Mechanical Engineering - Theory of machines - Discussion

At critical speed, the resonance occurs at the shaft, which means actual and natural frequency are equal. At resonance, vibration is maximum, there will be a danger of breakage.

The Critical speed of a shaft is also called as Whirling speed. It is defined as the speed at which a rotating shaft will tend to vibrate violently in the transverse direction if the shaft rotates in a horizontal direction. In other words, the whirling or critical speed is the speed at which resonance occurs.