Study On Dynamic Properties Of Gas Foil Journal Bearings

Gas foil bearings are constructed with corrugated bump foil as an elastic foundation and a smooth top foil. The air is used as a lubricant of the bearings. And the rotor is supported by the film pressure which generated in the clearance between the top foil and the rotor once the rotor achieves a certain speed. Therefore, the bearing could achieve a non-contact suspension. A large structural damping is generated due to the relative motion between the bumps and the top foil, and between the bumps and the housing. The damping could inhibit the development of the self-excited and low-frequency whirl of the bearing-rotor system, in order to enhance the stability of the system.A spring model of multi-degree of freedom has been used to analyze the elastic deformation of the foil, and compare the results with the shell model. Then, the film thickness equation and the Reynolds equation have been coupled to get the steady state performance of gas foil bearings. The dynamic characteristics of a gas foil bearing has been represented by a set of spring and damping coefficients which are functions of the eccentricity ratio, the rotating speed and the whirl frequency of the journal. In addition, these coefficients are the essential parameters, which could be used to determine the critical rotational speed or an unbalance response and investigate the stability. Two perturbation methods have been used to develop a mathematic model which could be used for the dynamic coefficients calculation of gas foil bearings. An elastic shell model and a spring model have been used to get the deformation of the bump foil and the top foil. The finite difference method and the finite element method have been used to get the dynamic characteristics of the gas foil bearings. The results have been compared to the published experimental data so that the model has been proved to be correct and accurate. The effects of the eccentricity ratio, the shaft speed and the whirl frequency on the dynamic coefficients have been also discussed.A bearing performance test-rig and a foil bearing have been designed. In addition, the critical rotational speed has been calculated. Therefore, the foundation work which has been done could be used as a reference for the design of gas foil bearings.