Vibration analysis (ISO 10816/20816) — predictive maintenance for motor and pump bearings

The guide on thermal imaging cameras covers one predictive-maintenance technique for electrical connections. Rotating machinery — motors, pumps, fans — has its own dedicated predictive-maintenance technique: vibration analysis, standardised in the ISO 10816 / ISO 20816 series. This article covers how the severity zones of that standard work, and why the specific bearing condition behind a rising vibration reading requires more than just a single overall number.

What is actually measured

A vibration sensor (typically an accelerometer, mounted on or near the bearing housing) measures mechanical vibration and — for the general severity assessment in ISO 10816/20816 — the signal is usually expressed as RMS vibration velocity in mm/s, over a defined frequency band (commonly roughly 10–1000 Hz). Velocity is used rather than raw acceleration or displacement because it correlates well, across a wide range of machine sizes and speeds, with the fatigue-relevant mechanical stress in the machine.

ISO 10816 → ISO 20816: same concept, renumbered standard

ISO 10816 (general guidelines plus machine-specific parts) has been progressively superseded since around 2016–2017 by the ISO 20816 series, without changing the underlying concept: overall vibration severity is assessed against zone boundaries that depend on the machine class (broadly by power rating and rigidity of the foundation) rather than a single universal number for all machines.

The four severity zones

Zone	Meaning
A	Vibration typical of a newly commissioned machine — the reference condition.
B	Acceptable for unrestricted long-term operation.
C	Not suitable for continuous long-term operation — the machine may run for a limited period until a convenient repair opportunity, but the condition should be investigated.
D	Vibration severity of a magnitude normally considered sufficient to cause damage to the machine.

Note: the specific mm/s RMS boundaries between these zones differ per machine class (small machines, medium machines on a rigid foundation, large machines, machines on a flexible/soft foundation) — this article covers the concept, not a single number to apply to every machine; the applicable machine-specific part of ISO 20816 (or the manufacturer's own limits) gives the correct boundary for a specific installation.

Absolute level versus trend: why a snapshot reading is not enough

A single vibration reading, compared only against the zone table, tells you where the machine stands today relative to a newly commissioned machine — but it says little about how fast the condition is changing. The same practical principle used for leakage-current trending and thermographic delta-T assessment applies equally to vibration: a machine that has run stably in Zone B for years and suddenly starts trending upward is a stronger and earlier warning sign than an absolute Zone-B/C boundary crossing on its own. A documented baseline measurement, taken shortly after commissioning while the machine is known to be healthy, is what makes a later trend meaningful.

Diagnosing the cause: characteristic bearing fault frequencies

An overall vibration velocity number flags that something has changed, but not automatically what. A frequency-domain analysis (FFT) of the same vibration signal distinguishes between fundamentally different root causes:

1× running speed — dominant peak typically indicates rotor unbalance.
2× running speed — often indicates misalignment (in combination with 1×).
Characteristic bearing defect frequencies — ball-pass frequency outer race (BPFO), ball-pass frequency inner race (BPFI), ball spin frequency (BSF) and fundamental train frequency (FTF), each calculated from the bearing's own geometry and shaft speed — a peak at one of these specific frequencies points to a defect on that specific bearing component (outer race, inner race, rolling element, or cage), well before the fault grows large enough to raise the overall broadband vibration level.

Relationship to electrical bearing damage (a different failure mode)

Bearing damage is not always mechanical in origin. The [guide on VFD bearing currents](/guides/nen-3140/lagerstromen-frequentieomvormer-geisoleerde-lagers-aardingsborstel) covers electrical discharge machining (EDM) pitting of bearing races caused by common-mode voltage from a variable-frequency drive — a distinct failure mechanism, detected by a different technique (shaft voltage or bearing current measurement, not vibration analysis). By the time EDM bearing fluting shows up clearly in the vibration spectrum, the electrical root cause has typically already been active for some time; the two techniques are complementary, not interchangeable.

Practical relevance

For a predictive-maintenance programme on critical rotating equipment (pumps, fans, motors), a baseline vibration measurement at commissioning, a periodic trend measurement against the machine's own baseline, and an FFT-based diagnosis once the trend crosses into Zone C give a substantially earlier and more specific warning than waiting for an absolute vibration alarm alone — analogous to how a delta-T trend gives an earlier warning in thermographic inspection than a single absolute temperature reading.

Common mistakes

Judging condition from a single absolute reading without a baseline or trend — a machine can already be degrading steadily while still sitting inside Zone B on any individual measurement.
Mounting the accelerometer inconsistently between measurements (magnetic base at a different angle or location, loose contact) — this changes the high-frequency response of the measurement and invalidates a trend comparison.
Applying the zone boundaries for a rigidly mounted machine class to a machine on a flexible/soft foundation — the correct boundary for that class is measurably different, and using the wrong table produces an unjustified pass or an unjustified alarm.
Confusing 1×/2× running-speed vibration (unbalance, misalignment) with a bearing-specific defect frequency — these point to entirely different corrective actions (balancing/alignment versus bearing replacement) and misdiagnosis leads to the wrong repair.