White Etching Cracking Failures
By Cory Mittleider on 08/03/21
Background
In the early years of multi-MW (1.0MW and larger) wind turbines operators were surprised by a high failure rate of high-speed shaft (HSS) and high-speed intermediate shaft (HSIS or IMS) bearings. Many of these failures were occurring in less than 5 years of operation. Over the last several years many in the industry have investigated these failures including the bearing manufacturers, OEM’s, operators and more.
Over a decade later the causes of this problem are much better understood, and the operators and service providers are better equipped to identify and replace these bearings up tower.
Investigation
The cracks are typically found on the inner ring, and when caught early enough look like figure 1. If left to run longer the crack will continue through the entire ring and lead to additional damage to the gearbox housing, shafts and/or gears.
Fig. 1: Image of cracked and spalled inner ring
Viewing the cracks up close, as shown in figure 2, you can see the grind lines of the raceway are clean all the way to the crack. This confirms that the crack initiated below the surface, so the rings are cut and prepped for inspecting with a scanning electron microscope.
Fig. 2: Close up image of cracked raceway surface
When the cracked bearings are sectioned and inspected by microscope, elaborate crack networks are found underneath the surface, as seen in figures 3 and 4. These areas show up as a brighter white reflection leading to this failure being named White Etching Cracking or WEC for short. Some also call this White Structure Failure or WSF. These white areas within the cracks are comprised of broken ultra-fine ferrite pieces, essentially an iron sand.
Cause of failure
The WEC failures are the result of a specific combination of events described below.
Bearing slip
Roller bearings require a minimum amount of load to ensure rolling of the rollers during operation. If this load is insufficient or there is a sudden change in rotational speed this can lead to slipping contact between the raceway and rollers as shown in the below video.
This scenario occurs frequently in Wind turbine gearboxes when bringing the turbine online which creates a high speed but low load scenario on these bearings. NREL (National Renewable Energy Laboratory) investigated this effect in the cylindrical roller bearings (CRB) in the HSS-RS and HSS-GS positions of their 1.5MW test turbine. The cage is driven by the rollers and therefore is an accurate way to identify if the rollers are in fact rolling. The tables below show a comparison of the theoretical cage speed (assuming full rolling contact) compared to the measured cage speed during a high speed, low load scenario while bringing the turbine online. These results show that in this scenario the cage is going at 75-80% slower than it would with full rolling contact, indicating a massive amount of slip between the rollers and raceway. Figure 5 is from SKF’s presentation at NREL DRC in 2018. [1]
Fig. 5: Calculated vs measured cage speed under high speed/load load
Lubrication film
Ideally during operation, the oil in the gearbox provides full separation between the roller surface and the raceway surface, called full film or hydrodynamic lubrication. In certain cases, this is not possible and leads to either mixed lubrication or worse boundary lubrication. In these scenarios the asperities are allowed to contact one another. Figure 6 shows a closeup representation of the surface asperities of a roller and raceway and what these different lubrication cases look like.
Fig. 6 Boundary vs Mixed vs Hydrodynamic lubrication
These poor lubrication conditions can be caused by many things such as incorrect viscosity, contamination, or simply degraded oil. In any case, even with good oil, the sliding action of the roller in the raceway from the bearing slip described above will also act to wipe away oil reducing this layer thickness.
The combination of mixed or boundary lubrication leading to contact of asperities while the rollers are sliding leads to smearing damage of the raceways. This action opens a fresh wound on the raceway surface with a negative charge.
Hydrogen
In addition to the base oil there are many additive packages and even some water suspended in the oil. When the smearing damage described above occurs it creates localized high pressure and temperature spots which can strip away a positively charged hydrogen ion from the oil. This positively charged hydrogen ion is attracted to the negatively charged surface created by the sliding damage above.
Fig. 7: Hydrogen generation leads to weakened bonds and subsurface failure in gearbox bearings
The hydrogen ions bond to the steel and migrate below the surface. The hydrogen weakens the bonds between the steel molecules and with the continuous over-rolling of these areas the bonds break initiating a crack. Once the crack is started the continuous cycling of the applied load to the area in addition to potentially more hydrogen generation and bonding weaking more bonds will grow the crack further until it reaches the surface and eventually the entire bearing ring is cracked through. Figure 7 shows this process.
Results
The result of the combination of these events leads to the WEC failures in Wind gearboxes. These typically occur in the HSS and HSIS positions. Argonne National Labs has also stated that there is a causal link between the sliding found in these bearings and WEC failures with additional contributing factors from certain oil additives and even electrical discharge. [2]
When these failures are detected, they must be replaced. Waiting too long to replace bearings with initial cracking failures will lead to more damage within the gearbox as the wear continues leading to a bearing ring that is completely cracked through.
