| Gas foil bearings are widely used in high-speed rotating machinery because of their oil-free lubrication,high speed,low power consumption,high and low temperature resistance and other characteristics.However,sub-frequency vibration often occurs in the rotor system supported by gas foil bearings,which makes the system unstable,and even damages the equipment.Aiming at this problem,this paper introduces flexible pivot tilting pad structure of the gas foil bearing,it provides a new solution to the stability problem of rotor system supported by gas foil bearings,and forms flexible pivot tilting pad foil bearings(FPTPFB)supported by flexible pivot tilting pads and bump foil in series to reduce the cross stiffness of bearings,reduce the instability caused by fluid dynamic pressure effect,and improve the stability of rotor system.Based on the theoretical and experimental study of flexible pivot tilting pad foil bearings(FPTPFB),the concept of "micro-friction" of gas foil bearing supporting rotor system is put forward,and the related inference of "micro-friction" phenomenon is given,which points out the direction to solve the problem of serious separation between theoretical research and engineering application in gas foil bearing field.According to the "soft" support reducing cross foil gas dynamic pressure bearing stiffness,improve bearing adaptability,lower bearing between the rotor and the thinking of "friction",a new type of multi-mesh gas foil bearing is presented and its related theoretical and experimental research is carried out.In addition,breakthroughs were made in the technical indexes of gas foil bearing: The DN value of radial gas foil bearing reached 4.5million(mm.rpm),and the bearing capacity of 50-99 mm thrust gas foil bearing reached1460 N at 25 KRPM.The main research contents and achievements of this paper are as follows:The theoretical models of flexible pivot tilting pad and bump foil were established respectively,and the air-elastic coupling model of flexure pivot tilting pads foil bearing was established by coupling them with each other.The hysteresis curve of the push-pull experiment of bearing support structure was measured through the static push-pull test platform to verify the correctness of the theoretical model of bearing structure.The influence of rotor eccentricity,attitude Angle,ultimate load and dynamic coefficient on static performance of flexible tilting foil bearing is studied by using this model.And compared with the traditional three lobes foil bearing(TLFB).It is proved that the flexible pivot tilting pad structure is not only beneficial to improve the stability of the bearing,but also to improve the ultimate load of the bearing.In addition,the influence of design parameters such as pivot offset,flexure web tilting stiffness,bearing nominal clearance and preload on the performance of flexible pivot tilting pad foil bearing is analyzed,which lays a theoretical foundation for the research of the new bearing.By coupling the rotor motion equation,the unsteady Reynolds equation,the foil deformation model and the flexible pivot tilting pad motion equation,an accurate prediction model of threshold velocity and sub-synchronous vibration amplitude was established,and a nonlinear time domain orbit simulation method was proposed to evaluate the nonlinear dynamic performance of the FPTPFB supported rotor system.The nonlinear dynamic response of the system was analyzed by using bifurcation diagram,axis trajectory diagram and waterfall diagram,and compared with the predicted results of the rotor system supported by TLFB,the ultra-high stability of the FPTPFB was verified.In addition,the influence of bearing and rotor system parameters on the stability of FPTPFB,such as pivot offset,rotor mass and rotor unbalance displacement,is analyzed,which provides theoretical guidance for the design of the new type of bearing.A high-speed motor rotor test rig was designed and built.TLFB was used to verify the reliability of the test rig and the stable operation of DN value of 4.5 million(mm×RPM)was completed,which laid a reliable foundation for subsequent experiments in this paper.In addition,the influences of the pivot offset and the length of the flexible beam on the rotor subsystem were studied on the high-speed motor test rig.The results were compared with the theoretical model to verify the correctness of the nonlinear theoretical model of the rotor-bearing system supported by FPTPFB.Experimental and theoretical results show that increasing the flexibility of bearings is beneficial to improve the stability of the rotor system,but the greater flexibility of bearings will reduce the stability of the rotor system.In addition,the influence of rotor system parameters,such as rotor mass and unbalance displacement,on the stability is also studied,which accumulates a large number of