How to read a vibration, when that number means something

The right quantity, and the right band

On a rotating machine you can measure acceleration, velocity, or displacement: it’s the same vibration told three ways. Acceleration weights the high frequencies, displacement the low ones. Velocity sits in the middle: over a wide span of frequencies, a constant level corresponds to comparable stress. The standard’s criterion is this: you read on the bearings, or right next to them, and take as the severity index the highest of the broadband RMS velocities recorded. That velocity is the quantity of choice is established engineering practice before it is a prescription. RMS means energy-averaged. Broadband means that everything falling inside the band ends up in the number, without distinguishing the lines of the spectrum. What comes out is a single value: it tells you how much the machine vibrates, not what it has.

Broadband does not mean the whole band. The value is calculated between 10 Hz and 1000 Hz. Below 600 rpm the band starts at 2 Hz, otherwise the first rotational harmonic falls outside the count. Below 120 rpm the velocity scale no longer applies. If the instrument integrates up to 3 kHz instead of up to 1000 Hz, the mm/s figure comes out equal or higher: you are adding energy the scale does not consider. With an accelerometer on a non-rotating part, the signal must be integrated to velocity, and the instrument’s response must be flat at least across that band.

The four zones, and why the boundaries aren’t the same for everyone

The standard doesn’t give a single limit. It gives four evaluation zones, A, B, C, and D, separated by three boundaries. The boundary values change with the machine’s group and with the type of support. The descriptions below are restated in our own words: the full text is in the standard, available from UNI, the Italian standards body.

Zone A. The range where a newly commissioned machine typically falls.

Zone B. Compatible with long-term operation, without restrictions on use.

Zone C. Not suitable for continuous long-term operation: you carry on for the time it takes to organize the intervention.

Zone D. A level high enough to potentially damage the machine.

There is a second criterion, and in day-to-day work it’s the more useful one. An increase or a decrease in the broadband value exceeding 25% of the B/C boundary is to be considered significant, especially if sudden, and opens an investigation. Watch the reference: it’s 25% of the B/C boundary, not of the starting value. The standard gives the 25% as a guideline: on a machine you know well, experience may suggest a different value.

Zone B does not mean “compliant.” The zone boundaries are not acceptance specifications, and the standard says so explicitly: acceptance of a new machine is always a matter of agreement between manufacturer and customer. Nor are they the alarm and trip levels, which the standard treats separately. The correct sentence is “it falls in zone B according to the criteria of ISO 20816-3.” There is no such thing as ISO 20816-3 certification: it is an evaluation standard, not a certification scheme.

Only two groups, and how a machine ends up in one

Before looking at any number you must assign the machine to a group. There are two. If you’ve seen four somewhere, that was the 1998 edition: groups 3 and 4 were the pumps, removed in 2009.

Group	How to recognize it	Typical construction
Group 1	Above 300 kW nominal. Electrical machines with shaft height H of 315 mm and above	Typically sleeve bearings
Group 2	Above 15 kW and up to and including 300 kW. Electrical machines with H from 160 to below 315 mm	Typically rolling-element bearings, above 600 rpm

The shaft height H is the distance between the rotation axis and the mounting plane, per ISO 496 as referenced by the standard, and on a standardized motor you read it from the designation. On an electrical machine, never use power alone as the criterion. What counts is the nominal power of the rotating machine, not the thermal or process output delivered by the plant. A machine with 50 kW of output can have a motor well below 15 kW, and in that case it is out of scope.

Rigid or flexible support, not decided by eye

Rigid and flexible do not describe how massive the foundation is. They compare two frequencies: the one at which the machine shakes its own support, normally the rotational frequency, and the lowest natural frequency of the machine-plus-support system, in the direction you are measuring. If the second sits above the first with a margin on the order of a quarter, in that direction the support is rigid. Otherwise flexible.

If the class cannot be derived from drawings and calculation, it is determined by testing. Typical cases indicated by the standard: medium and large electric motors, especially slow ones, rigid; vertical units and machines above 10 MW, flexible.

The same machine, two different classes. A support can be rigid vertically and flexible horizontally: it’s common, not an exception. Each measurement is compared against the class of its own direction. A single column for all directions leads to misclassification.

The zone boundaries for group 2, the medium machines

Zone 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 values, band from 10 Hz to 1000 Hz, measured radially on bearings, pedestals, and housings, axially 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 a group 2 machine on a rigid support, and it is the boundary between B and C for a group 1 machine in the same condition. Same number, opposite verdict.

Honesty about the numbers. Here you have the velocity boundaries for group 2 only. The standard’s table is broader, covering group 1 and the displacement values, and is not reproduced because it is protected by copyright: it is consulted at UNI. If these values are going into a specification or an alarm threshold, check them against the official copy and verify the edition.

What this scale does not cover

The scope is precise: coupled industrial machines, above 15 kW nominal, between 120 and 30,000 rpm, measured in situ. Inside it are, among others, steam turbines and generators up to 40 MW, rotary dynamic compressors, and electric motors with a flexible coupling. Plenty of what you find on a shop floor stays outside. And there is one thing this scale never does: tell you what has broken.

The machines left out, and where to look instead.

• Rotodynamic pumps: ISO 10816-7:2009, above 1 kW. They left this part of the standard in 2009.
• Rotary positive-displacement compressors, screw compressors for example, and submersible motor pumps: out of scope. Reciprocating compressors: ISO 20816-8.
• Reciprocating machines, and machines rigidly coupled to a reciprocating one: ISO 10816-6. Wind turbines: ISO 10816-21.
• Gas turbines above 3 MW: ISO 20816-4. Steam turbines and generators above 40 MW at 1500, 1800, 3000, or 3600 rpm: ISO 20816-2; at other speeds they stay here.
• Hydroelectric and pumped-storage plants: ISO 20816-5.
• Fans: criteria generally applicable only above 300 kW, or if not on flexible supports. For the others, the manufacturer-customer agreement and ISO 14694 apply.

The case that gets mistaken most often: the pump. On a pump unit with a separate motor and a flexible coupling, the two sides are evaluated under different standards: the motor’s bearings under ISO 20816-3, the pump casing under ISO 10816-7, which has its own thresholds. Applying the group 2 boundaries to the pump casing is technically incorrect, even though many do it.

For the comparison with the scale to be valid

To check before passing judgment.

1. Measure on the bearing, on the bearing housing, or on a structural part that responds to the dynamic forces. Never on fan cowls, thin sheet metal, or covers.
2. Two orthogonal radial directions for each bearing support. A single transducer is allowed with caution: it may not capture the maximum in that plane. On vertical or inclined machines, add the position with the highest reading.
3. Machine at steady state: rotor and bearings at operating temperature, nominal speed, voltage, load, flow, and pressure. During run-up, coast-down, and passage through a resonance these boundaries do not apply.
4. Variable speed or load: measure in every condition the machine stays in for extended periods, and take the highest value.
5. Comparisons over time: same position, same orientation, equivalent conditions. If you suspect vibration transmitted from outside, also measure with the machine stopped: act on the environment when that reading exceeds the lower of 25% of the value measured in operation and 25% of the B/C boundary.