Vibration is the to and fro motion of an object from its mean position of rest. The actual distance moved in a particular direction is called as the Vibration Displacement. This Displacement is measured in mils, mm or microns. The movement from mean position to crest OR mean position to trough can be called as Displacement peak value, whereas - the movement from crest to trough can be called as the Displacement peak to peak (pk-pk) value.

When you observe Rotary Machines in the real world - you will never see large displacement in a high speed machine running at say 6000 RPM. However, there is a good chance of seeing good displacement in a slow speed machine running at say 600 RPM.

Let us examine Displacement for a Pure Vibration which is Sinusoidal in nature (single frequency and single amplitude). Let us assume frequency to be 100 Hz (6000 CPM and called FHz) and Vibration to be 1 g pk (called g pk). The velocity peak (called Vpk) comes out to be (Vpk=(1561 x gpk) / FHz) = 1561 x 1 / 100 = 15.61 mm/s pk. The Displacement pk-pk (D pk-pk) comes out to be (D pk-pk = (318 x Vpk) / FHz) = 318 x 15.61 / 100 = 49.63 microns pk-pk. Now, if the frequency was 10 Hz (600 CPM) in place of 100 Hz for the same velocity of 15.61 mm/s pk - then Displacement would be = 318 x 15.61 / 10 = 496 microns pk-pk. So, we can see that reduced vibration frequency leads to better Vibration Displacement measurement - thereby proving the point that for slow speed machines or low frequency vibrations - Vibration Displacement is the right measurement