Measurement of Rotordynamic Coefficients for a Hydrostatic Radial Bearing

Abstract

Measurement of rotordynamic coefficients is presented for a pair of hydrostatic radial bearings, including direct and cross-coupled stiffness and damping. Two different hydrostatic configurations were tested: (1) an externally fed bearing 74.7 mm (2.95 in.) in diameter with a nominal direct stiffness of approximately 210 MN/m (1.2 million lb/in.) and (2) an internally fed bearing 54.6 mm (2.15 in.) in diameter with a nominal direct stiffness of approximately 88 MN/m (0.5 million lb/in.). Each bearing had 6 equally spaced hydrostatic pressure pockets, stationary for the externally fed bearing and rotating for the internally fed bearing. Also, both bearings had extended exit regions to provide additional damping. The top rotational speed was 22,700 rpm and the maximum axial Reynolds number was 50,000 using a freon derivative, Freon-113, as the working fluid. The test apparatus was a “synchronous rig” as an intentionally eccentric journal was used as the sole source of excitation. Data reduction was done by performing a matrix solution to separate damping from stiffness. Results show the internally fed bearing to be 20 percent less stiff than predicted, and to have a significant amount of damping which agrees well with predictions. The internally fed bearing was found to be approximately 60 percent less stiff than predicted, and to be roughly neutral in terms of damping, as predicted.