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

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bearings suffer from instabilities. There is usually some sort
of resonant response when the spinning frequency corresponds to
the natural frequency of the rotor in its bearing constraints.
However, very often the critical factor is a vibration called 'half-
speed whirl'. Whirling arises because when a rotating cylinder
is displaced from its equilibrium position the restoring force
from the bearing does not act straight back towards the centre but
at an angle towards the direction of rotation. Half-speed whirl
occurs at a frequency exactly half the rotor running speed. Although
the amplitude of the whirling motion may be small, this vibration
is sharply resonant where the natural frequency of the rotor is
close to this value. Then it invariably leads to rapid failure of
the bearing supports.
Any imbalance of the rotor gives rise to an out-of-balance
disturbance at the rotation frequency. Although machining toler-
ances are important, the inclusion of a sample can often cause a
drastic rotor imbalance. The most reliable systems are those in
which these factors are not critical.
4.2.5 THE DRIVING JETS
Assuming that a rotor is efficiently supported, its rotation
speed is determined by the velocity of the gas passing through the
driving jets. Using simple convergent nozzles the maximum exit
velocity is limited to the sonic velocity of the gas in the region
of the nozzle exit(53). Higher rotation speeds may in principle
be achieved by using supersonic gas nozzles, but the design and
manufacture of these involves advanced engineering concepts.