On the Film Thickness of Grease-Lubricated Contacts at Low Speeds

The contribution of the thickener to the thickness of the lubricating film in grease-lubricated contacts is investigated. Four different types of greases were tested in a ball/spherical roller-on-disc machine, where the film thickness was measured using the interferometry method, varying the temperature, load, and slip. The test results show that there are two lubrication regimes. Below a transition speed the grease thickener plays an important role where the film thickness increases with decreasing speed. At higher speeds the film thickness is primarily governed by the base oil. This transition speed is a function of the temperature and not of the load and slip. At higher speeds the grease film thickness can be calculated using the base oil viscosity. The electrical capacitance method was applied to measure the film thickness at ultralow speed in a real bearing, showing that the single contact results are indeed applicable to a full rolling bearing.