| Environmentally acceptable lubricants(EALs)have the advantages of nonpolluting and naturally degradable,which can improve oil pollution emission caused by the use of mineral oil,and more meet the requirements of green navigation of ships.However,under low temperature and high-pressure conditions,viscosity of EAL is lower than ordinary mineral oil and hard to form a thick enough oil film,which can lead to high-temperature failure when it functions as lubricating medium of a stern shaft.To improve lubrication performance,adding viscosity modifiers to EALs has become an effective solution to oil problems.Lubricants containing viscosity modifier can form a certain thickness of oil film under boundary lubrication conditions.The oil film is mainly composed of high molecular compounds in viscosity modifier,showing nonNewtonian hydrodynamic properties dominated by stress couple.In view of this,EALs containing polymer compound additives are regarded as couple stress fluids,and the research is carried out for the lubrication of stern shaft to explore the lubrication performance and mechanism of stern dynamic pressure under the coupling effect of multiple parameters.The main research contents and results of this paper are as follows.(1)Based on the theory of coupled stress fluid lubrication,the stern lubrication model considering the couple stress effect interval and variable couple stress parameters is established,and the discretization and solution methods of the model equations are given.The influence of different couple stress fluid thickness coefficients on lubrication parameters such as oil film pressure distribution,attitude angle,load carrying capacity and friction coefficient is discussed,and the relationship between oil film thickness and couple stress effect area is summarized.The influence of variable couple stress parameter on the lubrication performance of misaligned bearing with and without oil film viscosity pressure characteristics is compared and analyzed.The results show that,compared with Newtonian fluid,the maximum oil film pressure increases and the oil film bearing area decreases under static load bearing lubrication;with the increase of couple stress parameters,the high pressure area is more concentrated;when the film thickness coefficient of couple stress fluid is 4,the bearing lubrication performance is close to the steady state.(2)Considering the influence of surface roughness and couple stress effect of misaligned journal,the mixed hydrodynamic lubrication model of couple stress fluid is derived based on the theory of couple stress fluid and the theory of mean flow.Stribeck curve is served to reveal the lubrication state transition process of the journal.The G-T micro-convex contact model is used to calculate the micro-convex combined force,and the equilibrium relationship among external load,micro-convex support force and oil film support force is discussed according to different lubrication states.On this basis,the study of couple stress effect and surface roughness on lubrication performance and lubrication state of misaligned journal is carried out.The analysis results show that when the couple stress parameter is small,the external load is shared by the oil film force and the micro-convex body support force under low speed conditions,and the critical speed of dynamic pressure lubrication and mixed lubrication of the bearing is reduced compared with that of Newtonian fluid due to the couple stress effect.(3)The hydrodynamic hybrid lubrication model of the stern bearing is established considering the fluid thermal effect of couple stress fluid,which involves the coupling solution of various model equations(generalized Reynolds equation,energy equation,solid heat conduction equation,load balance equation),viscosity-temperature equation and variable couple stress parameters in the pressure field and temperature field.The model equations are uniformly discretized by the finite difference method,and the calculation of the oil film combined force is completed by the Simpson integral method.Based on the model,the influence of thermal effect on lubrication performance under different couple stress parameters is discussed,and the misalignment angle,surface roughness and couple stress are coupled to carry out the study of lubrication performance of thermal fluid multi-parameter mixture under different misalignment angles.The results show that couple stress effect reduces the maximum oil film pressure and the maximum temperature inside the oil film;with the increase of the couple stress parameter,the effect of the misalignmengt angle on the maximum temperature of the oil film and couple stress parameter gradually decreases.(4)Taking into account the axial motion of the journal,the dynamic pressure lubrication model of the misaligned j ournal considering the axial motion is established,and the influence of the coupling influence of the axial motion and the couple stress effect on the lubrication performance of the bearing is analyzed in depth.It is found that,contrary to the couple stress effect,the minimum oil film thickness decreases with the increase of axial speed,while the maximum oil film pressure and friction coefficient increase.Compared with the axial velocity,the couple stress effect has a greater effect on this friction coefficient.The couple stress effect effectively reduces the local oil film pressure increase caused by misalignment angle,and curbs the trend of maximum oil film pressure position moving toward the end face and reduces the edge effect.(5)Based on the experimental phenomenon that couple stress fluid can reduce the friction coefficient and increase the oil film carrying capacity,four quinazolines with different molecular structures were designed and synthesized,and the compounds were blended with two base oils as additives to form new lubricants.The four-ball wear test was conducted to compare the friction reduction performance of the lubricants with different molecular structure additives,to verify the couple stress effect and to select the lubricants with the best performance.The above-mentioned lubricants with better performance were applied to sliding bearings,and the friction coefficients of bearing oil films under different loads and journal linear speeds were experimentally tested,and the results were compared and analyzed with the values calculated by the theoretical model.The results show that the lubricant containing compound additive with long chain-like molecular structure has the best friction reduction performance under film conditions compared with base oil and other molecular structure additive lubricants,and it obviously enhances the oil film load carrying capacity and shows couple stress fluid fluid characteristics.In the experiment of sliding bearing lubrication,under the heavy load and low speed conditions,the experimentally measured friction coefficient is consistent with the calculated results of the dynamic pressure model based on the couple stress effect,which proves the accuracy of the theoretical model. |
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