Rotordynamic evaluation of a tangential-injection hybrid bearing

Abstract

: : Issued also on microfiche from Lange Micrographics. For future high-speed cryogenic turbomachinery, ball bearings will be replaced by hybrid bearings that combine hydrostatic and hydrodynamic effects. Hybrid bearings develop cross-coupled stiffness coefficients due to fluid rotation, leading to predictions of onset speeds of instability and potential limitations in their range of application. Injecting fluid into a bearing recess against rotation, versus the customary radial injection, can reduce the circumferential flow and the cross-coupled-stiffness coefficients, and increase the margin of stability. Test results are presented here for a hybrid bearing with against-rotation injection. The bearing has a 76.4 mm diameter with L/D = 1, and Cre = 0.001 . Data are presented for 550C water at three speeds out to 25000 rpm and three pressures out to 7.0 MPa. Compared to a radial-injection hybrid bearing, experiments show injection against rotation enhances stability, yielding reductions of cross-coupled stiffness coefficients and whirl frequency ratios. However, increased flow rate and a drop of effective stiffness with increasing speed adversely affect the bearing's performances. The prediction code developed by San Andrłs (1995) includes angled-orifice injection. The code correctly predicts trends, but at low speed, measured cross-coupled stiffness larger negative values.