Theoretical Analysis And Experimental Investigation On Rotordynamic Performance Of A Rigid Rotor Supported On Porous Tiliting Pad Bearings

Gas lubricated hydrostatic bearings are characterized by high stiffness,high precision,low friction,low heat,non-pollution,and can adapt to extreme working conditions,and have been widely used in the semiconductor industry,aerospace industry,precision measurement instruments and other fields.In recent years,turbocharging machinery,refrigeration compressors,blowers and other mechanical equipments have increasingly higher requirements for power density,which requires the rotor to operate at high speed.For the traditional rolling bearings,due to the increase of the rotational speed,the bearings wear seriously and the life of the bearings reduce rapdaly.For the liquid lubricated bearing,the friction loss of the bearing increases with the increase of rotational speed,and the liquid temperature rises,resulting in severe heat.Therefore,the traditional rolling bearings and oil lubricated bearings are difficult to meet the high speed working conditions.Gas lubricated hydrostatic bearings are a choice of high speed rotating machinery.At present,gas lubricated hydrostatic bearings are mostly used in heavy load and low rotational speed conditions,and the researches in bearings with high rotational speed are less.The hydrostatic journal bearings cause strong sub-synchronous vibration at high rotational speed,which affects the application of journal hydrostatic bearings at high rotational speed.The porous tilting pad bearings(PTPBs)which can improve the high speed stability is proposed.The research contents and achievements of this paper include the following:The test rig for measuring the permeability of porous materials is set up and the permeability of graphite materials with different densities is measured and the effect of permeability on material properties is analyzed.Based on the lubrication theory of porous materials and the motion equation of tilting pads,the theoretical model of PTPBs is established.Newton iteration method and finite difference method were used to solve the static characteristics of the bearings,and the perturbation method was used to solve the dynamic characteristics of the porous tilting pad bearing.The predicted load capacity and gas flow of porous tilting pads are compared with results in references and verifies the reliability of the theoretical model.The mechanism of hydrostatic effect and hydrodynamic effect of PTPBs is analyzed,the reasonable range of bearing clearance is defined,and the effects of bearing clearance,air supply pressure,radial stiffness and rotational stiffness of the tile on bearing eccentricity,gas flow,dynamic stiffness and damping coefficients are discussed.In order to more accurately predict the motion of the jouranl center,the nonlinear model of the PTPB is established by using the time-domain trajectory method and coupling the motion equation of the journal center and tilting pads and the gas flow equation.The influences of external pressurized gas on the stability of the PTPB is discussed,and the influence of supply pressure,bearing clearance,radial stiffness and rotational stiffness of tilting pads and air supply mode on the nonlinear stability of the bearing are analyzed.The study on the nonlinear characteristics of PTPBs is helpful to understand the influence of external pressurezied gas and bearing parameters on the working state of the bearing,and provides a theoretical basis for the design of the PTPB under different working conditions.Based on the analysis of the static,dynamic and nonlinear characteristics of the porous tilting pad bearing,a bearing-rotor test bench supported by the porous tilting pad bearing is built.The influence of air supply pressure on rotor suspension height and gas flow of porous tilting pad bearing is analyzed.The effects of air supply pressure,air supply modes and unbalanced mass on the dynamic resp onse characteristics of bearing-rotor system were studied through the deceleration experiment.A test rig of the rotor supported on the PTPBs is designed and constructed.The influences of supply pressure on rotor suspension height and gas flow of PTPBs ar e analyzed.The effects of supply pressure,air supply modes and unbalanced mass on the dynamic response characteristics of bearing-rotor system were studied through the deceleration experiment.The dynamic model of PTPB-rotor system is established.The model considers the rigid rotor model,the flow equation of the gas in the porous material and the bearing clearance,the motion equation of the tilting pads and the rotor.By means of rotor center trajectory,poincare map,fast Fourier transform and bifurca tion diagram,the causes and influencing factors of sub-synchronous vibration are discussed.The influences of rotor mass,supply pressure,the preload and pad installation mode on dynamic response are analyzed.