Project Planning And Management
Professor David Harrison
Project Planning & Management Assignment
Air Foil Bearing Review
All Air Foil Bearings (AFB) in general fit in one of two categories, hydrostatic and hydrodynamic. Hydrostatic bearings require a pressurised air feed in order to lift of the foil surface but offer improved performance on start up and rundown  while developing no more drag than and equivalent hydrodynamic bearing. Hydrodynamic bearings generate their air film purely through the rotation of the shaft. This independence of an external air supply has a significant impact on cost reduction but comes at the expense of increased wear at run-up and run-down . An exception to this is a novel hybrid air foil bearing was recently proposed by Kim and Lee  which helps reduce wear during start up and shut down by combining a magnetic bearing with a hydrofoil. This allows the journal to be lifted clear of the top foil at start up and rundown then operated hydrodynamically once lift off speed has been exceeded. It also allows operation above the first bending critical speed by providing a method of increasing the stiffness and damping on request. The benefits of Air Foil Bearings over traditional bearings has driven recent developments in the technology as their use was initially limited to small, low load high speed machines . Benefits include low power consumption, lubricant free operation, long maintenance free life and the ability to withstand very high temperatures and high levels of vibration. They are however limited to a certain number of start/stops determined by the type of bearing and the amount of static load they are carrying due to rubbing during run up and run down. This inherent limit does not halt the demand for greater loads and speeds in oil free machines which must be met by the manufactures. To meet these demands, the last 10 years has seen a large amount of research being carried out by specialist companies and universities to understand the complexities of their operation when highly loaded and to overcome the problems these loads produce. By moving from compression sprung foils to bump foil, Kumar and Kim have shown that loads can be significantly increased at speeds as low as 10,000 rpm  but the problem of their comparatively low stiffness and damping still remains and is worsened by their dependence on frequency and amplitude. Lee et al propose using a viscoelastic material between the bumps and top foil to introduce sufficient damping to the system for operation at speeds above the first bending critical speed . Stiffness and damping related problems have also been tackled by Hou et al who have developed a type of air foil bearing with uniform circumferential stiffness and adequate damping to eliminate the possibility of self excited half speed whirl  although it has been shown by Lee et al that the majority of the rotor response is synchronous. Many of the vibration problems ultimately relate back to the inability to accurately predict bearing coefficients theoretically. Part of the problem is that most models consider a uniform circumferential stiffness  despite experimental results showing ripple traces on the foils after they have been in use  due to the deference in deflection at points of the foil directly supported by bumps and those straddling bumps. Recent work by Andres and Kim has also shown that the dynamic behaviour of air foil bearings evidences Duffing oscillator type motion which further complicates the accurate prediction of their behaviour . A new mathematical model was proposed by Iordanoff et al which considers the friction between the top foil and bump foil as friction in contrast to the accepted method of adding it as equivalent damping could help improve the accuracy of predicted damping coefficients. Their model also found...
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