On The Theory Of Thermo Rheological Lubrication And Its Application In EHL

Thermal effect,rheological effect,roughness effect and transient effect are all important aspects of advanced elastohydrodynamic lubrication(EHL)theory.The aim of this thesis is to clarify the individual or combined influences of these effects upon the tribo-characteristics of lubricating films under the operating conditions we are interested,based on the thermo-rheological lubrication theory.The study was carried out from simpler problems to complicated problems.First of all,the lubricant was assumed to be a Newtonian fluid.The effect of temperature distributions under four types of surface temperature boundary conditions on the load carrying capacity of nominal parallel gaps was investigated.That is,the load carrying mechanism of thermal wedge was explored.It is pointed out that the contribution of thermal effect to the load carrying capacity of the film comes from two aspects ? the thermal density wedge and the thermal viscosity wedge,the role of them,however,are difficult to be distinguished strictly.Usually the effect of density wedge is quite weak although it is always positive.The effect of viscosity wedge,however,plays a dominant role and sometimes can be as strong as the effect of a geometry wedge.By further analysis it is revealed that the thermal viscosity wedge comes not only from the surface temperature difference(STD)as indicated by Cameron for more than fifty years,but also from the film temperature gradient(FTG),which is independent of STD.In addition,through various numerical solutions of hydrodynamic lubrication for some typical plain bearings,the contributions of thermal wedge and geometry wedge on the load carrying capacity were discussed.Secondly,aimed at most non-Newtonian flow models,the physical meaning of the generalized viscosity was clarified,and an improved generalized Reynolds equation was derived.By introducing a new expression of the constitutive equation for the Eyring shear-thinning model,the generalized viscosity of the lubricant can be determined directly from the Newtonian viscosity and shear strain rate in the numerical procedure.The efficiency of rheological analysis,therefore,has been increased remarkably.Through the analysis of a number of numerical examples and the study of the scale effect,it is shown that the generalized viscosity as well as the friction coefficient is determined by both effects of thermal-thinning and shear-thinning.The effect of shear-thinning takes place at small slide-roll ratio under heavily loaded conditions,whereas the effect of thermal-thinning becomes stronger at large slide-roll ratio.A new formula for calculating the viscosity of oil from its density was proposed based on a hypothesis that “the same density,the same viscosity for the same lubricant”.The viscosity predicted by the new formula agrees well with experimental data when squalane was taken as an example of lubricant.Furthermore,with the Dowson-Higginson's density-pressure relation as a bridge,the new formula can unify the Barus' piezoviscous formula,the Doolittle's free-volume viscosity formula,and the Roelands' piezoviscous formula in the form,so that a deeper insight on the relationship between density and viscosity has been achieved.The new viscosity formula has been successfully employed in the thermal Eyring EHL analysis for point contacts.The numerical results of friction coefficient agree well with experimental data,implying that the correctness of the proposed viscosity-density formula is acceptable and the hypothesis for viscosity and density is hold reasonably.In the last two chapters of this thesis,the work for steady-state lubrication problems done in the previous chapters were extended to the transient thermal Eyring EHL problems with high speed rough surfaces under slide-roll conditions.Simple surface topography,i.e.,a single transverse ridge or groove was studied firstly.The influences of thermal conductivity of contact bodies,the magnitude and sign of the slide-roll ratio,and the relative position of the roughness,etc.,on the perturbed film were investigated for point contacts.Then,it is assumed that on both contact surfaces there exist a series of transverse ridges.The interaction of ridges was discussed deeply.Meanwhile,the correlations of the ridge wavelength and the contact size with friction coefficient were explored,the knowledge in the micro-EHL regime,therefore,was enriched.This is helpful not only for academic purpose,but also for engineering design.