Effects of thermally induced inhomogeneous shear and surface thermal boundary conditions on the shear stress in sliding elastohydrodynamic contacts

Abstract

Numerous research work has shown that signifi cant thermally induced cross-fi lm inhomogeneous shear or thermal shear localisation may be developed in sliding elastohydrodynamic lubrication (EHL) contacts with pronounced consequences. This paper uses the theoretical framework established in previous research to further analyse the effects of the shear localisation on the lubricant shear stress and thus the EHL traction. Results obtained suggest that the shear localisation signifi cantly accelerates the thermally induced reduction of the shear stress in sliding EHL contacts. The study also shows dramatic reduction of the shear stress in EHL contacts with one-insolated surface, which is signifi cantly attributed to the more intensifi ed thermal shear localisation near the insolated surface. The practical signifi cance of the dramatic shear stress reduction is discussed.

Modeling and Analysis of the Meshing Losses of Involute Spur Gears in High-Speed and High-Load Conditions

“…The theory suggests a possibility to significantly reduce the meshing losses in gear transmission by reducing the thermal conductivity of the tooth surface of one of the two gears. Based on some results from the theory, Chang [27] speculated that a reduction of as much as 50% may be achieved. This speculation is analyzed using the model developed in this paper.…”

“…As a result, the shear stresses and thus the friction and meshing loss are much reduced. Nevertheless, the use of an insulated gear to much reduce the meshing loss speculated in Chang [27] is seen to be more or less a theoretical fantasy as there are hardly any materials or surface coatings that can offer such perfect insulation. A small amount of heat conduction through the surface would generate sufficient thermal leakage to diminish almost all the benefit.…”

“…A number of studies showed or suggested that, under high-load and high-sliding conditions, large thermally induced shear localization may be developed across the EHL film to significantly reduce the shear stresses thus EHL traction. Furthermore, Chang [27] showed that if one of the contact surfaces is insulated, the localization will be developed at the insulated surface due to high local temperature. As a result, the localization can be significantly more pronounced than the problem with two conductive surfaces and, in conjunction with the higher overall temperature, the shear stress in the lubricant can be much lower.…”

“…The two equations are coupled to each other. The coupling may become very pronounced under high-load and high-sliding conditions such as in gear meshing, leading to the development of thermal shear localization across the EHL film [22,27]. A good solution of them should ensure a quality calculation of the shear stresses and thus the meshing friction that is reasonably accurate.…”