µPIV Measurement of Grease Velocity Profiles
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 semi-solid material, which prevents it from flowing/leaking out from the lubricated system and gives it sealing properties, protecting the system against contaminants. Unlike oil, grease has a much more complicated rheology, which makes it more difficult to model and understand grease flow. Grease acts as a lubricant reservoir, and understanding grease flow is essential in order to model and predict how grease is transported within e.g., a rolling element bearing housing, a sealing arrangement or replenishment of a gear mesh.
Three greases with different rheological behaviors (NLGI 2 grease, NLGI1 grease and NLGI00 grease) have been used in two kinds of test rigs: a straight channel with different restrictions and a rotating shaft with two narrow gap sealing-like restrictions.
In the first test rig two types of flow restrictions were applied into a straight channel in order to simulate flow of grease near a sealing pocket. In the case of a single restriction, the distance required for the velocity profile to fully develop when going from a wide to a narrow gap is approximately the same as the initial height of the channel. In the corner pocket before and after the restriction, the velocity is very low and part of the grease is stationary. For the channel with two flow restrictions, this effect is even more pronounced in the “pocket” between the restrictions. Clearly, a large part of the grease is not moving since the yield stress of the grease is not exceeded. This condition particularly applies to the cases with a low-pressure gradient and where high consistency grease is used. In practice this means that grease is not replaced in such “pockets” and that some aged/contaminated grease will remain there.
A test rig comprising of a rotating shaft with two narrow gap sealing-like restrictions (a so called Double Restriction Seal, DRS) was designed to simulate the a labyrinth type of seal. Two different gap heights in the DRS have been designed to compare grease flow. It is shown that partially yielded grease is detected in the large gap geometry and fully yielded grease in the small gap geometry. Grease shear thinning behavior and wall slip effects have been detected and discussed. 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.