False Brinelling
By Nathan Stang on 5/06/22
Brinelling or True Brinelling, named after James Brinell the inventor of the Brinell hardness scale, is a common form of damage found in bearings. The Brinell hardness scale measures the surface hardness of a material by indentation of a small ball into said material. The dimensions of the indentation created at the given force indicate the Brinell hardness of a material. Similarly, brinelling damage is the indentation of a surface though contact that exceeds the elastic limit of the material. Brinelling occurs mainly from excessive loading or impacts. False brinelling however while similar has different underlying causes and symptoms. False brinelling is attributed mainly to small oscillatory movements or vibration. See figure 1 below to visualize the differences between true and false brinelling. Notice the true brinelling on the left moves or pushes material by yielding it. This creates mounds of plastically deformed material or shoulders, while the end result of false brinelling is the creation of a divot in the material from wear along with metal particulates. [1] False brinelling can be distinguished from true brinelling because it lacks the mounds on either side.
Fig. 1: Ture brinelling (left) vs False brinelling (right)
Causes
The rolling elements in bearings when operating as intended will not make direct contact with the bearing raceways. The lubrication in the bearing separates the rolling elements from the raceways. By preventing this metal to metal contact the bearing’s surface material’s lifetime is significantly extended. See figure 1 below. Although bearing raceways may appear smooth, they have microscopic peaks and valleys of material. Notice when the lubrication layer is hydrodynamic the peaks of each surface are not touching, but as the lubrication layer worsens contact begins. When the material peaks contact each other, they break creating debris due to the high localized contact stresses present. These debris then further contribute to wearing the contact surfaces accelerating false brinelling.
Fig. 2: Lubrication boundary layer depiction
Oscillatory movements or vibrations can cause the lubrication layer in bearings to degrade or thin due to the repetitive motion continuously pushing lubricant out of the rolling elements path. During normal bearing operation this isn’t usually an issue because continuous rotation allows fresh grease to circulate throughout the bearing. False brinelling in “normal” bearing applications is typically a result of transportation, or other situations where the bearings are subjected to oscillatory/vibration motion while the bearing is not rotating. [2] Some applications such as wind turbine blade/pitch bearings are subjected to oscillatory motion or loading with minimal bearing rotation during normal operation. This makes wind turbine blade bearing susceptible to false brinelling even when operating as intended.
False Brinelling and Fretting Corrosion
Fretting corrosion is a very similar type of damage to false brinelling. In fact, it is so similar some consider it to be another name for false brinelling. Fretting corrosion occurs primarily during dry contact conditions while false brinelling occurs during boundary to mixed lubrication conditions. [1] False brinelling creates metal particles that without sufficient protection from lubrication will be allowed to oxidize. As the particles oxidize, they become more abrasive, and will accelerate wear further causing fretting corrosion to occur. [3]
False Brinelling in Wind Turbine Blade Bearings
Wind turbine blade bearings are an especially demanding application and can be exposed to many different failure modes. Due to the lack of bearings rotation in pitch bearings along with the repetitive loading as the turbine rotates these bearings can be subjected to false brinelling damage. Figure 3 below displays some minor false brinelling damage found in a wind turbine pitch bearing after around 5 years in operation.
Fig. 3: False Brinelling
In the above design of bearing, a 2-row 4-point slewing bearing, the ball contact area can travel up and down the raceways as the turbine rotates. This is because 2-row 4-point bearing have a load dependent contact angle. This changing contact angle can damage the lubrication layer in the affected area leading to false brinelling. Figure 4 below displays this phenomenon.
Fig. 4: 45° contact angle (left), contact angle under load (right)
If the turbine pitch system does not sufficiently rotate the bearing during operation, and instead oscillates or holds the bearing at approximately the same pitch angle false brinelling will occur. During low wind conditions this type of bearing loading is especially prominent as pitching activity is often decreased during low wind operation.
False Brinelling Mitigation
False brinelling unfortunately cannot be avoided by changes in design, but control changes and lubricant changes can be made to minimize the effects. All bearings will be subject to false brinelling if they are subject to small oscillatory movements or loading while not sufficiently rotating or circulating fresh lubricant. Control changes can be made to decrease the severity of false brinelling in wind turbines by performing “service pitching”. This involves rotating the bearing a larger number of degrees after it has been operated at the same pitch angle for extended periods. This repairs the lubrication layer in the bearing by circulating fresh grease thought the bearing. Although it is often difficult to alter the turbine pitch control system’s programing to accomplish this effect, additionally control changes can affect turbine output. Because false brinelling is so closely related to the lubrication layer the lubricant used is also a key factor in preventing false brinelling. Ensuring the bearing is properly lubricated with a lubricant fitted to the application is key in false brinelling mitigation.
Fretting Corrosion or False Brinelling?
[1] Godfrey, D. 2003. Fretting Corrosion or False Brinelling? Tribology & Lubrication Technology 59 (12) 28–30.
[2] “False Brinelling,” NSK Americas. [Online]. Available: https://www.nskamericas.com/en/services/troubleshooting/damage-by-type/false-brinelling.html. [Accessed: 22-Dec-2021].
The rusting of steel surfaces in Contact
[3] Tomlinson, G. A. (1 July 1927). "The Rusting of Steel Surfaces in Contact". Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences. 115 (771): 472–483. Bibcode:1927RSPSA.115..472T. doi:10.1098/rspa.1927.0104. ISSN 1364-5021.