Designing Bearings for Wind Turbines

Werner Göbel, senior business manager,
Renewable Energy, SKF


Main gearboxes in wind turbines can exhibit the phenomenon of torque inversion; where the load transfer in rolling bearings changes and leads to a displacement of the shafts relative to each other. These relative movements become bigger with increasing elasticity of the whole construction and with increasing bearing clearance. Due to the inertia of the construction parts that are in motion, the shaft displacements lead to undesired additional loads on the bearings.

The major shaft displacements are axial displacements. In roller bearings, this increases the probability of axial skidding of the rolling elements, which can lead to wear. Wear reduces the service life, and in the worst case, this can lead to total bearing failure. Skidding of rolling elements is not damaging, as long as there is a sufficiently thick lubricant film to prevent direct metallic contact of rolling elements with the bearing rings and the cage. But, the development of such a separating lubricant film becomes more difficult due to:

  • the bigger the degree of irregularity of the skidding movements
  • the lower the speed, and
  • the lower the lubricant viscosity.

The required minimum viscosity for a separating lubricant film increases according to the dimension of the plant due to decreasing speed.

The degree of irregularity of the skidding movements is determined predominantly by the amplitudes and frequencies of the vibrations acting on the bearings, but also by the degree and mode of deformations, which are forced on to the bearings by the environment. Both criteria are assessed and considered within the framework of conventional bearing design, however, not in great detail, since the classical design tools allow this only to a limited extent.

A new look at the problem
The following is a comparison of the different concepts of the classical and the new dynamic bearing design and their respective solutions.

For a long time, rotor shafts have been almost exclusively supported by two spherical roller bearings, or alternatively one spherical roller bearing (picture 1), and the gearbox itself. In both variants, except for a few exceptions, the end of the rotor shaft is mounted into the hollow input shaft of the gearbox. The shafts are then fixed to each other by means of a (shrinkage) coupling, which clamps both ends together. In the first variant, the gearbox mass is supported by the rotor shaft, and the torque support at the gearbox housing takes up only the bearing reaction forces resulting from the torque. In the second variant the torque support has to bear also the gearbox mass and part of the rotor load. In this case, the proportionate rotor load is transmitted via the planet carrier bearing into the torque support.

Due to the flexibility of the machine frame and the interplay of numerous manufacturing tolerances, it is practically impossible in this design to align the housing in a way that the bearing seatings are aligned to the shaft within narrow limits, as it is required for most roller bearings. As deviations may be considered bigger, it is only self-aligning bearings, such as spherical roller bearings, or SKF CARB toroidal bearings, that could be used without problems.

In the past, there have occasionally been problems due to the displacement of the non-locating bearing, as it occurred in the spherical roller bearing by displacement of the outer ring in the housing. Thus, a loose fit is required. Due to the wind turbulences, the direction of the load acting on the outer ring does not remain absolutely stable, but varies at least slightly. This increases the risk of fretting corrosion, which eventually impedes the displacement of the non-locating bearing, and in the worst case reduces the service life of housing and bearing. This problem could be solved after the introduction of the SKF CARB toroidal bearing, as the displacement of the non-locating bearing - similar to cylindrical roller bearings - occurs within the bearing, and therefore, the outer ring can be firmly located in the housing. In the case of the classical design, the ideal construction requires that the rotor shaft is supported by an SKF CARB toroidal bearing as the non-locating bearing, and a spherical roller bearing as the locating bearing.

With the classical design method, there are still quite some improvements that can be made in order to minimize additional loads caused by the system vibrations (blade passage frequency, natural frequencies of tower and blade) and the skidding movements within the bearing. The spherical roller bearings that are fitted in wind turbines, are radial bearings, which can also accommodate axial forces. Especially in a flexible environment, radial bearings need at least a small radial clearance. The clearance increases with the size of the bearing.

