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

4.4.4 COMMENTS
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For completeness we include at this point a few general
comments about the foil bearing rotor system.
It was found that at speeds up to 7 kHz the foil bearing
rotors were extremely reliable. Every rotor constructed as
described in Section 4.4.2 rotated stably even with full-sized
liquid samples. Not all the conical rotors would rotates even
on air. Although it is not a normal requirement of any NMR
rotation system, it is worth noting that the Bernoulli rotors
would not spin with liquid samples of any reasonable volume.
Because of the heating effect upon rotation, use of the foil
bearing system appears to be limited to those cases where the
temperature of the sample is relatively unimportant. As the
attainment of rotation rates above 7 kHz was limited only by the
failure of the alumina axles, silicon nitride might well prove
a more suitable axle material. Its tensile strength is typically
the same as alumina, but its density is only about 2.6 gm/cm3
compared to 3.8 gm/cm 3.
The coefficient of thermal expansion in
silicon nitride is half that of alumina(64) so upon rotation both
the induced thermal and mechanical stresses should be less. If
the use of this or any other material does result in a consider-
able improvement in performance it might prove possible to use a
much simpler rotor design in a foil bearing, such as a single
hollow tube.