Research On Lubrication Of Main Bearing Of Marine Low Speed Engine Considering Dynamic Thermal/Mechanical Properties Of Bearing Material

The main bearing is the core part of the marine low speed diesel engine,and the performance of the crankshaft-main bearing friction pair directly affects the reliability of the low speed engine.There is still a certain difference between the performance of the domestic main bearing and the performance of the imported main bearing.It is difficult to explain the mechanisation of the difference only from the material characteristics or the dynamics of the moving parts.The basic problem has become a difficulty that the industry needs to solve urgently.This dissertation aims to reveal the lubrication law of the main bearing more comprehensively through considering the bearing material properties,friction pair lubrication and crankshaft dynamic characteristics.Taking the main bearing of low speed diesel engine as the main research object,a thermal-mechanical coupling micro contact model of crankshaftmain bearing friction pair which considering the dynamic thermal/mechanical characteristics of the bearing material is established,then a transient thermal elastohydrodynamic lubrication model of the bearing which considering the dynamic thermal/mechanical properties of the material is established.The transient thermal elastohydrodynamic lubrication model explores the influence of the dynamic characteristics of the crankshaft on the lubrication characteristics of the bearing.The model provides technical support for the accurate and real-time evaluation of the lubrication performance of the main bearing in the marine low speed engine and the improvement of the lubrication design level of the main bearing.The main work of this dissertation is as follows:By conducting the thermal/mechanical performance analysis experiments of materials,this study investigated the changes in material properties and mechanical properties of the marine low speed engine main bearings over a wide strain rate range(low and high strain rates)and operating temperature range.Based on the static tensile test,split Hopkinson pressure bar test,and thermal properties test data of marine low speed engine main bearing alloy material,this dissertation considered the influence of temperature and strain rate on thermal/mechanical properties,and constructed an accurate mechanical constitutive model of bearing material at wide strain rates to cover the thermal properties at different operating temperatures.The thermal/mechanical behaviors of bearing materials in the sliding contact process under different working conditions were simulated.The results indicated that the thermal/mechanical properties of the main bearing materials for low speed marine diesel engines are significantly affected by temperature and strain rate.Considering the precise thermal/mechanical properties of the marine low speed engine main bearing material,a micro contact mechanics model of the crankshaft bearing material including the thermal mechanical coupled effect was constructed to explore the influence of the working condition parameters and material property parameters on the contact mechanics behavior.The changes of the contact characteristic parameters(normal contact force,sliding friction coefficient and surface temperature rise,etc.)with the overlapping depth,relative sliding speed,and material properties were obtained.Also,an empirical formula for the contact characteristics of the rough surface of the crankshaft bearing shell was established based on statistical methods.Considering the transient dynamic characteristics of the crankshaft,including the thermal mechanical coupled micro contact characteristics,deformation,thermal effects and other thermal fluid solid multi field coupled effects of bearing materials,a transient mixed thermal elastohydrodynamic lubrication analysis method considering thermal mechanical coupling effect for main bearing was constructed considering the influence of temperature change effects of material properties such as elastic modulus,coefficient of thermal expansion,specific heat capacity,thermal conductivity on bearing deformation calculation and transient energy analysis.Compared with the traditional main bearing lubrication analysis method,the influence of thermal mechanical coupling effect on the lubrication characteristics of main bearing was explored.The research results indicate that as the load increases,the proportion of dynamic thermal/mechanical properties of bearing material in the thermal effect of bearing lubrication gradually increases,from 0.77% under light load to 4.62% under heavy load.For the marine low speed diesel engine main bearing,the lubrication characteristics of main bearing under different working conditions were explored based on the above transient thermoelastic hydrodynamic lubrication analysis method considering thermal mechanical coupling effect.Under rated operating conditions,the effects of radial clearance,oil viscosity,and rotational speed on bearing lubrication characteristics(oil film thickness,oil film pressure,and bearing temperature)were investigated.In response to the harsh starting conditions of marine low speed engines,the effects of factors such as radial clearance,oil inlet temperature,and starting time on the lubrication performance of the main bearing were studied.The research results indicated that shortening the starting time can reduce the temperature rise and micro convex contact force during the starting process to a certain extent,while increase the rate of temperature rise and have adverse effects on the thermal stability of the bearing.For the crankshaft main bearing friction pair of marine low speed diesel engine,the threedimensional(longitudinal,bending and torsional)vibration transfer matrix of the crankshaft was derived based on the continuous beam theory,and the dynamic model of the crankshaft main bearing system was constructed.The three-dimensional forced vibration response of the crankshaft during the operation of the real engine was obtained,and then the vibration environment boundary of the main bearing lubrication analysis was obtained.In that case,the influence of crankshaft vibration on the lubrication characteristic parameters(oil film thickness,oil film pressure and bearing temperature)of main bearing was investigated.The results showed that the vibration response of the crankshaft of the marine low speed engine had a significant impact on the lubrication performance of the main bearing,and the lubrication performance of the fourth bearing are the most severe in the marine low speed disesel engine studied in this dissertation.