Reading Vibration Severity: a Practical Guide to ISO 10816 / 20816

A single number — vibration velocity in mm/s RMS — tells you whether a machine is healthy, drifting, or about to fail. Here's how to read it the way the standards intend.

Why velocity, and why RMS

Vibration can be expressed as displacement, velocity, or acceleration. For overall machine-health severity in the common 10 Hz–1 kHz range, the international standards settle on velocity because it correlates well with the energy and fatigue damage a machine experiences across a wide speed range. And they use RMS (root-mean-square) because it reflects the sustained energy of the signal rather than a single peak.

So the headline severity number is velocity RMS in mm/s, measured broadband over roughly 10–1000 Hz. That's the value the ISO 10816 series — now superseded by ISO 20816 — uses to classify machine condition.

The A / B / C / D zones

The standard sorts a measurement into one of four evaluation zones:

• Zone A — vibration of newly commissioned machines. Excellent.
• Zone B — acceptable for unrestricted long-term operation.
• Zone C — unsatisfactory for long-term operation; run only short-term while you plan a fix.
• Zone D — severe; vibration is capable of causing damage. Investigate/stop.

The actual mm/s boundaries between zones depend on the machine's size and how it's mounted (rigid vs. flexible support). As a representative example, here are the boundaries for a medium-size machine (roughly 15–300 kW) on a rigid foundation:

Bigger machines and flexibly-mounted machines tolerate higher numbers; small high-speed spindles are often held tighter. Treat the table as a starting reference and always confirm the class and limits that apply to your machine.

How to measure it well

• Mount solidly on the bearing housing, in line with the load path — a loose or padded mount under-reads.
• Measure in the right band (about 10 Hz–1 kHz) and report the RMS overall, not a single peak.
• Be consistent — same location, axis, and speed every time, so trends are comparable.
• Trend, don't spot-check. A machine doubling from 1.0 to 2.0 mm/s is still "Zone B," but the change is the early warning.

From a raw spectrum to a verdict

Knowing the number is one thing; getting it reliably is another. The GrayVolt Vibration Analyzer streams a live FFT spectrum and overall velocity RMS to your phone, so you can read severity in the field and watch it trend over time — no analyzer cart, no laptop. Pair it with the once-per-rev tach and you can also tell why it's high: 1× usually means imbalance or misalignment, while bearing-defect frequencies show up higher in the spectrum.