What are the possible causes when a gear set fails early?
When a gear set fails early, four explanations compete: the gear set was rated for less than it was asked to carry, it was not built to the design it was rated against, it was installed so the load never landed where the design assumed, or it was lubricated in a way the contact could not tolerate. Each of those four explanations for an early gear set failure implicates a different party and a different document trail.
The evidence on the gear teeth constrains the field of explanations for an early gear set failure but rarely settles it. The separation is made by testing each candidate explanation against the record that should exist for it.
What does a gear rating calculation under ANSI/AGMA 2001 or ISO 6336 actually claim?
A gear rating calculation under ANSI/AGMA 2001 or ISO 6336 claims a stress margin at an assumed cycle count under an assumed load; ANSI/AGMA 2001 and ISO 6336 are load-capacity methods, not guarantees. Each method computes a contact stress for surface durability and a bending stress at the tooth root, compares them against allowable values for the material, and applies factors — application, overload, dynamic, load distribution, size — that convert a nominal torque into a design condition.
When a gear set fails early, the question about its ANSI/AGMA 2001 or ISO 6336 rating is seldom whether the arithmetic was correct but which inputs were chosen, because the factors carry more judgment than the equations.
What does the service factor in a gear rating assume?
The application or service factor in a gear rating encodes what the driving and driven machines are expected to do — uniform load, moderate shock, heavy shock — which makes the gear service factor an assumption about someone else’s machine. The gear service factor is often picked from a table early in a project by someone who never saw the installation. A conveyor, a crusher and a reciprocating compressor are not the same duty.
Where a gear set failed inside its calculated life, the gap between the duty class assumed in the service factor and the duty class observed is frequently the largest discrepancy in the file.
Does a gear tooth failure point to the bending side or the surface side of the rating?
It depends on how the gear tooth failed, because the bending strength half and the surface durability half of a gear rating fail differently. A root crack propagating across the tooth section from a fatigue origin at the fillet engages the bending strength side, and with it fillet geometry, root finish and residual stress. A pitting flank engages the surface durability side, and with it contact stress and hardness.
A gear tooth lost in a single ductile overload, with no fatigue precursor, engages neither side of the gear rating. That is a question about what produced the torque spike, and the answer is usually outside the gearbox.
How are heat treatment and case depth checked on a failed carburized or nitrided gear?
Heat treatment and case depth on a failed carburized or nitrided gear are a manufacturing question, and they are measured, not argued: a metallographic section through flank and root, a microhardness traverse establishing effective case depth, and comparison against the drawing callout. Heat-treat certifications are the paper side of the same case-depth question.
For a carburized or nitrided gear, the gear rating assumes a case of specified depth and hardness over a core of specified strength. A case too shallow for the applied contact stress lets cracking initiate at the case-core transition rather than where the rating calculation contemplated. A case too deep or too brittle changes root behavior.
How do you check whether a failed gear met its accuracy class?
Whether a failed gear met its accuracy class is checked by metrology on the failed gear set, which measures involute profile, lead, pitch and runout deviations directly. The gear drawing calls out an accuracy class — under ISO 1328 or the equivalent AGMA classification — bounding those deviations, and those tolerances are what permit the load-distribution factor in the gear rating to be as favorable as assumed.
The complication is that wear also changes gear tooth profile, so a metrology measurement taken after a gear failure must distinguish a manufacturing deviation from one the failure produced. That is why the undamaged teeth and the original inspection chart both matter.
How can you tell whether a gear set failed because of how it was installed?
The contact pattern on the gear teeth is the test for installation: a wear band concentrated at one end of the face, or at the tip, is an installation signature independent of the gear itself. Misalignment, bore or housing error, soft foot and incorrect backlash all produce the same outcome — the load lands on part of the face width instead of all of it — and the gear tooth records this.
Where a gear set was checked with marking compound at commissioning, that record is directly comparable with the as-worn band. Where a gear set was not checked with marking compound at commissioning, the absence is itself relevant to the standard of care.
How do you find out whether lubrication contributed to a gear failure?
Whether lubrication contributed to a gear failure is found by testing the oil in the gearbox for viscosity and additive state and comparing it with the purchase records for what was bought, the oil-analysis history for how it trended, and the manual for what was specified. Where those four sources disagree, the disagreement is the finding. That gear lubrication evidence is easy to obtain.
A gear lubricant should be treated as a specification, not a habit, because the lubricant is part of the gear design. Viscosity grade, additive package, filtration target and sump temperature are specified for reasons the gear rating relies on, and field substitutions made for availability or cost are common and rarely documented.
How do you find out what load a failed gear set actually carried in service?
The load a failed gear set actually carried is reconstructed from the data modern drives record, which is far more than anyone reads. Motor current, torque, speed, starts per hour, trip history and process data together reconstruct a load spectrum that can be set against the one the gear rating assumed. Methods for service life under variable load exist within the ISO 6336 series.
A gear set run for years at a torque the rating never contemplated is a different case from one that saw a single severe event, and the distinction is usually recoverable from data rather than testimony.
How are the four candidate causes of a gear failure separated, and how is that work challenged?
No single test resolves the attribution of an early gear set failure; the four candidates — rating, manufacture, installation and lubrication — are separated by their own evidence. The metallurgical section addresses manufacture, the metrology addresses geometry, the contact pattern addresses installation, the oil addresses lubrication, and the recorded duty addresses whether the gear rating was ever the right rating. Gear failure attribution work that survives is work in which each excluded candidate was excluded on stated evidence.
The predictable challenges to a gear failure attribution are that the rating was recalculated with inputs chosen after the fact, that profile deviations on a worn gear were attributed to manufacture, that the assumed duty came from a nameplate rather than a log, and that lubricant condition was inferred from the specification.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.