Bearing Failure

Micropitting on Bearings Failures

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

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

What is micropitting?

Micropitting refers to the formation of tiny, micro-scale craters (or pits) on the contacting surfaces of a bearing due to surface distress caused by excessive stress, or when the lubrication film is not thick enough to separate high-points, known as asperities, on the bearing roller and raceways. This phenomenon is often a result of mixed lubrication conditions or inadequate lubricant performance, which leads to direct metal-to-metal contact and surface wear.

Micropitting is a less severe but related failure mode compared to macropitting. Both are driven by Hertzian fatigue but differ in severity. While micropitting alone is unlikely to cause immediate failure, it can evolve into macropitting if left unaddressed. Visually, micropitting has a matte, frosted appearance with micro-pits that resemble small black holes.

Causes of micropitting in bearings

Several factors can increase the risk of micropitting in bearing applications, especially in low-speed, high-torque environments like wind turbines. These include:

Inadequate lubrication: Changing lubricant formulations or performance degradation can cause micropitting. When the lubricant fails to develop a sufficient oil film, high asperities come into direct contact, concentrating stress and leading to surface distress. High-quality lubricants on micropitting prevention can play a crucial role in reducing this failure mode.
Poor surface finish: During the manufacturing process, any oversights in grinding or polishing can lead to rough surfaces. Increased surface roughness introduces more asperities, which contribute to abrasive wear and the likelihood of micropitting.
Poor element load distribution: Design flaws can lead to uneven load distribution across the rolling elements, increasing localised stress. For instance, excessive axial loads in main bearings may lead to rough surfaces and micropitting.
Surface defects: Discrete surface defects, such as debris dents or fretting corrosion, can act as nucleation points for micropitting by increasing local stress concentrations.
These conditions often result in surface distress, small micro-cracks, and the liberation of material, eventually forming micropits.

Appearance of micropitting

Though individual micropits may not be easily visible, micropitting typically occurs systemically across bearing surfaces, creating a frosted, granular appearance. The pits appear as small black holes under close inspection, especially when intense lighting is used. Micropitting generally remains shallow, around 10µm deep, but can spread significantly across the bearing surfaces.

Progression of micropitting

The evolution of surface roughness due to micropitting may or may not be progressive. In some cases, the geometry of the contact area changes for the better, leading to a self-arrest of the micropitting. This can happen when lubrication conditions improve or when the rough surface caused by the micropits evens out. On the other hand, if left unchecked, micropitting can degrade bearing geometry, concentrate loads, and disrupt the lubricant film, leading to further damage and progression into macropitting.

Detectability of micropitting

Method

Detection Efficiency

Notes

Borescope inspection

Easy

Micropitting is readily observed and distinguishable with a borescope.

Visual inspection

Medium

Micropitting is detectable to the naked eye in most cases, though a borescope may be necessary for areas with limited visibility.

Vibration analysis

Hard

Early-stage micropitting is difficult to detect via vibration analysis, though it may be detected as it evolves into macropitting.

Oil debris sensor

Hard

Micropitting 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 analysis

Hard

Micropitting 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.

SCADA data

Not applicable

SCADA data does not aid detection of micropitting.

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Prevention and mitigation

Mitigating micropitting involves several measures:

Check lubricant quality: Maintaining the correct lubricant viscosity and ensuring adequate additives are present can prevent metal-to-metal contact.
Optimise surface roughness: Verifying that bearing surfaces are within specification can prevent the formation of asperities and reduce the likelihood of micropitting. If necessary, super-finishing or other corrective actions can help.
Improve load distribution: Modifying bearing geometry to ensure even load distribution can reduce localised stress. This may involve considering system-wide factors like shaft deflections and bearing clearances.

Lastly, installing an efficient filtration system can help remove any debris that could exacerbate micropitting and prevent the progression to macropitting.

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Severity rating

Rank 1 Normal wear of surface asperities, with little to no formation of micropits.

Detection:

Visual, borescope

Recommended action:

None – run turbine as normal

Rank 2 Micropitting covering up to 25% of the bearing surfaces.

Micropitting – bearing failure appearance

Detection:

Visual, borescope

Recommended action:

None – run turbine as normal

Rank 3 Micropitting covering 25-50% of the bearing surfaces. Even beyond 50% coverage, it remains S3 unless macropitting develops.

Micropitting – bearing failure severity rating 3

Detection:

Visual, borescope

Recommended action:

Run turbine and increase inspection frequency – look to identify any progression into macropitting.