On Start-up Behavior Of Hemispherical Spiral-grooved Gas Bearings

The hemispherical spiral groove air bearings has been widely used in the high-speed rotating gyroscopes.The stability and reliability of the air bearing have a significant effect on the precision of the gyroscopes.However,during the start-up and shut-down of the air bearing,the hydrodynamic air film is insufficient to withstand the thrust load,leading to the contact between the rotor face and the air bearing surface.Consequently,wear is generated on the air bearing surface.Failures of the bearings are likely to occur due to the wear during the contact.In this study,the dynamic characteristics for the two different starting states of the bearing are studied.The main content of the research includes:The start-up model of the hemispherical spiral groove air bearings is established.The model considering both hydrodynamic fluid flow and asperity contact is solved.The asperity contact pressure is obtained by the Greenwood-Williamson(GW)contact model.The hydrodynamic pressure is calculated by modified Reynolds equation considering rough surface and gas rarefaction effect.During the start-up process,the dynamic load,motion parameters,surface contact force,etc.of the hemispherical spiral groove gas dynamic pressure bearing are studied,and the characteristics of the air bearing force and the contact force in the horizontal and vertical start-up states are revealed.The influence of geometric parameters,bearing surface conditions,machining errors and other factors on the vertical and horizontal starting process are explored.According to the change of the contact force during the start-stop process,the start-stop wear analysis model of the gas bearing is established.Researches on the asymmetric load state and motion trajectory of the hemispherical spiral groove gas bearing in the horizontal start-up state have been carried out.The influence of bearing structural parameters,surface roughness and machining errors on the dynamic load and contact characteristics of the bearing during horizontal start is discussed.The results show: during the horizontal start process,the dynamic air force gradually increases,and the dynamic contact force gradually decreases.When the rotor is separated from the stator,the dynamic load remains unchanged.The maximum value of surface air pressure during horizontal start-up is closely related to the position of minimum film thickness.Different structural parameters have different effects on the horizontal start-up process.The smaller the surface roughness,the faster the separation time between the rotor and the stator.But the trajectory of its center of mass finally crosses at one point.During horizontal start-up,the sphericity error has little effect on the dynamic characteristics of the bearing.The analysis of the periodic symmetrical load and linear motion state of the hemispherical spiral groove gas dynamic pressure bearing in the vertical starting process is carried out.The effects of different surface roughness,different structural parameters and geometric errors on the dynamic load and contact characteristics of the bearing are studied.Finally,the wear evolution law of the bearing surface after repeated start-stop is explored.Research indicates: During the vertical start of the bearing,the rotor surface pressure presents a periodic distribution,and the dynamic load trend is consistent with the trand when the bearing is started horizontally;Structural parameters have an important influence on the vertical start-up process,and its change rule is different from the horizontal start-up change rule.The influence law of surface roughness is consistent with the influence law of horizontal starting,but the sphericity error has a greater influence on the dynamic characteristics and contact characteristics of the bearing during the vertical starting process.The greater the sphericity error,the shorter the time for the rotor to separate from the stator,but the greater the change in the contact width.In the vertical start-stop wear analysis,as the number of start-stops increases,the change rate of the maximum wear depth on the rotor surface gradually decreases,and finally remains stable.