Critical speed (vibration)

A critical speed is a shaft rotational speed at which the shafting or rotor running frequency, or one of its blade-rate harmonics, coincides with a structural natural frequency, producing resonant lateral, torsional, or axial vibration. For a propeller shaft the first lateral (whirling) critical and the torsional criticals of the shaft-engine system are checked against the operating range, and class rules require a margin so that no continuous-running critical falls inside the normal speed band. Blade rate equals shaft rpm times blade number, the main forcing of stern and deckhouse vibration. Designers shift criticals out of the running range by changing bearing spans, shaft diameter, or blade count.