Characterization and Measurement of Hybrid Gas Journal Bearings

Abstract

This paper concentrates on the study of hybrid gas journal bearings (bearings with externally pressurized mass addition). It differs from most work in that it goes back to “basics” to explore the hydrodynamic phenomena in the bearing gap. This work compares geometrically identical bearings with 2 configurations of external pressurization, porous liners where mass-addition compensation is varied by varying the liner’s permeability, and bushings with 2 rows of 6 feedholes where the mass-addition compensation is varied by the feedhole diameter. Experimentally, prototype bearings with mass-addition compensation that spans 2 orders of magnitude with differing clearances are built and their aerostatic properties and mass addition characteristics are tested. A dimensionless time and mass compensation constant are suggested that are thought to be more ontologically meaningful for presenting extendible results. The findings indicate that the defined maximum achievable static bearing efficiency, over 2 orders of magnitude of mass addition compensation, is almost entirely determined by the bearing geometry and external pressurization configuration so long as the clearance is tuned to the mass compensation (larger mass compensation requires larger clearance). A “near surface effect” is discovered in feedhole bearings leading to a region of negative stiffness. This is analyzed by dissecting the pressure waves in the bearing gap and is attributed to the pressure plume emanating from the feedholes nearest the minimum bearing gap height.