Abstract: Some experimental studies are described about the film thickness between two parallel surfaces in a step bearing by a custom-made test apparatus. In the experiments, the lubricated parallel contact consists of a step-slider and a transparent glass disc. The film thickness is measured under different entrainment speeds and loads. The phenomenon of cavitation is observed. The results show that the variation of film thickness versus speed is divided into two regimes. In regime I where the speed is low, the film thickness is very thin and independent of speed；While in regime II, as the speed increases the film thickness shows significant increase and the film thickness versus speed displays a linear relationship in a log-log scale. The critical velocity dominating the transition from regime I to II depends on the lubricants and the applied loads. Some theoretical analyses are made about this problem, Same results are gotten. In regime I the load carrying capacity is generated mainly by local hydrodynamic effect caused by surface roughness, while in regime II the load carrying is by the hydrodynamic effect of geometry of the step bearing.

Key words: Cavitation, Film thickness, Local hydrodynamic effect, Surface contact, Surface roughness