New combination bearings for wind turbine rotor shafts

Schaeffler's new bearing arrangement combines the excellent characteristics of tapered roller bearings as locating bearings and cylindrical roller bearings as non-locating bearings. In addition, the design of the bearing housing enables the required angular adjustment to be made in order to compensate for any misalignment between the bearing positions.

Normally the main shafts of wind turbines are supported by two spherical roller bearings used as a combination of locating and non-locating bearings, with separate bearing housings. Spherical roller bearings are able to compensate for any angular misalignment of the shaft and support the high radial and axial loads incurred in wind turbines.

However, the design of spherical roller bearings is such that relatively high radial internal clearance acts on the locating bearing side. This results in high axial internal clearance that is typically six to seven times higher than the radial internal clearance. If high axial loads occur at low speeds - during a sudden gust of wind, for example - the spherical roller bearing moves axially. This leads to sliding movements between the rolling elements and raceway that can damage the bearing over time. Axial displacement must occur via the bearing housing on the non-locating bearing side. This requires significant forces that increase the load on the bearing.

Reduced axial clearance

Schaeffler's new tapered roller and cylindrical bearing solution avoids the high axial clearances that are found in spherical roller bearings.

Double-row tapered roller bearings in 'X' or 'O' arrangements make excellent locating bearings. The bearings can be set with very little clearance or even with preload. The contact angles of the rows of rolling elements in the bearing are optimally adjusted to match the load conditions. The distribution of loads on both rows of rolling elements is therefore more uniform. The bearing supports the high axial forces acting on the shaft without any lateral sliding movement. Damage to the rolling bearings is therefore prevented and, at the same time, tighter guidance of the shaft and rotor is possible.

In the second position, a double-row cylindrical roller bearing is used as the optimum non-locating bearing of the main shaft. Here, the radial internal clearance is also reduced. The load distribution of the rows of cylindrical rollers is also more uniform. Axial length compensation no longer occurs via the bearing housing but via the rolling elements within the bearing instead, which means the bearing is subject to significantly reduced loads.

Angular adjustment

An angular adjustment facility is also required for the bearing position of this bearing arrangement in order to compensate for shaft deflections. In a conventional arrangement this is normally achieved by means of a spherical roller bearing.

If tapered roller bearings and cylindrical roller bearings are used, static angular misalignments are compensated for by the spherical surfaces between the outer ring and housing. Both bearing outer rings have a spherical outside surface that is located in the concave locating bore in the housing. These spherical surfaces slide on one another in a manner similar to that found in spherical plain bearings. Coating the surfaces improves the sliding characteristics and prevents fretting corrosion.

Calculations and simulations have been completed and comprehensive test stand runs have already started. Several renowned wind turbine manufacturers have already expressed an interest in the new bearings. Schaeffler will begin volume production in the Spring of 2011.

For more information about the new combination bearings for wind turbine rotor shafts, call Schaeffler UK's Marketing Department on +44 (0)121 313 5870 or email [email protected].