Study On Film-forming Mechanisms And Optimal Profile Of Roller Friction Pairs Considering Surface Roughness And Boundary Film Properties

In this dissertation,the tribological performances of roller friction pairs(RFP)under different lubricants are investigated from the viewpoint of elastohydrodynamic lubrication which can improve the performance of RFP.The main work is to establish the mixed lubrication model of RFP.This model incorporates the influnces of the surface roughness and boundary film properties.The scuffing problem,the fatigue life problem and the profile design problem of RFP are deeply inveatigated using this model.In addition,film-forming characteristics of grease-lubricated RFP under low-middle-speed working conditions are also studied.First of all,most of RFP are under mixed lubrication in harsh conditions,such as high-speed and heavy-load working conditions.In order to accurately predict the contact and lubrication performances of RFP under such conditions,a new model which incorporates asperity elasto-plastic deformation and the boundary film properties is established.This model can effectively give the limited operating conditions under specific conditions.Using this model,three parts of work is completed.First,the mixed lubrication model of RFP is verified with theoretical and experimental results,and the paramertrical analysis is conducted.The results show that as the surface roughness,hardness,sliding-rolling ratio and the friction coefficient of boundary film increase,the scuffing resistance decreases.Second,the coupled model which includes mixed lubrication model of RCP,quasi-static model of roller bearings and fatigue life model of roller bearings is established to study the effect of lubrication on fatigue life.The results show that as the misalignment angle increases,the fatigue life of roller bearings sharply decreases,and the crown design of roller can effectively improve the fatige life of roller bearings.Third,the method of the optimal profile design taking account of lubricating factor is presented.The central film thickness formula and the optimal profile coefficient formula of RCP are simulated and can be directly applied to design the roller profile.Then,the self-developed oscillated and rotated optical elastohydrodynamic lubrication(EHL)test rigs are applied to study the film-forming capacities of grease under different working conditions.The results show that the type of thickener and the content of thickener have significant influence on film-forming capacities under low-speed and fully-flooded grease conditions.Different from point contact,due to the distinction of contact pressure between the middle and the end of the roller,lots of thickener fibers exist at the middle of the roller;the logarithmic profile is helpful to reduce the thickener fibers since the axial pressure gradient is increased;under the middle-speed or high-speed conditions,the degree of starvation increases with increasing base oil viscosity,rolling speed and the crown drop of the roller.Reducing the replenishment time by reducing the gap between the rollers also can increase the degree of starvation.In addition,when the grease is pushed aside by the passage of rollers,if there is not enough time for bleeding oil to fully replenish the track under high-speed conditions,the film thickness at the end of the contact area is lower than that at the end of the contact area which cause the middle area partly starves,while the end region is under fully-flooded lubrication conditions.The new theoretical model and simplied roller profile design method are presented by the results from this dissertation.These results can be used to guide the dimensional design of RFP and the chose of lubricant which further improve the life and reliabity of RFP.