What Unit is Right for My Application?

MEASURE	UNITS	FREQUENCY RANGE	GENERAL USE
Displacement	mils-pk to pk	0-10 Hz	Relative Motion in Bearings / Casings
Velocity	IP - 0 to pk or RMS	10 Hz - 1 kHz	General Machine Condition
Acceleration	gs - 0 to pk	above 1 kHz	General Machine Condition, Gears / Bearings

In order to assess the severity of vibration, an analyst must select an engineering unit to be used to quantify the amount of vibration present in a machine. Depending on the dominant frequency of vibration, an analyst would use the amplitudes of displacement, velocity, and acceleration to measure vibration severity. The table above shows the units, frequency range, and applications that each measure is appropriate for.

Note, these are common artificial boundaries used in the vibration analysis industry, overlap of usage is not uncommon.

Displacement:    Relative shaft displacement relates to stress/motion and is measured by a noncontact Proximity Probe; these can illustrate the relative motion between the mounted probe tip and the target shaft. This will allow the user to determine the extent of bearing clearance taken up by vibration while a shaft is running, or if the probes are mounted axially, it will provide the relative displacement of the shaft due to thrust or thermal expansion.

Velocity:  For general application monitoring of medium-frequency (10 Hz - 1,000 Hz) vibrations, velocity is the standard amplitude unit of measurement. Velocity is related to machine fatigue/energy, it is a popular unit of measurement for medium-frequency vibration because a single value for peak velocity or rms can be used in rough assessments of machine condition without the need to consider frequency.  An analyst has the choice of using a velometer to obtain a velocity signal, or they could integrate the output of an accelerometer to velocity with their data collector or data analysis software.

Acceleration:  Acceleration is the best unit of measurement to use for any vibration frequency greater than 1,000Hz; it is tied to force and is often used for high-frequency vibrations like those caused by rolling element bearing defects and gearmesh. Monitoring high-frequency vibrations will allow an analyst to detect early signs of bearing and machine defects and provide them with the most amount of time to plan and schedule machine maintenance/repairs.