The Calculation, Explained
Why velocity, and what the zones mean
For rotating machines in the 10–1,000 Hz range, vibration velocity correlates best with the dynamic stresses and fatigue the machine experiences — displacement overstates slow machines, acceleration overstates fast ones, velocity treats the energy fairly across the range. ISO 20816 (successor to ISO 10816) grades the measured RMS velocity on bearing housings into four zones:
- Zone A — as-new condition. Zone B — acceptable without restriction. Zone C — tolerable for limited periods; find the cause and plan the fix. Zone D — likely to cause damage; act.
The boundaries depend on machine size and duty (groups) and on whether the foundation is rigid or flexible — a machine on a flexible skid is allowed more motion because the foundation itself participates. The conversions below the verdict assume the reading is dominated by the running-speed component, as overall readings usually are:
Assumptions and applicability
- The reading is a broadband RMS velocity on the bearing housing, in the 10–1,000 Hz band, at steady operating condition — not a start-up transient, not a shaft-relative proximity reading (those are judged by ISO 20816's shaft-vibration parts and different limits).
- A zone verdict is a screening judgement, not a diagnosis. A machine can sit in Zone B overall while a single spectral line — a bearing defect frequency, a 2× misalignment peak — is already announcing a failure. Overall level first, spectrum second, always.
- Trend beats threshold: a machine that has doubled its Zone B reading in a month deserves more attention than a stable machine sitting higher. The zones are the floor of the conversation, not the ceiling.
- Boundary values here are the standard ISO 20816-3 tabulated values for industrial machines above 15 kW; gas and steam turbines have their own parts of the standard (20816-2/-4/-5) with different boundaries.
Worked example (the defaults above)
A Group 1 machine on a rigid foundation reading 4.5 mm/s RMS: the B/C boundary for its group is exactly 4.5, so it sits at the threshold of Zone C — restricted operation, corrective action to be planned, not a machine to file and forget. At 1,480 rpm the same motion is about 41 µm peak (82 µm pk-pk, 3.2 mils) of displacement and roughly 0.10 g peak of acceleration — the three languages of the three instruments on your route, saying one thing.
References
ISO 20816-1 and ISO 20816-3 — Mechanical vibration: measurement and evaluation of machine vibration (successor series to ISO 10816).
Related reading: The Maintenance Mix — Why No Single Strategy Survives Contact with a Real Plant.
Frequently Asked Questions
What vibration level is acceptable for rotating machinery?
Per ISO 20816, bearing-housing velocity in Zone A is as-new condition and Zone B is acceptable for unrestricted long-term operation. For a large machine on a rigid foundation Zone B ends at 4.5 mm/s RMS; above that the machine enters Zone C, restricted operation, and corrective action should be planned.
What is the difference between Zone B and Zone C in ISO 20816?
Zone B means the machine can run indefinitely without restriction. Zone C means the vibration is not suitable for continuous long-term operation: the machine may run for a limited period while the cause is found and a fix is planned. Zone D is vibration likely to cause damage.
Should vibration be measured in mm/s, displacement or acceleration?
For general machine condition in the 10 to 1,000 Hz range, velocity in mm/s RMS is the fairest severity measure and the basis of ISO 20816 zones. Displacement suits low-speed machines, acceleration suits high-frequency bearing and gear defects. They are convertible at a known frequency, which this calculator does.
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