On the film thickness in grease lubricated deep groove ball bearings
Pramod Shetty is a PhD student in the Department Surface Technology and Tribology. (Co)Promotors are prof.dr.ir. P.M. Lugt and dr. J.A. Osara from the Faculty of Engineering Technology.
Most rolling element bearings use grease as a lubricant. The service life of a bearing is determined by the bearing fatigue life and grease life. Both are influenced by the quality of lubrication, which is strongly determined by the thickness of the lubricating film. The optimal selection of a lubricating grease and/or the prediction of grease and/or bearing life can only be done properly if the film thickness can be determined. However, until now, there is no equation to predict film thickness in grease lubricated bearings. In practice, the equations that were derived for oil lubrication are used.
In this thesis, the film thickness immediately after the churning phase is studied under various conditions on different bearings and greases. An improved electrical capacitance method is developed to measure the film thickness by using an electrical model of the bearing, including the effect of starvation. It is shown that the film thickness immediately after churning is determined by the dynamics of the flow of lubricants in and around the contacts and not by oil released by the grease after churning (bleed). It is observed that the film thickness in a grease lubricated bearing is almost constant at higher speeds.
It is demonstrated that the level of starvation under pure axial load can be well described using only the product of base oil viscosity, half contact width, and linear speed (ηbu). It is also shown that bearing size/geometry has little to no influence in determining the level of starvation in the ηbu concept. The film thickness study in radially loaded bearings and bearings under combined (axial+radial) loads showed that there is an additional replenishment at the lower loaded zones. This is due to a larger ‘gap’ between the ball and the groove in the low load zone causing a reduction in resistance to viscous flow, thereby increasing the flow of lubricant towards the contacts.
Finally, a master curve is created that can be used to calculate the film thickness in grease lubricated ball bearings under radial, axial and combined loads.
