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

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the manufacture of which was described in Section 4.3.3. Rotors
constructed in this way (shown in Figure 4.6) have been run at
speeds up to 9 kHz without instability. However at speeds above
7 kHz the axles are subject to mechanical failure.
The cause of these failures is not fully understood. The
tensile strength of this material is approximately 2650 kg/cm2
while the radial stress predicted by equation (4.2) is 500 kg/cm2
at 7.5 kHz. Attempts to reduce the stress concentration by rounding
off the outside edges of the axles and the internal edges between
the axles and body failed to improve the situation. The rotors
became warm after they had been running for some time. Thermal
stresses produced in the axles, as determined by simple theory
would appear to be an order of magnitude too small to explain fail-
ure, but experimental evidence lends weight to such an explanation.
The rotors became hotter at higher rotation speeds and failure
occasionally occurred even after the rotors had been spinning at
a stable speed for several minutes.
4.4.3 THE SUPPORTING RIG
In the original design the bearing tapes were positioned about
the rotor axle by three pairs of aluminium pins and tensioned by
weights. Other methods of tensioning such as clamping the tapes
around rubber bushes and the use of simple springs generally
proved inferior especially at higher rotation speeds. Once the
bearing tapes had 'bedded in' the degree of tensioning failed
to affect the performance of the rotors to any great extent.
Normally the rotors started more easily and ran slightly faster
(62)
(63)