Study On Tribo-Dynamic Characteristics And Fatigue Life Prediction For Helical Gear With Oil Lubrication

Helical gears are widely employed in high-power transmission due to the advantage ofsteady operating performance and heavy-load capability. With the increasing requirementfor high speed and heavy load, the meshing performance, high efficiency and fatigue life ofinvolute gear become critical and gradually are focused on by many researchers. Under thecondition of high speed and power, the gear tooth exhibits strong non-linear behaviors. Dueto the high shear effect of sliding velocity, amount of heat generates from tooth surface,which greatly reduces the viscosity of lubricant and leads to more complicated lubricatingcondition. Therefore, the author aims to build up a tribo-dynamic model that covering thecoupled influences of elastohydrodynamic lubrication (EHL) characteristics, the thermaleffects and dynamic behaviors. Based on this pressure-solving model, a new fatigue lifemodel is proposed including the factors of residual stress, the hardness gradient and thecrack growing process in different propagation stage.First, a two-dimensional finite line contact EHL model is built and a steady model ofthe non-uniform load distribution is proposed for helical gear based on the minimum elasticpotential energy criterion, which is combined with the mixed EHL to obtain the film shapeand pressure distribution in contact area. In numerical solution, a unified discrete formationis established to determine the asperity contact region, and the multilevel integrationmethod is introduced to speed up the calculation of surface simultaneously. Besides, theGauss-Seidel iteration and multilevel method are used to obtain the lubricatingcharacteristics. Accordingly, the influence disciplinarianof asperity roughness, direction anddensity on local pressure and film distribution are revealed.When the gear is working in high speed or heavy torque conditions, the high shearbehaviors of oil film and asperity frictions in contact region will generate amount of heat.Under the condition of elastohydrodynamic lubrication, the shear heating effect at inlet willdecrease the viscosity of oil film, which would lead to less volume of lubricant beingentrained by rolling velocity. Consequently, the film thickness becomes thinner. Therefore,the energy equation is combined with moving point heat source method using the heatpartition coefficient to obtain the temperature distribution in mid-layer film and tooth surfaces. Accordingly, the heat transferring mechanism in different meshing position isanalyzed.Due to the profile and assembly error, the dynamic transmission error and loadingimpact are unavoidable during rotating process especially under high speed conditions. Todiscover the coupled relationship between the lubricating performance and dynamicbehaviors, a tribo-dynamic model is built up considering the influences of time-varyingmeshing stiffness, the transient film squeezing effect, the surface topography and frictiontorque. In numerical solution, the film temperature is taken as the most outer iterative cycle.In order to uncouple the dynamic motion equation and the EHL model, an analyzed modelof film elastic deformation is proposed to simplify the calculation of film stiffness. Besides,based on the assumption that the relative displacement satisfies the periodicalboundarycondition, the dynamic meshing force is solved. Finally, the solved dynamic pressure andtemperature in contact region are obtained in dynamic model.In terms of the gear failures, the contact failures such as pitting and case crushing aremost common forms. The contact failure occurs at the spot where either the material defectsor shear stress peak exists. The maximum shear stress in subsurface is employed by authorto evaluate the fatigue life base on the material inherent properties. Crack initiation modelfor gear contact failure is built according to the risk accumulation theory. In the modelingof short crack propagation, the non-linear and discontinuouscharacteristics are taken intoaccount. Besides, a unified equation for long crack propagation covering various crackswith different scales is proposed to predict the whole fatigue life of gear pair. Consequently,the whole contact fatigue life is predicted by combining the three parts of the fatigue life indifferent stage.