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13/10/2010
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With bearing problems representing the single biggest reason for wind turbine failures, Paul Fanning looks at some of the latest methods being employed to combat the problem.
 The wear on bearings is the single biggest technological problem facing the windpower industry, regularly leading to failure and stoppages. For this reason, a number of suppliers have developed or are developing solutions to reduce these failure rates significantly.
One such solution comes from Schaeffler, whose rolling bearing for wind turbine gearboxes is not only resistant to the effects of slippage, but actually prevents slippage from occurring in the first place.
Slippage is a huge problem for rolling bearings. The life and performance of a rolling bearing will suffer if it is overloaded or under-loaded. Because rolling bearings are designed with a minimum load requirement in order to function optimally, if f this minimum load is not met, slippage will occur. This means that the rolling elements not only rotate, but also slide on the bearing raceways. Eventually, this can lead to surface damage such as increased wear or smearing, particularly in critical lubrication conditions.
The FAG tube roller bearing is a conventional cylindrical roller bearing that incorporates three cylindrical rollers with a slightly increased diameter. These three rollers have a hollow design and are supported by a support roller in the inner bore. These rollers with a slightly increased diameter generate preload in the bearing and drive the bearing cage (and therefore the entire roller set) at low loads.
At higher loads, the bore of the cylindrical rollers ensures sufficient deflection of the larger rollers so that that they are not overloaded. This means that the load is evenly distributed to all the rolling elements, similar to a conventional rolling bearing.
The support roller in the inner bore of the tube roller has a slight clearance to ensure that the roller is less rigid (more flexible) at higher loads. At the same time, this also prevents excessive deflection of the tube roller.
NSK presented a new innovation at HUSUM WindEnergy event – a hybrid bearing with ceramic balls that boasts a long life and also acts as an electrical insulator. This function is needed to prevent electricity from passing through the power train and to minimise the associated damage, when current peaks or circulating currents (electrical corrosion) occur.
NSK's ROBUST bearings with ceramic balls have gained a reputation as extremely stressable, long-life, high-precision bearings in the machine-tool industry. Now, NSK is utilising this experience for larger bearings – such as those used in wind turbines – and has ramped up its production capacities for high-precision ceramic components accordingly.
Another tack has been taken by Cooper Roller Bearings. The King's Lynn-based company has elected to address the issue of ease of repair by the use of its split roller bearings. The company recently received a grant of £256,250 from the Department of Energy and Climate change to, "develop and demonstrate the use of split bearing technology in large scale wind applications." Cooper is the world's largest manufacturer of split mounted roller bearings.
The advantage of the split roller bearing over the solid bearing is reliability and how easy it is to replace. If you put wind turbines 70 kilometres offshore - which is being planned for the North Sea at the moment - it is very difficult to service them.
Using these bearings is that you don't need to take anything off the shaft to replace the bearing, whereas if a fixed bearing goes, it is necessary to send a crane out to take the whole rotor off, which is both expensive and time consuming. Using split roller bearings, however, only a small work boat is needed.
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Author
Paul Franning
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