When looking at a vibration signature, it helps to think in terms of multiples of the rotating speed. Because not all plant equipment operates at the same speed, this simplifies analysis. Rotor unbalance, for instance, usually shows up at rotating speed. Mechanical problems — such as a bent shaft, bad coupling, or oversized bearing housing — tend to appear at 2 x rpm.

Vibration frequencies at higher multiples of the rotating speed correspond with the number of components in a specific rotating part, such as the number of balls in a bearing. Other sources of vibration frequencies at multiples of the rotating speed may include fan blades, impeller vanes, rotor bars or stator slots, or some combination thereof.