ISO 10816 and 20816 in practice: vibration limits and what they mean

Anyone who monitors vibration on a motor, pump or fan eventually runs into the same question: how much vibration is too much? The answer industry has relied on for decades comes from ISO 10816-3, replaced in 2022 by ISO 20816-3, which kept the same values for vibration measured on the housing and brought the evaluation of rotating and non-rotating parts together in a single document.

This article explains what the standard measures, how it classifies machines, what the numbers are and, above all, what a "limit" actually means in practice. It is written for the maintenance manager who has to configure alarms and for the technician who has to justify a shutdown.

What the standard measures

The standard evaluates RMS vibration velocity in millimeters per second, measured on the bearing housing over a band of 10 Hz to 1,000 Hz (2 Hz to 1,000 Hz for machines running below 600 rpm). It is a single number per point and per direction that sums up all the vibration energy in that band. It is not the spectrum; it is the overall value.

It applies to industrial machines above 15 kW, from 120 to 30,000 rpm: electric motors, pumps with the impeller on their own shaft, fans, compressors and medium-sized turbines. Pumps with the impeller mounted directly on the motor shaft have their own standard, ISO 10816-7.

Measurements are taken on the bearings or on the housing that supports them, in two perpendicular radial directions (vertical and horizontal on horizontal machines) and also axially when there is a thrust bearing. Fan covers and thin sheet metal do not count: they vibrate on their own.

Groups and support type

The standard sorts machines by size and by support stiffness, because the same vibration level means different things in each case.

• Group 1: power above 300 kW, or electric motors with a shaft height of 315 mm or more. These usually have fluid-film (sleeve) bearings.
• Group 2: 15 kW to 300 kW, or shaft height between 160 and 315 mm. These usually have rolling-element bearings and run above 600 rpm.

A support is considered rigid when the first natural frequency of the machine-plus-foundation system, in the measured direction, is at least 25% above the main excitation frequency, which is almost always the running speed. Everything else is flexible. A medium-sized motor anchored to a concrete base is usually rigid; a machine on isolators or on a light steel frame is usually flexible. The same machine can be rigid vertically and flexible horizontally, in which case each direction is evaluated against its own class.

The four zones

• Zone A: vibration typical of a newly commissioned machine.
• Zone B: acceptable for unrestricted long-term operation.
• Zone C: unsatisfactory for long-term continuous operation. The machine can run for a limited period until there is an opportunity for remedial action.
• Zone D: severe enough to cause damage to the machine.

The numbers (ISO 20816-3:2022, Tables A.1 and A.2)

Zone boundaries in RMS velocity, mm/s:

• Group 2, rigid support: A/B 1.4 · B/C 2.8 · C/D 4.5
• Group 2, flexible support: A/B 2.3 · B/C 4.5 · C/D 7.1
• Group 1, rigid support: A/B 2.3 · B/C 4.5 · C/D 7.1
• Group 1, flexible support: A/B 3.5 · B/C 7.1 · C/D 11.0

Notice that the boundaries rise as the machine gets larger and the support gets softer. A 75 kW motor on a rigid base enters zone C at 2.8 mm/s; a 500 kW machine on springs only enters zone C at 7.1 mm/s. Apply the wrong limit and you get either false alarms every day or silence until the breakdown.

What a "limit" means

The standard itself makes the point: the zone values are not acceptance criteria. Acceptance of a new machine is an agreement between manufacturer and buyer, and historically it falls in zone A or B, never exceeding 1.25 times the A/B boundary.

For monitoring, the standard defines two levels of action:

• Alarm: a warning that a given value has been reached or that a significant change has occurred. The recommendation is to set the alarm at the machine's baseline plus 25% of the B/C boundary, without exceeding 1.25 times that boundary. A machine that has always run at 1.0 mm/s can have its alarm set below 2.8.
• Trip: the value above which continued operation causes damage. It sits in zone C or D, never above 1.25 times the C/D boundary.

Why the trend matters more than the absolute value

The standard has a second criterion, and it is the most ignored one. Criterion II deals with change in vibration relative to an established reference, under the same operating conditions and at the same point. When the increase or decrease exceeds 25% of the B/C boundary, the change is considered significant, even if the machine is still in zone B, and an investigation should be opened. For a Group 2 motor on a rigid support, that is a change of 0.7 mm/s.

The reason is simple: a machine that went from 1.2 to 2.0 mm/s in three weeks is telling a story, and the story matters more than the fact that 2.0 is still "acceptable." A wearing bearing, misalignment that crept in after a replacement, a support that worked loose: all of them show up first as a trend, not as a limit being exceeded. The standard also notes that sudden changes deserve more weight than gradual ones. With one reading a month, the trend has three points per quarter; with continuous readings, the slope shows up within days.

How we configure it in practice

On our platform, each sensor is assigned the machine class (group and support type) and the thresholds come preset: entering zone C triggers an alert, entering zone D triggers an alarm, and only a change of level sends a notification. The manager can override the limits per sensor, which is the right approach whenever there is a known baseline. The trend is calculated over the history and projects, with a stated confidence level, when zone D would be reached.

What to do now

1. Record the power, shaft height and support type of every critical machine. Without them, there is no way to pick the right table.
2. Record the baseline of each point with the machine in a known, good condition. That is what gives Criterion II its meaning.
3. Set alarms from the baseline, not just from the table.
4. Look at the trend every week. A stable machine in zone B means peace of mind; a machine climbing within zone B is something to act on.

If you want to know what your plant needs, talk to one of our engineers.

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Sources: ISO 20816-3:2022, Mechanical vibration — Evaluation of machine vibration by measurements on rotating and non-rotating parts — Part 3 (Tables A.1 and A.2, clauses 5.2, 5.3, 6.2, 6.3 and 6.5); ISO 10816-3:2009 (superseded by the former); ISO 10816-7:2009 for rotodynamic pumps.