Gearbox failure root cause analysis
In shortGearbox failures most often start with lubrication problems (wrong, low, degraded or contaminated oil), overload or shock, misalignment, or bearing damage. Oil analysis, the condition of the gear teeth and bearings, and the operating history usually point to the real cause.
Common failure causes
| Cause | What happens | What to check |
|---|---|---|
| Lubrication | Wrong grade, low level, old or contaminated oil | Oil level and grade, oil analysis, change interval, breather and seals |
| Contamination | Water or particles damage surfaces | Oil analysis for water and particles, seal condition, magnetic plug debris |
| Overload or shock | Teeth or shafts are overstressed | Load history, jams, emergency stops, brake and coupling behaviour |
| Misalignment or installation faults | Uneven loading wears gears and bearings | Alignment records, foundation, coupling condition |
| Bearing failure | Wear, fatigue or lubrication starvation | Vibration trend, bearing temperature, noise, debris |
| Heat | Oil degrades and clearances change | Operating temperature, cooling, ambient and load |
Evidence to collect
- Oil sample from the sump, and the magnetic plug and filters if fitted.
- Photos of gear teeth contact pattern and any pitting, scuffing or breakage.
- Vibration, temperature and noise history before the failure.
- Maintenance records: oil changes, inspections, seal replacements.
- Operating history: loads, jams, stops and any recent changes.
Worked 5 Whys (illustrative)
| Step | Answer |
|---|---|
| Problem | A gearbox failed with damaged bearings and worn gear teeth. |
| Why 1 | The bearings and gears wore faster than expected. |
| Why 2 | Oil in the gearbox was contaminated with water. |
| Why 3 | A seal had leaked and the breather was blocked. |
| Why 4 | Seals and breathers were not checked on the routine. |
| Root cause | The inspection routine did not cover seals and breathers. |
Prevention
- Use oil analysis and act on trends in water, particles and wear metals.
- Include seals, breathers and oil level in routine inspections.
- Record loads, jams and stops that could cause shock loading.
- Check alignment after any repair or replacement.
- Use the manufacturer's lubricant and interval as the baseline.
Frequently asked questions
What is the most common cause of gearbox failure?
Lubrication problems, including contamination, are among the most common. Overload, misalignment and bearing damage are also frequent.
What can oil analysis tell me about a gearbox?
It can show water, particles and wear metals, which point to contamination and to which parts are wearing.
Should I rebuild or replace a failed gearbox?
That depends on the damage, cost and your spares strategy. Whichever you choose, find the root cause first, or the replacement may fail the same way.
Run your own root cause analysis.
Describe the failure or add a photo or file, and RapidRCA builds the 5 Whys, fishbone, actions and report, using the ROOTS method.
Start an RCA
Describe the failure or add a photo or file, and RapidRCA builds the 5 Whys, fishbone, actions and report, using the ROOTS method.
Start an RCA
Last updated 14 October 2026. These guides explain a method and give examples. Always follow your equipment manufacturer's manual, your site procedures and applicable regulations.