| Because of its many outstanding advantages compared with rolling bearingssuch as high speed, pollution free, high precision turning, long operating life, limitenvironment adaptability like high and low temperature, the hydrodynamic gas foilbearings have vast potential for future development and important valuableapplication in those fields including avigation, aerospce engineering, cryogenics andrefrigeration. The elastic surface self-adaptivity and foil structural dry fricition notonly make the bearing run in high speed and have strong rotating ability, but alsomake it difficult to establish theoretical model of bearing foil structure. In this paper,the internal friction effects in bearing foil strucure are investigated. Firstly, the foilbearing theoretical model is established and numerical algorithm analytical solutionis presented. Based on it the effects of internal friction in foil structure on static anddynamic characteristics of foil bearing are analyzed. Furthermore, the experimentalstudy of static and dynamic charactristics of foil bearing and dynamic performanceof bearing-rotor system provides a new method for research of frictioncharacteristics of foil structure in gas foil bearing.The theoretical calculation model of bump foil stiffness including foilstructural internal friction effects is developed by finite element method and theeffects of Coulomb friction on stiffness of bump foil are studied. The results indicatethat Coulomb friction force can enhance the stiffness of bump foil and the stiffnesscharacteristic of bump foil Coulomb friction model is more complex than linearspring stiffness model that in literatures.On the basis of establishing the foil structural finite element model with theconsideration of shear stiffness in top foil and foil structural friction effects, thecompressible gas lubricated Reynolds equation and film thickness equation aresolved together by using finite element mothod and finite difference method, thenthe effects of shear sthiffness in top foil and Coulomb friction in foil structure onbump foil static performance are studied. The accuracy of foil structural model andnumerical algorithm analytical solution are verified via comparing numerical resultswith test data from NASA.Differential equations for calculation of bump foil bearing dynamiccharacteristics are derived from air Reynolds equation, air pressure, gas filmthickness and bump foil deformation by using perturbation method. The effects ofCoulomb friction in foil structure on bump foil stiffness and damping coefficientsare studied via comparing of dynamic characteristics of bump foil bearing withdifferent foil structural friction coefficients. The practical bump type gas foil journal bearing-rotor system model is built.The critical speeds of rotor system are solved and the effects of foil structure frictionon critical speeds of rotor system are studied by solving rotor system frequencyequation combined with stiffness and damping coefficients of bearing at variousspeeds. The hydrodynamics of bearing-rotor system are analyzed by using Newmarkmethod. It shows that the vibration values at bearing supports decrease with theincreasing of rotor speed. Although some low frequency components occur at highrotating speed, but the shaft orbit has good repeatability and gas foil bearing-rotorsystem has good rotating stability at high speed.A bump type gas foil journal bearing performance test-bed is designed and built.A test method for studying effects of foil structure friction on performance of bumptype gas foil journal bearing is proposed by changing the roughness of bearinghousing cylindrical hole surface. The bearing performance including bearing staticstiffness, lift-off speed, limiting load capacity and dynamic coefficients are test andit indicates that reducing roughness of bearing housing cylindrical hole surface canweaken the bearing static stiffness, decrease the lift off speed, reduce the bearingsurface wear at start and stop stage and extend bearing life. Furthermore, decreasethe foil structural friction force can enhance bearing limiting load capacityeffectively and it can provide a new idea to improve the capacity pf gas foil bearing.A test-bed is developed for bump type gas foil bearing-rotor system. Thevibration frequency and foil temperature measurement can both determine thelift-off speed. The effects of friction between bump foil and bearing housing ondynamic performance of rotor system are analyzed. Reducing roughness of bearinghousing cylindrical hole surface can effectively eliminate high frequency vibrationand improve the stability of rotor system and it provides a new approach to improvethe stability of foil bearing-rotor system. |
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