When two surfaces form a wedge, and one surface moves relative to the other surface,
pressure is generated between the surfaces due to the hydrodynamic action of the fluid which carries load.
Eccentricity occurs at the center of the rotor and the bearing by self-load of the rotor, thereby production Con-vering Region.
Hydrodynamic pressure is produced in the conversing region by the rotational force of the rotor.
The airfoil bearing supports the rotor by using that hydro-Dynamic pressure.
The rotor emerges and rotates at 2,000 rpm or higher, and the friction loss that occurs during operation is close to zero.
Even though, the principle of an air bearing is simple, but application is complex.
Usually running radial clearance between the shaft and the bearing is less than 0.012mm
for a 50mm diameter shaft running at 36,000 rpm.
But the shaft growth due to temperature and centrifugal force could be 0.05mm.
Hence a bearing can not be made to work at various speeds and temperatures.
In addition, damping is required to suppress any whirl instability,
and there could be misalignment between various rotating parts and stationary parts.
These problems are resolved by foil bearings.
Thrust bearings withstand axial loads in a rotating machinery.
In a journal bearing the wedge action comes from eccentricity between the center of the rotating shaft
and the center of the bearing itself, whereas in a thrust bearing the wedge is built in taking into account any deflection due the axial load.
There are multiple radial springs which transfer the load to the housing.
Foils between the springs deflect under pressure forming the wedge required for the hydrodynamic action.
Both journal and thrust bearings apply a small amount of preload on the shaft when the machine is not running.
The foil face which is touching the shaft is coated for lubricity during startup and shutdown.
Most commercial aircraft use Teflon-S made by DuPont which is good up to 250℃.