Any rotating shaft will have lateral (or flexural) and torsional natural frequencies. If the shaft is subjected to a force at any of these frequencies, the amplitude of vibration will be particularly large. The dynamic forces in a rotating machine will generally occur at multiples of rotational frequency and at certain speeds one of the forcing frequencies will coincide with a natural frequency. These speeds are known as critical speeds.
The most significant critical speed is usually the speed at which the rotational frequency coincides with the first flexural natural frequency of the shaft.