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Optimised bearings improve reliability of wind turbines

Optimised bearings improve reliability of wind turbines Liu Su of The Timken Company examines the general failure modes of wind turbine gearbox bearings, and looks at the solutions to the problem.

As the transmission part that connects the rotor and generator, the wind turbine gearbox transforms the low-speed input of the rotor into an output that the medium- or high-speed generator needs. It is one of the major components of the wind turbine. Because of the complexity of working conditions and the high reliability required by the wind turbine gearbox, its design, application (particularly the bearing selection), installation and operation become even more important.

Any inappropriate selection or improper installation and use of the bearing will lead to a variety of bearing damage and failure modes, or might even damage other components of the gearbox. Such damage and failure will result in shutdown of the wind turbine directly or indirectly, which will affect productivity and bring unplanned costs of replacement and maintenance. However, there is a wide range of effective solutions to the following common failure modes.

There are a number of designs of wind turbine gearboxes, but essentially they consist of a planetary stage and a parallel stage. So let's look at a relatively common design example, which uses a planet carrier as input with fixed ring gear, transmitting the output from the sun gear to the parallel stage. We can then analyse the common modes of bearing failure and the corresponding solutions.

The planet carrier bearing
The selection and application of the planet carrier bearing is related to the design of the rotor. Currently full complement cylindrical roller bearings are commonly used. If spherical roller bearings are used as rotor bearings, whether 3-point mounting design with a single spherical roller bearing or the 4-point mounting design with two spherical roller bearings, the rotor and the planet carrier could be back and forth axially due to the existence of radial and axial clearances in spherical roller bearing when the turbine brakes or in other conditions that the direction of axial loading alternates.

At this point, if you choose cylindrical roller bearing as the planet carrier bearing, because of a certain relative displacement space in the axial direction between its inner and outer rings, the axial movement from the rotor will transfer to the cylindrical roller bearings of the planet carrier; and if the movement is large enough, there is impact on cylindrical roller bearings.

Moreover, since the ring gear is fixed with the gearbox housing, the axial movement of planet carrier along with planetary gear will cause wear to the surfaces of planetary gear teeth.

Solutions: Single row tapered roller bearings mounted with spread to each other address impact of the rotor axial movement upon the planetary gear by preloading the tapered roller bearings. What's more, the load zone of the preloaded tapered roller bearing can be optimised to reduce the raceway stress and improve the rigidity of the planetary gear stage, and can also take additional axial force which passed into the planet carrier.

The planetary gear bearing
The common type of planetary gear bearing is composed of a pair of double-row cylindrical roller bearings. Insufficient interference fit or lack of contact surface between the bearing outer ring and the bore of the planetary gear due to deformation of the gear will cause the outer ring creeping and wear.

For the helical planetary gear design, the planetary gear endures the same amount of axial forces but with opposite direction when it meshes with the sun gear and ring gear, the overturning moment of which results in larger loads upon the outer two rows of rollers while smaller upon the middle two rows. Uneven load distribution among the four rows of rollers makes some differences in their actual lives; once designed with the same target life, the outer two rows will have fatigue spalling prior to inner rows.

Solution: An integrated flex pin planetary gear assembly provides one of the best solutions to improve the reliability of planetary stage. The gear and bearing outer ring are integrated seamlessly to prevent the failure of the outer ring, and to provide more internal space for designing more and larger rollers to improve the capacity. Further, the load zone can be optimised by preloading two rows of tapered rollers to reduce the stress and risk of roller skipping, thus achieving a more even load distribution. The flex pin design allows flexible deflection during the operation of the planet gear to ensure proper mesh of the teeth surface, particularly for the design of multiple planetary gears. Those features allow a more even load distribution among the planetary gears, and reduce precision requirements for machining and assembling.

High speed shaft bearing
The combination of cylindrical roller bearing and four-point contact ball bearing is commonly used in high speed shaft applications. In the case of high speed and low load, roller skipping and raceway smearing are common for the cylindrical roller bearing while smearing and spalling might appear in ball bearing.

Solution: Wear resistant cylindrical roller bearings and single row tapered roller locator bearings address the issue. The wear resistant cylindrical roller bearings can prevent smearing constantly - avoiding raceway damage caused by impurities of lubricant and poor lubrication in operation throughout their life cycle. And the single row tapered roller bearings can take radial and bidirectional axial load, with pure rolling characteristics that can minimise the possibility of smearing.

With the rapid development of the wind power equipment industry, especially with multi-megaWatt wind turbines just around the corner, the reliability of wind turbines and components will become an ever greater focus of research and development. With improved gearbox bearing arrangements, designers will be able to optimise the gearbox for a significant improvement in the reliability of the gearbox and the turbine as a whole.
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