What is critical speed or resonant frequency?
All rotors have a resonant frequency. The resonant frequency is the speed in cycles per minute at which a rotor will vibrate if struck in such a way as to create a vibration. Think of a ruler hung off the edge of a desk and then struck. If most of the ruler is off the desk, the frequency will be low and the amplitude will be high as there is a lack of rigidity. If very little of the ruler extends beyond the desk, the frequency will be high and amplitude will be low since the rigidity increases. The critical speed of a shaft is based on the resonant frequency and will most likely deflect at this speed even if well-balanced.
