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Vibration displacement is the total distance a vibrating object moves from its rest position. It is measured in micrometers (μm) or mils (1 mil = 25.4 μm) and can be expressed as peak (0-peak) or peak-to-peak.

Displacement is the most important vibration parameter for low-frequency machinery — typically below 10 Hz (600 RPM). At low frequencies, velocity and acceleration values become very small even when physical movement is significant.

Peak displacement (0-peak) measures the maximum excursion from the center rest position in one direction. Peak-to-peak displacement measures the total range of motion — from the maximum positive to the maximum negative excursion.

For sinusoidal vibration, peak-to-peak equals exactly twice the peak value: d_p-p = 2 × d_peak. Most specifications use peak-to-peak values.

To convert velocity (mm/s RMS) to displacement (μm peak-to-peak), use:

d_p-p = v_RMS × √2 × 1000 / (π × f)

where f is frequency in Hz. The √2 converts RMS to peak, dividing by (π × f) integrates velocity to displacement, and × 1000 converts mm to μm. If you know RPM instead of Hz, divide RPM by 60 first.

For sinusoidal vibration, these parameters are related through frequency via calculus. Velocity is the derivative of displacement: v_peak = 2πf × d_peak. Acceleration is the derivative of velocity: a_peak = 2πf × v_peak = (2πf)² × d_peak.

Higher frequencies amplify acceleration relative to displacement. A small displacement at high frequency produces large acceleration, while large displacement at low frequency may produce negligible acceleration.

Use displacement for low-frequency vibration monitoring, typically below 10 Hz (600 RPM). Large slow-speed machines such as paper mill rolls, cooling tower fans, wind turbines, and reciprocating compressors require displacement monitoring.

Velocity (mm/s RMS) is the standard for general machinery in the 10–1000 Hz range (ISO 10816). Acceleration is preferred above 1000 Hz for bearing defects and gear mesh.