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Polishing

Polishing – Gear failure

Polishing is a light form of abrasive wear. It occurs when fine-scale particles within the lubricant scratch active gear tooth surfaces. These scratches are on a microscopic scale, meaning the overall visual appearance is actually a bright, mirror-like, polished wear patch. The process is identical to smoothing an object with sandpaper: fine-scale abrasives remove local high-points (asperities). Polishing wear typically repeats across multiple teeth, and on mating teeth as well. Generally, mild polishing results from fine abrasive particles produced by asperity breakdown during gearbox run-in. However, more severe polishing may also occur later in the gearbox life. Regardless, polishing wear is typically non-progressive and not a cause for concern. It is considered acceptable wear and tear which will not likely in itself affect the gearbox lifetime.

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Causes

Polishing is caused by very fine abrasive particles in the lubricant. 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.

The fine abrasive particles that cause polishing can be produced in a 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

Polishing results in shiny, bright, reflective, mirror-like wear patches on the surface of gear teeth. It occurs in discrete patches which may appear smooth and flattened as opposed to the typical involute shape. As 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 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 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.

Detectability

MethodDetection EfficiencyNotes
Visual inspection✓✓Polishing should be observable on visible gears.
Borescope inspection✓✓✓Polishing should be observable on visible gears.
Vibration analysisVibration will likely not detect polishing as it typically affects all teeth equally.
SCADA dataSCADA does not aid detection of polishing.
Oil debris sensorPolishing is a result of very fine-scale abrasive particles. These are likely too small to be of concern for oil debris sensors.
Oil sample analysisPolishing is a result of very fine-scale abrasive particles. These are unlikely to be of concern in an oil sample analysis.

Discussion

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.

Severity Rating

RankDescriptionDetectionRecommended Action
1Mild polishing. Light markings, which do not obscure original machining marks.Visual, borescopeNone – run turbine.
2Moderate polishing wear patch(s) with a mirror-like appearance. Obscures original machining marks. Possibly accompanied by burned oil.Visual, borescopeNone – run turbine.
3N/AN/AN/A
4N/AN/AN/A
1
Example of rank 1 polishing (a gear failure)
2
Example of rank 2 polishing (a gear failure)
3
Progresses to other failure modes
4
Progresses to other failure modes
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