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Effect of rheology on the flow motion of lubricating greases
How well the contacts in a machine element are re-lubricated is strongly dependent on the lubricating grease flow behaviour, implying an understanding of the relation between grease flow and grease rheology is important. During re-lubrication the flow occurs both in free-surface and wall-bounded geometries; the latter is e.g. present as the grease flows through the seal (Figure 5).
The grease flow motion is heavily influenced by its yield- and shear thinning properties. For grease flow in the channel with one flat restriction, Figure 6 (a), all the greases show similar velocity profiles with a difference in maximum velocity. The maximum velocity for a constant driving pressure is reduced with the NLGI grade, i.e. a higher consistency of the grease contributes to a larger flow resistance. It was also observed that there is a flow several orders of magnitude smaller than the bulk flow over the restriction in the corner before the restriction, as indicated by red arrows in Figure 7. Within this corner area, it is observed that the NLGI 00 grease at high velocity exhibits small circulation (a vortical flow pattern).
Figure 5. Illustration of grease flow (arrows showing flow direction) as a bearing is relubricated with grease. Fresh grease is supplied to the bearing housing and grease transversely flows through the bearing, the housing, and the seal to force out the used grease. Courtesy of SKF ERC.
To simulate the flow through typical sealing geometries, a straight channel with restrictions has been used; see Figure 6. Micro Particle Image Velocimetry (µPIV) has been used to visualize the flow [6,7]. µPIV is a non-intrusive method where tracer particles are mixed with the grease and the flow motion of the particles is captured by a high speed camera. A pulsed laser is used as light source. The motion of the particles (and consequently the motion of the grease) is tracked by a correlation algorithm comparing the position of the particles in a pair of images separated by a small time step. To illustrate the influence of grease rheology on the flow, lithium greases with NLGI grade from 00 to 2 have been considered.
Figure 7. Close up view of the corner area before the flat restriction; NLGI 00 grease.
Figure 6. (a) Channel with one flat restriction, and (b) channel with double restriction. Dimensions are in mm.
Figure 8. The averaged vector field for (a) NLGI 2 grease at low pressure gradient, (b) NLGI 2 grease at high pressure gradient, and (c) NLGI 00 grease at high pressure gradient.
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LUBE MAGAZINE NO.126 APRIL 2015
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