Research On The Start-stop Wear And Thermal Slip Behavior Of The Heavy-load And High-speed Coated Hydrodynamic Journal Bearings

Affected by special operating conditions,the hydrodynamic bearings used in high-power heavy-duty gas turbines own compact size and micro relative clearance,and often start up and shut down frequently under heavy applied load but without hydrostatic supporting system,which are easily cause bearing seizures and worn failures.Nowadays,self-lubrication coatings are becoming an alternative way to improve the friction performance of the bearing.The slip properties at lubricantcoating interface are quite different from those of traditional bearing friction pairs.However,the lubrication and friction performance of the coated bearings during the start and stop processes has not been systematic researched yet.In this paper,the contact behavior of the bearing during the start-stop process under applied load is analyzed,and the effect of the slip effect at lubricant-coating interface on the hydrodynamic and thermal behavior is investigated.The thermal properties of the coated tilting pad journal bearings during start-stop process are conducted to explore the safe start-stop operating conditions.Based on Greenwood-Williamson(GW)contact model,a mixed lubrication model considering both contact of surface roughness and hydrodynamic lubrication during startup and shutdown of plain journal bearings is established.The transient model is solved by finite element and Newmark methods,and compared with the experimental data.The transient contact behavior of the surfaces under the mixed lubrication regime is analyzed,and the contact time,lift-off speed,transient hydrodynamic pressure,contact pressure and minimum film thickness are given.The influences of surface roughness,acceleration,and relative clearance on the startup and shutdown process are discussed.Based on the two-component slip model that considers both critical shear stress and slip length,a thermoelastohydrodynamic(TEHD)lubrication model considering wall slip is established.The fluid control equation,energy equation,heat conduction equation and deformation equations are iteratively calculated by upwind finite element method.The heat conduction characteristics among the film,coating,interlayer and basement is presented.The simulation results are compared to the experimental and ANSYS results to verify the effectiveness of the model.Then the influence of operating conditions on the thermal slip behavior is analyzed.The influence of position and shape of the slip zone on the hydrodynamic force and friction coefficient are systematically analyzed.Firstly,the optimal slip zone of the partial wall slip zone for high eccentricity ratios(heavy load)in isothermal case is studied.With considering operating conditions,the selection criteria of slip zone parameters under constant load is given.Then the influence of the coating(slip)position on the thermal behavior,minimum film thickness and maximum hydrodynamic pressure under constant load conditions is analyzed.The selection criteria of the coating position are given.Finally,the influences of coating thickness and material parameters on the thermal behavior of the coated bearings are discussed.Aimed at six coated tilting pad journal bearing(CTPJB)used on marine gas turbine,the experimental and theoretical research on both the steady-state and startstop process are conducted.A high-speed and heavy-duty journal bearing test rig suitable for different journal sizes is built,and a TEHD model for CTPJB is established.The wear characteristics and thermal characteristics of the PTFE,graphite and silver(Ag)coatings during the startup and shutdown for micro-clearance bearings are tested.The influences of bearing clearance,coating material,operating conditions on the THED behavior of Ag-coated bearings are studied by simulation and tests.Then the influence of relative clearance,acceleration and initial temperature on the temperature rise during startup and shutdown is explored.The appropriate operating conditions for safe startup and shutdown of the bearing are proposed.