Bearing Failure

Bearing Polishing

Polishing is a light form of abrasive wear. It occurs when fine-scale particles within the lubricant scratch bearing contact surfaces.

Polishing wear and abrasive particles: an overview

Polishing is a light form of abrasive wear that occurs when fine-scale abrasive particles within the lubricant scratch the polished surface of bearing contact areas. These microscopic scratches result in a bright, mirror-like finish or polished wear patch. The effect is similar to smoothing an object with sandpaper, where fine abrasives remove high points or asperities.

In turbine components, particularly during the running-in process, this wear is considered acceptable wear. Polishing typically occurs across gear teeth, bearing raceways, and rolling elements. Mild polishing often arises from abrasive wear particles produced during the initial turbine run-in. However, more severe polishing can occur later, especially when abrasive wear phenomena become more pronounced.

Causes of polishing

Polishing is triggered by fine abrasive particles present in the lubricant. These particles, often resulting from common wear modes like asperity breakdown or ceramic material wear, pass through the contact surfaces, creating localized abrasive wear scratches.

The production of these abrasive wear particles may happen through various processes:

Running-in process: during turbine run-in, microscopic surface roughness is worn down, producing small particles.
Later operational life: contaminants such as metal shavings, sand, or rust may generate abrasive wear, which eventually leads to polishing.
Black oxide wear: bearings with black oxide coatings are susceptible to block wear as the thin coating (1-2 µm) can wear away during turbine operation due to particle accumulation in the lubricant.

Appearance of polishing wear

Polishing creates shiny, reflective surfaces with a bright, mirror-like finish on contact areas. These polished machine surfaces appear smooth but may still show signs of wear depths or other abrasive wear phenomena under magnification. The resulting mirror-like finishes can often obscure the original machining marks.

Moderate polishing: Polishing may obscure machining marks and produce varnish buildup (dark brown oil deposits) due to the frictional heat generated during the process. This is considered acceptable wear.
Black oxide wear: Corrosive compounds may accelerate the removal of the black oxide coating, leading to tribochemical attack, adhesive wear, and potential surface distress like micropitting.

Progression of polishing wear

Polishing is generally non-progressive and is not a cause for concern. Antagonistic wear may occur if extreme-pressure (EP) additives are insufficient, leading to more severe conditions such as abrasive wear. After the polishing process reaches a stable state, the bearing surfaces will achieve optimal performance.

However, failure to address lubricant contamination or implement proper filtration (such as a 10 µm filter) may allow abrasive compounds to circulate, leading to further wear progression. In such cases, the wear can evolve into micropitting or chemical wear.

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.

Detection methods

Method

Detection Efficiency

Notes

Visual inspection

Easy

If accessible, polishing should be observable with the naked eye.

Borescope inspection

Easy

Polishing is readily observed and distinguishable on bearings with a borescope.

Vibration analysis

Hard

Vibration will likely not detect polishing as it typically affects the bearing surfaces equally.

Oil debris sensor

Hard

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

Hard

Polishing is a result of very fine-scale abrasive particles. These are likely too small to be of concern for oil sample analysis.

SCADA data

Not applicable

SCADA data does not aid detection of polishing.

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Prevention and mitigation

Anti-wear lubricant additives and aggressive EP additives can help mitigate polishing wear. Additionally, using compatible buffing compounds, such as Brown Tripoli Polishing Compound – Medium Course Compound, can aid in achieving high-gloss finishes without exacerbating abrasive wear. Implementing a polishing process with proper lubrication management and film protection may prevent excessive surface roughness or gear polishing wear.

For black oxide bearings, maintaining the integrity of the oxide layer is crucial for protection against tribochemical attack, corrosion, and other failure modes. If the coating is worn away, metal polish or softer metals may be exposed, leaving the bearing susceptible to additional wear.

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Severity rating

Rank 1 Mild polishing. Light markings, which do not obscure original machining marks.

Detection:

Visual, borescope

Recommended action:

None – run turbine as normal.

Rank 2 Moderate polishing wear patch(s) with a mirror-like appearance. Obscures original machining marks. Possibly accompanied by burned oil/reddish-brown oil varnish marks.

Detection:

Visual, borescope

Recommended action:

None – run turbine as normal.