Industrial gearbox — Failure Modes & Failure Rate
Power-transmission unit reducing motor speed to driven-equipment speed (or stepping up where needed). Failure modes split between the gears themselves and the bearings / seals supporting them — for service life prediction, AGMA 2001 or ISO 6336 give the gear-tooth fatigue calculation, while the bearings dominate ISO-281-style life.
Failure modes
Tooth pitting (surface fatigue)
- Root causes
- Hertzian contact-stress fatigue at the pitchline; lubricant film breakdown allowing metal-to-metal contact; hard particulate contamination grinding the surface.
- Detection
- Oil analysis (wear-particle counts, ferrous content); vibration trending shows gear-mesh sidebands; visual inspection during scheduled maintenance.
- Mitigation
- Oil-film thickness verified by EHL calculation at design stage; gear surface hardness (HRC 58-62 for case-hardened gears); ISO 4406 cleanliness target on the lubricant; gear-mesh vibration monitoring per ISO 10816.
Tooth bending fatigue / breakage
- Root causes
- Cyclic root-fillet stress exceeding the bending-fatigue endurance limit; sub-surface inclusion acting as crack initiator; sustained shock loading exceeding design.
- Detection
- Vibration step-change at the moment of breakage; broken-tooth signature in the gear-mesh spectrum; oil-debris pickup on the magnetic plug.
- Mitigation
- Bending-stress design margin per AGMA / ISO 6336 with appropriate material grade; shock-load damping (fluid coupling, soft start); torque-limiting on impact-prone applications.
Bearing failure
- Root causes
- Same as a standalone bearing — see <a href="/failure-modes/ball-bearing">ball-bearing failure modes</a>. Gearbox bearings often run hotter and dirtier than free-shaft bearings, so failure rate tends toward the higher end of generic data.
- Detection
- Vibration trending; bearing-housing temperature; oil-analysis particle count.
- Mitigation
- API 613 (special-purpose gear unit) bearing-life requirements; lubrication-system reliability for force-fed gearboxes; condition monitoring.
Lubricant breakdown / loss
- Root causes
- Oil-cooler tube leak draining the sump; oil overheating breaking down additive package; water contamination through a failed breather or seal.
- Detection
- Sump level alarm; oil-temperature alarm; periodic oil analysis.
- Mitigation
- Sump-level switch wired to trip the drive; oil cooler with redundant flow path; desiccant breather; barrier seals on shaft penetrations.
Coupling / shaft failure
- Root causes
- Misalignment-induced fatigue; key-shear from torque reversal; corrosion or fretting at shaft fits.
- Detection
- Vibration spectrum; shaft alignment check (laser); visual inspection at maintenance.
- Mitigation
- Laser alignment at installation; shaft-key dimensions per AGMA / ISO; torsional-vibration analysis for variable-speed drives.
Typical applications
Speed reduction on motor-driven pumps, fans, compressors, conveyors, mixers; turbine output gearing; mill drives; crane and hoist mechanisms; vehicle drivetrains.
How to model in a fault tree
For machinery FTA the gearbox is usually decomposed into gear-side and bearing-side branches because mitigations and detection signals differ. For SIS-protected drives where loss of the gearbox is the initiating event for a hazard (e.g. mixer stops in a runaway exotherm), the failure-to-rotate mode is the relevant λ_DU and proof-test interval (run-cycle test) directly affects PFD. For redundant trains (e.g. boiler-feed pump A and B with separate gearboxes), apply Beta-factor CCF for shared lube-oil supply or shared cooling water.