Articles
A bearing solution configuration comprising tapered roller and cylindrical roller bearings has been developed for supporting the main rotor shafts of wind turbines.
The combination, from Schaeffler, features angular adjustment, enabling narrow axial guidance of the wind turbine rotor shaft, which means that shaft displacement and the effects of vibration are significantly reduced, said the company.
The solution employs the 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.
Up to now, the main shafts of wind turbines are normally 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 that occur 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 in an axial direction. This leads to sliding movements between the rolling elements and raceway, which 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.
Schaeffler’s tapered roller and cylindrical bearing combination prevents high axial clearances that are found in spherical roller bearings. Double row tapered roller bearings in ‘X’ or ‘O’ arrangements are ideal 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, which means the bearing is subject to significantly reduced loads.
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 solution, this is normally achieved by using 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 similar to spherical plain bearings. Coating the surfaces improves the sliding characteristics and prevents fretting corrosion.
Schaeffler said that calculations and simulations have been completed and comprehensive test stand runs have already started. Several wind turbine manufacturers have already expressed interest in the bearing solution. Schaeffler will begin volume production from Spring 2011.