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Abrasive wear

Abrasive wear – Gear failure

Abrasive wear is the scratching or gouging of active gear tooth surfaces resulting in the creation of metal wear debris within the gearbox. It can be caused by inadequate lubrication leading to asperity contact between gear teeth, 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, but usually arrests provided any wear debris is removed from the lubricant. However, 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 to lead to tooth failure within a short period.

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

Click here for: Abrasive wear – Bearing failure

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 mating gear teeth. As the asperities come into contact, some are broken off and become wear particles. Two-body abrasion can also be caused when particles embedded in one gear-tooth (such as those from indentations) abrade the mating gear tooth.
  • 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 bearings). As such, three-body abrasion may be indicative of separate failure modes elsewhere in the gearbox.

Appearance

Scratches or gouges on the active gear tooth surface oriented in the direction of sliding which are bright in colour. The exact appearance of the markings varies depending upon the properties of the wear particles. If caused by an asperity or embedded hard particle on one gear tooth (two-body abrasion), the markings may be consistently positioned on the mating gear teeth. If due to wear debris in the lubricant (three-body abrasion), the markings will be randomly dispersed across the gear teeth (often referred to as particle mesh).

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 mating gear teeth. This will likely arrest if the asperities are smoothed out and provided 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 abrasion results not only 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 tooth failure in a short period of time. However, in severe cases the tooth thickness may be reduced significantly with the tooth-tip reduced to a sharp edge. As it leads to material being displaced from the gear teeth, abrasive wear may develop into micropitting.

Detectability

MethodDetection EfficiencyNotes
Visual inspection✓✓✓Visual inspection of some parallel stage gears for abrasive wear marks will be possible . Filter magnet for evidence of debris.
Borescope inspection✓✓✓Allows close-up view and enables identification of abrasive wear marks in less accessible locations.
Vibration analysisDue to the small amounts of material being shed during abrasive wear alone, vibration is unlikely to pick it up.
SCADA dataSCADA data does not aid detection of abrasive wear.
Oil debris sensor✓✓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✓✓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.

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.

Severity Rating

RankDescriptionDetectionRecommended Action
1Either no or some superficial two-body abrasive wear marks due to asperity break down during run-in. The scratches are not deep enough to remove considerable amounts of the material. Damage arrests after run-in period.Visual, borescopeNone – run turbine as normal
2Evidence of wear particles causing three-body abrasion or an embedded particle causing two-body abrasion after run-in. Numerous marks with some depth. Can be progressive.Visual, borescopeContinue to monitor during routine inspections. Seek to identify source of debris particles.
3Substantial scores with some depth across multiple parts of the gear teeth. Emergence of micropitting around the abrasive scratches. Reduction of tooth thickness – perhaps to a sharp edge. Progressive.Visual, borescopeThorough inspection to identify source of debris particles. Consider lubricant change to minimise consequential damage.
4N/AN/AN/A
1
Example of rank 1 abrasive wear (a gear failure)
2
Example of rank 2 abrasive wear (a gear failure)
3
Example of rank 3 abrasive wear (a gear failure)
4
Progresses to other failure modes
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