Study On Characteristics Of The Magnetorheological Fluid Lubricated Floating Ring Bearing

The rotor system is commonly used as a conveyor to provide power to various mechanical systems in modern industry.Therefore,the efficient and steady working condition of the rotor system is fundamental and crucial.When agitated by mass imbalance,external shock,unexpected vibrations would be induced in the rotor system.Consequently,the research field of rotor vibration control has always been one of the active areas of study.Since rotors are supported on various types of bearings,among myriad control strategies,applying “active” or “smart” bearing in the rotor system is one of the most common methods.This kind of method seeks to,by some measures,alter the characteristics of the bearing itself,then to tune or suppress the rotor vibration.This study originates from the National Program on Key Basic Research Project(973 Program): “Nonlinear coupling mechanism in large-scale equipment dynamics”.In this dissertation,a type of hydrodynamic lubrication bearing using smart fluid is proposed.This kind of bearing utilizes the structure of a floating ring bearing(FRB)with magnetorheological fluid(MRF)as lubricant,while an external magnetic field excitation system is installed outside the bearing to provide magnetic fields to the bearing.The magnetic fields will change the properties of the MRF in the bearing,then the dynamic properties of the bearing and finally the behavior of the rotor.To analyze the feasibility of this type of bearing,this dissertation conducts studies both theoretically and experimentally.By categorizing the MRF into the Herschel-Bulkley(HB)fluid,study of the HB fluid in lubrication is carried out.Due to the nonlinear relationship between shear rate and shear stress in the HB model,the study abandons the direct application of the ordinary Reynolds equation,but rather starts from the fundamental momentum conservation equation and continuity equation of fluid.By introducing the HB model into these equations,an implicit Reynolds equation is obtained.With this equation,yield stress,shear thinning effect,and core formation of the HB fluid in lubrication is studied,along with characteristics of the pressure and shear stress distribution in the lubricant film.Moreover,this equation is introduced into the FRB to study the two films in it created by the floating ring.The results demonstrate that due to the lower shear rates in the outer film,the shear thinning effect in it is limited,which establishes the theoretical foundation of MRF lubrication in the FRBs.The semi floating ring bearing(sFRB),whose outer film is a squeeze film,is also investigated.The outer film of the sFRB functions as a squeeze film damper,and by using MRF,its damping property becomes adjustable through changing external magnetic field strength.The Timoshenko beam theory is utilized to model the rotating shaft,and combining the bearing theory,a compound FRB-rotor system dynamic model is established.The finite element method and the Newmark- method are then adopted to solve the equations for the transient responses of the rotor system to study the imbalance responses when the rotor is at constant speed or speeding up or under shock impact.The results theoretically prove the abilities of the MRF lubricated FRB in rotor vibration control.The MRF lubricated FRB is designed and manufactured.Considering the characteristics of the MRF,larger clearances in the FRB are chosen.Outside the FRB,a magnetic field excitation system including four coils and poles is attached.This system provides the bearing with constant magnetic fields.To allow the magnetic field to penetrate the bearing and reach the MRF,aluminum alloy and bronze are chosen to make the FRB.The bush and floating ring are specially made of tin bronze considering its abradability.A magnetorheometer is designed and manufactured to test the self-made MRF for its shearing properties.Using this magnetorheometer,properties of the MRF are proved to be fit to the HB fluid rather than the original Bingham fluid when external magnetic field is applied.With the test results,parameters of the HB model are identified for the self-made MRF,providing actual parameters for theoretical calculations.On the other hand,to verify the ability of the proposed MRF lubricated FRB to change its apparent viscosities under magnetic fields,a bearing test rig is designed and set up.This test rig is to test the stiffness and damping properties of both the inner and outer film of the FRB,under testing conditions of different bearing loads,rotational speeds and magnetic field strength.The experimental results show that with intensifying magnetic field strength,the stiffness and damping coefficients of the bearing outer film increase.The bearing test rig is further modified and improved to set up a cantilever rotor test bench to investigate the performances of the MRF lubricated FRB in rotor configuration.The test results demonstrate the ability of this type of bearing in rotor vibration control while at the same time enhancing the stability of the FRB itself.This dissertation proposes a novel type of bearing,and by means of theoretical analysis,calculations and experiments,its feasibility in rotor vibration is verified.