Study on the load-carrying mechanism of a parallel step bearing

In this paper, the load-carrying mechanism of a parallel step bearing was studied from two different viewpoints, i.e. step bearing and parallel bearing. It has a history of about 100 years that Rayleigh step bearing can give optimum load-carrying capacity for certain geometry. As to parallel bearing, several mechanisms have been proposed: the thermal wedge, the viscosity wedge, the thermal distortion, and the local hydrodynamic effect induced by surface irregularities. In this research, a custom-built fixed-incline slider-bearing test rig was used to measure the lubricant film thickness between a stationary parallel step slider and a rotating transparent disc. Film thickness was measured by interferometry at different sliding speeds. It is worth noting that both temperature and humidity were held constant in the experiments. Experimental results show that the measured curve of film thickness versus speed can be divided into two main distinct regions. One is the lower speed region, where the lubricant film is very thin and less dependent on the sliding speed. In addition, cavitation often occurs in lower speed region. The other is the higher speed region, where the lubricant film is thick and curves of film thickness versus sliding speed present linear relationship in the double logarithmic coordinate system. For different viscosities and loads, the experimental results are quite similar in spite of diverse transition speeds from lower speed region to higher speed region. Theoretical analyses indicate that the film-formation behavior in lower speed region can be attributed to the local hydrodynamic effect caused by the real surface irregularities on the smooth land of the step slider, while the load-carrying mechanism in higher speed region agrees well with the classical lubrication theory of Rayleigh step bearing.