Engine & Turbine Technology
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.
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. Tey 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. Te 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 Te 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 Generator bearings must fulfil a challenging set of application-related requirements as well as being non-conducting. Te 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.
Te 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 fully ceramic balls. Tey have outstanding performance characteristics, such as heat resistance, a longer operating life, a lightweight design and limited thermal expansion. Te optimised design gives these bearings a long service life and makes them very reliable. NSK’s ball
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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. 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). Tanks 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.
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. Te 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.
Fault simulation NSK has numerous state-of-the-art test benches for research of this kind. Tis enables the dynamic loads acting on wind turbine bearings to be simulated. Te 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 2MW turbine’s gearbox under simulated start- up and braking conditions. ●
For more information, visit
www.nskeurope.com
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