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Micropitting

Micropitting – Gear failure

Micropitting refers to the formation of very small, micro-scale craters (or pits) on the surface of an active gear tooth flank 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 mating gear teeth 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 – Bearing failure

Causes

Gear operation is inherently prone to micropitting due to the repetitive Hertzian contact between the tooth involute surfaces, hence the need for lubrication to separate the contacts and lessen the stress. As Hertzian Fatigue is one of the two failure modes that gearing is designed against, any micropitting which developed towards the end of the gearbox 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 case carburized gears, grinding is required  to obtain the final part geometry. Oversights during this stage (see grinding burn and machining marks) can result in a poor surface finish with increased roughness. This roughness consists of an increase in the number and size of asperities.
  • Poor Tooth Load Distribution – Due to poor design, the tooth load may be unevenly distributed. This means higher contact stress will exist in one or more areas compared to elsewhere across the tooth flank, for example at the root, tip, or shifted excessively to one side (see edge loading).
  • 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 localized 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 gear tooth flank. 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 multiple gear teeth 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 gear tooth, 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 gear run-in where contacting asperities are normally worn down by abrasive wear. In such cases, the geometrical changes caused by the micropitting 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 gear geometry. As the liberated material wears away the gear surface, the tooth shape 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 accuracy of the gear mesh, resulting in decreased efficiency and increased noise and vibration.

The development of micropitting is concerning and undesirable, yet even with substantial tooth coverage (>50%) it seldom affects the long-term performance of the gearbox. 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 easily on gear teeth, 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, especially if repeating on multiple teeth.
Vibration analysis✓✓It is generally accepted that vibration analysis cannot detect early stage micropitting, though in some cases the gear mesh frequencies will develop indications. If it progresses to macropitting then vibration analysis is more effective at picking-up tooth defect frequencies.
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 gearboxes. 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. Nonetheless, it has become more common in conjunction with the industry move to case hardened, carburised gears (larger pits tend to form on softer, through hardened gears).

Micropitting can be mitigated against by checking lubricant viscosity and additives. It should also be verified that gear tooth 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 gear tooth microgeometry to improve tooth load distribution. Optimising microgeometry may require consideration of gearbox deformation, bearing clearances and shaft deflections to predict and correct tooth load distributions. Tooth modifications can include tip relief, crown or lead slope typically on the scale of microns.

A proper lubrication filtration system (10um filter or better) can mitigate against micropitting 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
1N/AN/AN/A
2Micropitting covering up to 25% of the gear tooth.Visual, borescopeNone – run turbine as normal
3Micropitting covering 25-50% of the gear tooth. 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
Not applicable
2
Example of rank 2 micropitting (a gear failure)
3
Example of rank 3 micropitting (a gear failure)
4
Progresses to other failure modes
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