Bearing Failure

Abrasive wear - bearing failures

Abrasive wear is the scratching or gouging of bearing rollers or raceways resulting in the creation of metal wear debris within the gearbox.

Also referred to as:
Scratching, Gouging, Scoring, Cutting, Ploughing, Particle Mesh

Overview

Abrasive wear can be caused by inadequate lubrication leading to asperity contact between rollers and raceways, or by contaminants within the lubrication itself such as dirt or wear particles from other gearbox components.

It is common whilst asperities are smoothed out during the run-in period. However, it usually arrests provided any wear debris is removed from the lubricant. Wear particles which are not removed from the lubricant will continue to multiply and damage can progress into micropitting.

That said, abrasive wear alone will not lead to bearing failure within a short period of time.

Causes

There are two main types of abrasive wear, with different causes:

Two-body abrasion – Occurs when the lubrication film is not developed enough to separate microscopic localised high-points, known as asperities, on contacting roller and raceway surfaces. As the asperities come into contact, some are broken off and become wear particles. Two-body abrasion can also be caused by particles embedded in one or more of the raceways or rollers (such as those from indentations) abrading any contacting components. This is why abrasive wear is often referred to as scratching, gouging, scoring, cutting or ploughing.
Three-body abrasion – Results from foreign contaminants in the lubricant. These include sand, scale, rust, machining chips, grinding dust, weld spatter, weld debris and wear particles. These can originate from the bearings themselves or other components (such as gears). As such, three-body abrasion may be indicative of separate failure modes elsewhere in the gearbox.

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Appearance

The exact appearance of the markings varies depending upon the properties of the wear particles. Generally, one sees banded scratches or gouges ploughed circumferentially around the bearing roller or raceway. This is caused either by an asperity, embedded hard particle, or by a loose piece of debris being caught and pushed or slid in-front of the rotating component.

Progression

Abrasive wear may or may not be progressive.

Some mild two-body abrasion can be expected during gearbox run-in where asperities due to poor surface roughness from manufacturing intolerance scratch or gouge the bearing surfaces. This will likely arrest if the asperities are smoothed out and any contaminants are removed from the lubricant through filtration. The lubricant should be replaced after a suitable run-in period. Under the same circumstances, mild two-body abrasion due to hard embedded particles may also arrest.

Three-body abrasive wear, which can not only result from foreign wear particles but can progress as a result of wear particles in the lubricant produced by previous two-body abrasion. This will likely multiply and progress unless the wear debris is removed from the lubricant.

Overall, abrasive wear is unlikely to progress to bearing failure in a short period of time. However, as it leads to material being displaced from the bearing, the surface distress caused by abrasive wear will accumulate and its progression will accelerate and eventually develop into micropitting unless the debris is removed from the lubricant.

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Advanced sensing

Detectability

Method

Detection Efficiency

Notes

Visual inspection

Medium

Access for visual inspection of bearings with the naked eye is very restricted, apart from, to an extent, the main bearing. A borescope should be used instead. Filter magnet for evidence of debris.

Borescope inspection

Hard

Allows close-up view and enables identification of abrasive wear marks on accessible areas of bearings.

Vibration analysis

Easy

Due to the small amounts of material being shed during abrasive wear alone, vibration is unlikely to pick it up.

SCADA data

Easy

SCADA data does not aid detection of abrasive wear.

Oil debris sensor

Medium

Abrasive wear does not shed much debris in early stages. Rather it is generally caused by debris from failures modes elsewhere. Debris sensors will provide confirmation debris is present, but it will not indicate the source of the wear.

Oil sample analysis

Medium

Abrasive wear does not shed much debris in early stages. Oil analysis will provide confirmation debris is present, but will not indicate the source of the wear.

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Discussion

It is best practice to identify the source of abrasive wear and whether it is two or three-body. This is achieved via thorough gearbox inspection, as well as oil filter and magnet checks. Oil sample analysis to establish the extent and material composition of debris present in the lubricant is useful. It is also important to ensure vents, breathers and seals are functioning correctly. Regular visual inspections of the gearbox, along with regular lubricant analysis can help mitigate against abrasive wear through early detection.

Other methods for preventing abrasive wear include ensuring proper run-in procedure is followed. This includes replacing the gearbox oil after a suitable run-in period to allow the asperities to be broken down. Regular oil maintenance will mitigate against progression of abrasive wear. Properly flushing and replacing the oil, or using an inline filter, will ensure the asperities are removed from the gearbox.

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Severity rating

Rank 1 Either no or some superficial two-body adhesive wear marks due to asperity break down during run in. The scratches are not deep enough to remove considerable amounts of material from the bearing. Should arrest after run-in.

Detection:

Visual, borescope

Recommended action:

None – run turbine as normal.

Rank 2 Evidence of wear particles causing three body-abrasion or an embedded debris particle causing two-body abrasion after run-in. Numerous marks with some depth. Can be progressive.

Detection:

Visual, borescope

Recommended action:

Continue to monitor during routine inspections. Seek to identify source of debris particles within.

Rank 3 Emergence of micropitting around the abrasive wear scratches. Substantial scores, with some depth, across multiple parts of the bearing. Progressive.

Detection:

Visual, borescope

Recommended action:

Thorough inspection to identify source of debris particles. Consider oil change to minimize consequential damage.

Rank 4 Progresses to other failure modes

Detection:

N/A

Recommended action:

N/A