Abstract
The film thickness of a ball-on-disc contact lubricated with four greases of different formulations was measured under different operating conditions until starvation. Two polymer-thickened greases and two lithium-thickened greases, formulated with base oils of different nature and/or viscosity, were tested. The central film thickness was measured under constant operating conditions (load, temperature, slide-to-roll ratio) varying only the entrainment speed. In a separate test, the film thickness was measured over time with all operating conditions set to constant. Pictures of the film thickness profile across the contact area were also registered. The results were compared with the fully flooded results. The coefficient of friction (COF) was measured in a ball-on-disc contact under equal operating conditions and the results were correlated with the film thickness findings. The different grease formulations and the influence of the operating conditions on the film thickness and COF were discussed. The polymer thickened the greases, promoting lower COF and higher film thickness, especially when there is thickener material crossing the contact which happens quite often for these greases.
Highlights
Starvation in oil lubricated contacts was first visually observed using optical interferometry by Wedeven et al [1] in 1971, followed by Chiu et al [2], and later, by Pemberton and Cameron [3].Wedeven et al [1] observed that if the inlet region is properly flooded with lubricant, the inlet pressure build-up will generate a lubricant film whose thickness will no longer increase even if the lubricant supply increases—the fully flooded condition [1]
Using optical interferometry to visually determine when starvation occurs in a ball-on-disc contact, Kaneta et al [13] found that starvation takes place quicker with greases formulated with high viscosity oils
Central Film Thickness Measured With Varying Entrainment Speed
Summary
Starvation in oil lubricated contacts was first visually observed using optical interferometry by Wedeven et al [1] in 1971, followed by Chiu et al [2], and later, by Pemberton and Cameron [3]. Cann et al [7] performed film thickness measurements over time under constant operating conditions and observed that the degree of starvation (ratio between the starved and the fully flooded film thickness) increases with rolling speed, base oil viscosity and thickener content, but it decreases with temperature. Using optical interferometry to visually determine when starvation occurs in a ball-on-disc contact, Kaneta et al [13] found that starvation takes place quicker (meaning, at lower entrainments speeds and in smaller time of operation) with greases formulated with high viscosity oils. They found that starvation occurs more with higher speeds because the time for replenishment between over-rollings is shorter, hindering replenishment. The study of grease lubricated contacts is of utmost importance for rolling bearing technology
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