In order to increase the reliability, availability and, therefore, the economy of wind turbines, Schaeffler Technologies has revised its portfolio of spherical roller bearings for main rotor bearing supports by introducing a range of design measures.

Using operating data from approximately 10,000 wind turbines, Schaeffler investigated the reliability of spherical roller bearings, which are used as the main bearing in many applications. The engineers discovered a typical pattern of damage and developed appropriate countermeasures. The result is spherical roller bearings optimised for use as the main bearing with revised micro and macro geometry. The development of an asymmetrical spherical roller bearing also represents a significant step towards greater robustness.

Sturdy design for reduced wear

The bearing supports of the rotor shaft are of central significance in wind turbines. The locating bearing is subject to especially high axial loads. With this in mind, Schaeffler has optimised its tried and tested standard spherical roller bearings in order to meet the specific requirements for use in wind turbines.
The new measures focus on two areas. With regard to microgeometry, developers have improved surfaces in order to reduce friction, and thus the application of energy and wear. In addition, they have adjusted osculation to transmit rolling-element normal forces across a large surface, reducing contact pressure. The rolling elements have also been specially profiled.
Adjustments in macrogeometry include a reduction in the internal bearing clearance, which has improved operating clearance and decreased the rolling bearing forces and the axial sliding of the drive train. The use of a firm centre rib for the locating bearing supports of the wind turbine rotor and increased axial rigidity also reduce the axial displacement of the drive train. Moreover, the firm centre rib helps distribute the contact pressure along the roller. With these measures, Schaeffler has been able to increase the bearings’ robustness against wear, which is caused especially by axial displacement. The bearings have been subjected to comprehensive testing to obtain Schaeffler’s ‘X-life’ seal of approval, with the necessary validation steps certified by Germanischer Lloyd (certificate GL-CER-002-2015).

Improved load distribution

The asymmetrical bearing design represents yet another major step. It significantly increases the axial load carrying capacity and thus the service life of the main bearings in wind turbines. It allows a greater contact angle on the row of bearings supporting the axial load, and a flatter contact angle on the row of bearings that mainly support radial loads. This results in better load distribution, decreased contact pressure and a significant reduction in axial sliding distance. The figure below shows the axial displacement in relation to the external axial force for the various bearing designs. The curve shows a difference in the Fax/Frad ≈ e load range. Since locating bearings in wind turbines are often operated in this range, the calculated accumulation of the axial distances shows significant differences. The higher the value, the greater the risk of fatigue close to the surface.