Reliability
Thermography, vibration, ultrasound or oil: what each technique catches
A guide to predictive maintenance techniques: the P-F curve, what each technique detects, the basic vibration signs and where to start.
Thermography, vibration, ultrasound, oil analysis. Each predictive technique sees a different kind of problem, at a different point in the life of the failure. Picking the wrong one for the equipment means paying for inspections and still getting caught off guard by the breakdown.
The P-F curve: how much time you have
Almost every failure gives signs before it happens. Point P is the moment the problem becomes detectable; point F is functional failure, when the equipment stops doing its job. The interval between the two is the time you have to plan the intervention.
For a bearing, the typical sequence goes roughly like this:
- Abnormal friction from poor lubrication: ultrasound picks it up.
- Early raceway damage: high-frequency vibration picks it up; oil analysis, when the bearing is oil-lubricated, shows wear particles.
- Greater damage: vibration shows up clearly in the regular spectrum.
- Heat: thermography and a thermometer pick it up.
- Audible noise, smoke, smell: sensory inspection picks it up. The breakdown is very close.
The earlier the technique detects it, the more time you have to plan. A rule of thumb: the inspection interval should be well below the P-F interval, usually half or less. If the failure takes four weeks from the first sign to the breakdown, a monthly inspection may arrive too late.
What each technique catches
| Technique | Catches well | Arrives late or misses |
|---|---|---|
| Thermography | Loose connections in panels, busbars and terminals; phase imbalance; damaged thermal insulation; stuck steam traps; a bearing that's already hot | Early failure in rotating equipment: heat is a late consequence |
| Vibration | Unbalance, misalignment, looseness, bearing and gear defects | Electrical panel problems; static equipment |
| Ultrasound | Bearing lubrication (too little and too much), compressed air and steam leaks, electrical discharge in medium-voltage panels | Detailed diagnosis of misalignment or unbalance |
| Oil analysis | Internal wear in gearboxes, hydraulic systems and diesel engines; water and particle contamination; degraded oil | Grease-lubricated equipment; electrical failures |
| Sensory inspection | Leaks, noise, smell, visible looseness, loose bolts, dirt | Almost everything at an early stage; depends a lot on who's inspecting |
Vibration: the basics every technician should know
You don't need to be an analyst to read the most common signs. They show up at multiples of the shaft speed (1x is the running speed itself):
- 1x running speed, strong in the radial direction: unbalance.
- 2x running speed, with high axial vibration: misalignment.
- Several multiples (1x, 2x, 3x...) and a reading that changes when you tighten the base: mechanical looseness or soft foot.
- High frequencies that aren't whole multiples of running speed: bearing defect. The envelope technique helps separate them from the rest.
A trend is worth more than a single reading. Always measure at the same point, in the same direction and with the equipment under the same load condition. A value that has doubled since the last measurement needs attention, even if it's still below the alarm.
Thermography in practice
- Inspect panels with the circuit under load, preferably close to normal load. A lightly loaded panel hides loose connections.
- Compare like phases with each other: a connection much hotter than its neighbors on the same circuit is suspect, even if the absolute temperature looks low.
- Follow the safety rules for opening an energized panel; when possible, use inspection windows.
- On rotating equipment, use thermography as a complement: it confirms there's a problem, vibration tells you which one.
Where to start
You don't need to buy everything at once. An order that works for most teams:
- Sensory inspection with a route and a checklist. It costs little and catches a lot, as long as it's done with a defined route, records and frequency, not "when there's time".
- Thermography on electrical panels. It can be contracted once or twice a year; the payback is usually quick.
- Vibration on rotating equipment with criticality A. A portable data collector on a route, or fixed sensors on the most critical units.
- Oil analysis on large gearboxes, hydraulic systems and diesel engines.
- Ultrasound for precision lubrication and hunting compressed air leaks, which often pay for the instrument with the energy saved alone.
A technique is only worth it if the measurement turns into action. Every anomaly found needs to generate a work order with a deadline, and the work order needs to be done before point F. A thermography report filed away with no work order is money thrown away.
A good example of where this makes a difference is the bearing: many causes of early failure show up first in ultrasound and vibration, weeks before the heat.
In taskfield, inspection routes live in the technician's app, with the points, the measured values and photos. A reading out of range can open the corrective work order on the spot, linked to the equipment.
Summary
- Each technique detects the failure at a different point on the P-F curve; the earlier, the more time to plan.
- Inspect at intervals well below the P-F interval, usually half or less.
- Thermography is the main technique for panels and connections, but arrives late on rotating equipment.
- Vibration: 1x means unbalance, 2x with high axial means misalignment, high frequencies mean bearings.
- Start with route-based sensory inspection, panel thermography and vibration on critical equipment.
- Every anomaly found has to become a work order that gets done before the failure.
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