| As the key supporting component of an aeroengine,the lubrication performance of mainshaft bearing directly affects the working performance of the engine.With the increasingly thrust-weight ratio requirements of aeroengines,the demands for the service conditions of bearing become more and more rigorous.The bearings directly bear the extreme working conditions,such as high-speed,high-temperature,heavy-load,and large fluctuation conditions,and in such severe working conditions,the bearings inevitably operate in a mixed lubrication state,and even more severe boundary lubrication and dry friction state.The poor lubrication may easily induce surface failures such as skidding damage,friction,wear,and scuffing,which greatly reduce the reliability and service life of bearings.Therefore,it is meaningful to study the thermal mixed lubrication behavior of aeroengine mainshaft bearing in these extreme conditions.In the present work,the aeroengine mainshaft angular contact ball bearing is taken as the research object.The mixed elastohydrodynamic lubrication(TEHL)behaviors of bearing under typical working conditions are studied through modeling and simulation.Firstly,a mixed TEHL model for point contact problems that can simulate various lubrication states is established based on the semi-system method and column sweeping method.Then,considering the effects of time-varying contact characteristics and solid lubricating coating of bearing,the non-steady lubrication and solid-liquid composite behaviors are analyzed with bearing dynamic characteristics.Finally,the influences of lubrication and friction on the dynamic characteristics of bearing are considered to investigate the coupling behaviors of bearing quasi-dynamics and mixed TEHL,and the reliable lubrication technology for mainshaft bearing is developed and established.A mixed TEHL model in point contact with consideration of contact geometry,dynamic change of roughness,thermal effect,lubricant rheological properties,and transient effect is established by combing the united transient Reynolds equation and transient three-dimensional energy equation for fluid and heat conduction equation for solid.The efficient and stable numerical solution is systematically studied.The model can provide a stable numerical solution to simulate the entire transition from the boundary,mixed,and full film lubrication under severe conditions.Based on the proposed model,the influences of slid-roll ratio,load,velocity,and surface roughness on the oil film,pressure,and temperature are systematically investigated.Furthermore,the calculation method for the prediction of friction based on the proposed model is put forward by introducing the non-Newtonian rheological model,and the frictional characteristics in the mixed lubrication state are obtained.According to the non-steady state conditions of variable speed and load for mainshaft bearings,a time-varying mixed TEHL model considering the factors of time-varying velocity,load,and contact geometry is built based on the above mixed TEHL model.The validity of the present model is demonstrated by comparing the previous research results in variable speed and load.On this basis,combined with the actual working conditions of bearing,the mixed TEHL behaviors of bearing under the non-steady conditions are studied based on the bearing dynamic analysis results.The effects of load,speed,structural parameters under the steady conditions,and acceleration under the start-up and shut-down conditions on the transient mixed TEHL behaviors are obtained.The mixed TEHL model of multi-layered coating-substrate system is established by considering the influence of the surface solid lubricating coating on the lubrication of bearings.Based on the dynamic characteristics analysis of bearings,the lubrication and tribological behavior of the coating-substrate system of bearing under different working conditions are studied.Effects of surface roughness,coating material properties,boundary lubrication friction coefficient,and other factors on the bearing lubrication performance and subsurface stress are comprehensively analyzed to clarify the tribological behaviors and failure mechanisms.It provides theoretical guidance for the tribological design of solid lubricating coatings and their engineering applications in bearings.Experimental research on the tribological performance of point contact is carried out on the ball-on-disk sliding-rolling tribological test rig.The friction coefficients under different slide-roll ratios,loads,entrainment velocities,and inlet lubricant temperatures are measured.Based on the experimental data,the existing limiting shear stress model for lubricating oil is modified.The validity of the theoretical model lubrication is demonstrated by comparing the experimental and theoretical results under different working conditions.For the limitation of elastohydrodynamic lubrication and friction calculations in traditional bearing dynamic analysis,a more accurate tribo-dynamic model is established with the bearing quasi-dynamic analysis and mixed TEHL analysis by coupling the minimum film thickness and friction coefficient.The dynamic characteristics and lubrication characteristics of the bearing in the coupling state are revealed. |
