Research On The Aerodynamic And Squeeze Gas Levitation With The Consideration Of Surface Characteristics

Non-contact levitation technologies have great values to be broadly used in the field of medical machineries,precision experimental equipment,aerospace,etc.These technologies bring great opportunities and challenges for the development of machinery industry.Gas levitation technologies have the advantages of low friction,simple structure,easy maintenance and great heat dissipation.Hence,they are considered as the ideal non-contact levitation technologies to be used in the conditions of high precision,high rotational speed,high temperature,etc.Although gas levitation technologies have many advantages,the lower gas viscosity causes their load-carrying capacity being not ideal.On the other hand,the gas levitation technologies require high-precision manufacturing.This causes the performance of the levitation will be easily influenced by the surface accuracy.In view of the above questions,this paper proposes the method of forming the grooves on the relative moved surfaces to improve the performance of the aerodynamic and squeeze gas levitation technologies.Meanwhile,the models of the surface roughness and surface defects were formed to analyze the influence of the surface accuracy on the aerodynamic gas levitation technology.The main contents and results of this dissertation are as follow:The micro spherical spiral grooved gas bearings were chozen to study the aerodynamic gas levitation characteristics in micro scale.Basing on the shape and structure characteristics of the micro spherical bearings,the proper slip flow velocity boundary was used to obtain the rarefaction effected Reynolds equation.As the gas film discontinuous which caused by the grooves will influence the solving precision,three steps were used.The first step was obtaining the Reynolds equation in the oblique coordinate basing on twice coordinate transformations.The second step was doing a surface integral for the Reynolds equation.The Green's theorem was used to convert the surface integral to line integral.This step could realize the Reynolds equation being reduced to first order equation.The third step was obtaining the discrete Reynolds equation by using the eight-node difference method.The Newton iteration method was used to obtain the gas film pressure.As every pressure node were obtaind basing on the around eight gas film thickness nodes,this numerical method could significantly decrease the influence of the discontinuities of gas film thickness on the calculation precision.The accuracy of the calculated results were compared and proved by the experimental results.The dynamic Reynolds equation was obtained by the perturbation method and solved by using the same method of solving static Reynolds equation to obtain the dynamic coefficients.The analyzing results could find that the slip flow will decrease the load capacity of the spherical bearings.The load capacity and the dynamic coefficients were significantly influenced by the spiral grooves.The spherical spiral grooved gas bearings were chozen to study the temperature characteristics of the aerodynamic gas levitation technologies.The energy equation in the spherical coordinate system was obtained basing on the shape characteristics of the spherical bearings.The final pressure and temperature distribution was obtaind by coupling solving the Reynolds equation and energy equation with considering the coupling effect of the temperature,pressure,gas viscosity and density.There was no need to decrease the order of the energy equation.Hence,the mesh grid of the gas film thickness had to be modified to have the same size as the gas pressure.This could ensure the temperature,gas pressure and gas film thickness have the uniform mesh grid.To analyze the thermohydrodynamic characteristics of micro gas bearing,the gas rarefaction effect on the gas viscosity-temperature should be included.Meanwhile,the surface roughness modeled basing on the fractal theory was coupled into the gas film thickness.The results indicated that the surface roughness will increase the friction torque to enhance the influence of temperature.The gas viscosity-temperature which influencd by the rarefaction will decrease the bearing load capacity when considering the influence of temperature.The line velocity will be significantly increased when the bearing becomes to macro scale.Correspondingly,the increasing of temperature becomes more significantly.The temperature rising could be significantly suppressed by controlling the internal energy distribution characteristics and the energy flowing characteristics.The manufacturing defects of lithography are easily formed in th e vertical direction of bearing axial.The gas-lubricated micro flexure pivot tilting pad bearing was chosen to analyze the effect of manufacturing defects on the performance of aerodynamic gas levitation technologies in micro scale.The Bow and Taper defects,which are the very common in lithography,were considered.Meanwhile,the gas rarefaction effect was also coupling considered.The motion equations of the pads with the influence of the gas pressure were modeled,and the motion characteristics of the pads were coupled into the gas film thickness.The static deformations of the pads were considered when analyzing the static characteristics of the bearings.The perturbations of the pads in their directions of motions were considered when analyzing the dynamic characteristics of the bearings.It has to consider the motions of the pads changing over the time when analyzing the nonlinear characteristics of the bearings.The size relations between the defects and bearing clear ance were changed by changing the maximum height of the defects,pad preload and mass of the rotor.The predicted results show that the defects will decrease the bearing load-carrying capacity,dynamic coefficients and stability.The bearing performances could be enhanced by the pad preload.However,increasing the pad preload will make the bearing become more sensitive to the defects.The surface groove effect on the squeeze gas levitation technologies was studied.The compressible Reynolds equation coupled with the mode shape of the radiator plate was presented in the cylindrical-coordinate system.The modal analyzing of the radiator was performed to ensure the amplitude of the radiator being maximum.The second resonant frequency of the radiator was chosen by FEM and experimental analyze to make sure the radiator has high energy output.Meanwhile,the second resonant frequency was not too high which could prevent the ultra-high energy inputting to destroy the system.If the grooves were machined on the radiator,the resonance frequency of the radiator will be changed with the various of the groove characteristics.To ensure the comparability,the grooves were machined in the surface of the reflector.The squeezing gas Reynolds equation was firstly transformed the coordinate and reduced order.Then,the eight-node difference method was used to disperse and analyze.The numerical and experimental results indicated the pressure distribution was influenced by the mode of the radiator and the surface grooves of the reflectors.The radial grooves drive the inner high pressured gas flowing out to decrease the load-carrying capacity.The circumferential grooves could reserve the high pressured gas to increase the load-carrying capacity.Changing the groove number,depth and width could obtain better levitation performance.The circular radiator was replaced by the rectangular rail to obtain the gas levitated linear rail with the coupling of squeezing and aerodynamic effect.The Reynolds equation including gas squeezing effect,gas aerodynamic effect and gas acceleration characteristics has been proposed.The grooves were also machined in the surface of the levitated plate to analyze the groove effect.The resonance frequency of the rail was calculated basing on the FEM and compared with the experimental results.In the experimental study,it has to make sure the rail being vibrated at the resonance frequency to obtain highly pure traveling and standing waves.The plate was experimental tested to obtain the results of the levitation height for standing wave and the relationship of time and displacement for traveling wave.The results were compared with the numerical result.It could be obtained that the levitation and transportation abilities can be improved when the length of the groove vertical to the rail wave propagate direction.Different groove parameters effect on the levitation and transportation characteristics were detailedly studied when the length of the groove vertical to the rail wave propagate direction.These parameters were groove width,number,depth and length.The results indicate that these parameters all have the optimal values to obtain the best levitation and transportation capability.In conclusion,the surface characteristics have multiple effects on the performance of gas aerodynamic and squeeze levitation technologies.The grooves could improve the gas aerodynamic effect to improve the load capacity and temperature characteristics of the gas aerodynamic levitation technologies.Basing on the characteristics of the surface wave of the squeezing radiator and rail,the grooves could also improve the levitation and transportation characteristics.However,the surface roughness and machining defects will significantly weaken the performance of the aerodynamic gas levitation technologies.Hence,machining regular grooves in the surface is a simple and potential method to improve the performance of gas aerodynamic and squeeze levitation technologies.Meanwhile,it has to ensure machining precision to decrease the effect of surface roughness and defects.