How Bad Is 4.5 mm/s? Reading Vibration Severity Without Guessing

A maintenance fitter reads 4.5 mm/s RMS off a bearing housing and asks the only question that matters: is that bad? The honest answer is that the number alone cannot say. On a 22 kW pump bolted to a stiff concrete plinth, 4.5 mm/s sits right at the edge of the zone where long-term damage becomes likely. On a large turbine-generator set on a flexible foundation, the same 4.5 mm/s is comfortably inside normal long-term operation. Same reading, opposite verdicts.

That is not a flaw in the standard — it is the whole point of it. ISO 20816 (the successor to ISO 10816) does not publish one limit; it publishes a scheme of severity zones whose boundaries depend on the machine group — broadly its size and type — and on whether its support is rigid or flexible. Reading severity without guessing means knowing which column of the table your machine lives in before you look up the row.

The zone scheme is deliberately plain. Zone A is the vibration level typical of a newly commissioned machine — what the machine should read on day one. Zone B is acceptable for unrestricted long-term operation: nothing to fix, keep trending. Zone C is unsatisfactory for continuous long-term running — the machine may be operated for a limited period, but you should be planning the remedial action. Zone D is severe enough that damage is likely: vibration at this level is normally considered sufficient cause to act now.

Two details keep people honest. First, the value being judged is the overall broadband velocity, in mm/s RMS, typically evaluated over roughly 10 to 1,000 Hz and measured on non-rotating parts such as bearing housings — not a single spectral peak, not a peak or peak-to-peak value, and not a reading from a hand on the casing. Second, the boundaries are guidance for machines in general, agreed by committee across thousands of machine types. Your OEM's limits, your contract acceptance levels, and the applicable edition of the standard itself always take precedence over any summary table — including the ones in this article.

Pick a machine group and a support type below, then drag the velocity slider — or tap directly on the chart. The bands redraw as you switch class and mounting, and the verdict updates in the words the zones actually use. Watch what happens to 4.5 mm/s as you flip from a medium rigid machine to a large flexible one: the reading does not change, but its meaning does.

One rule before you use it on a real machine: the boundary values in this checker are indicative. They follow the widely published band structure associated with ISO 20816-3 for these machine groups, but they are not a reproduction of the standard, and machine groups beyond the two shown here have their own tables. Confirm the class, the support type, the evaluation band and the exact boundaries against the applicable edition of ISO 20816 and your OEM documentation before you base a maintenance decision on them.

The physics behind the shifting boundaries is not mysterious. Larger machines have heavier rotors, larger bearing clearances, and casings whose stiffness relative to the rotating forces is proportionally lower — so a given housing velocity represents a less alarming internal state than the same velocity on a small, tight machine. The zone tables encode decades of fleet experience about what level of housing vibration each class of machine sustains without distress.

Mounting matters for a similar reason. A rigid support — a machine bolted to a massive foundation whose natural frequency sits well above running speed — transmits shaft forces into the housing with little amplification, and small housing motions can reflect meaningful internal loads. A flexible support lets the whole machine move more freely, so higher housing velocities are normal and tolerable. The support classification is a property of the installed system, not the machine alone: the same motor reads differently on a plinth than on isolation springs, and the standard classifies it differently too.

Severity zones tell you how urgent a machine is; they say nothing about what is wrong with it. Zone C could be unbalance, misalignment, looseness, a resonance sitting near running speed, or a bearing well into its final stages — five faults with five different remedies, all hiding behind one amber number. The zone is the trigger to look; the spectrum, the phase readings and the time waveform are how you find out what you are looking at. Treat the band chart as the triage nurse, not the surgeon.

And on any machine you already own, your own trend beats the table. A representative example, not a specific customer: a 75 kW pump has read a steady 3.1 mm/s since commissioning — Zone C on paper for its class — while the motor beside it has climbed from 0.9 to 2.2 mm/s over three months and still sits comfortably in Zone B. The table says worry about the pump; the trend says investigate the motor first, because a machine that has tripled its own normal is telling you something a static boundary never can. The zones are the backstop for machines with no history; the trend is the early warning for machines with one. This is also why the most valuable measurement you will ever take is the boring baseline on day one.

A severity verdict is only as good as the reading underneath it. That means overall velocity in mm/s RMS over the evaluation band, from a properly mounted accelerometer on the bearing housing — same point, same mounting, same sample rate and averaging every visit, with a measurement chain you can trace back to a calibration certificate. Change the mounting or the band and you have not measured a different machine state; you have measured a different number, and the zone chart will happily mislead you with it.

If severity charts are new to you, the free TIERA 101 primers at 101.tieraonline.in cover this ground properly — Vibration 101 walks through amplitude measures, the displacement–velocity–acceleration triad and reading a machine against ISO 20816, and Measurement Setup 101 covers mounting, settings and repeatable routes. They are free primers, not accredited ISO certification. When you want formal, examined credentials, the TCAT analyst programme on our services page (/services) runs proctored examinations at exams.tieraonline.in. And whatever you read here: the checker above is indicative — the applicable edition of ISO 20816 and your OEM's limits govern, so confirm against the standard itself before you act on a boundary.

Reading a zone chart takes a minute; owning a trustworthy trend takes a measurement chain. A PhonoVibe DAQ with a TIERA accelerometer gives you a calibrated, factory-certificated chain in a carry bag, and TVIB computes overall velocity in mm/s RMS over your chosen band, alongside the spectrum you will need the moment a reading crosses into amber.

TVIB's band cursors and alarm bands let you set limits from your own baseline rather than a generic table — the day-one Zone A reading becomes the reference, and rising energy in the bands you care about becomes the early warning. If you want help choosing sensible bands and levels for your fleet, talk to us; we set up measurement programmes, we do not certify ISO compliance.