How to read a vibration, when that number means something

In brief

• ISO 20816-3 measures how much a machine vibrates, not what is wrong with it.
• The number is the broadband RMS velocity, between 10 and 1000 Hz, read on the bearings.
• The four zones A, B, C, D change with the machine group and with the support.
• Zone B does not mean “compliant”, and there is no ISO 20816-3 certification.

The right quantity, in the right band

On a rotating machine you can measure acceleration, velocity or displacement: the same vibration told three ways. The standard uses the broadband RMS velocity, read on the bearings or right next to them, and takes the highest value found as the severity index.

• RMS means averaged in energy.
• Broadband means that everything falling within the band ends up in the number, without distinguishing the lines of the spectrum. Out comes a single value: it says how much the machine vibrates, not what is wrong with it.
• The band is 10-1000 Hz. Below 600 rpm it starts at 2 Hz; below 120 rpm the velocity scale no longer applies. If the instrument integrates up to 3 kHz the number comes out equal or higher, because it adds energy the scale does not consider.

The four zones

The standard does not give a single limit but four zones, separated by three boundaries that change with group and support.

Zone	What it means
A	Where a newly commissioned machine normally falls
B	Compatible with unrestricted long-term operation
C	Not suitable for continuous operation: you carry on for the time it takes to organize the intervention
D	High enough to damage the machine

The second criterion, the most useful day to day. An increase or decrease in the value exceeding 25% of the B/C boundary is significant, especially if sudden, and opens an investigation. The reference is the B/C boundary, not the starting value.

Zone B does not mean “compliant”. The boundaries are not acceptance specifications, nor alarm levels: acceptance of a new machine is a matter of agreement between manufacturer and customer. The correct sentence is “falls in zone B under the criteria of ISO 20816-3”. There is no ISO 20816-3 certification.

Only two groups

Before looking at any number, assign the machine to a group. There are two: groups 3 and 4 were pumps, removed in 2009.

Group	How to recognize it	Typical construction
1	Above 300 kW rated; electrical machines with shaft height H of 315 mm and above	Sleeve bearings
2	Above 15 kW and up to 300 kW; electrical machines with H from 160 to below 315 mm	Rolling-element bearings, above 600 rpm

What counts is the rated power of the rotating machine, not the output of the plant: a 50 kW-output machine can have a motor below 15 kW, and in that case it is out of scope. On an electrical machine never use power alone: the shaft height H (ISO 496) is read from the motor designation.

Rigid or flexible support

It does not describe how massive the base is. It compares two frequencies: the one at which the machine shakes its support, normally the rotation, and the lowest natural frequency of the machine-plus-support system, in the direction you are measuring. If the second is above the first by a margin of about a quarter, the support is rigid; otherwise flexible. If it cannot be derived from drawings and calculation, it is determined by test.

Typical cases: medium and large electric motors, especially slow ones, rigid; vertical units and machines above 10 MW, flexible. The same machine can be rigid vertically and flexible horizontally: each measurement is compared with the class of its own direction.

The boundaries for group 2

Boundary	Rigid support	Flexible support
A / B	1.4 mm/s	2.3 mm/s
B / C	2.8 mm/s	4.5 mm/s
C / D	4.5 mm/s	7.1 mm/s

RMS velocities, 10-1000 Hz band, radial measurement on bearings and housings, axial on thrust bearings, at steady state. For group 1 the boundaries are higher: 4.5 mm/s is the upper limit of zone C for group 2 on a rigid support, and the B/C boundary for group 1. Same number, opposite judgment.

The full table of the standard (group 1 and displacements) is protected by copyright and is available from UNI. Before putting these values in a specification or an alarm threshold, check them against the official copy.

What this scale does not cover

The scope is precise: coupled industrial machines, above 15 kW, between 120 and 30,000 rpm, measured in situ. And there is one thing the scale never does: say what has broken.

Out of scope, and where to look instead:

• Rotodynamic pumps: ISO 10816-7.
• Rotary positive-displacement compressors and submersible motor pumps: out of scope. Reciprocating compressors: ISO 20816-8.
• Reciprocating machines: ISO 10816-6. Wind turbines: ISO 10816-21.
• Gas turbines above 3 MW: ISO 20816-4. Steam turbines and generators above 40 MW: ISO 20816-2.
• Hydraulic power and pumping plants: ISO 20816-5.
• Fans: generally only above 300 kW; for the others, agreement between the parties and ISO 14694.

The most frequent mistake: the pump. On a pump-plus-motor set with a flexible coupling the two sides are evaluated with different standards: the motor bearings with ISO 20816-3, the pump casing with ISO 10816-7. Applying the group 2 boundaries to the pump casing is incorrect, even if many do it.

For the comparison to be valid

1. Measure on the bearing or its housing. Never on fan guards, thin sheet metal or covers.
2. Two orthogonal radial directions for each bearing support; on vertical or inclined machines add the position with the highest reading.
3. Machine at steady state: rated temperature, speed, load, flow and pressure. During start-up, shutdown and resonance the boundaries do not apply.
4. Variable speed or load: measure in all the conditions in which the machine runs for long periods, and take the highest value.
5. Comparisons over time: same position, same orientation, equivalent conditions. If you suspect vibration transmitted from outside, measure with the machine stopped too.