Whirling of shafts occurs due to rotational imbalance of a shaft, even in the absence of external loads, which causes resonance to occur at certain speeds, known as critical speeds.

1. CRITICAL
OR
WHIRLING SPEED
(of Shaft)
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2. Definition
The speed at which the shaft runs so that
the additional deflection of the shaft
from the axis of rotation becomes
infinite, is known as critical or
whirling speed.
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5. Cause Behind Whirling of shaft
Whirling Speed is due to the
unbalanced forces acting on a
rotating shaft.
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6. Derivation
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11. Notes :
1. When the centre of gravity of the rotor lies
between the centre line of the shaft and the centre
line of the bearing, e is taken negative.
On the other hand, if the centre of gravity of the
rotor does not lie between the centre line of the
shaft and the centre line of the bearing (as in the
above article) the value of e is taken positive.
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12. 2. To determine the critical speed of a shaft
which may be subjected to point loads,
uniformly distributed load or combination of
both, find the frequency of transverse
vibration which is equal to critical speed of
a shaft in r.p.s.
The Dunkerley’s method may be used for
calculating the frequency.
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13. 3. A shaft supported is short bearings (or
ball bearings) is assumed to be a simply
supported shaft while the shaft
supported in long bearings (or journal
bearings) is assumed to have both ends
fixed.
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14. For safety purpose it is advised to avoid
that critical speed range if possible
otherwise transition through this speed
should be instantaneous.
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15. Thank You
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