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Brinelling

Brinelling – Bearing failure

Brinelling (true brinelling) refers to the permanent deformation of a bearing surface, commonly the rolling surface of the bearing raceways, caused by excessive load on the bearing. It commonly occurs when there is a static overload or shock load on the bearing, when the contact stress exceeds the yield strength of the material. The appearance is a depressed ellipse, or line contact depending on the rolling element type, with the original surface finishing marks still visible inside the depression. True brinelling and false brinelling are commonly confused with each other due to similar appearance. False brinelling, a form of fretting corrosion, is a different failure mechanism; wear of the surface due to vibration rather than an overload of the bearing. Brinelling is also distinct from indentations which are discrete and caused by debris, whilst brinelling is plastic deformation over a substantial part of the contact footprint caused by overload of the bearing itself.

Also referred to as: True Brinelling, Overload, Loading Marks

Causes

True brinelling is caused by an excessive load applied to a bearing, so that the contact stress exceeds the elastic limit of the ring or rolling element material. This excessive load can be either a static overload of the bearing or a shock load experienced during operation and can be caused either by a single overload event or a series of overloads. A static overload can be experienced when the turbine rotor is locked during maintenance, or when the wind speed is not sufficient enough to rotate the rotor, and there is a high load applied to the bearing. These high loads could be caused during maintenance work, or could simply be from the rotor weight if the bearing has not been properly designed to withstand it. Excessive preloading or mishandling during transport or installation can also cause brinelling.

Appearance

True brinell marks are a permanent deformation of the rolling surfaces of a bearing. The deformation marks are typically shaped either elliptic, round or rectangular depending on the shape of the Hertzian contact patch, which changes for different bearing types. Although identical in shape to a false brinell mark (see Fretting Corrosion), the original surface finish can still be seen in the depressions of a true brinell mark. Brinell marks in bearings are commonly seen in groups, as a single brinell mark will be created from each rolling element in the load zone where the contact stress exceeds the yield stress of the material. There may also be raised edges observed around the brinell mark as material around the edges are pushed upwards as the depression is created. Micropitting may be visible around the edges of the brinell marks.

Progression

The plastic brinell depressions will create local stress concentration areas, and when overrun by other rolling elements during continued operation, cracks could initiate in the raceway surfaces. Potential sharp raised edges formed at the edge of the brinell marks can cause macropitting to occur when overrun during continued operation due to the increased cyclic stress at the edge of the brinell mark. This will typically occur downstream of the brinell mark, in the rolling direction. Also, a reduction in the oil film thickness may be seen due to the depressions allowing oil to fall into them. This can lead to increased wear particles in the oil due to more boundary contact between surface asperities, causing consequential damage to other components.

Detectability

MethodDetection EfficiencyNotes
Visual inspection✓✓If accessible, brinelling will usually be visible with the naked eye. Minor cases with only slight brinelling marks may be difficult to see without a borescope.
Borescope inspection✓✓✓Brinelling will be readily observed and distinguishable with a borescope.
Vibration analysis✓✓✓Brinelling large enough to be of concern will cause a repeating impulse which can be easily detected using vibration analysis.
SCADA dataSCADA data does not aid in detection of brinelling.
Oil debris sensorAs no material is shed during brinelling, an oil debris sensor will not detect brinelling.
Oil sample analysisAs no material is shed during brinelling, an oil sample analysis will not detect brinelling.

Discussion

In order to avoid brinell marks during mounting, make sure to apply the mounting load on the race that is being mounted. For example, apply the mounting load to the inner ring when mounting a bearing to the shaft, and the outer ring when mounting the bearing in the housing. This will make sure there is no overload of the bearing between the raceways and rolling elements which could create brinell marks. It is very important to ensure a bearing is specified that has the correct static and dynamic load capacity for the application and running conditions. Otherwise, a static overload of the bearing could occur during turbine downtime due to the static weight of the rotor and blades.

Severity Rating

RankDescriptionDetectionRecommended Action
1N/AN/AN/A
2N/AN/AN/A
3Permanent deformation depression in the shape of the Hertzian contact patch between the rolling elements and raceways (ellipse, round or rectangle). Material is pressed into the surface so the original surface finishing marks are visible. May see raised edges around the brinell mark.Borescope, vibrationRun turbine. Increase inspection frequency to monitor for progression. Consider scheduling replacement.
4N/AN/AN/A
1
Not applicable
2
Not applicable
3
Example of rank 3 brinelling (a bearing failure)
4
Progresses to other failure modes
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