Solutions
There are tens of thousands of wind turbines installed across the country where fundamental redesign of the gearbox is not possible, so it is important to understand the options available to be applied or installed in these existing applications.
Without being able to change the loading, or bearing design used on the shaft operators are mostly left to options such as improved lubrication options, cleaning, and monitoring and when a bearing does fail replacement with improved bearing technology. Below is a summary of bearing related options to help with these problems.
Black Oxide
The earliest and simplest attempt to solve the problem was black oxide coating (a.k.a. BOC). Often called a coating but more accurately it is created not by coating the bearing, but by a special process that converts (oxidizes) the steel surface of the bearing. This is usually applied to the rings as well as the rollers of the bearing as shown in figure 8.
Fig. 8: Exploded view of black oxide gearbox bearing
As it relates to WEC failures BOC has 2 primary advantages. First it increases oil adhesion to the surface increasing lubrication film which can help avoid mixed and boundary lubrication conditions longer than an uncoated bearing. Second, when the surface is oxidized it is in a more chemically stable state, so generated hydrogen is less likely to attach to the steel and migrate within.
The disadvantage of black oxide is that it is comparatively softer than the rest of the bearing. Over time this converted material can wear away eliminating the benefits it was originally used for. This running in and wear away process can happen quite quickly in Wind applications.
Read more about Black Oxide here
Case Hardening
Figure 9 shows the difference in hardening profile between through hardened and case-hardened bearings. Case hardened bearings are often used in tough applications when the standard, through hardened, bearing types are found to be inadequate. You can read more about through hard vs case hardened bearings here.
Fig. 9: Comparison of through hard to case hard bearing
Case carburizing is the most common method of case hardening, but there are other options offered by different manufacturers such as case carbonitriding.
As it relates to WEC failures case hardened bearings have 1 primary advantage which is the compressive residual stress whereas through hardened bearings typically have a neutral residual stress. If hydrogen does start to generate and migrate into the bearing material weakening the atomic bonds within the steel the compressive residual stress resists the crack initiation and propagation.
Case hardened bearings combined with black oxide offer an even better improvement than black oxide alone in performance against WEC failures.
Special materials
In addition to case hardening heat treatments, special materials can help prolong or completely prevent WEC failures. There are 2 options most prominently available in the market for Wind applications: Super Tough and Anti-White Structure Tough.
Super Tough
NSK’s Super Tough (a.k.a. STF) bearings were developed from the ground up with a specially designed metal alloy combined with case carbonitriding heat treatment. This combination provides several important characteristics to the finished bearing including high retained austenite at the raceway surface, higher surface hardness of the raceway compared to either through hard or case carburized bearings, and an improved carbide distribution leading to improved load carrying and long life.
In addition to a significant increase in life in WEC conditions, Super Tough bearings offer the best service life available in poor lubrication conditions as shown in figure 10.
Fig. 10: Life of various bearing technologies with contaminated lubrication
Anti-White Structure Tough
NSK’s Anti-White Structure Tough (a.k.a. AWS-TF) bearings are targeted specifically for the WEC failures in the Wind industry. After researching the failures, they developed a new alloy specifically aimed at the root cause of the WEC failures as described above. The alloy used in these bearings prevents hydrogen attachment and migration within the bearing in turn preventing WEC failures better than any other bearing technology available. [5]
Fig. 11: Life of various bearing technologies in WEC conditions
Conclusions
With a thorough understanding of the cause of failure operators now have tools available to improve bearing life in their gearboxes, and when they do encounter a failure there is now much better bearing technology available for replacement bearings.
NREL has been a major contributor to solving the WEC problems that face the industry, through their own research, collaborative research, and hosting a yearly forum to present and discuss this research in the Drivetrain Reliability Collaborative (DRC). They are now also working on models to better predict these failures before they even occur.
Resources
[1] Vaes, D. (2018, February 21). Uptower measurements to understand roller slip in HS-S bearings. Lecture presented at NREL DRC in CO, US, Golden. here
[2] Zeroing in on the no. 1 cause of wind turbine gearbox failures. (2020, October 13). Retrieved July 28, 2021, from here
[3] Mittleider, C. (2020, October 06). What is Black Oxide?, Retrieved July 28, 2021, from malloywind.com/articles/boc
[4] Mittleider, C. (2018, August 15). Through Hardened vs Case Hardened Bearings, Retrieved July 28, 2021, from malloywind.com/articles/th-ch
[5] NSK. (n.d.). AWS-TF Anti-white Structure Bearings [Brochure]. Author. Retrieved July 28, 2021, from here
Press release about DRC here
History of NREL presentations at DRC here
If you have any questions please give me a call at 605-357-1076
-Cory Mittleider