The Film Behaviors of Grease in Point Contact During Microoscillation

Abstract

Microoscillation is a typical case of transient motion, which occurs in many machine elements, including rolling or sliding element bearings, cams, and gears. Wear is easy to occur on the surface of such elements, particularly at the end point of the stroke, where the surfaces are momentarily static. In the present work, an experimental investigation is conducted to explore the grease film behavior of point contact lubrication during microoscillation in the case of pure rolling or pure sliding. The technique of relative optical interference intensity was used to monitor the variation of the grease film thickness and the motion of the grease in the contact area through analyzing the captured interferograms. Experimental results indicate that a crescent-shaped grease film can form along the motion direction in the contact area under microoscillation. The grease film is formed in the inlet region, and the film thickness remains while moving in the Hertzian contact area. In the case of pure rolling, the crescent-shaped grease film and the initial entrapped grease film are carried by a tow effect of moving interfaces in the contact area. However, in the case of pure sliding, there are relative motions in the sliding direction at the two interfaces of the grease/ ball and the grease/disk in the Hertzian contact area. The shape of the entrapped grease remains almost unchanged while moving in the Hertzian contact area. During the repetition of microoscillations, the crescent-shaped grease film thickness drops gradually.

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

“…5(a) and 5(l), the grease film shapes as well as the midsection film thickness were not identical. The upward entrapment was mainly owing to the large or small aggregation of the thickener fiber, other than the squeeze effect [4,5,8]. If we compare Fig.…”

“…Li et al [7] conducted an experimental investigation to explore crescent-shaped grease film in rolling and sliding microoscillation. They [8] also observed the transient grease film variation in a reciprocating motion during one working cycle. Wang et al [9] studied the grease film distribution in a reciprocating sliding motion experimentally on a ball-disk test rig using an optical interferometric technique.…”

“…Except the works of Li et al [7,8] and Wang et al [9], all of the above experiments were carried out under a steady-state condition. The existence of thick grease film with the aid of thickener fiber at low speeds has become widely accepted.…”

Grease film evolution in rolling elastohydrodynamic lubrication contacts

“…The use of optical EHL apparatus is beneficial to reproduce the evolution process of grease films [46,47]. Under a single charge of fresh grease, the variation in contact conditions with time, from fully flooded to different starvation levels, can be recorded.…”

Microbubble Phenomenon in the Grease Lubricating Film Induced by Micro-Oscillation

“…Li et al [15] recently presented a detailed review of lubrication under non-steady state conditions, but most of the studies reviewed were carried out for liquid oils. Li et al [15,16] investigated the lubricating behavior of grease lubricant films under nonsteady state conditions, including swaying motions (acceleration/deceleration) and micro-oscillation, using ball-on-plate optical interferometry based on the technique of relative optical interference intensity. Experimental results of such studies indicate that grease lubrication under non-steady state conditions is very complex, and the characteristics of grease lubricant under these conditions are as yet far less well understood as those under steadystate conditions.…”