The thin films in concentrated contacts will be subject to high shear at high speeds, causing an overall temperature rise of mechanical components. The elevated temperature will result in both degradation of lubricant and the drop of viscosity that weaken the film formation. During the temperature rise, the transitions of the lubrication regime from full film to mixed or boundary lubrication will occur. A poor lubrication regime will hence increase the risk of lubrication failure. The purpose of this paper was to reproduce the influence of ambient temperature on film formation and lubricant regime transition in a model test device.
Using the ball-on-disc optical single-contact optical elastohydrodynamic lubrication (EHL) apparatus, contact lubrication states were observed at different ambient temperatures. The experiments were conducted under pure rolling and fully flooded conditions. The results showed that the lubricant viscosity significantly decreased as ambient temperature rised, resulting in an overall film thickness reduction and a weak film formation ability. There existed a critical entrainment speed beyond which the film thickness would increase with entrainment speed. The critical entrainment speed also increased with ambient temperature raised. At elevated temperatures, the contacts would be in boundary and mixed lubrication regions at a large speed scope. At low loads and elevated entrainment speeds, it clearly showed that the contacts transited from EHL to HL (hydrodynamic lubrication). This could be recognized from the interferometrical images in which losed the typical EHL feature due to the recovery of contact surface deformation.
Considering the pressure-viscosity effect of lubricant and deformation effect of contact surfaces, the contacts could also be divided into four regimes, i.e. IR (Isoviscous-rigid) regime, IE (Isoviscous-elastic) regime, PR (Piezoviscous-rigid) regime, and PE (Piezoviscous-elastic) regime. With the increase in entrainment speed, the contacts presented a trend of lubrication regime transition from PE to IR, especially for the low load, while with the temperature and load increasing, the contacts would approach to PE regime.
It was found that slopes of the film thickness curve (film thickness with entrainment speed) became larger at higher temperatures. Theoretically, the slope of the film thickness curve should always be constant since only the inlet lubricant viscosity became lower induced by high temperatures. This mainly attributed to the temperature difference between the contact surfaces and the open heating system. At the present test stage, a circulating oil supply method was used, i.e., the heated lubricant was fed into an oil reservoir, and the steel ball partially immersed into the reservoir. Then the lubricant was entrained into the contact region through the ball rotation. The problem with this lubricant supply method was that only the lubricant and steel ball was heated without glass disc heating. This asymmetry heating system affected the film formation. The higher of test temperatures, the larger of the surface temperature difference and the higher the slope of the film thickness curve. Moreover, the open heating system would cause continuous heat dissipation to the surrounding air, which resulted in an overall film thickness reduction than that of full system heating.
To validate the experimental results, full numerical simulation was conducted. The numerical results showed that when the surface temperature difference was introduced, the slope would indeed become larger, which had a similar trend as the experimental observations. The calculated film thicknesses were lower than those of experimental ones due to the heat dissipation in experiments.
This paper confirmed that heating method and surface temperature differences had an obvious effect on film formation and lubrication states. Further investigation under sliding-rolling conditions would be conducted.
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