Analysis Of The Flow Field For Gas Bearing Working In Vacuum Environments

Gas bearing is widely used in microelectronics manufacturing and ultra-precision positioning device because of low friction,high precision,no pollution,long life span and others.With development of the next-generation lithography exposure,laser interferometry measuring and semiconductor detection technology,ultra-precision motion positioning requires not only higher positioning accuracy,but also required to be carried out in vacuum or ultra-clean environment.But in a vacuum environment,aerostatic gas bearing performance changes significantly compared with in the atmosphere.Currently,there are a few researches on lubrication theory of gas bearing in vacuum environment.The most research is based on single continuous flow and never considering the influence of transition flow and molecular flow to gas bearing internal flow field.To solve the problem,single supply hole and plate type aerostatic gas bearings which is applies to vacuum environment are taken as research objects.Some theoretical analysis and experimental research of internal flow field have been done.Mainly,this paper has finished jobs as follows,Firstly,the flow field of gas bearing in vacuum environments is divided into three parts,the entrance area,bearing area and sealing area.Based on the principle of gas film lubrication,D-T model and first order slip model is used to study this issue.Analysis has been made by D-T model with dimensionless parameters.It shows that the relative carry capability is improved by increasing dimensionless entrance pressure and entrance area radius or by reducing the ratio of bearing radius and film thickness.First order slip model obtains that increasing boundary Knudsen number will make carry capability decline.Secondly,a vacuum boundary condition of numerical simulation model has been established.COMSOL is used to simulate the internal flow field of three parts.It's believed that the entrance area flow is complex,but the proportion is relatively small.The bearing area is the main source of bearing capability.The seal area needs to use molecular flow module to simulation.The impact of field to load capability and seal effect is considered.Thirdly,numerical calculation is done to customized experimental gas bearing,and its performance test apparatus is built.Pressure distribution of film and film thickness is measured on the equipment when different load,taking gas pollution to vacuum chamber into account.Experiment result shows that gas film thickness is increased in vacuum condition,and gas leakage increases first and then decreases.Lastly,combining the experiment results with theoretical analysis and software simulation results,theory and simulation method is summed up.Comparison shows that the first order slip model result fit better with experiment and simulation results.The optimization method for design of gas bearing applied to vacuum condition is also presented.The increase of pump speed can improve seal effect,but economic cost must be considered when a reasonable pump is chosen.For two exhaust groove bearing,the fist and second seal width should be allocated reasonable.By research of this issue,it's desirable to obtain performance calculation methods and reducing gas leakage to improve gas bearing sealing methods,mainly considering the rarefied effect.It's aimed to provide some theoretical support for gas bearing applied in vacuum environments and to promote the development and progress of gas bearing.