The fine abrasive particles that cause polishing can be produced in a several ways:
- A degree of mild abrasive wear is expected during turbine 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 bearing operational life, small misalignments, fine abrasive contaminants (sand, rust) or wear particles can lead to polishing.
As a black oxide coating is only typically 1-2 um thick, it is common for the black oxide coating to wear from the bearing rolling elements and raceways during operation, due to small particles passing through the contact zone.
When of moderate severity, patches of polishing may obscure the original machining marks on the bearing. 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. An accompanying reddish-brown surface may be visible next to the patches of polishing wear due to oil varnish.
During black oxide wear, the black oxide coating will be removed resulting in the bare metal of the bearing being exposed and visible.
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.
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 abrasive particles. It will most likely arrest after the polishing progresses enough to result in a satisfactory contact. 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 turbine lifetime or performance.
As a result of the black oxide coating being removed on a black oxide bearing, the benefits of the coating are no longer achieved. This will make the bearing more susceptible to tribochemical attack, corrosion, adhesive wear and surface distress such as micropitting.
In some cases, polishing wear may develop later in the turbine 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.
|Visual inspection||✓✓||If accessible, polishing should be observable with the naked eye.|
|Borescope inspection||✓✓✓||Polishing is readily observed and distinguishable on bearings with a borescope.|
|Vibration analysis||✓||Vibration will likely not detect polishing as it typically affects the bearing surfaces equally.|
|SCADA data||✓||SCADA data does not aid detection of polishing.|
|Oil debris sensor||✓||Polishing is a result of very fine-scale abrasive particles. These are likely too small to be of concern for oil debris sensors.|
|Oil sample analysis||✓||Polishing is a result of very fine-scale abrasive particles. These are likely too small to be of concern for oil sample analysis.|
A small amount of mild polishing is inevitable in turbines during run-in. Indeed, manufacturers anticipate and account for it within their designs. However, polishing can also become prevalent on older bearings: 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. Localised bands of polishing are common on planet bearings due to regular uneven load sharing in the standard 4-row planet bearing configuration.
Polishing wear can be mitigated against using anti-wear additives. A proper lubrication filtration system (10um filter or better) will also help prevent the circulation of adhesive particles around the gearbox.
Black oxide bearings can offer good protection of the bearing surfaces against tribochemical attack, adhesive wear, moisture corrosion, micropitting and other failure modes. Particularly, bearing manufacturers recommend black oxide bearings for cylindrical roller bearings for both the parallel and planetary shaft stages and also for tapered roller bearings on the high speed shaft stage. However, as the coating is only 1-2um thick, it is relatively common for the coating to be eventually worn as small particulates pass through the bearing loaded zone removing some of the coating.