Turbine components: bearings

Photo courtesy of Timken

Photo courtesy of Timken

Where are bearings used in a wind turbine?

Although bearings are used many places throughout a nacelle, such as in the yaw and pitch bearings and on the generators, those on main shafts and gearboxes are most problematic.

In main-shaft bearings, modular wind-turbine designs commonly use spherical-roller bearings (SRB) units that are more than 1m in diameter. Designers often select single SRB designs, one supported by a single main bearing and two torque arms that carry gearbox reaction loads.

At least two factors contribute to early failures in main-shaft bearings. One is high thrust load on a radial SRB. While there is no official maximum limit, a conventional ratio of permissible thrust-to-radial load for two-row spherical roller bearings is between 0.15 and 0.20. The second is an inadequate lubricant film generation. Generally, operating conditions for the main shaft’s bearing are not ideal for lubricant-film generation. With a max operating speed of about 20 rpm, the bearing surface speed and lube-film generation may be insufficient to separate the roller-to-race asperities.

A tapered roller bearing (TRB) main-shaft design with preload can also improve the powertrain performance. TRBs help ensure system stability and rigidity, load sharing between rows, and predicted roller-to-race interactions. This design allows several configurations.

In one, a widespread single tapered roller bearing offers an economic design that can preload an entire system with two dissimilar TRBs. An upwind and downwind bearing series can then accommodate the application load by adjusting the contact angle and bearing capacity as needed. With a widespread effective center, the bearings are usually more compact.

Another configuration on the large diameter, tapered-roller bearing use a spacer between cone races. This has become an appealing option based on its field performance and ease of assembly. Steep race angles create high-tilting stiffness in a short axial space to counteract applied pitch and yaw moments. Separate bearing components can include seals and grease to simplify handling and installation. A factory set preload ensures a properly mounted setting.

In a third configuration, a single preloaded tapered roller bearing, offers a high-load capacity and manages the combination radial-thrust loads as compared to a single SRB. The single preload ensures load sharing across both bearing rows and tolerates greater system misalignment as compared to the design with the space between cone races.

Bearings are also used in wind turbine gearboxes. One service company says certain gearbox models show a particular design weakness: use of a four row, cylindrical bearing with a through-hardened race in the planet gears. This is not an optimal bearing configuration for the application. The company adds that when re-manufacturing such a gearbox, it is best to use a case carburized, inner-race bearing with coated rollers. The coating here is also the tungsten carbine version.

In addition, altering clearance across the individual rows improves load sharing, thereby allowing for a longer performing bearing. This can also prevent common case-crack failures due to debris that collects in the bottom of a gearbox. It is also necessary to use case carburized and coated bearings in high speed and intermediate positions. These simple upgrades can greatly enhance the overall performance of the gearbox, leading to a significant life extension.

Gearbox reliability has improved remarkably in the last 10 years thanks to a better understanding of bearing loads. One company warranties its rebuilt gearboxes for five years, versus as little as two years for some OEM warranties. Trends in the redesign of main shaft and gearbox bearings have been driven by unexpected failures in these units. The unplanned replacement of a main-shaft bearing can costs operators up to $450,000 and have an obvious impact on financial performance.

By: Paul Dvorak, Windpower Engineering & Development

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