experimental data for the design of FPTPFBs and rotor systems.The influence of clearance and misalignment parameters on the stability of rotor system supported by TLFB and FPTPFB is studied.Experimental results show that bearing clearance has little effect on system stability under the condition of good alignment,but it has obvious effect on system stability under the condition of misalignment.Then,the influence of rotor misalignment on the stability of the rotor system supported by TLFB and FPTPFB is studied under the conditions of different bearing clearances.According to the experiments,it is found that the contact friction between bearing and rotor plays an important role in the rotor system supported by flexible gas foil bearings.And the concept of "micro-friction" is further defined.By means of experimental verification and full-spectrum data analysis,it is deduced that the subsynchronous vibration of the rotor system supported by gas foil bearings is produced by the combined action of tangential force generated by "micro-friction" and tangential force generated by fluid dynamic pressure effect.Under the condition of non-ultra-high speed,the possibility of system instability caused by sub-synchronous vibration excited by "micro-friction" is much greater than that caused by dynamic pressure effect of fluid.It provides a new and practical method for the study of gas foil bearing,and also provides ideas and methods for the follow-up life study of gas foil bearing supporting rotor system.According to the idea that "flexible" support is beneficial to reduce the cross stiffness of gas foil bearing,improve the adaptability of bearing and reduce the "micro friction" between bearing and rotor,a kind of multi-mesh gas foil bearing with simpler process and better performance was proposed.Using beam elements to build a theoretical model of the network structure,Combined with the model of top foil shell element,the static and dynamic characteristics of reticulated gas foil bearings with different mounting angles under different speeds and loads were calculated by coupled with Reynolds equation,and the rationality and effectiveness of the theoretical model were verified by comparing with the results of push-pull experiment.It provides a theoretical basis for the application of the bearing.In the experimental study of bearing characteristics of thrust bearing,the bearing capacity of 50-99 mm thrust gas foil bearing reaches 1460 N at 25 KRPM,achieving a breakthrough in technical indicators.Furthermore,it is found that "micro-friction" is common in the ultimate bearing process of thrust gas foil bearing.Improving the wear resistance of bearing coating and bearing cooling can effectively prolong the "micro-friction" stage of thrust bearing,and then improve the ultimate bearing capacity of thrust bearing.The multi-mesh gas foil bearing has a good ultimate bearing capacity not weaker than that of bump foil bearing.In high-speed motor rotor experiment platform,comparing the bump type and multi-mesh type gas foil bearing rotor system,suggests that multi-mesh type gas bearing rotor system has better stability,and it is verified that the instability sub-frequency is mainly caused by "micro-friction" excitation under non-hypervelocity conditions.The multi-mesh gas foil bearing has good performance,simple manufacturing process and good consistency,and has a very good application prospect.In summary,in this paper,the flexible pivot tilting pad structure is introduced into the bump type gas foil bearing to form the flexible pivot tilting pad and elastic bump foil series supporting FPTPFB.Adjusting the pivot offset and flexible beam length can significantly adjust the flexibility of bearing support structure.Through the theoretical model and experimental research with different bearing parameters on the flexible pivot tilting pad foil bearing(FPTPFB)and the bearing rotor system is studied,and compared with bump type gas foil bearing,find suitable flexible supporting can significantly improve the stability of rotor system,and found that the flexible pivot tilting pad structure under extreme conditions can reduce the bearing wear deformation compensation,Further,by means of experiment and data analysis,the root cause of sub-frequency is studied,and the concept of "micro-friction" is defined,which provides a new and practical method and thought for the research of gas foil bearing.Then,a kind of reticulated gas foil bearing with flexible support structure,simpler process and excellent performance was proposed.The theoretical modeling and characteristic analysis of the multi-mesh radial and thrust gas foil bearings were carried out in detail.The stability and bearing characteristics of the multi-mesh radial and thrust gas foil bearings were experimentally studied. |
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