Vibration Analysis for Beginners: What the ISO 10816 Limits Actually Mean
The short answer
ISO 10816 and its successor ISO 20816 grade a machine's overall vibration velocity, measured in mm/s RMS, into four zones. Zone A is a newly commissioned machine, zone B is acceptable for unrestricted long-term running, zone C is unsatisfactory for long-term running and should be planned for repair, and zone D is severe enough to cause damage. The boundary values change with machine size and mounting, which is why a number with no class attached tells you nothing.
The numbers people quote
For the common case of medium machines, roughly 15 kW to 300 kW on rigid foundations, class II:
| Zone | Velocity, mm/s RMS | What it means |
|---|---|---|
| A | up to 1.4 | Typical of a new machine |
| B | 1.4 to 2.8 | Acceptable for unrestricted long-term operation |
| C | 2.8 to 7.1 | Unsatisfactory for long-term operation. Plan a repair |
| D | above 7.1 | Severe. Damage is likely |
For small machines up to 15 kW, class I, the boundaries are lower: 0.71, 1.8 and 4.5. For large machines on flexible foundations, class IV, they are higher: 2.8, 7.1 and 11.0.
Two consequences follow, and both are missed constantly. A 3 mm/s reading is a planned repair on a 30 kW pump and unremarkable on a large flexible-mounted machine. And a machine that has moved from 0.8 to 2.4 mm/s is in trouble even though it is still inside zone B, because the trend matters more than the band.
Measure the same way every time
An overall velocity number is only comparable with itself. Fix the measurement point, the direction, the mounting method and the machine's operating condition, and record them. A magnet-mounted accelerometer and a hand-held probe on the same bearing will not agree, and neither will readings at 40 and 90 per cent load.
Take horizontal, vertical and axial at each bearing. The relationship between the three is diagnostic on its own: high axial usually means misalignment or a bent shaft, high horizontal relative to vertical often means looseness or a resonance.
Four patterns worth knowing
Unbalance. A dominant peak at 1x running speed, mostly radial, roughly equal horizontally and vertically, and it rises with the square of speed. The most common finding and the easiest to correct.
Misalignment. 1x and a strong 2x, with meaningful axial vibration. If the axial reading is more than about half the radial, look at alignment before anything else.
Looseness. A run of harmonics, 1x, 2x, 3x, 4x and beyond, often with raised noise between them. Frequently found after a repair that fixed a symptom without tightening what caused it.
Rolling element bearing wear. Energy at the bearing defect frequencies, which are not integer multiples of running speed, and a rising high-frequency noise floor long before the overall velocity moves. This is why overall velocity alone misses early bearing faults, and why envelope or demodulated measurements exist.
Where this fits in maintenance
Vibration is one input. Thermography finds electrical and mechanical heat, oil analysis finds wear metals and contamination, and motor current signature analysis finds rotor bar and eccentricity faults. A programme that uses one technique finds one class of fault.
The vibration analysis course covers measurement, spectra and the fault patterns; the machinery fault diagnosis course puts them together with the other techniques; and infrared thermography covers the electrical half.
Frequently asked questions
Is ISO 10816 replaced by ISO 20816? ISO 20816 is the current series and consolidates the older parts. The zone concept and the practical limits are the same, and most Indian plants still quote 10816 in their standards. Quote whichever your site standard names.
Velocity, displacement or acceleration? Velocity for general machine condition between roughly 10 Hz and 1 kHz, which is what these limits use. Displacement for low speed and shaft-relative measurements, acceleration for bearing and gear faults at high frequency.
Can I do this with a phone app? For a rough trend on a badly behaving machine, sometimes. For a number you will act on, no: the sensor mounting and the frequency response are the measurement, and a phone controls neither.
