Condition monitoring
Vibration monitoring explained: RMS, ISO 10816 and the faults behind the numbers
A practical vibration monitoring guide: RMS velocity vs acceleration, ISO 10816-3 severity zones, sensor placement and how to recognise common machine faults.
Why vibration is the best early indicator
Every rotating machine vibrates. As components wear, loosen or move out of alignment, the forces inside the machine change, and so does the vibration. Those changes usually show up well before a rise in temperature, an audible noise or a drop in performance. That's why vibration monitoring is the foundation of predictive maintenance for motors, pumps, fans, compressors and gearboxes.
Velocity, acceleration and displacement
The same vibration can be described three ways. Each one highlights a different part of the frequency range:
| Measure | Typical unit | Most sensitive to | Use it for |
|---|---|---|---|
| Displacement | µm, mm | Low frequencies | Slow-speed machines, shaft movement in sleeve bearings |
| Velocity | mm/s RMS, in/s | Mid frequencies (roughly 10–1000 Hz) | Overall machine severity: imbalance, misalignment, looseness |
| Acceleration | g | High frequencies | Bearing defects, gear mesh, lubrication problems |
RMS (root mean square) is the effective energy of the signal and is the standard basis for severity limits. Peak and peak-to-peak values capture short impacts that RMS can smooth out.
ISO 10816-3 severity zones
ISO 10816-3 gives broadband RMS velocity limits for industrial machines above 15 kW, measured on bearing housings. It has been revised as ISO 20816-3, but the zone concept and many of the limits are widely used. Machines are grouped by size and by how stiffly they are mounted:
- Zone A: newly commissioned machines
- Zone B: acceptable for unrestricted long-term operation
- Zone C: unsatisfactory for long-term operation. Plan corrective action.
- Zone D: severe enough to cause damage
| Machine group | Foundation | A / B | B / C | C / D |
|---|---|---|---|---|
| Group 2: 15–300 kW | Rigid | 1.4 mm/s | 2.8 mm/s | 4.5 mm/s |
| Group 2: 15–300 kW | Flexible | 2.3 mm/s | 4.5 mm/s | 7.1 mm/s |
| Group 1: 300 kW–50 MW | Rigid | 2.3 mm/s | 4.5 mm/s | 7.1 mm/s |
| Group 1: 300 kW–50 MW | Flexible | 3.5 mm/s | 7.1 mm/s | 11.0 mm/s |
Always confirm limits against the current standard and the OEM's guidance for your machine.
Fixed zones are a good starting point, but they don't tell you about change. A machine that has run at 1.2 mm/s for two years and is now at 2.4 mm/s is still in zone B, yet its vibration has doubled. That's why AnalytIQ pairs ISO zones with anomaly detection against each machine's own baseline.
Crest factor, kurtosis and high-frequency energy
Early bearing damage produces short impacts that barely move overall RMS. Three indicators help catch them:
- Crest factor = peak ÷ RMS. It rises when sharp impacts appear on a smooth signal.
- Kurtosis measures how "spiky" the signal is. A healthy machine sits near 3. Higher values suggest impacting.
- High-frequency acceleration picks up the energy from rolling-element defects and poor lubrication, often weeks before velocity rises.
As damage spreads, crest factor and kurtosis can actually fall again while RMS rises. That's why trends matter more than any single reading.
Recognising common faults
| Fault | Typical signs |
|---|---|
| Imbalance | High radial vibration at 1× running speed, similar on both bearings |
| Misalignment | High 1× and 2× with a strong axial component, heat at the coupling |
| Mechanical looseness | Many harmonics of running speed, erratic readings, direction-dependent |
| Bearing defects | Rising high-frequency acceleration, crest factor and kurtosis, later temperature |
| Lubrication problems | Raised high-frequency noise floor that drops after re-greasing |
| Pump cavitation | Random broadband high-frequency vibration, noise like gravel |
Sensor placement that gives reliable data
- Mount on the bearing housing, as close to the load zone as possible, not on covers or guards.
- On larger machines, measure both the drive end (DE) and non-drive end (NDE). Comparing the two helps locate the fault.
- Use a stud or a flat, clean surface for magnetic mounts. Inconsistent mounting is the most common cause of bad trends.
- Keep the orientation fixed and documented, so X, Y and Z mean the same thing every time.
- Record the running state. Readings taken while the machine is stopped or idling shouldn't count toward its health.