Bearing Failure

Electrical Pitting Bearing Failures

Electrical erosion is the removal of material from bearing contact surfaces by means of an unintended electric current or voltage. There are two distinct types of electrical erosion: fluting and pitting - this page specifically addresses electrical pitting in bearings.

Also referred to as:
Excessive Voltage, Spark Erosion, Lightning Damage, Beading

Electrical pitting – bearing failure severity rating 3

Overview

Electrical pitting, not to be confused with electrical fluting, is often called excessive voltage due to the common cause of an excessive voltage through a bearing, and is most commonly seen on both rolling elements and raceways. Pits are caused as a result of current flowing through the bearing to ground due to a potential difference.

In wind turbines, the excessive voltage is often caused by lightning strikes. Ensuring adequate grounding and using insulated bearings where necessary can help protect bearings from electrical erosion. And although electrical erosion is a severe failure mode, it is not common in wind turbine bearings.

Causes

Electrical erosion is caused by unintended currents flowing through Hertzian contact points. This usually occurs between the rolling elements and raceways of a bearing.

Electrical pitting in bearings is commonly caused by excessive voltage such as from a lightning strike or occasionally a static build up. This can cause current to flow through turbine bearings if there is not an adequate grounding path.

As the current flows between bearing components, sparks may create pits by removing material from the surface of the bearing. This material is removed due to localised melting and welding together before being pulled apart and vaporised or washed away.

The properties of the material around the localised melting zone are often affected due to tempering/rehardening which can cause points of high stress concentration micropitting or crack initiation.

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Appearance

Pits caused by electrical erosion tend to have a conical shape with a melted surface appearance. The craters, which have a diameter up to ~100um, tend to be ordered in a bead like procession in the rolling direction of the bearing.

The pits are usually duplicated on both the rolling element and the raceway due to the contact areas melting and welding together before being torn apart.

Progression

All forms of electrical erosion are considered severe, however there can still be some progression.

Over time, the pits can develop fluting or progress into severe adhesive wear, micropitting and macropitting. This is caused by the change in material properties around the pits due to tempering/rehardening and the stress concentrations as the Hertzian contact patch passes over the surface damage.

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Detectability

Method

Detection Efficiency

Notes

Visual inspection

Hard

Difficult to observe visually due to access limitations on generator bearings. If the micro-craters are large enough and there is access to other bearings then it may be possible to observe visually.

Borescope inspection

Medium

Difficult to observe with a borescope due to access limitations on generator bearings. However, if accessible a borescope will be able to detect electrical erosion.

Vibration analysis

Medium

Noise and vibration of high intensity can be an early indication of electrical erosion so it can usually be detected using vibration data.

SCADA data

Hard

SCADA data does not usually aid detection of electrical erosion.

Oil debris sensor

Hard

Electrical erosion does not shed much debris so cannot usually be detected using oil debris sensors.

Oil sample analysis

Hard

Electrical erosion does not shed much debris so cannot usually be detected using oil sample analysis.

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Discussion

Electrical erosion can be a very severe failure mode if found in a bearing due to the removal of material from the critical contacting surfaces and the high intensity vibration usually experienced. In wind turbines, electrical erosion is most commonly seen in generator bearings, main bearings and pitch bearings. Also, the current discharge can damage the lubricant properties causing further damage to components.

To prevent electrical erosion of bearings and other issues, generator manufacturers incorporate a slip ring and brush system to provide a low resistance path to ground for any charge that may build on the rotor shaft. This grounding system may degrade over time if the ground ring becomes oxidized, the brushes become worn or the brush holder spring does not have enough pressure to hold the brush firmly against the ring. Maintenance of the system is critical and often prescribed bi-annually.

Several solutions exist for generators where electrical erosion is a persistent problem. Solutions are listed in order of increasing expense and effectiveness:

Improving grounding system maintenance practices and frequency
Resurface ground ring
Upgrade grounding brush and/or brush holder
Replace bearings with aluminium oxide outer or inner ring coating to increase electrical resistance
Replace deep grove ball bearings with hybrid bearings that use non-conductive ceramic balls

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Severity rating

Rank 1 N/A

Detection:

N/A

Recommended action:

N/A

Rank 2 N/A

Detection:

N/A

Recommended action:

N/A

Rank 3 Conical shaped pits, up to 100um diameter and ordered in a bead like procession. Duplicated on both the rolling element and the raceway

Detection:

Borescope, vibration

Recommended action:

Monitor regularly for signs of progression or violent vibration and consider scheduling replacement soon. If persistent, improve mitigation against electrical erosion.

Rank 4 N/A

Detection:

N/A

Recommended action:

N/A