A Shear Thinning Model Based On The Time Of Recovery And Its Application In EHL

A primary way to enhance the transmission efficiency and to reduce energy loss is to suppress the friction of rubbing surfaces.Even to this day,the friction features of lubricants confined in contacts are still hot topic both in industry area and scientific researches.The friction of lubricants is sensitive to its viscosity and state under high pressure,high shear strain rate,and high temperature conditions.Thus,the rheological properties of lubricants are complex and tough to be evaluated.From this consideration,this dissertation will focus on the explorations of the rheological behaviors of lubricants theoretically and experimentally,so that to develop a new rheology model to predict accurately the rheology characteristics of lubricating oils.In this dissertation,the availability of a popular assumption employed in rheology studies in the past decades was first discussed.In this assumption the shear strain rate was defined as the ratio between sliding speed and film thickness.The scale effect on lubrication was analyzed with consideration of non-Newtonian and thermal effects.By analyzing the temperature and the generalized viscosity of the lubricant in highly pressured contacts,an unexpected prediction was found,which has not been reported in the literatures yet.Consequently,the influence of shear strain rate on the generalized viscosity was introduced.The results indicate that the widely used assumption will become invalid for large scale contacts or at other status with high temperature gradients.Especially,for the polymeric oils with high viscosity,this assumption will deliver inaccurate rheological information even at small scale contacts,misleading our understanding of lubricant rheology.Regarding to the limitations of the rheology models which are acceptable to some extend but still in open argument and discussion,exploratory work was conducted to develop a new rheology model that is able to describe the rheological features for extensive lubricants.In present work,the contact,passage and recovery of molecule interactions in the sheared fluid film were simulated by a ball-rod configuration.Based on the recovery time of the ball-rod deformation,a new semi-analytic rheology model was established.By comparing with the noted Eyring and Carreau-Yasuda models,it is found that the three models are consistent in their forms.But the new model is more favorable due to its proper number of rheology parameters.Furthermore,the new rheological equation was combined with the mathematical model of thermal elastohydrodynamic lubrication(EHL)for point contacts.The rheological features of three types of lubricants,i.e.bio-oil of squalane,polymeric oil of PAO 650 and mineral oil of Shell Turbo 33 were then obtained through numerical simulations.The results show a good agreement between the friction coefficients versus slide-roll ratio predicted theoretically and measured experimentally,suggesting that the new proposed rheological model is accurate enough and beyond the limitation of oil types.To further validate the applicability of the new rheological model,thermal EHL solutions in opposite sliding point contacts as well as in the contacts of two tapered rollers were achieved elaborately.It is found that for the thick polymeric oil in the ball and glass contacts under opposite sliding conditions,abnormal surface dimple produced by thermal viscosity wedge will be predicted by the numerical solutions.Moreover,with increase in the magnitude of ball speed,the dimple will be amplified and the EHL pressure spike shifts to left and becomes the main pressure peak eventually.By comprehensively analyzing the effects of shear thinning,thermal thinning as well as thermal viscosity wedge,the mechanisms of lubrication behaviors were then rationally acknowledged.Moreover,extensive experiments were conducted with three oils in ball and glass contacts.The agreements between experiments and numerical predictions were achieved through qualitatively comparisons.This further validates both the reliability of the new model and the experiments.In addition,using the new model,the thermal rheological behavior in modified tapered roller contacts was also analyzed by considering the moment equilibrium.When the load was applied not through the contact center,the roller will skew,resulting in local high stress at one end,whilst this situation will be improved to some extent by increasing the convexity of rollers.Therefore,it is beneficial to avoiding local lubrication failure and partial wear by proper modifications of the generatrix of rollers.Finally,a test rig was developed to simultaneously measure the film thickness and friction coefficient under various slide-roll ratios.Using this test rig,the fiction properties of standard reference oil was analyzed.Moreover,the friction behaviors of three PAO oils with different chain structures and four oils with the same grade but different structures were compared.The results show that synthetic oils contribute to the lower friction coefficient.The results also point out that the friction behavior of oils strongly depends on the molecular structures.