CLASSIFICATION OF LUBRICANTS ACCORDING TO CAVITATION CRITERIA

Cavitation in lubrication liquids has long been known to be detrimenta l to components in hydraulic systems. Damage has been detected in jour nal bearings, especially under severe dynamic loading, gears, squeeze film dampers and valves.These findings have led to intensive studies o f metal resistance to cavitation erosion, in order to minimize the dam age. Results of these studies have been:(a) classification of known ma terials according to their resistance to cavitation erosion; (b) devel opment of new materials and processes to increase their durability. On e of the main achievements in this respect was the establishment of th e ASTM G32-92 Standard Method of Vibratory Cavitation Erosion Test. Ho wever, very little was done with respect to the liquid phase, e.g. the lubricants. As a consequence there is no standard procedure for testi ng of lubricants for their cavitation properties and no relevant speci fications in national and international standards. This study includes theoretical and experimental investigations. The theoretical approach examines the lubricant in elastohydrodynamically lubricated (EHL) con tacts. Using numerical simulations, based on Reynolds equation and ela stic deformation theory, the pressure profile and film shape have been computed. It is further investigated how the operating conditions aff ect the properties, e.g. ''cavitation energy'' of zones of sub-ambient pressure values and if a correlation between these results and cavita tion erosion criteria can be found. The experimental approach includes testing of 20 liquid lubricants, belonging to the following four grou ps: mineral oils, mineral-based oils, bio degradable oils and syntheti c oils. Testing was performed by vibrating a standard aluminium tip in each oil and periodically recording the gravimetric results. These re sults enabled the classification of the lubricants according to their cavitance, which is inversely proportional to the mass of solid materi al eroded by a cavitating liquid under controlled conditions. The resu lts of both approaches can be combined into an engineering tool in the future. This tool may serve the designer to improve the use of existi ng lubricants and the lubrication industry as an aid for the developme nt of new lubricants with increased cavitance in hydraulic systems.