NNR IN RAPIDLY ROTATED METALS By - Nottingham eTheses ...

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gap then diverges and the pressure is reduced. The bearing
formed in this way always acts perpendicularly to the surface of
the rotor so the system is free from the problems of fractional
speed whirl. Because the foil is flexible the bearing is un-
affected by a certain degree of misalignment or imbalance in the
rotor
(60).
A preliminary investigation into the feasibility of attain-
ing high rotation frequencies by using a three pad foil bearing
rotor system was undertaken by the Gas Bearing Advisory Service
(61)
at Southampton University. Considerable success was achieved
with metallic rotors and a cylindrical titanium rotor (shown in
Figure 4.4) reached a rotation speed of 15 kHz when propelled by
helium. A second type of rotor with smaller diameter bearing
axles, placed at the extremities attained a speed of
11 kHz.
However with non-metallic bearing surfaces the rotors failed to
run reliably if at all.
4.4.2 THE ROTOR
Experience with foil bearings has shown that the use of a
polished bearing surface is a most important criterion for their
reliable operation. Larger diameter bearings tend to be slightly
less susceptible to small surface irregularities
- the mean bear-
ing pressure is lower so there is greater clearance between the
foil and the rotor. Against this the rotationally induced stresses
are also greater so the rotor arbors have to be stronger mechanically.
Various non-metallic materials have been used for the rotor
axles including several types of tufnol, nylon, glass, glass fibre
and ceramic. Those rotors which-did run either became unstable
)