Start from failure modes
A failure mode is the specific way an asset fails: an outer race spall, a broken rotor bar, a loose connection, cavitation. A failure mode and effects analysis (FMEA, described in IEC 60812) lists them per asset class, with their causes, effects and how they can be detected. For PdM, three questions per failure mode are enough to start:
- How often does it happen, and what does it cost?
- Does it give a warning, and how long is the P-F interval?
- Which measurement detects it earliest and most reliably?
The main techniques
| Technique | What it measures | Detects well | Limitations |
|---|---|---|---|
| Vibration | Acceleration, velocity or displacement of bearing housings or shafts | Unbalance, misalignment, looseness, bent shaft, bearing and gear defects, resonance, rubs, belt problems | Needs correct sensor, mounting and settings; very slow speeds are hard; electrical faults only indirectly |
| Ultrasound | High-frequency sound and stress waves (typically 20–100 kHz, often around 40 kHz) | Lubrication starvation and over-greasing, early bearing distress, slow-speed bearings, valve passing, compressed air and gas leaks, steam traps, electrical arcing, tracking and corona | Little diagnostic detail for gear or alignment faults; directional and sensitive to background noise |
| Motor current and voltage signature analysis (MCSA, ESA) | Supply current (and voltage) spectra, usually from current transformers in the motor control centre | Broken rotor bars, static and dynamic eccentricity, supply unbalance and harmonics, stator degradation (with voltage), load-side effects such as belt, coupling and pump problems | Needs stable load (rotor faults usually above about half load); VFD supplies complicate analysis; mechanical faults less clear than with vibration |
| Oil and wear-debris analysis | Wear metals, particle counts, water, viscosity, oxidation, additives; online sensors or lab samples | Gear and bearing wear in lubricated systems, contamination, wrong or degraded oil, water ingress | Only for oil-lubricated assets; sampling point and practice affect results; lab turnaround |
| Infrared thermography | Surface temperature images | Loose or corroded electrical connections, overloaded circuits, failing insulation, refractory damage, blocked coolers, misaligned couplings, steam traps | Needs line of sight and load; emissivity and reflections; usually late for bearings |
| Temperature | Contact temperature on bearings or windings | Overheating, lubrication failure in late stages | Late indicator; useful as a safety net, poor for planning |
| Process and performance data | Flow, pressure, power, speed, efficiency from existing control systems | Pump and compressor wear, fouling, efficiency loss, operating off the best efficiency point | Needs good instrumentation and context; ambiguous on cause |
Failure mode to technique matrix
Use this as a starting point. ● = primary (detects early and reliably), ○ = supporting (confirms or detects later).
| Failure mode | Vibration | Ultrasound | MCSA / ESA | Oil | Thermal | Temp |
|---|---|---|---|---|---|---|
| Unbalance | ● | ○ | ||||
| Misalignment | ● | ○ | ○ | ○ | ||
| Mechanical looseness, soft foot | ● | ○ | ||||
| Rolling bearing defect (standard speed) | ● | ● | ○ | ○ | ○ | ○ |
| Rolling bearing defect (slow speed) | ○ | ● | ○ | ○ | ||
| Lubrication starvation or over-greasing | ○ | ● | ○ | ○ | ○ | |
| Sleeve bearing wear, oil whirl | ● | ● | ○ | |||
| Gear wear, tooth damage | ● | ○ | ○ | ● | ||
| Broken rotor bars | ○ | ● | ○ | |||
| Air-gap eccentricity | ○ | ● | ||||
| Stator winding degradation | ● (ESA) | ○ | ○ | |||
| Loose or corroded electrical connection | ○ | ○ | ● | |||
| Arcing, tracking, corona | ● | ○ | ||||
| Cavitation, flow problems | ● | ● | ○ | |||
| Belt wear, sheave misalignment | ● | ○ | ○ | ○ | ||
| Lubricant contamination, water | ● | |||||
| Resonance, structural weakness | ● |
By asset type
| Asset | Dominant failure modes | Recommended techniques |
|---|---|---|
| Induction motor (DOL) | Bearings, unbalance, misalignment, rotor bars, eccentricity, stator, connections | Vibration + temperature; MCSA/ESA for medium and large motors; thermography of terminals and supply |
| Motor on variable speed drive | As above, plus bearing electrical erosion (fluting) and resonance at certain speeds | Vibration with speed-aware data collection; ultrasound for bearing currents; drive diagnostics; MCSA with caution |
| Centrifugal pump | Bearings, seal, cavitation, impeller wear, misalignment, running off best efficiency point | Vibration (include vane pass); process data (flow, head, power); ultrasound for cavitation and seals |
