NSK: Current trends and development projects for wind turbine bearings

Even higher expectations regarding service life and reliability twinned with pressure on costs: this is the picture currently emerging for bearings in the wind energy industry. With the help of examples, the following article explains how one of the world’s leading suppliers develops new generations of bearings in these circumstances.

At first sight, the business news from the wind energy industry appears contradictory. Last year, turbines with a total output of 40 GW were installed around the world and the industry is expected to keep growing in the future. In fact, a whole new market is being born with the emergence of offshore technology. At the same time, however, leading manufacturers of wind turbines and suppliers who specialise in wind power are announcing harsh cost-cutting programmes. Experts also expect to see further consolidation amongst suppliers.

NSK wind industry
Considerable cost pressure impacting on the sector
The explanation for this apparent contradiction is that although the industry is growing, it had anticipated even stronger growth. This means that it now has substantial excess capacity. The result is considerable pressure on prices, which is also affecting suppliers. For one of the world’s leading manufacturers of bearings for wind turbines, this means that research and development projects must keep striving to meet the industry’s extremely high expectations for bearings’ service life in adverse operating conditions with highly dynamic stress. At the same time, however, the R&D team must take into account the costs associated with any new development or enhancement at an early stage and produce a clear cost-benefit analysis.

Where does the greatest potential for innovation lie?
Of course, wind power companies also welcome innovations which cut costs without compromising on bearings’ reliability. This is particularly important as experts agree that the global competition among turbine manufacturers is set to become even more intense in the future.

With all this in mind, suppliers have to look very closely at which areas have the greatest market potential for developing new generations of bearings. NSK has identified multi-megawatt turbines with an output of 2 to 4 MW as one such area. The company is already well positioned in this segment: in recent months, leading manufacturers have presented new turbines equipped with NSK bearings.

Cylindrical roller bearings with higher load ratings
NSK has already optimised cylindrical roller bearings for the planet wheels of turbines in the 3 MW class, running two development projects simultaneously in this field. The first project aimed to improve the internal design. Without changing the bearing diameter or the number of rollers, the team optimised the micro-geometry’s design and production engineering while keeping the macro-geometry the same – resulting in a 15% increase in the dynamic load rating. This leads to a 70% longer calculated service life. As well as being suitable for use in planet wheels, these bearings are ideal for other applications on the ‘fast’ side of the wind turbine gearbox.

The second project was dedicated to fundamentally redesigning the bearing’s internal design. The number of rollers was increased, meaning that the cross section of the cage was reduced. It still has to withstand the same loads, however. The project team achieved this by optimising the cage’s geometry without impairing its rigidity. An FEM analysis and extensive test bench trials were conducted to back up the optimisation work. As well as a higher dynamic load rating, these bearings have an enhanced static load rating.

Customers will soon be able to choose between the two options outlined above. It should be noted, however, that NSK always modifies its bearings to cater for the specific application scenario and/or turbine type.

New cage design for tapered roller bearings
NSK’s tapered roller bearings are often used for the planetary carriers in the input stage of wind turbine gearboxes and for the rotor shaft. They can also be found in the so-called integrated rotor bearing, which pushes the gear unit and the hub together.

In the past, NSK used cages made from pressed, thick metal plate for these bearings. However, the ever-growing bearing diameter forced the company to find alternative solutions, so it developed a welded cage for diameters of 1,000 mm or more. The cage fulfils the highest technical specifications, but it is very complex to produce and requires advanced welding skills. For this reason, NSK is currently testing a plastic segment cage, which is cheaper to produce and can also be manufactured with a high level of reproducibility.

Improved raceway surface design
The raceways of bearings are subject to a great deal of strain, especially on the fast-moving shafts. For example, if the turbine is switched on to run idle at full speed, slippage can occur which may damage the surface of the bearings. Slippage damage of this kind is a known problem in the industry, but its cause has not yet been identified, despite in-depth research. Although NSK produces a huge number of bearings, it is comparably rare for the company’s components to suffer from this problem. NSK attributes this to its special surface finishing and cage location. Nevertheless, the company’s research and development department is continuing to work on further optimising the surface design.

Generator bearings with ceramic balls
Generator bearings must fulfil a challenging set of application-related requirements as well as being non-conducting. The idea is to prevent electricity from passing through the power train and minimise the associated damage, e.g. when current peaks or circulating currents (electrical corrosion) occur. NSK offers electrically insulated bearings with ceramically coated outer rings specifically to cater for these requirements. It has now further enhanced this technology.

The bearings previously used have a permanent ceramic coating, which is applied to the steel surface using plasma spraying. As an alternative, the company also supplies bearings with outside diameters of up to 340 mm as hybrid bearings with ceramic balls. They have outstanding performance characteristics, such as heat resistance, a longer operating life, a lightweight design and limited thermal expansion. The optimised design gives these bearings a long service life and makes them very reliable. NSK’s ball production process benefits from the company’s experience of manufacturing high-performance hybrid bearings for machine tools.

Advances in materials engineering too
Endurance tests have shown that the rolling fatigue life of carbon-chromium alloy steel used for bearings is significantly affected by non-metallic inclusions. In response to this, NSK has developed a new material in conjunction with a steel producer – Z steel – which contains far fewer non-metallic inclusions, such as sulphur and oxides. Tests under operating conditions show that the service life of bearings made from Z steel is up to 1.8 times longer than that of bearings made from conventional, vacuum-degassed steel.

Service life up to ten times longer
Many wind turbine bearings – cylindrical roller bearings, tapered roller bearings, spherical roller bearings, deep-groove ball bearings and angular-contact ball bearings – use the Super Tough steel material developed by NSK (Super-TF). Thanks to use of a different steel alloy and heat treatment designed to optimise the material’s martensite and residual austenite content, Super-TF bearings last up to ten times longer than components made from conventional bearing steels.

Causal research into damage
In-depth materials development work also pays off when the industry seeks to clarify why particular types of damage occur. Damage of this kind usually only affects bearings from NSK comparatively rarely. White structure flaking is one such problem whose cause has not yet been identified with absolute certainty. The damage profile itself is well known because similar damage was observed some twenty years ago in generator bearings within the automotive industry. It is possible that current passage is also the cause in the context of wind power, but researchers have been unable to prove this to date. A specialist conference on the subject generated plenty of insights but did not produce a clear picture of the cause. NSK is working on reproducing the mechanism of the damage in order to gain a better understanding of the processes at work in high-performance bearings.

Test bench for fault simulation
NSK has numerous state-of-the-art test benches for research of this kind. This enables the dynamic loads acting on wind turbine bearings to be simulated. The test benches are used to calculate the parameters for new generations of bearings, to simulate faults and to generate damage profiles. At present, the developers are testing cylindrical roller bearings and tapered roller bearings for the ‘fast’ side of a 2 MW turbine’s gearbox under simulated start-up and braking conditions. The aim of these tests is to develop even better detailed designs and improved materials. Development work is also under way to reduce costs. Given the very high cost pressure in the wind energy industry, clients are delighted whenever NSK succeeds in offering the same bearing properties at a lower price.

source: NSK