dynamic response at a frequency of 12,000 samples per second. Test bearing elements undergo multiple forced dynamic tests, and the corresponding time responses are transformed into the frequency domain and averaged. Bearing stiffness, damping, and inertia force coefficients are then identified from an impedance matrix in the frequency domain. Dynamic tests for an open ends squeeze film damper at various journal center static (offset) positions are presented for three levels of impact loads. In general, the identified damping coefficients correlate well with theoretical predictions. Also presented are dynamic tests for a plain journal bearing with a length-to-diameter ratio (L/D) of 0.30. The difference between the experimental journal static operating position and the predicted journal static operating position is attributed to localized thermal distortion. The identified stiffness and damping coefficients follow the same trends as theoretical coefficients, but compare poorly.