Rotordynamic evaluation of a roughened-land hybrid bearing

Hybrid bearings represent an attractive alternative to ball bearings for use in highspeed cryogenic turbopumps. However, the internally-developed cross-coupled forces can generate instabilities responsible for a speed limitation of the machine. To reduce these forces and raise the onset speed of instability, the use of deliberately-roughened stators, already successfully tested for liquid ''damper'' seals, is investigated. Rotor-dynamic results are presented for a five-pocket orifice-compensated hole-pattern-land hybrid bearing tested with water at high speed and high pressure. Experimental data show a good prediction of leakage pow rate and direct damping but a significant improvement in stability compared to a conventional smooth-land hybrid bearing, resulting in an elevation of the onset speed of instability Comparisons between measurements and predictions from a code developed by San Andres (1994) shows good predictions for flowrate and direct damping but an overprediction for the direct and cross-coupled stiffness coefficients by about 30 and 50 percent, respectively. The use of the Moody friction-factor model is thought to be mainly responsible for the poorer predictions of stiffness coefficients.