Judging vibration levels: standards, alarm bands and trends
A reading means little until it is compared with a limit and with its own history. Standards give the first, a trend gives the second, and the trend matters more.
Two kinds of measurement, two ways to judge
| Machine | Measured by | Judged on |
|---|---|---|
| Fluid-film bearings, heavy casing, large relative motion | Proximity probes on the shaft | Shaft vibration, usually peak to peak |
| Rolling bearings and general machines | Casing or bearing-cap sensors | Casing vibration, usually RMS velocity |
The four bands
International standards for machine vibration group the overall level into bands: new or recommissioned machines, acceptable for long-term running, acceptable only for a limited period (surveillance), and likely to cause damage (shutdown). The limits depend on the machine class, its power and speed, and how it is mounted. Use the standard that applies and the machine maker's own limits where they are stricter.
Peak, peak-to-peak or RMS
Peak-to-peak is the full swing of the signal, peak is half of that, and RMS is the effective energy of the signal. For a clean sine wave, peak is 1.414 times RMS and peak-to-peak is 2.83 times RMS. Always check which one a limit or a reading refers to before comparing them.
Trend beats absolute value
- Take a baseline when the machine is known to be healthy and loaded normally.
- Compare each reading with the baseline and with the previous reading.
- A steady rise over weeks is often more important than a single reading above a limit.
- Set alarms on the change as well as on the level.
- A sudden drop can also be a warning: a sensor fault, or a part that has stopped moving.
Make the alarm useful
An alarm that rings every week is ignored. Set the alert level at a margin above the normal range for that machine, the alarm higher, and review them after every change. Keep the speed and load with every reading, because both change the vibration.