Condition monitoring
Spectrum reading cheat sheet: sensors, resonance and fault frequencies
Start from turning speed, sort peaks into sub-synchronous, synchronous and non-synchronous, then match patterns. A one-page guide.
Order of work
Walk the machine, then record turning speed at the measurement point before any reading. List harmonics of turning speed. Other peaks of interest and their harmonic relations come after that.
| Sensor | Best at | Typical range |
|---|---|---|
| Displacement | Low frequency and shaft movement | Low frequency to several hundred Hz |
| Velocity | Low and high data equally; the common choice | About 10 Hz to 1 kHz |
| Acceleration | High frequency, rolling bearings and gearboxes | Up to tens of kHz depending on mounting |
| Where the peak sits | What it usually is |
|---|---|
| Below 1x (sub-synchronous) | Belts, oil whirl, rubs, loose bearing in housing, cage frequency of a bearing |
| At 1x and exact multiples (synchronous) | Unbalance, misalignment, looseness, bent shaft, blade or vane pass, gear mesh |
| Other non-integer (non-synchronous) | Rolling element defects, electrical, other machines, resonance, belts, cavitation, pipe noise |
| Fault | Clue |
|---|---|
| Unbalance | Only 1x; amplitude rises with speed squared; little axial |
| Offset misalignment | High 2x, radial |
| Angular misalignment | High 1x, often axial |
| Inner race defect | BPFI with running-speed sidebands |
| Outer race defect | BPFO with little or no sideband; rough guide: roller count times running speed times 0.4 |
| Rolling element defect | Peaks around 2 x BSF with cage-frequency sidebands |
| Gear mesh | GMF = running speed in Hz times number of teeth; sidebands at turning speed that grow as wear develops |
| Belt | 2x belt frequency dominates; need at least six revolutions of data |
Resonance
Every structure has natural frequencies. When a forcing frequency is near one, vibration is amplified. A rotating speed that excites it is a critical speed. Find it with an impact (bump) test with the machine off. Light-weight designs and variable speed drives make resonance more common.
Spectrum, waveform and trend
The spectrum shows frequencies; the waveform shows impacting and modulation; the trend shows growth. Use all three.
Takeaway. Sort peaks by their relation to turning speed first. That one habit removes half the guesses.
Put it to work in the tool