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 motionthe shaft displacements lead to undesired additional loads on the bearings.
The major shaft displacements are axial displacements. In roller bearingsthis increases the probability of axial skidding of the rolling elementswhich can lead to wear. Wear reduces the service lifeand in the worst casethis can lead to total bearing failure. Skidding of rolling elements is not damagingas long as there is a sufficiently thick lubricant film to prevent direct metallic contact of rolling elements with the bearing rings and the cage. Butthe 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
- 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 bearingsbut also by the degree and mode of deformationswhich are forced on to the bearings by the environment. Both criteria are assessed and considered within the framework of conventional bearing designhowevernot in great detailsince 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 timerotor shafts have been almost exclusively supported by two spherical roller bearingsor alternatively one spherical roller bearingand the gearbox itself. In both variantsexcept for a few exceptionsthe 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) couplingwhich clamps both ends together. In the first variantthe gearbox mass is supported by the rotor shaftand 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 casethe 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 tolerancesit is practically impossible in this design to align the housing in a way that the bearing seatings are aligned to the shaft within narrow limitsas it is required for most roller bearings. As deviations may be considered biggerit is only self-aligning bearingssuch as spherical roller bearingsor SKF CARB toroidal bearingsthat could be used without problems.
In the pastthere have occasionally been problems due to the displacement of the non-locating bearingas it occurred in the spherical roller bearing by displacement of the outer ring in the housing. Thusa loose fit is required. Due to the wind turbulencesthe direction of the load acting on the outer ring does not remain absolutely stablebut varies at least slightly. This increases the risk of fretting corrosionwhich eventually impedes the displacement of the non-locating bearingand 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 bearingas the displacement of the non-locating bearing – similar to cylindrical roller bearings – occurs within the bearingand thereforethe outer ring can be firmly located in the housing. In the case of the classical designthe ideal construction requires that the rotor shaft is supported by an SKF CARB toroidal bearing as the non-locating bearingand a spherical roller bearing as the locating bearing.
With the classical design methodthere are still quite some improvements that can be made in order to minimise additional loads caused by the system vibrations (blade passage frequencynatural frequencies of tower and blade) and the skidding movements within the bearing. The spherical roller bearings that are fitted in wind turbinesare radial bearingswhich can also accommodate axial forces. Especially in a flexible environmentradial 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 bearingthe axial clearance can bedepending on the bearing series3.8 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 bearingthis bearing also assumes the function of the locating bearing andas it is usually placed at the wide diameter of the shaftthe required load capacity is reached with a bearing series 230.
Howeverin comparison to series 240the 230 series has a smaller thrust angleand therefore is in the upper range with respect to the axial clearance. In contrastbearings of series 240 are in the lower rangeie axial clearance is up to 35 per cent lower.
Moreoverbearings of series 240 have a considerably higher load capacity.
In the design variant with two rotor bearingsthe performance capability of series 240 allows the location of the locating bearing at the thinner end of the shaftwhich leads to another reduction of axial clearance by 15 per cent. In practicethis would mean that the function of the locating bearing is taken over by a spherical roller bearings 24096 instead of a 230/600which cuts the axial clearance by half.
Stiffer arrangements
Howeverthese 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 conceptwhich is sometimes also referred to in the technical literature as single bearing concept. This refers to a solutionin which one single rolling bearingsimilar to the azimuth bearingcan also accommodate tilting movements. Thereforethis 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 timethis was still done with a triple-row cylindrical roller bearinga 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.
New bearing concepts
By the application of modern simulationanalysis and calculation methodsbetter designs can be developed.
Firstin 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 operationthe higher the risk of wear and early failure.
Secondthe failed bearing design had a further handicap. In both axial rowscylindrical rollerswhich due to their geometry have a natural tendency to roll in a straight directionare forced to follow a circular raceway around the bearing centre. This causes friction at the rolling elements.
Both criteria lead to wearand 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 modifiedin a way that it is better suited to accommodate the stresses resulting from the application.Howeveronly by the application of dynamic simulation can the interaction between rolling elementsrings and cage be seen and quantified. SKF’s self-developed software for bearing simulationBEASTis an extremely powerful design tool and was used in the search for a solution. In particularBEAST gives information about the cage function and interaction that no other design tool can give.
The results have shown that the measures to increase the bearing stiffness can be ideally realised 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°.
Due to the taper designthe rollers roll without the 'circular raceway generated’ frictionmentioned earlier. The friction occurring 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 additionthe bearing shoulder is designed in a way that is favourable from the tribological point of viewin order to ensure that an ideal lubricant film can develop in the roller-shoulder contact area.
The decisive advantagehoweverlies in the flexible cageby 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 bearingsproposed by SKF in the early designs for wind turbines. At that time sophisticated dynamic analysis was not available and verificationby long term field trial and error testingwas 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 gearboxor the generatorwhich means less stresses and longer life on those components.
Werner Göbel is Senior Business EngineerRenewable EnergySKF. www.skf.com
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