Critical speed (nk) is the rotational speed at which acting dynamic forces cause a machine component (e. g. shaft, rotor) to vibrate at its natural frequency (also referred to as intrinsic frequency, fi) and can even result in resonant vibrations throughout the entire machine and pump set. This effect has the potential to damage fast rotating machinery but can be minimised when such rotational speeds are passed through quickly. See Fig. 1 Critical speed
Fig. 1 Critical speed: Schematic amplitude profile of the rotational frequency induced vibration of a rotor as a function of rotational speed. The critical speed nk (n in min-1, w und f in s-1) occurs in the operating range where the rotational frequency coincides with one of the natural frequencies fi (f1, f2, f3).
In a broader sense, critical speed is also known as the rotational speed at which the frequency of a pulsating torque coincides with the natural torsional frequency of the shaft assembly.