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Free tool · ISO 20816-3

Which vibration zone is your machine in?

Enter power, support type and measured RMS velocity: the calculator tells you which ISO 20816-3 zone the machine is in and where to set alarm and trip limits, the way the standard prescribes. Free, no sign-up.

kW
Above 300 kW is Group 1, 15 to 300 kW is Group 2.
Support type
Rigid if the lowest natural frequency of machine and foundation is at least 25% above the running frequency. If unsure: if vibration on the foundation is much lower than on the bearing housing, it is rigid.
mm/s RMS
RMS value 10–1000 Hz on the bearing housings; use the direction with the highest reading.
mm/s
The value after commissioning or overhaul. Needed to calculate the correct alarm limit.

The calculation runs in your browser: no data is sent unless you request the report.

Your result will appear hereFill in at least power and measured velocity.
What the zone cannot see

A machine in zone A can have a bearing that is failing.

Vibration severity stops at 1000 Hz: it describes unbalance, misalignment and looseness well. Bearing defects live higher, between 1 and 6 kHz, and barely move the RMS value.

VBox4AI measures RMS velocity on three axes, classifies the zone per the standard, learns the baseline by itself and sets limits the ISO way. On top, it watches the high band where bearing faults show up.

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How it works

Four steps, as in the standard.

  1. Group

    From power: Group 1 above 300 kW, Group 2 from 15 to 300 kW.

  2. Support

    Rigid or flexible picks the column in the table. A flexible support allows higher amplitudes.

  3. Zone

    The measured RMS value is compared with the A/B, B/C and C/D boundaries.

  4. Limits

    Alarm = baseline + 25% of the B/C limit, up to 1.25 × B/C. Trip no higher than 1.25 × the C/D limit.

The values used

ISO 20816-3 zone boundaries

RMS velocity in mm/s, measured on non-rotating parts of the machine.

BoundaryGroup 1RigidGroup 1FlexibleGroup 2RigidGroup 2Flexible
A / B2.33.51.42.3
B / C4.57.12.84.5
C / D7.111.04.57.1

These are general guidelines: specific machine types may have different limits, and the manufacturer's values take precedence.

FAQ

Frequently asked questions

What is the difference between ISO 10816-3 and ISO 20816-3?

ISO 20816-3 (2022) replaced ISO 10816-3. It simplified the machine groups by removing the old groups 3 and 4: pumps are now treated as normal rotating machines. The zone boundaries for groups 1 and 2 are the familiar ones.

Why shouldn't the alarm sit on the B/C boundary?

Because the standard says otherwise: the alarm is set at the healthy-machine baseline plus 25% of the upper limit of zone B, and should not exceed 1.25 times that limit. A machine running at 1.2 mm/s can double its vibration and still stay below the B/C boundary: with the alarm on the boundary, nobody notices.

Which machines does it apply to?

Industrial machines above 15 kW running between 120 and 30,000 rpm, measured on the bearing housings. Below 15 kW the calculator gives an indication using Group 2 limits, for reference only.

Where and how should I measure?

On the bearing housings, in horizontal, vertical and axial directions, as RMS velocity in the band from about 10 to 1000 Hz. Use the highest value; the dominant direction is already diagnostic information.

Does zone A mean the machine is healthy?

It means overall vibration is low. It does not see incipient bearing defects, which appear as high-frequency impacts: for those you need high-band kurtosis and envelope analysis, with a sensor that reaches that band.

This calculator applies the general criteria of ISO 20816-3 for guidance. For formal use, the text of the standard and, where available, the machine manufacturer's limits apply.

Read more: ISO 20816-3 and bearing diagnosis · White paper: ISO 10816 and ISO 20816