Due to the relatively small thrust angle of the spherical roller bearing, the axial clearance can be, depending on the bearing series, 3.8 to 6.5 times higher than the radial clearance. As in the case of the wind turbine design described before with only one spherical roller bearing, this bearing also assumes the function of the locating bearing and, as it is usually placed at the wide diameter of the shaft, the required load capacity is reached with a bearing series 230. However, in comparison to series 240, the 230 series has a smaller thrust angle, and therefore is in the upper range with respect to the axial clearance. In contrast, bearings of series 240 are in the lower range, i.e. axial clearance is up to 35% lower. Moreover, bearings of series 240 have a considerably higher load capacity.

In the design variant with two rotor bearings, the performance capability of series 240 allows the location of the locating bearing at the thinner end of the shaft, which leads to another reduction of axial clearance by 15%. In practice, this would mean that the function of the locating bearing is taken over by a spherical roller bearings 24096 instead of a 230/600, which cuts the axial clearance by half.

Stiffer arrangements but unsatisfactory solutions
However, these solutions are not ideal and there is a need for stiffer bearing arrangements. Contributing to stiffer arrangements are :

  • increase of the thrust angle
  • reduction of bearing clearance up to bearing preload
  • elimination of the self-aligning function

Some of these measures were thought to be solved by using the moment bearing concept, which is sometimes also referred to in the technical literature as single bearing concept. This refers to a solution, in which one single rolling bearing, similar to the azimuth bearing, can also accommodate tilting movements. Therefore, this single bearing was thought to be sufficient to support the rotor. This concept was used in wind turbines more than 10 years ago. At the time, this was still done with a triple-row cylindrical roller bearing (picture 2); a design that had been used for a long time in the case of azimuth bearings. However this solution was unsuccessful in wind turbines and the bearings failed prematurely.

Dynamic analysis leading to new bearing concepts
By the application of modern simulation, analysis and calculation methods, better designs can be developed.

First, in wind turbines the bearing environment is not necessarily stiff enough that the required rotor support and dimensional stability are achieved. The bigger the deformations to which a bearing is subjected to in operation, the higher the risk of wear and early failure.

Second, the failed bearing design had a further handicap. In both axial rows, cylindrical rollers, which due to their geometry have a natural tendency to roll in a straight direction, are forced to follow a circular raceway around the bearing center. This causes friction at the rolling elements.

Both criteria lead to wear, and thus increase the load acting on the cage.

The material dimensional stability and its effect on performance can be investigated by means of FEM calculations that also take into consideration effects of the operating environment. This analysis provides much information from which the bearing design can be modified, in a way that it is better suited to accommodate the stresses resulting from the application.

However, only by the application of dynamic simulation can the interaction between rolling elements, rings and cage be seen and quantified. SKF's self-developed software for bearing simulation, BEAST, is an extremely powerful design tool and was used in the search for a solution. In particular, BEAST gives information about the cage function and interaction that no other design tool can give.

SKF - new solutions
The results have shown that the measures to increase the bearing stiffness can be ideally realized by a bearing concept based on a double-row taper roller bearing in conjunction with a flexible cage and a thrust angle up to 45°. (Picture 3). Due to the taper design, the rollers roll without the "circular raceway generated" friction, mentioned earlier. The friction occuring in taper roller bearings at the bearing shoulder remains small due to a small roller angle. The combination of a small cross section with a large pitch diameter leads to this small roller angle also for bearings with a large thrust angle. In addition, the bearing shoulder is designed in a way that is favourable from the tribological point of view, in order to ensure that an ideal lubricant film can develop in the roller-shoulder contact area. The decisive advantage, however, lies in the flexible cage, by which the total friction moment of the bearing is considerably lower than in all other variants designed so far.

Another design that would be successful are hub bearing arrangements with taper roller bearings, proposed by SKF in the early designs for wind turbines. At that time sophisticated dynamic analysis was not available and verification, by long term field trial and error testing, was not considered economical by the wind turbine industry.

An added advantage of the new SKF solutions is that they also transmit considerably smaller external vibrations to the gearbox, or the generator, which means less stresses and longer life on those components.

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