Gear Failure

Abrasive Wear

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

What is abrasive wear?

There are two primary types of abrasive wear: two-body abrasive wear and three-body abrasive wear.

1. Two-body abrasive wear: This occurs when asperities (high points on surfaces) come into direct contact due to inadequate lubrication, breaking off to form abrasive particles. These particles then abrade the mating surfaces. This form of wear can also result from indentations on one gear tooth that scratch or gouge the opposing tooth.
2. Three-body abrasive wear: In this case, wear is caused by external contaminants like sand, dirt, and metallic debris that get trapped in the lubricant, turning it into an abrasive medium. These contaminants circulate through the gearbox and damage surfaces as they move.

Another type of wear to consider is adhesive wear, which occurs when surfaces adhere to each other under pressure and subsequently tear apart. While distinct, adhesive wear can sometimes accompany abrasive wear, especially in severe cases of poor lubrication.

Causes of abrasive wear

The most common causes of abrasive wear include:

Inadequate lubrication: When the lubrication film is too thin, asperities on gear teeth make contact, generating abrasive particles.
Contaminated lubricant: Lubricant contamination from dirt, rust, or other debris introduces foreign particles that cause three-body abrasive wear.
Manufacturing roughness: Poor surface finish or original machining marks on components can exacerbate wear during the run-in period.

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Appearance of abrasive wear

Abrasive wear manifests as scratches or gouges on the gear teeth or bearing surfaces, typically oriented in the direction of sliding. In cases of two-body abrasive wear, the scratches are usually localised on specific gear teeth, while three-body abrasive wear causes more random patterns, often described as particle mesh.

Progression of abrasive wear

Abrasive wear can lead to micropitting, a progressive failure mode characterised by small pits on gear surfaces. During the gearbox run-in period, mild two-body abrasive wear is common as asperities are worn down. However, if wear particles are not removed from the lubricant, they can accumulate and cause further damage, leading to micropitting.

Preventing abrasive wear

To prevent abrasive wear, it is crucial to:

Conduct regular gearbox inspections.
Perform oil analysis to detect contaminants and identify wear debris in the lubricant.
Replace the gearbox oil after the run-in period to ensure that abrasive particles and contaminants are removed.
Use inline filters to continuously remove particles from the lubrication system.

In modern gearboxes, advanced condition monitoring techniques, such as belt condition monitoring systems and vibration analysis, can help detect early signs of abrasive wear. Identifying issues early through regular maintenance and advanced sensing can save significant time and money by avoiding extensive damage and reducing the likelihood of micropitting or other failure modes.

Detectability

Method

Detection Efficiency

Notes

Borescope inspection

Easy

Allows close-up view and enables identification of abrasive wear marks in less accessible locations.

Visual inspection

Easy

Visual inspection of some parallel stage gears for abrasive wear marks will be possible . Filter magnet for evidence of debris.

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.

Vibration analysis

Hard

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

SCADA data

Not applicable

SCADA data does not aid detection of abrasive wear.

Conclusion

Abrasive wear is a critical type of wear that affects gearboxes, with two primary forms: two-body and three-body abrasive wear. By understanding the causes—such as lubricant contamination and inadequate lubrication—along with regular maintenance practices like gearbox inspection and oil replacement, the progression of abrasive wear can be mitigated, ensuring optimal gearbox performance and longevity.

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Severity rating

Rank 1 Either 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.

Detection:

Visual, borescope

Recommended action:

None – run turbine as normal.

Rank 2 Evidence 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.

Detection:

Visual, borescope

Recommended action:

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

Rank 3 Substantial 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.

Detection:

Visual, borescope

Recommended action:

Thorough inspection to identify source of debris particles. Consider lubricant change to minimise consequential damage.

Rank 4 Progresses to other failure modes

Detection:

N/A

Recommended action:

N/A