A gear tooth does not fail without a reason recorded in its wear pattern. Contact fatigue, bending fatigue, and scuffing each leave a different signature on the flank and the root.
Start a conversation with our AI Research Concierge, already scoped to gear failure. Pick a starting point, or describe your situation directly.
Gear failures divide cleanly along the two places load is carried: the tooth flank, where rolling and sliding contact generates Hertzian stress, and the tooth root, where bending stress is highest. A flank that pits is telling you the contact stress or the lubricant film exceeded what the surface could sustain over its design cycles; a root that cracks and eventually snaps a tooth is telling you the bending fatigue limit was exceeded, often from an overload event, a resonance, or a manufacturing defect in the fillet. Scoring and scuffing are a third story entirely — a momentary breakdown of the lubricant film under high sliding velocity that welds and tears asperities rather than fatiguing them. A tooth-contact pattern, read correctly, will often show whether a gear set was ever properly meshed at all before it started failing by any of these mechanisms.
Gear failure mechanisms separate by where on the tooth they initiate and what stress state produced them.
Subsurface Hertzian shear stress at the pitch line initiating cracks that propagate to the flank surface, removing material as pits once cycles accumulate past the design rating.
Cyclic bending stress concentrated at the root fillet initiating a crack that propagates across the tooth section until it snaps off.
Momentary breakdown of the lubricant film under high sliding velocity and contact pressure, welding and tearing asperities across the flank in a characteristic radial pattern.
Contaminant particles or insufficient lubrication progressively removing material from the flank profile, altering the tooth form until the mesh itself degrades.
A single load spike beyond the ultimate strength of the material snapping a tooth with little or no fatigue precursor — typically from a shock load, jam, or foreign-object event.
Fine-scale surface-initiated fatigue from a marginal lubricant film at the asperity level, frosting the flank and preceding macropitting if left unaddressed.
Gear investigations combine the physical evidence on the tooth with the geometry and metallurgy that produced it.
Gear failures rarely stay contained to the gearbox:
The mesh pattern, debris location, and undisturbed gear positions are the evidence. Further teardown before documentation destroys the contact-pattern record that identifies the mechanism.
Pitting is subsurface-initiated fatigue, producing discrete craters with a fatigue-crack origin beneath the surface, typically at or below the pitch line. Scoring is a surface event from lubricant-film breakdown, producing radial scratching and welding marks across a broader band of the flank, usually most severe near the tip and root where sliding velocity is highest. The two point to different fixes — pitting toward load, material, and life, scoring toward lubricant film condition and surface finish.
Yes. A shock load, a jam, or a foreign-object event can fracture a tooth in a predominantly ductile overload mode with no fatigue precursor, distinguishable from a fatigue-driven tooth loss that shows progressive crack growth and beach marks. The practical question is usually what produced the momentary overload, which often points outside the gearbox entirely.
Tooth-contact pattern analysis — using marking compound, or reading the as-worn wear band on a failed set — shows whether load was distributed across the full face width and tooth height as designed. An off-center, single-edge, or tip-loaded pattern indicates misalignment, incorrect backlash, or a housing or bearing problem present from installation, independent of any issue with the gear material itself.
Often, yes. Micropitting frosts the flank at a fine scale, and if the lubricant film condition is not corrected it tends to progress into macropitting and eventually a worn-out or spalled flank. Catching it early through oil analysis and borescope inspection is generally far less costly than waiting for it to develop into a tooth failure.
The AGMA rating calculation and the gear drawing establish what the set was designed to carry. Heat-treatment and inspection records from manufacturing establish whether it was built to that design. Operating and duty-cycle logs establish whether it was run within that envelope. Comparing all three against the physical evidence on the failed gears is what separates the three possible causes.
Technical briefings from our work in this area.
When a gear set fails early, four explanations compete — the rating, the manufacture, the installation and the lubricant. Each is a different party and a different document trail.
readMesh position, debris distribution, the oil and the as-worn contact band are all recoverable on the day of a gearbox failure and largely gone a week later. Most of that loss is well-intentioned.
readThree of the most common gear flank findings are routinely reported under one another's names. Each points to a different remedy, a different party and a different set of records.
readTell us what happened. We will triage it and connect you with the right expert — usually within one business day.