Causes of gear polishing
Figure 1 – The damage to gears caused by abrasive particles
Gear lubricants contain very fine abrasive particles, and as these small particles pass through the load zone, they scratch away localised high-points on the active gear tooth surfaces. This results in a smoother, polished area as illustrated in Figure 1.
These abrasive particles that cause gear polishing can be produced in several ways.
- A degree of mild abrasive wear is expected during gearbox run-in, where asperities due to microscopic surface roughness are worn down. The small abrasive particles this produces will circulate in the lubricant and can cause polishing.
- Later in a gearbox’s operational life, small misalignments, fine abrasive contaminants (sand, rust) or wear particles can lead to polishing.
Appearance
Gear polishing results in shiny, bright, reflective, mirror-like wear patches on the surface of gear teeth. Notably, it occurs in discrete patches which may appear smooth and flattened as opposed to the typical involute shape.
Because gear polishing is caused by fine-scale abrasion, it only appears on active (loaded) gear flanks. Furthermore, polishing wear marks typically repeat across multiple teeth, as well as in corresponding areas on mating gear teeth. Under magnification, polishing wear ceases to be mirror-like and more closely resembles conventional abrasive wear scratches.
When of moderate severity, patches of polishing may obscure the original machining marks on the gear tooth. Moderate gear polishing is also often accompanied by an oil varnish. This forms due to the frictional heat generated by the polishing action burning the lubricant. It has a caked-on, dark brown appearance.
Note: Its super-smooth, mirror-like appearance means polished areas are often instinctively assumed to be the desired condition. Rather, to inexperienced observers, it is machining marks which seem to indicate an issue. This is of course not the case.
Progression
Polishing is typically not progressive. Nor is it considered cause for concern.
A degree of mild polishing is typical during run-in as contacting asperities are worn down, producing fine abrasives. It will most likely arrest after the polishing progresses enough to result in a satisfactory contact. Indeed, some initial polishing will actually improve the load distribution. Proper run-in procedure involves an oil change, which would remove these abrasives. As such, it is considered acceptable wear and tear which will not have an immediate, or even certain, effect upon the gearbox lifetime or performance.
In some cases, polishing wear may develop later in the gearbox life. In such cases, it is more likely to be of moderate severity and arise due to similar factors to conventional abrasive wear. If adhesive particles are not removed from the lubricant, polishing will progress. This will either be into abrasive wear if the particles grow in size, or into micropitting.
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A small amount of mild polishing is inevitable in wind turbine gearboxes during run-in. Indeed, manufacturers anticipate and account for it within their gearbox designs. However, polishing can also become prevalent in older gearboxes: the lubricant breaks down, and misalignment and abrasive wear particles (of all sizes) become more likely. In the latter case, it is more likely that moderate polishing damage will develop.
Anti-wear lubricant additives can be used to mitigate against polishing wear. A proper lubrication filtration system (10um filter or better) will also help prevent the circulation of abrasive particles around the gearbox.
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