Film thickness in a grease lubricated ball bearing

The film thickness for grease lubricated bearings is normally calculated using the base oil viscosity, where it is assumed that the bearing is running under fully flooded conditions. It is well known that this is not accurate since grease lubricated bearings are usually running under starved lubrication conditions leading to thinner films. Single contact measurements have shown that, in the case of starvation, the film thickness decreases with increasing speed. It is shown in this paper that this effect seems to be very small though. The film thickness measurements, for three types of thickener material and base oil, show that the film thickness is almost independent of speed. To quantify this starvation effect, the film thickness can be expressed as the ratio between real film thickness and calculated film thickness using the base oil viscosity, h(g)/h(ff). The measurements in this paper show that h(g)/h(ff) > 1 for very low speeds but decreases with speed asymptomatically reaching a constant value. It is shown here that h(g)/h(ff) depends on speed, load and temperature and on the grease properties. It is not only the base oil viscosity that determines the film thickness. It is also governed by other grease properties that are today not known.