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Indentations are formed when metallic debris in the oil film is trapped between, and rolled over by, contacting gear surfaces. This produces extremely high localised contact stresses which can cause the debris particle to permanently deform and dent the gear surface. The metal debris which produces these indentations can come from a variety of sources:
- Poor cleanliness during gearbox manufacture or assembly can leave behind debris particles which will damage the gears during operation.
- Damage from handling and transportation.
- Poor cleanliness standards during up-tower maintenance or inspection can lead to metal debris particles being introduced into the gearbox.
- Most commonly, indentations are secondary damage due to component failure elsewhere in the gearbox generating debris – such as bearing macropitting. The debris is then transported through the gearbox by the lubricant.
Indentations produce a cavity or pit of varying size and depth, usually with soft corners on the sides of the pit (compared to the sharp edges of macropitting). As a result of the deformation of material, the edges of the pit are raised above the normal profile of the contact surface, as illustrated in below:
These raised edges act as a ring of increased stress which will be quickly polished, producing a distinctive “halo” around the indentation pit.
The exact appearance of an indentation will vary based on the properties and size of the material that was originally trapped between the loaded contact surfaces. Ductile materials will deform significantly, creating an indentation with soft, gradual edges. Brittle materials with high hardness will produced indentations with sharper edges due to the lack of compliance under load. The bottom of the pit will be relatively flat, which distinguishes it from the rough bottom characteristic of macropitting.
Provided there is no known source of debris generation, a single or small number of randomly dispersed indentations may not be cause for concern. A large amount of indentations on gear teeth would signify damage elsewhere and be cause for concern. It is possible that indentations will not progress. However, the high stress concentrations around the edges of an indentation promote the initiation of contact fatigue damage. This often means micropitting or point surface origin macropitting develops around the indentations. These types of damage are progressive failure modes which can lead to functional failure of the gears. Furthermore, brittle materials may fracture as they indent the gear surface, producing smaller particles which additionally cause three-body abrasive wear.
|Visual inspection||✓✓||The naked eye can detect most indentation marks on gears easily, provided the inspection opening provides a line of sight. Otherwise a borescope is required.|
|Borescope inspection||✓✓✓||Indentations are readily observable and distinguishable with a borescope.|
|Vibration analysis||✓✓✓||Indentations large enough to be of concern will cause a repeating impulse which can be easily detected using vibration analysis.|
|SCADA data||✓||SCADA data does not aid detection of indentations.|
|Oil debris sensor||✓✓||Indentations result from large pieces of debris within the lubricant, which would likely be detected by an oil debris sensor.|
|Oil sample analysis||✓✓||As indentations are directly linked to the cleanliness of the lubricant, an oil sample analysis would likely identify the presence of debris particles.|
Indentations are distinct from three-body abrasive wear as they result from the over-rolling of debris, rather than debris scratching the gear during sliding contact. As gears are primarily in sliding, rather than rolling contact, indentations are more common in bearings.
To mitigate against the occurrence of indentations, the following steps should be taken:
- Use oil filtration to ensure suitable oil cleanliness where large debris particles are removed from circulation.
- Maintain strict cleanliness standards while performing gearbox maintenance or inspections that require opening a gearbox cover.
- Maintain strict cleanliness standards during the gearbox manufacture and assembly.