Shear Propertiesof Grease Films Under Different Sliding Rollin Gratios

The objective of this thesis is to observe the shear properties of grease films under different sliding rolling ratio(SRR)conditions.Grease lubrication is primarily used in rolling bearings.However,the grease will give different rheological responses to shear rate and shear duration due to its viscoelasticity and thixotropy,which correspondingly changes the grease films and friction coefficients.Thus the lubrication mechanisms are complex.In rolling bearings,the grease subjects different shears in micro and macro scales,depending on the locations and running conditions.The oil bleeding and replenishment caused by shear will affect the lubrication states.Therefore,the evolutions of grease films under different SRRs are experimentally observed.The purpose of this work is to explore the lubrication mechanisms determined by constitution of grease and running parameter.The main contents include: I.A ball-and-disc point contact test rig is established to simultaneously measure film thickness and friction coefficient,which includes the improvement of the friction measurement unit,the calibration of friction and the dynamic measurement technique of film thickness and friction.II.Shear properties of FVA3 reference oil under different SRRs.The 3D friction coefficient maps with speed and SRR is obtained at different loads.The lubrication states are then clearly recognized and the effects of shear thinning,limiting shear stress and thermal on friction coefficients are verified.The acceptable agreements between experiments and numerical predictions are obtained,indicating the test rig is available to analysis the shear properties the EHL films.III.Shear properties of base oil films under different SRRs.The evolution of film thickness and friction coefficient with glass revolutions are observed.The influences of base viscosities on film thickness and friction coefficient are verified.Under minimal oil supplying conditions,film thickness decay and increase in friction coefficient induced by starvation are observed.IV.Influence of base oil viscosity on shear properties of grease films.It is found that there exists a proper SRR which makes grease film reach an equilibrium shear state,under which the grease film is high than that of base oil.Hence a lower friction coefficient is obtained by grease film.The centrifugal force improves the lubricant replenishment whilst it will induce the film distortion.V.Shear properties of grease film under different consistencies.It is found that the consistency significantly affects the grease film thickness and friction coefficient.For the grease with high consistency,the grease replenishment is restricted due to the high yield shear stress and high apparent viscosity.Hence,the grease film thickness reduces and friction coefficient gradually increases.By contrast,the good fluidity of grease with low consistency contributes to the film formation and consequently a lower and constant the friction coefficient is achieved.The thesis provides a simultaneousmeasurement approach of film thickness and friction.The rheological properties of EHL films,the lubrication state,the film formation mechanisms and its shear properties can be analyzed base on the measurement results.The work is beneficial to understand of the tribological properties of EHL films.