Lubricating grease: Experiments and modeling of wall-bounded and free-surface flows
Lubricating grease is commonly applied to lubricate e.g. rolling bearings, seals and gears. Grease has some clear advantages over lubricating oil: it is a semisolid material, which prevents it from flowing/ leaking out from the bearing system and gives it sealing properties, and it also protects the system from contaminants and corrosion. Due to its consistency, lubricating grease has many additional advantages over lubricating oil: it does not require pumps, filters and sumps. However, the rheology of grease makes it more difficult to measure and study its flow dynamics. This study focuses on the influence of rheology on grease flow in different geometries involving a straight channel with restrictions, concentric cylinder geometry, and free-surface flow on a rotating disc. To better understand grease flow in bearings and seals, two types of flow restrictions were applied into the straight channel in order to simulate the flow of grease near a seal pocket. In the case of a single restriction, the horizontal distance required for the velocity profile to fully develop is approximately the same as the height of the channel. In the corner before and after the restriction,the velocities are very low and part of the grease is stationary. For the channel with two flow restrictions, this effect is even more pronounced in the narrow space between the restrictions. Clearly, a large part of the grease is not moving. This condition particularly applies in the case of a low-pressure gradient and where high-consistency grease is used. In practice this means that grease may be locally trapped and consequently old/contaminated grease will remain in the seal pockets.
A configuration comprising a rotating shaft and two narrow gap sealing-like restrictions (also called Double Restriction Seal, DRS) was designed to simulate a sealing contact. Two different gap heights in the DRS have been used to compare the grease flow. It is shown that partially yielded grease flow is detected in the large gap geometry and fully yielded grease flow in the small gap geometry. For the small gap geometry, it is shown that three distinct grease flow regions are present: a slip layer close to the stationary wall, a bulk flow layer, and a slip layer near the rotating shaft. The shear thinning behaviour of the grease and its wall slip effects have been determined and discussed. Free-surface flow of grease occurs in a variety of situations such as during relubrication and inside a rolling element bearing which is filled to about 30% with grease in order to prevent heavy churning. Here the reflow of lubricant to the bearing races is a key point in the lubricant film build-up, and centrifugal forces have a direct impact on the amount of available grease. Understanding of the free-surface flow behaviour of grease is hence important for the understanding of the lubrication mechanism. Adhesion and mass loss are measured for greases with different rheology on different surfaces and temperatures. It is shown that the critical speed at which the grease starts to move is mostly determined by grease type, yield stress and temperature rather than surface material. A developed analytical model covers a stationary analysis of the flow resulting in solutions for the velocity profile of the grease as well as a solution for the thickness of the viscous layer remaining on the disc.