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Blowin’ in the wind – Wind Turbines benefit from Schaeffler bearings
White etching cracks signify damage within the microstructure of steel, drastically shortening the reliability and operating life of bearings in wind turbines. However, thanks to Schaeffler UK, help is at hand via a wide range of innovative solutions.
Wind power is one of the fastest growing sustainable technologies – with 93GW of new capacity installed in 2020 – a record year for the industry. Today, there is 743GW of wind power capacity worldwide, helping to avoid over 1.1 billion tonnes of CO2.
Yet this level of adoption is not sufficient to make sure the world achieves net zero by 2050. In fact, countries across the world need to be installing wind power three times faster over the next decade to meet this target and avoid the worst effects of climate change.
Minimising total cost of ownership
As more wind turbines are installed, the issues that could hinder their operational effectiveness are under increasing scrutiny. Obviously, if a wind turbine is not being used, it cannot produce clean energy, while maintenance, repair and operations (MRO) costs can soar, particularly in the case of offshore wind farms. The reliability of components therefore plays an essential role in minimising TCO, maximising operational efficiency, achieving regulatory compliance, and enhancing worker safety.
The development of larger, multi-megawatt wind turbines has resulted in significantly higher forces, torques, and loads. With such a wide range of bearings performing different functions within a wind turbine’s structure, white etching cracks (WECs) have become a major cause for concern.
WECs are structural changes in the material that appear below the surface of the bearing – the material is weakened, and micro-cracks form the inner or outer ring, drastically shortening reliability and operating life. These cracks are known to occur in both through-hardened and case-hardened rolling bearings.
WECs are not caused by influences that could have an effect on component fatigue such as contact pressure, insufficient level of steel purity or overloading. In fact, it is still not completely clear what causes WEC to occur. However, according to recent research, additional stresses in the form of dynamics, mixed friction, and the effects of electrical systems are most likely creating the conditions for them to form.
Research and development
Schaeffler has been producing bearings for wind turbines for more than 30 years, including high-performance bearing solutions for rotor shafts, gearboxes, generators, and wind tracking systems.
Now the number one global manufacturer for bearings for this sector, Schaeffler’s experts work in close cooperation with wind turbine developers, manufacturers, and operators to develop a range of innovative solutions aimed to drive the resistance of bearings to WECs, preventing premature failure.
To further their knowledge and best practice expertise, Schaeffler has invested in comprehensive test facilities for creating WECs, enabling them to analyse causes and concept solutions. This research has enabled them to reduce the risk of WECs occurring by optimising the design of bearings and selecting the most appropriate lubricants and other materials for individual bearings components.
Schaeffler uses state-of-the-art calculation and simulation software program to ensure that the optimum design can be developed for a range of bearing applications. This is considered as part of its service – starting from a single rolling bearing and its components, through to the adjacent construction and the entire power transmission system. It also includes a range of remote condition monitoring systems and diagnostic programs. Through this wealth of expertise, Schaeffler is now the leading provider of wind turbine MRO services worldwide.
A solutions provider
The solution that Schaeffler recommends – which has been statistically verified – for the efficient and cost-effective reduction of WEC damage is the through-hardening of the bearings, in combination with a black oxide coating on the inner ring, outer ring, and rolling elements – with Schaeffler experts developing an ideal coating system that significantly increases a bearing’s resistance to WECs.
The Durotect B coating system has been developed from conventional black oxide finishes to give increased performance, minimising the risk of damage caused by slippage, improving run-in behaviour, increasing corrosion resistance, and the enhancing the level of protection against WECs.
In fact, research has shown that WECs have occurred in less than 100 out of 1 million through-hardened bearings with black oxide or Durotect B coating that were produced and delivered by Schaeffler for use in wind turbines since 2005. This corresponds to an incredibly low failure rate of less than 0.01%.
For applications that require high load ratings, Schaeffler also offers rolling bearings made from Mancrodur carbonitrided steel and coated with Durotect B. For those who aren’t familiar, carbonitriding involves subjecting the bearings to a special heating process in which the bearing surface is enriched with carbon and nitrogen, increasing surface hardness and wear resistance.
Strength and durability
Similarly, Cronidur special steel with high chromium content can be used to completely eliminate the risk of WECs. Cronidur 30 is a martensitic through-hardened steel that, due to its alloy composition – carbon, nitrogen, chromium, and molybdenum – exhibits exceptional corrosion resistance, strength, durability, and high temperature hardness.
An inspection programme carried out by Schaeffler to determine individual solutions for different applications and additional stresses showed that no cases of WECs occurred in bearings with Cronidur 30.
Bearings for wind turbines can also suffer damage due to the passage of electrical current, so in order to prevent this, Schaeffler has developed a ceramic insulation coating. Commonly used for wind turbine generators, Insutect A is applied using the plasma spray method, then sealed and applied to the outer ring (J20AA) or inner ring (J20C).
Avoid costly unplanned downtime
Another method of avoiding premature failure of your bearings is to ensure that the main bearing arrangement and its lubrication are perfectly in sync. For example, 10MW offshore wind turbines that are intended to be operational for a minimum of 25 years, costly maintenance work must be kept down.
This is essential, as the cost of dismounting bearings and replacing components can incur exponential costs. The rental costs of boats, cranes, and other equipment for replacing a main shaft bearing alone, can be as high as €800,000.
In these circumstances, any unplanned downtime can last up to 20 days. If the customer loses 2.2 million kilowatt hours during those 20 days, costs can rise to a further €200,000.
Therefore, the savings achieved by avoiding unplanned production stoppages can reach approximately €1 million, proving that the correct amount of grease for the main shaft bearing is truly essential.
Take the best foot forward
Machine elements in modern mechanical systems are subjected to increasingly harsh operating conditions and wind turbines are no exception.
WECs continue to be a cause of the premature failure of rolling bearings, and while research from Schaeffler into what causes this to happen remains ongoing, the good news is that there is a practical and proven coating system that significantly increases a bearing’s resistance.
For more information on Bearing & Lubrication solutions and to get in touch with one of our ERIKS Bearings Specialists or Lubrication Engineers, please contact your local ERIKS Service Centre, who will be happy to discuss your options.
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