Theoretical And Experimental Study On Flexure Hydrodynamic Gas Bearing-rotor System

Gas bearing is becoming the best choice for the high-speed and high power density rotating machinery because of its oil-free,low power loss and extremely high DN value.Hydrodynamic gas bearing use the ambient gas as lubrication medium,which simplified the bearing structure and improved the reliability of the rotating machinery.However,high requirement of manufacturing precision,poor stability and low load capacity prevented the development of the hydrodynamic gas bearing.Flexure hydrodynamic gas bearing has a layer of compliant support structure,thus reducing the requirement of bearing manufacturing precision.This dissertation studied two kinds of flexure hydrodynamic gas bearing,which are damped flexure pivot tilting pad gas bearing and gas foil bearing,from the standpoint of optimizing the bearing structure and improving the bearing stability.Damped flexure pivot tilting pad gas bearing was proposed by introducing the metal mesh into the flexure pivot tilting pad gas bearing to improve the bearing stability because the metal mesh can dissipate large amount of vibration energy.The load capacity of the tilting pad gas bearing is limited because of the discontinuous pad.However,bump-type gas foil bearing can acquire extremely high load capacity by designing the compliant structure appropriately because of its continuous gas film.This dissertation presented a comprehensive theoretical and experimental study on this two kinds of flexure hydrodynamic gas bearing.Moreover,the rotordynamic performance of the gas bearing-rotor system is becoming more and more important as the development of gas bearing equipments.This dissertation studied the rotordynamic performance of the flexure hydrodynamic gas bearing-rotor system theoretically and experimentally.The main content of this paper is shown as bellows:This dissertation studied a damped flexure pivot tilting pad gas bearing by introducing the metal mesh into the flexure pivot tilting pad gas bearing.The theoretical model of the damped flexure pivot tilting pad gas bearing was been built by coupling the theoretical model of the flexure pivot tilting pad gas bearing and metal mesh ring.The static performance and dynamic coefficients of the new bearing were predicted based on the theoretical model.The theoretical model was demonstrated by comparing the predicted results with the published results.This dissertation studied the effect of metal mesh density,pad radial stiffness,metal mesh ring interference,bearing clearance and pad preload on the dynamic coefficients of the bearing.The predicted results demonstrated that metal mesh can improve the damping of the bearing effectively.The bearing theoretical model,and the bearing static performance and dynamic coefficients prediction method are significant in designing the bearing.The orbit simulation method,which couples the rotor motion equation,compressible transient gas Reynolds equation,and pad-metal mesh motion equation,was adopted to build the nonlinear prediction model of the damped flexure pivot tilting pad gas bearing.The nonlinear prediction model was demonstrated by the theoretical and experimental results in the published paper.The nonlinear simulation results confirmed that the metal mesh ring can improve the bearing stability.The effect of metal mesh density,destabilizing force,pad radial stiffness,pad preload and bearing clearance on the nonlinear performance of the bearing was studied.The nonlinear study on the damped flexure pivot tilting pad gas bearing is useful in extending the application area of the new bearing design.The bearing bifurcation performance was studied based on the nonlinear prediction model.The bifurcation characteristics of different bearing parameters and operating conditions was studied by using the rotor trajectories,FFT plots,Poincaré maps and bifurcation plots.The bifurcation characteristics of bearing operating speed,rotor mass,unbalance radius,pad radial stiffness and pad preload were studied.The numerical results revealed that rich and complex nonlinear behaviors were contained in the rotor responses.The rotor experiences T–periodic motion,KT-periodic motion or quasi-periodic motion with the variations of different system parameters.Large amount of useful simulation results were accumulated in this study.The theoretical model of bump-type gas foil bearing was built by coupling the compressible gas Reynolds equation,2D plate top foil model and bump stiffness model.The static performace and dynamic coefficients of the bearing were predicted based on the theoretical model.An oil-free turbocharger test rig supported by gas foil bearings was designed and the rotordynamic model of the bearing-rotor system was built based on the theoretical model of the gas foil bearing.The final test rig design was acquired after iteratived rotordynamic analysis and rotor-bearing structure adjustment.An oil-free turbocharger test rig was built to demonstrate the design.The successful speed-up and coast-down tests confirmed the feasibility of the design of the oil-free turbocharger test rig.The effect of bearing welding spot on the rotordynamic performance of the gas foil bearing-rotor system was studied through experiments.The experiment results showed the oil-free turbocharger test rig exhibits the best rotordynamic performance when the bearing welding spot located at 90°.A 5 degree of freedom(5-DOF)model of gas foil bearing-rotor system which considering the effect of thrust gas foil bearing was built.The theoretical model couples the radial gas foil bearing model,thrust gas foil bearing model and rigid rotor model.The modal stiffness and modal damping for the symmetric and asymmetric bearing-rotor system were predicted in frequency domain.The effect of axial force on the modal stiffness and modal damping was studied.The nonlinear responses of the symmetric and asymmetric gas foil bearing-rotor system based on the 5-DOF model was simulated.The effect of axial force on the nonlinear stability of the gas foil bearing-rotor system was studied.The simulation results showed that the thrust gas foil bearing and axial force exhibit significant influence on the rotordynamic performance of the bearing-rotor system.In summary,this dissertation studied two kinds of flexure hydrodynamic gas bearing,which are damped flexure pivot tilting pad gas bearing and bump-type gas foil bearing.The static,dynamic and nonlinear prediction models of the damped flexure pivot tilting pad gas bearing were built.The effective of improving the bearing damping by introducing the metal mesh into the bearing was demonstrated.This dissertation provided an analysis tool and accumulated large amount of useful data for designing the damped flexure pivot tilting pad gas bearing.A theoretical model of gas foil bearing was built,which is helpful in analyzing the rotordynamic performance of the gas foil bearing-rotor system.An oil-free turbocharger test rig which was supported by gas foil bearings was built to study the rotordynamic performance of the gas foil bearing-rotor system.The design was demonstrated by successfully operating this test rig.A 5-DOF theoretical model was built to analysis the rotordynamic performance of the gas foil bearing-rotor system when considering the effects of thrust gas foil bearing and axial force.The 5-DOF is a completely new analysis tool in the rotordynamic analysis of the gas foil bearing-rotor system.