Research Of Static Characteristic For Externally Pressurized Gas Bearing By Highter Supply Pressure

In gas lubrication, the mechanical moving parts are supported by the gas film between clearances, the width of which is from several micrometers to several decade micrometers. Compared with rolling bearings and liquid lubrication bearings, the gas bearings possess merits:high speed, high precision, low friction, long lifetime and etc. However, the gas supply pressure, load capacity and gas film stiffness of traditional gas bearings are low to use within low load precise support. But the development of the sophisticated industry technology needs high load gas lubrication system urgently.The cause of low gas supply pressure in traditional gas bearing is that the velocity of gas between the clearances becomes supersonic from subsonic when increasing the gas supply pressure and the width of clearances. When the velocity becomes supersonic, the pressure will decrease steeply and will sometimes be below atmospheric pressure. Because of the complexity of flow process, the research on supersonic phenomenon in aerostatic bearings was generally based on one dimension simplified equations for a long period of time. With the interaction between shock wave and boundary layer, the result from the one dimension analysis on shock wave structures deviates, to a large extent, from the practice. So the further study on the mechanism of supersonic flow in the clearances under high gas supply pressure or big clearances is necessary, which establishes theoretical foundation for the development of high supply pressure, high load gas bearing.In this paper, the theoretic analysis, numerical calculation and experiment are combined with to conduct. Based on software Fluent, the exact flow field model was set up, the numerical simulations of the flow in the clearances were done; the flow characteristics and the interaction between shock wave and boundary layer were studied in detail. By analyzing the flow state of the gas in the clearances, the simplified mathematical model was set up which described the flow field essential properties; the essential flow field properties were qualitatively analyzed. The structure of gas bearings were improved, and the load capacity of them is improved. The main research works are shown as below:â‘ The mathematical model of the whole flow field was built up considering the effects of inertia term and viscosity term, the viscosity-temperature equation was introduced to perfect its model. The laminar flow model in the CFD software was conducted to exactly simulate the flow field in the small clearances. For the flow field in the big clearances, the algorithm is qiute different in different regions of the flow field. The whole flow field including the gas supply hole is divided into two parts:laminar section and turbulent one. The interaction between oblique shock wave and boundary layer, property of gas film boundary layer and change of oblique shock wave structure were studied with the increase of gas film clearance and supply pressure.â‘¡The thrust gas bearing was chosed as a research subject, the dimensional analysis was conducted to derive the energy equation when the velocity near the gas supply hole was fast; the relation expression between temperature and Mach number was derived. The control equations considering the effect of initial force were set up; the pressure correction algorithm was used to calculate the change of viscosity force and initial force with both gas supply pressure and gas film clearance changing together. The effects of viscosity force and initial force on the pressure distribution in aerostatic bearings were studied as well. The result demonstrated that its inertial force stengthens and the pressure nearby the entrance drops more quickly with the increases of supply gas pressure and film thickness.â‘¢Based on the difference of the flow in clearances, the phase models were built up including the gas supply hole. The condition was introduced that the flow rate does not change under the supersonic flow, the simplified formula of pressure, velocity, Mach number and Reynold number were deduced and solved with self-defined programme, which provided theoretical reference for the prediction of the relationship between pressure and shock wave. The result showed that the maximum upswing pressure guadually increase with the increases of supply pressure and hole radius, and the decrease of film thickness.â‘£The structure of gas bearing with high load capacity was designed under higher gas supply pressure or bigger gas film clearance working condition, that is, by increasing the area of its throat, the through-flow areas were modified to gain the pressure in the film. The simulation was conducted under different structure shape, different size and different working conditions; the method and principle of bearing design were achieved in different working states, and the change law of the flow field properties were derived as well. The test of static pressure and characteristics of bearings proved the reliability of the theory model and the calculation method.