Theoretical and experimental investigation of the effect of oil aeration on the load-carrying capacity of a hydrodynamic journal bearing

Abstract

It is widely assumed that the presence of air bubbles in the lubricating oil of a hydrodynamic bearing gives rise to a reduced load-carrying capacity, because of the high compressibility and low viscosity of the air and its tendency, therefore, to upset the hydrodynamic effect. The aim of the work described in the current paper was to investigate the accuracy of this assumption by theoretical and experimental means, and also to provide quantitative data relating to the concentration of air bubbles and their size that are required for any discernible effect. The paper has the following three main contributions: (a) a theoretical model based on Reynolds equation, but modified to allow for the effect of aeration on lubricant viscosity and density, is proposed; (b) a novel method of injecting air bubbles into lubricating oil and for measuring their size and concentration was developed; and (c) an experimental hydrodynamic bearing test rig was implemented and run with both aerated and non-aerated lubricating oil, and in each case measurements of the load-carrying capacity for various operating speeds were made. The results from both theoretical and experiment work show that the presence of air bubbles in the lubricating oil leads to a slight decrease in bearing load-carrying capacity at high operating speeds. For normal operating speeds, however, (i.e. those resulting in eccentricity ratios greater than 0.6) results show that the presence of air bubbles has little effect on bearing load-carrying capacity.