NSK has been active in the wind power industry for decades and is one of the main suppliers of bearings for wind turbine gearboxes. We also focus on bearings for main rotor shafts and generators.
Wind turbines generate electricity in adverse, constantly changing conditions both onshore and offshore. The turbines are not readily accessible and designed for a 20-year service life. This places extremely high demands on all of the turbine’s components – especially the bearings. These must be able to withstand the continuous dynamic loading over the long term, even at high wind speeds. This means you need a partner with experience in this sector and a precise knowledge of bearings: NSK.
NSK bearings have gained a global reputation for outstanding quality built on their long life, reliability, resistance and carefully considered design.
NSK Wind Standard. In 2008, NSK played a pioneering role as the first manufacturer to create an in-house standard for bearings used in wind turbines: the Wind Standard U303. This standard specifies 100% traceability of components in each individual bearing all the way back to the material batch as well as all major process variables. It also defines NDT methods (non-destructive testing) to prevent grinding burns, surface cracks and structural faults in the material.
Our sales and application engineering expertise are pooled in the Wind Energy Team. The latest research findings from our technology centres flow into this pool of expertise. Large bearings for the rotor shafts of wind turbines can be tested under varying conditions on our test rigs.
The dynamic wind loads acting on the rotor place a high level of mechanical stress on the power train components of a wind turbine. The main bearing system has to absorb both radial loads and axial forces, including the rotor shaft’s bending moments. The rotational speed normally lies between 5 and 20 rpm. Smaller turbines run faster and larger ones rotate more slowly.
Today’s rotor shafts feature various bearing arrangements. Wind turbines up to 3 MW often have a combination of two spherical roller bearings. A so-called three-point support with only one spherical roller bearing is often used for smaller turbines; in this case the connected gearbox is used as the counter bearing.
A wind turbine’s main gearbox serves to convert low rotor speed into high generator speed. The gearboxes commonly used in megawatt-class turbines consist of one or two planetary stages and one or two spur-gear stages. Operating conditions for wind turbines make high demands of the bearings used for the individual shafts and gearbox components.
Constantly changing wind speeds mean that the bearings are subjected to strongly varying load conditions. The turbines are switched off when it is calm or there are strong winds and are either stationary or spin at very low speed. The resulting start-up and shutdown procedures lead to high loading. When the turbine is stationary, point loads occur that can cause false brinelling. Idling operation with rotation at very low speed is especially problematic, as the gearbox then runs extremely slowly and often without load. This prevents an adequate lubricant film forming, causing increased slippage of the bearing components instead of rolling friction.
Important parameters include the weight of the gearbox, which due to installation at a height of up to 120 metres needs to be as low as possible, and, of course, the costs. Optimum bearing concept and design is thus essential for competitive gearboxes.
Bearing design by experienced engineers using sophisticated calculation and simulation tools: Besides extreme and maximum conditions, load cycles, lubrication, deformation and thermal response are also taken into consideration. This allows a safe design to be created in order to achieve the desired service life using cost-optimised components.
HR taper roller bearings and cylindrical roller bearings from NSK are ideal for main gearboxes as they remain reliable despite changing loads and also have high load ratings. They can be arranged in various different constellations – as locating/non-locating bearings, adjusted bearing arrangements or floating bearings.
Long service lives using BOC (black oxide coating) and patented materials: Black oxide coating is the standard method to reduce the risk of white etching cracks (WEC) in bearings. For high demands the patented special material AWS-TF (Anti White Structure-Tough) is available to reliably prevent damage caused by white etching cracks. To increase the load rating and service life, our STF material (Super-Tough) has proved extremely successful, especially in conjunction with contaminated lubricants.
Operating conditions: sudden charge exchange caused by circulating currents
The role of a generator in a wind turbine is to convert mechanical energy into electrical energy. Factors such as magnetic asymmetry or the use of frequency converters often generate circulating currents which are dissipated in the space between the rotor and the stator. These currents travel via the bearings. The frequent and sudden charge exchange via the very thin lubricant film can cause damage to the raceway and the rolling elements (formation of microcraters). Electrical corrosion occurs if the bearing is subject to these discharges for a sustained length of time.
Conventional deep-groove ball bearings cannot withstand the mechanical loads in generators. However, ceramically coated deep-groove ball bearings and hybrid bearings from NSK are ideal, as they are not susceptible to electrical corrosion.
These coated deep-groove ball bearings have an insulating ceramic layer which electrically separates the bearing from the casing (stator). Although this layer is just a few tenths of a millimetre thick, it has a dielectric breakdown strength of several thousand volts.
With their ceramic rolling elements, hybrid bearings also serve to electrically isolate the stator and the rotor.
Ceramic-coated ball bearings
Operating conditions: high degree of static load
Yaw gears turn the nacelle into or out of the wind. They serve to position the equipment pod so that the whole rotor area is vertical to the wind. Pitch gears are needed to adjust the rotor blades to make maximum use of the kinetic energy.
Once the rotor blades or nacelle have been angled, the drives often stay in this position for several hours. During this time, the drives have to withstand constant load changes and vibrations.
NSK offers a wide range of taper and cylindrical roller bearings with high static load ratings which can be used to realise drives for any rated output.
NSKHPS Spherical Roller Bearings
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