Response of grease film at low speeds under pure rolling reciprocating motion

Abstract

Using an optical interferometric technique, the grease film distribution under a pure rolling reciprocating motion is observed on a ball-disk test rig, to explore the transient response of grease, which is expected to fill the contact with thickener fiber and run for a long time under steady-state conditions. It is found that the reciprocating motion reduces the accumulation of the thickener fiber gradually with time. The maximum film thickness forms around the stroke ends. After 1,000 working cycles, very severe starvation occurs so that either the central or minimum film thickness becomes nearly constant over one working period. Thus, the life of grease lubrication under a transient condition is far below that under steady-state conditions. However, it is also found that by selecting a smaller stroke length, the thickener fiber spreads out in the contact instead of being removed from the contact at the 1,000th working period. When increasing the maximum entraining speed of the reciprocating motion to a certain value, during which the thickener fiber is not expected to accumulate under a steady-state condition, severe starvation occurs very quickly, causing surface damage.

Variation of zero entraining velocity dimple in grease-lubricated reciprocating motion

“…This change exists as the surface speed varies within a range of 0.001-0.5 m/s in steady-state ZEV contact. That is to say, there is no critical entraining speed 9,10,12,13,31 under ZEV condition. Moreover, for experiments under transient condition, the disappearance of the thickener fiber is even faster, so it is not necessary to consider the influence of the thickener fiber on the film distribution.…”

Effects of slide-to-roll ratio and varying velocity on the lubrication performance of grease at low speed

“…As an important branch of transient EHL, reciprocation is a common form of motion. There have been a few experimental works for grease film distribution under transient conditions, such as the work by Li et al, 18,19 Wang et al, 20 and Han et al 21 Li et al 18,19 conducted an experimental investigation to explore crescent-shaped grease film in rolling and sliding micro-oscillation. They revealed the transient grease film variation in a reciprocating motion during one working cycle.…”

“…Due to the existence of the thickener, in the first several working periods, the variations of the film shape in the first and second stokes are different. Han et al 21 found that the reciprocating motion reduced the accumulation of the thickener gradually with time under the pure rolling condition. Another important point is that the thickener lumps affect the replenishment of grease.…”

Grease film evolution in rolling elastohydrodynamic lubrication contacts

“…8. The mechanisms of both shear degradation and starvation can induce film decay [52]. Shear degradation will cause both temporary and permanent apparent viscosity loss, hence reducing the film thickness, as shown for the initial film decay in Figs.…”