Investigation of the Causes of Static Instability in Externally Pressurized Gas Journal Bearings

Externally pressurized air journal bearings (air journal EPB’s) are lubrication-free, non-contact bearings that utilize air as the lubrication film. Unlike non-pressurized journal bearings that rely on rotation for hydrodynamic forces, the bearings generate a significant load bearing, restorative, hydrodynamic force independent of rotation. Empirical tests were performed measuring this restorative force and the mass flow out of the bearing-gap as a function of the eccentric displacement of the shaft from the bushing. Two different external pressurization configurations were examined. In one, the bearing-gap was pressurized via two rows of six feedholes located near the ends of the bearings (FH bearings). In the second, the bearing-gap was pressurized via porous liners (PL bearings). The so-called pressure compensation of the bearings was varied by altering the feedhole diameters for the FH bearings and changing the permeability of the PL bearings. The bearing clearance was varied by using shafts of varying OD’s. CFD calculations were then performed to simulate the empirical testing and were found to be in good agreement. The CFD analysis was then expanded over a wide design space for both the FH and PL bearings. The expectation was that the restorative force would continuously increase as the shaft was eccentrically displaced from the bushing reaching its maximum when the shaft was “grounded” so that the shaft contacted the bushing. However, the expanded CFD analysis showed a surprise result as it indicated that there were regions in the design space of the FH bearings where the maximum restorative force occurred at an eccentric displacement that placed the shaft near the bushing but did not ground it. Further eccentric displacement caused a decline rather than the expected increase in the restorative hydrodynamic force indicating a region of negative bearing stiffness. A reference was found that observed bearing load fall off (negative stiffness) with heavily loaded bearings (operating at high eccentricities) similar to our CFD findings. This area of operation was termed the “static instability region”. This paper presents further CFD analysis of the pressure wave in the bearing-gap at varying eccentricities. These waves are graphically dissected to show the characteristics that cause negative stiffness. The results indicate that PL bearings have no static instability and that the area of instability for FH bearings can be predicted.