| It is well known that the gearing contact fatigue strength design is based on Hertz theory,but this theory is only suitable for the frictionless,static dry contact between two smooth surfaces.However,the nearly all gear drives are operated under the state of lubrication.Therefore,the lubrication effect must been considered in the gearing contact fatigue strength design.It goes without saying that the lubricant's properties should been first paid attention to,viscidity is most important physical property of the lubricant and the measurement of viscidity is viscosity.Therefore,study of the effect of lubricant viscosity on geared fatigue life is completely essential.The meshing model of an involute cylinder gearing is first established.Then based on the thermal elastohydrodynamic lubrication theory,the surface pressure and subsurface stress distributions is obtained under the state of elastohydrodynamic lubrication,using multigrid method,multiple grid integration method and a sequential column sweeping scheme,respectively to solve the lubrication equations,the elasticity deformation and the temperature distribution.In order to discuss the influence of lubricant's viscosity on the contact stress,more than 80 sets of numerical calculations are completed under the light,medium and heavy load conditions,when other parameters of gearing and lubricant are constant and lubricant viscosity is changed in the certain range. The computational results show that there is a critical valueηCR,subsurface maximum principal shear stress(?)maxis decreased as the lubricant viscosityη0 rise whenη0≤ηCR;Whenη0＞ηCR,however,the subsurface maximum principal shear stress(?)maxis increased as the viscosity.It is necessary to indicate that although this conclusion counteracts in people's tradition knew,it actually tallies with predecessor's experimental results.It explains that the theoretical calculation of this paper is completely credible.The computational results also show that the amplitude of pressure spike is a quite important parameter.It and(?)maxassume the parabola relations,there is also a critical value PCR,When Pmax≤PCR,(?)maxis decreased as Pmaxrises. When Pmax＞PCR,(?)maxis increased as Pmaxrises.This is the reason that the excessive lubricant viscosity can increase the subsurface maximum principal shear stress.Based on the above results the paper considers that the lubrication factor have some limitations by the current design standard of gearing contact fatigue strength(ISO/TC60 423E).It demonstrates that the lubrication factor is monotonously going up as the viscosity rise.That is the higher viscosity is,the longer gear fatigue life is.Viscosity is not the higher the better for gear lubricating.Under a certain working condition,Selection of excessive lubricant viscosity will not only increase the cost,but also decrease contact fatigue life.The numerical calculations of current study are based on the full film EHL state,but fail to take into account of time-dependent nature in gear drive process. In addition,the results just raise questions about the applicable range of lubrication factor recommended by the international standards,but don't explore how to amend this factor. |
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