| Positive displacement pump, reciprocating compressor | Valves, bearings, crosshead, pulsation | Vibration with time-waveform and phase-referenced analysis; ultrasound for valves; pressure and temperature |
| Fan or blower | Unbalance (build-up), looseness, bearings, belts, resonance | Vibration (include blade pass); temperature |
| Gearbox | Gear wear, tooth cracks, bearings, lubrication | Vibration with high Fmax (≥ 3× gear mesh) and envelope; oil analysis; temperature |
| Screw or centrifugal compressor | Bearings, rotor contact, gear mesh (centrifugal), surge | Vibration (often wired protection on large units); oil analysis; process performance |
| Conveyor drive and pulleys | Gearbox, pulley bearings (slow), belt, idlers | Vibration on drive; ultrasound and thermography for slow pulley bearings and idlers |
| Cooling tower fan | Gearbox, long drive shaft, unbalance; wet and hard to access | Wireless vibration on gearbox and motor; oil sensor on gearbox |
| Kiln, dryer, mill, crusher (slow, heavy) | Support roller and trunnion bearings, girth gear, pinion, shell, refractory | Low-frequency vibration, ultrasound, oil analysis; thermal scanning of shell; process loads |
| Turbine and generator | Rotor dynamics, sleeve bearings, rubs, blade issues, electrical faults | Wired proximity probes and protection system; oil analysis; process performance; electrical tests |
| Switchgear, transformers, MCCs | Connections, insulation, overheating | Thermography (online where critical); ultrasound for partial discharge; oil analysis for oil-filled transformers |
By speed class
Shaft speed changes everything about vibration monitoring, because bearing and gear defects produce impacts at a rate proportional to speed, and the energy of those impacts falls sharply at low speed.
| Speed class | Range | What changes | Good practice |
|---|---|---|---|
| Very slow | below 150 rpm | Defect frequencies of a few hertz or less; tiny vibration amplitudes; long records needed | Low-frequency, high-sensitivity piezo sensors (about 500 mV/g); records of 10+ revolutions; ultrasound and oil as primary techniques; stud mounting |
| Slow | 150–600 rpm | Low amplitudes; longer records; envelope analysis remains effective | Piezo sensors preferred; check sensor noise floor at low frequency; ultrasound for bearings |
| Standard | 600–3,600 rpm | Most balance-of-plant machinery | MEMS or piezo; Fmax of 1–5 kHz for most faults, plus an envelope band for bearings; wireless works well |
| High | above 3,600 rpm | High gear mesh and blade frequencies; often sleeve bearings and shaft vibration | Wide-band piezo; proximity probes for sleeve-bearing machines; Fmax up to 10–20 kHz; stud mounting |
Motor current signature analysis in a program
MCSA and ESA measure the motor from the supply side, usually from the motor control centre. That makes them attractive for motors that are hard or unsafe to reach: submersible pumps, motors in hazardous areas, at height or in confined spaces. What a program needs to plan:
- Access point: clamp-on or installed current transformers on the supply cables (one phase for screening, three phases for unbalance), plus voltage taps for ESA. Online wireless current sensors can report periodically.
- Nameplate data: supply frequency, rated speed and current, number of poles, and ideally rotor bar and stator slot counts.
- Load: rotor bar sidebands appear at the supply frequency ± 2 × slip × supply frequency, and slip falls with load. Measure above roughly half load and at similar load each time.
- Resolution: sidebands can sit less than 1 Hz from a 50 or 60 Hz line, so records of 10 seconds or more are typical for rotor bar analysis.
- Supply type: direct-on-line motors are straightforward. On VFD supplies, the fundamental frequency varies and the waveform is noisy; analysis is possible but needs care.
Combining techniques
Techniques complement one another. A common pattern for critical assets: ultrasound or vibration detects a bearing problem first, oil analysis confirms wear debris, thermography or temperature shows when it is getting urgent. Agreement between two techniques raises confidence before an expensive intervention, and a disagreement prompts a closer look before acting.
Key takeaways
- Select techniques from failure modes, using a failure mode to technique matrix.
- Vibration is the backbone for rotating machinery, with ultrasound, motor current, oil and thermography covering its blind spots.
- Slow speeds need low-frequency sensors, long records, and usually ultrasound and oil analysis.
- Motor current analysis suits hard-to-reach motors, but needs stable load and long records.