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Micropitting

Micropitting – Bearing failure

Micropitting refers to the formation of very small, micro-scale craters (or pits) on the contacting surfaces of a bearing due to the surface distress caused by excessive stress and/or when the lubrication film is not developed enough to separate high-points, known as asperities, on bearing roller and raceways from contacting. As suggested by their names, micropitting and macropitting are two closely related failure modes. They denote different severities of failure driven by Hertzian Fatigue, with micropitting being the less severe; it alone is unlikely to cause component failure. Micropitting may or may not arrest, in the latter case it will most likely develop into macropitting. It has a matte, frosted appearance with the micro-pits appearing as small black holes.

Also referred to as: Grey Staining, Ghosting, Frosting, Glazing, Surface Distress, Peeling, Micro-spalling

Click here for: Micropitting – Gear failure

Causes

Bearing operation is inherently prone to micropitting due to the repetitive Hertzian contact between the rolling element and raceway surfaces, hence the need for lubrication to separate the contacts and lessen the stress. As Hertzian Fatigue is one of the failure modes that bearings are designed against, any micropitting which developed towards the end of the turbine design life would be considered normal contact fatigue of the steel. Discussed herein is rather the formation of unanticipated, premature micropitting caused by excessive surface stress and/or poor lubrication. There are multiple underlying reasons for these conditions to arise, the most common of which are:

  • Inadequate Lubrication – If the operator changes to a different lubricant brand or formulation, or there is a degradation in lubricant performance, it can cause micropitting. If the oil film cannot develop sufficient thickness, then the contact stresses are less distributed and metal-to-metal contact occurs.
  • Poor Surface Finish – During the manufacturing process for heat treated bearings, grinding is required to obtain the final part geometry. Oversights during this stage can result in a poor surface finish with increased roughness. This roughness consists of an increase in the number and size of asperities.
  • Poor Element Load Distribution – Due to poor design, the rolling element load may be poorly distributed. This means higher than expected contact stress will exist for one or more rolling elements. For main bearings, excessive axial load can lead to poor load distribution and micropitting on the bearing surfaces.
  • Surface Defects – A discrete surface defect either from manufacturing or operation (such as debris dents or fretting corrosion) creates a recessed or raised surface and therefore an increased localised stress, which micropitting will develop around.

The result of the circumstances outlined above is surface distress from metal-to-metal contact and the emergence of ultra-small cracks on the bearing surfaces. These micro-cracks propagate at a shallow angle before conjoining or returning to the surface, both of which eventually result in a very small amount of liberated material and the micro-scale craters (or pits) for which the micropitting failure mode is named.

Appearance

While individual micropits are not clearly visible to the naked eye, it is uncommon for them to occur in discrete, isolated areas. Rather, micropitting often occurs in a repeating or systemic fashion across bearing surfaces in which case it can be clearly identified as a continuous, fractured, matte grey, dull area. It has a granular, frosted appearance when viewed close up, with the pits appearing as small black holes. There will be a clear boundary between the damaged and undamaged areas. Whilst it may spread across much of the bearing raceway, micropitting is typically only 10µm deep.

 

Tip: Using intense directional lighting can aid in the identification of micropitting as the light will scatter off the micropits. It will highlight its frosted, fractured appearance which may at first appear as flat and shiny.

Progression

Micropitting may or may not be progressive and needs to be treated on a case-by-case basis.

Micropitting may emerge simply as a severe case of bearing run-in where contacting asperities are normally worn down by abrasive wear. In such cases, the geometrical changes caused by the micropits may lead to a satisfactory improvement in the contact and the progression will arrest. Micropitting may also arrest in situations where the lubrication is improved or the high stress area is successfully removed by the micropitting. This will be aided by a suitable filter removing any debris resulting from the material liberated during micropit formation.

On the other hand, as micropitting results in a loss of material it is equally likely to have a negative impact in its alteration of bearing geometry. As the liberated material wears away the bearing surface, the contacting surfaces will be changed, concentrating the load over a smaller area, disrupting the oil film, increasing the local stress and therefore enabling conditions for it to propagate. This will eventually have an impact on the rolling contact, resulting in decreased efficiency and increased noise and vibration.

The development of micropitting is concerning and undesirable, yet even with substantial bearing coverage (>50%) it seldom affects the long-term performance of the turbine. Micropitting is concerning rather for its potential to rapidly progress into other failure modes, such as macropitting.

Detectability

MethodDetection EfficiencyNotes
Visual inspection✓✓The naked eye can detect micropitting on bearing surfaces, the limitation is whether the inspection opening provides a line of sight, otherwise a borescope is needed.
Borescope inspection✓✓✓Micropitting is readily observed and distinguishable with a borescope.
Vibration analysis✓✓It is generally accepted that vibration analysis cannot detect early stage micropitting, though in some cases the bearing contact frequencies will develop indications. If it progresses to macropitting then vibration analysis is more effective.
SCADA dataSCADA data does not aid detection of micropitting.
Oil debris sensorMicropitting does not shed much debris in early stages. A debris sensor will provide warning once macropitting develops although it will not indicate the source of the wear.
Oil sample analysisMicropitting does not shed much debris in early stages. Oil analysis will provide warning once macropitting develops although it will not indicate the source of the wear.

Discussion

Micropitting is a common occurrence in wind turbine bearings. In large part this is due to challenges associated with the scale of components, the low speed high torque application, insufficient lubrication systems, variable speeds and non-torque loadings.

Micropitting can be mitigated against by checking lubricant viscosity and additives. It should also be verified that bearing surface roughness is within specification. If the latter is not found to be acceptable, corrective actions such as super-finishing could be considered. Another corrective action is to modify the bearing geometry to improve the load distribution. Optimising geometry may require consideration of a complete system deformation, bearing clearances and shaft deflections to predict and correct bearing load distributions.

A proper lubrication filtration system (10um filter or better) can mitigate against generated debris causing damage to other components, while periodic visual inspections will determine if any micropitting is progressing or if it has self-arrested. If the micropitting region continues to grow in area or depth then there is reason for further concern, especially if areas of macropitting form. The formation of macropits suggests that the contact stress is still excessive and the damage will progress.

Severity Rating

RankDescriptionDetectionRecommended Action
1Normal wear of surface asperities, with little to no formation of micropits.Visual, borescopeNone – run turbine as normal
2Micropitting covering up to 25% of the bearing surfaces.Visual, borescopeNone – run turbine as normal
3Micropitting covering 25-50% of the bearing surfaces. Even beyond 50% coverage, it remains S3 unless macropitting develops.Visual, borescopeRun turbine and increase inspection frequency – look to identify any progression into macropitting.
4N/AN/AN/A
1
No image available
2
Example of rank 2 micropitting (a bearing failure)
3
Example of rank 3 micropitting (a bearing failure)
4
Progresses to other failure modes
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