Research On Dynamic Characteristics Of Compliant Foil Gas Thrust Bearing-rotor System

Foil gas bearings are self-acting hydrodynamic compliant bearings which aresupported by elastic surface. They have several advantages, such as small mass,higher reliability, no scheduled mainenance, favourable flexibility of environmentand temperature, lower power loss, etc. Foil gas journal bearings and foil gas thrustbearings are its main structures. Since the1960s and1970s, with the technologicaldevelopment of aerospace, nuclear and cryogenic engineering, compliant foil gasjournal bearings have been applied to high speed rotating machineries, for instance,oil-free turbine, cryogenic expansion turbine and aeroplane’s air cycle machines, etc.compared with journal bearings, the development of compliant foil gas thrustbearings is slow, and there aren’t depth researches of the mechanics of thehydrodynamic film. Based on bump type compliant foil gas thrust bearings, thispaper analyzes the static and dynamic characteristics of them. Also the dynamiccharacteristics of thrust bearing-rotor system are studied.Firstly, the Reynolds pressure govening equation under cylindrical coordinateis established to analysze the gas thrust bearings. Given appropriate deformationmodel of the foil, the Newton-Raphson iterative Method and Finite DeferenceMethod are applied to solve the coupled Reynolds and lubricating film thicknessequation. Then, the static characteristics of compliant foil gas thrust bearings arestudied under non-tilting and tilting conditions of thrust disk. In contrast to thesolutions of rigid gas thrust bearings, it depicts that foil gas thrust bearings canobtain a comparative axial load capacity to rigid gas thrust bearings, also it needs alower driven force because of the small viscous frictional torque. The influences ofstructure, clearance and rotating speed parameters are studied in order to give areference for the static performance optimizing of compliant foil gas thrust bearing.Based on small perturbation method, the relationship between axialdisplacement, velocity and differential pressure is modeled. Using Finite DeferenceMethod, the dynamic coefficients are calculated. The pressure field under titlingcondition of thrust disk is taken into consideration, then the bending stiffnesscoefficients are gained. After analyzing the dynamic characteristics of rigid andcompliant foil gas thrust bearings, it indicates that the dynamic coefficients of foilgas thrust bearings are smaller than the results of rigid gas thrust bearings. Foroptimizing the dynamic performance of foil gas thrust bearing, finally parametricresearch is done.Based on the results of former dynamic characteristics, the suitable dynamiccoefficients are chosen for dynamic analysis of bearing-rotor system. The finite element model of thrust bearing-rotor system is established, then the critical speedsof axial and bending vibrations and unbalance responses of the rotor system arecalculated. The results declares that the the critical speeds of axial vibrations arelarge, but the first critical speed is smaller than limiting speed in general, theworking speed should be diffienent from it in a specific range. On the other hand,compliant foil gas thrust bearings could strengthen the stiffnesses of journalbearings.