At very high speeds, elastohydrodynamic (EHD) films may be considerably thinner than is predicted by classical isothermal regression equations such as that due to Dowson and Hamrock. This may arise because of viscous dissipation, shear thinning, frictional heating or starvation.In this article, the contact between a steel ball and a glass disc over an entrainment speed ranging from 0.05 m s −1 to 20 m s −1 was studied. Two sets of tests were performed. In the preliminary testing, the disc was driven at speeds of up to 20 m s −1 and the ball was driven by tractive rolling against the disc, its speed being determined using a magnetic method. After all possible explanations for the reduction in film thickness at high speeds were considered, it was shown that the results, which fall well below classical predictions, are consistent with inlet shear heating at the observed sliding speeds.Another set of tests was then performed, with both disc and ball driven separately, so that the accuracy of the shear heating theory for different types of oils and at different sliding conditions could be assessed. It was found that the thermal correction factor predicts the trend of film thickness behavior well for the oils tested and is particularly accurate at certain slide-roll ratios (depending on the type of oil). Experimental data were also used to obtain improved coefficients for the correction factor for different types of oil to achieve better prediction of film thickness at high speed throughout the whole range of slide-roll ratios.
When a concentrated contact is lubricated at low speed by an oil-in-water emulsion, a film of pure oil typically separates the surfaces (Stage 1). At higher speeds, starvation occurs (Stage 2) and the film is thinner than would be expected if lubricated by neat oil. However, at the very highest speeds, film thickness increases again (Stage 3), though little is known for certain about either the film composition or the mechanism of lubrication, despite some theoretical speculation.In this paper, we report the film thickness in a ball-on-flat contact, lubricated by an oil-in-water emulsion, at speeds of up to 20 m/s, measured using a new high-speed test rig. We also investigated the sliding traction and the phase composition of the film, using fluorescent and infrared microscopy techniques.Results show that, as the speed is increased, starvation is followed by a progressive change in film composition, from pure oil to mostly water. At the highest speeds, a film builds up that has a phase composition similar to the bulk emulsion. This tends to support the "micro-emulsion" view rather than the "dynamic concentration" theory.
Oil-in-water emulsions are the most widely used lubricants in metal rolling, as they provide a combination of cooling and lubricating properties. These avoid severe metal-to-metal contact by forming a separating (elastohydrodynamic) film between the metal strip and the rolls so as to prevent scuffing, limit wear and control friction.
At very high speeds, elastohydrodynamic (EHD) films may be considerably thinner than is predicted by classical isothermal regression equations such as that due to Hamrock and Dowson. This may arise because of viscous dissipation, frictional heating or starvation. In this paper, the contact between a steel ball and a glass disc was studied. The disc was driven at speeds of up to 20 ms−1, and the ball was driven by tractive rolling against the disc, its speed being determined using a magnetic method. It is shown that the results, which fall well below classical predictions, are consistent with inlet shear heating at the observed sliding speeds.
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