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.
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.
|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 data||✓||SCADA data does not aid in detection of brinelling.|
|Oil debris sensor||✓||As no material is shed during brinelling, an oil debris sensor will not detect brinelling.|
|Oil sample analysis||✓||As no material is shed during brinelling, an oil sample analysis will not detect brinelling.|
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.