A quick field assessment during inspections of open gearboxes: a fingernail drawn across a mark on the tooth flank either glides over it, suggesting polishing or staining, or catches, indicating a scratch, scuff, pit edge or step with measurable depth. It is not a measurement but helps sort cosmetic marks from damage that needs a photo, a replica or a closer look with a borescope.
Fingernail test catches on the scuff marks at the tips, so it's not just polishing, log it and pull an oil sample.
Also heard as
- nail test
- fingernail check
Related terms
Scuffing
Sudden adhesive damage where tooth surfaces weld and tear apart because the oil film broke down.
Micropitting
Very fine surface fatigue pits, too small to see one by one, that give tooth flanks a dull gray look.
Frosting
Shop word for the dull, frosted gray patches on gear teeth that turn out to be micropitting.
Pitting
Surface fatigue of gear teeth that breaks out small craters, typically on the flank just below the pitch line.