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.
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.
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.
|Visual inspection||✓✓||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||✓✓✓||Allows close-up view and enables identification of abrasive wear marks on accessible areas of bearings.|
|Vibration analysis||✓||Due to the small amounts of material being shed during abrasive wear alone, vibration is unlikely to pick it up.|
|SCADA data||✓||SCADA 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.|
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.