Indentations – Bearing failures

Contributing factors of bearing indentations

Indentations are formed when debris in the lubricant, trapped between a bearing roller and raceway, is rolled over. This produces extremely high localised contact stresses which can cause the debris particle to permanently deform and dent the roller or raceway. The debris which produces these can come from a variety of sources:

• Poor cleanliness during gearbox manufacture or assembly can leave behind debris particles which will damage the bearing during operation.
  Damage from handling and transportation.
  Poor cleanliness standards during up-tower maintenance or inspection can lead to 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.

Appearance of bearing indentations

Indentations produce a cavity or pit of varying size and depth, usually with rounded corners on the sides of the pit (compared to the sharp edges of macropitting). As a result of the deformation of the material, the edges of the pit are raised above the normal profile of the contact surface.

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 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.

Progression

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 number of indentations would signify damage elsewhere and be cause for concern. It is possible that they 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. Furthermore, brittle materials may fracture as they indent the bearing surface, producing smaller particles which additionally cause three-body abrasive wear.

How to detect bearing indentation

Discussion

Indentations are distinct from three-body abrasive wear as they result from the over-rolling of debris, rather than debris being pushed along and scratching the bearing surfaces in sliding contact. Indentations in bearings  are an indication that there may be another failure elsewhere. If there are multiple indentations, a through inspection should be performed to identify the source.

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.

Severity rating

Rank 1 Single or small number of isolated indentations, smaller than 1mm in diameter. No known debris source. May exhibit halo appearance around the edges of the indentation

Detection:

Borescope, vibration

Recommended action:

None – run turbine as normal.

Rank 2 Multiple indentations in a localised area. Indentations larger than 1mm in diameter. Halo appearance around the edges of the indentations. Perhaps some emergence of localised micropitting.

Detection:

Borescope, vibration, oil debris, oil analysis

Recommended action:

Run turbine, increase inspection frequency and carefully monitor vibration and oil for damage progression.

Rank 3 Indentation marks covering a significant area of roller and/or raceway. Emergence of micropitting around the indentations. Likely secondary damage from a separate failing component.

Detection:

Borescope, vibration, oil debris, oil analysis

Recommended action:

Identify source of indenting debris. Depending on condition, consider stopping turbine. Schedule for replacement to avoid consequential damage.

Rank 4 Progresses to other failure modes

Detection:

N/A

Recommended action:

N/A