Hydrodynamic Analysis of Hydrostatic Bearings With Runner Misalignment and Pad Damage

Abstract

Hydrostatic bearings are used in applications where surface speeds are low, or viscosities are insufficient to develop significant load capacity due to shear flow. They are also used in jacking applications for initial liftoff of rotors under low or no rotation conditions, especially for heavy rotors where significant babbitt damage would otherwise occur. Traditional hydrostatic bearing analyses assume isothermal lubricating flows. Analytical solutions also assume that the pressure in the pocket of the hydrostatic bearing is constant. This assumption is only approximately correct for low and zero operating speeds. Analytical solutions also assume that the runner and pad surfaces are parallel. The analytical solutions are not capable of capturing damage or misalignment effects. This paper describes a hydrodynamic analysis of a hydrostatic thrust bearing. The solution is based on a finite element solution to the generalized Reynolds equation. The finite element solution is applied in both the pocket and pad regions of the hydrostatic bearings. The analysis includes a flow loop balance that considers the effects of pressure losses in the lubricant supply piping, allowing for modeling of saturation effects in bearing load capacity. The flow loop balance for the lubrication supply is coupled with the bearing solution. This allows for pad loads to vary as a function of circumferential position in thrust bearings. The analysis was applied to the operation of a hydrostatic thrust bearing system for the HUSIR radio telescope at the Massachusetts Institute of Technology. Simplified models of pad damage and runner misalignment were considered in the analysis. The minimum film thickness and pressure profile was calculated. Runner misalignment reduced minimum film thickness by up to 80% when compared to a parallel runner under identical loading conditions. Runner damage equivalent to twice the nominal film thickness reduced the minimum film thickness by approximately 10%.