Three of the most common findings on a gear flank — micropitting, macropitting and scuffing — are routinely reported under one another's names, and the substitution is not cosmetic. Each points to a different corrective action, a different responsible party and a different question about whether the set was ever adequate for its duty. A frosted flank blamed on overload and a spalled flank blamed on the oil both send the analysis in the wrong direction. Distinguishing them is the first task on any failed gearbox, and it is done from the surface itself.
Three distresses that share a surface
All three appear on the working flank, all three remove material, and all three can be present on the same tooth at once. A gear pulled after long service often shows a frosted band, discrete craters and sliding damage together, and a report naming only the most dramatic of them has narrowed the case prematurely.
The discipline is to catalogue every distress present, record where on the tooth each appears, and only then ask which initiated and which followed.
Micropitting is a film condition at asperity scale
Micropitting is surface-initiated fatigue at the scale of individual asperities. The visible result is a matte or frosted grey band rather than discrete holes, and under magnification a field of very shallow craters measured in tens of micrometres. Under a loupe it is easily mistaken for staining or corrosion.
Its driver is specific film thickness — the ratio of the elastohydrodynamic film to the combined surface roughness. Where that ratio is marginal, asperities carry load directly. The levers are viscosity at temperature, surface finish and additive chemistry, and both ANSI/AGMA 925 and the ISO 6336 series give methods for evaluating film condition.
Macropitting is subsurface, and it starts where you would predict
Macropitting is classical Hertzian contact fatigue. Cracks initiate below the surface where shear stress peaks, propagate back toward the flank, and liberate a discrete crater with a fatigue origin identifiable in section or under SEM. Craters have defined edges and measurable depth, and they cluster at or just below the pitch line.
This is what the surface durability half of the rating is written to prevent. Macropitting well inside design life is therefore a direct challenge to the rating inputs: contact stress assumed, material specified, load spectrum actually seen.
Scuffing is thermal, instantaneous and directional
Scuffing is not fatigue at all. It is adhesion: the film collapses under combined sliding velocity and contact pressure, opposing surfaces weld, and the mesh tears them apart. The signature is radial, running up and down the tooth in the sliding direction, concentrated near the tip and root where sliding velocity is highest.
It can happen in minutes. A scuffed set does not tell you the gear was under-rated for cycles. It tells you a specific condition — a start-up, an overtemperature, a wrong-viscosity fill, a lost oil supply — took the film below what the contact needed.
Sequence matters as much as identification
Micropitting frequently precedes macropitting: once asperity-scale damage roughens the flank, the local stress field worsens and larger cracks follow. A set showing both may be one mechanism at two stages, or two independent problems, and the distinction changes the remedy.
Scuffing likewise alters the surface it leaves behind, and continued running will pit a scuffed flank. A pit inside a scuff band is weaker evidence of a contact-fatigue problem than the same pit on sound surface. Establishing order is often more informative than the identification itself.
What the terminology standards are for
ANSI/AGMA 1010 and ISO 10825 exist because these words were used loosely for decades. They give defined names and illustrated references for modes of gear tooth wear and failure, so an examination reported in that vocabulary can be checked against the same reference.
Informal usage — spalling for any material loss, scoring for any linear mark — is not necessarily wrong, but it is not verifiable. Where a matter is likely to be contested, the standard term with its defining characteristics stated is worth the extra sentence.
The evidence each identification requires
Micropitting is confirmed at magnification, not by eye, and ideally on an uncleaned surface. Macropitting is confirmed by sectioning through a crater to find the subsurface origin and crack path. Scuffing is confirmed by topography showing transfer and tearing rather than fatigue.
None of those confirmations survive aggressive cleaning. Wire brushing, blasting and solvent scrubbing all remove the fine detail the distinction rests on, and all three are routine in a shop trying to see what happened.
Where the identification changes the attribution
Micropitting points at lubricant selection, oil temperature, filtration and surface finish — responsibilities often shared between specifier, operator and manufacturer. Macropitting inside design life points at the rating calculation, the heat-treat quality, or the true load spectrum. Scuffing points at a discrete operating event or a lubrication fault.
Those are different parties and different documents. The mode named at the outset tends to determine which records are requested, and records not requested early are frequently gone later.
Where these opinions are challenged
Predictably: that the mode was identified visually without magnification, that the surface was cleaned before it was documented, that no section was taken to establish whether the origin lay above or below the surface, and that the film calculation used catalogue viscosity rather than the oil in the sump.
An examination recording each distress, its position on the tooth, the magnification used and the standard term applied will hold up. A single label applied to an entire gear set will not.
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.