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92 Electric Drives and Electromechanical Systems
























                FIG. 3.14 Cross section of an aerostatic bearing: the dimensions of the airgap has been greatly exaggerated.


             required to generate the supporting air film. In an aerostatic air bearing, Fig. 3.14,the
             two bearing surfaces are separated by a thin film of externally provided pressurised gas,
             normally air. The compressed gas is supplied by a number of nozzles in the bearing
             housing. The distance between the bearing surfaces is about 5e30 mm. As the object is
             supported by a thin layer of air, the friction between the shaft and its housing can be
             virtually zero.
                In machine tool applications, the lack of vibration and high rotational accuracy of
             an air bearing will allow surface finishes of up to 0.012 mmto beachieved. Theuse of
             an air bearing gives the system designer high rotational accuracy typically greater than
             5   10  8  m is achievable and will remain constant over time as there is no wear due to
             the absence of contact between the rotating shaft and the housing. Due to the low
             frictional drag, shaft speed of up to 200 000 rev min  1  with suitable bearings can be
             achieved. Unlimited life is possible due to the absence of metal to metal contact,
             provided that the air supply is clean, though special care is required to avoid damage to
             the bearings when the system is powered up and the air supply is not present.

             3.4.5   Magnetic bearings

             In a magnetic bearing the rotating shaft is supporting in a powerful magnetic field,
             Fig. 3.15. As with the air bearing gives a number of significant advantages, no wear
             between the rotating and stationary parts, thus particle generation is eliminated,
             therefore magnetic bearings are particularly suited to clean room applications. A mag-
             netic bearing can be submerged in the process fluid under pressure or operated in a
             vacuum without the need for seals. Operation through a wide temperature range, typi-
             cally 250 Cto  220 C is acheivable, hence magnetic bearings are widely used in


             superconducting machines.
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