Trends in the design of main shaft and gearbox bearings are driven by unexpected failures in these units. Unplanned main-shaft bearing replacement costs operators up to $450,000 and have an obvious impact on financial performance.
One gearbox 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 remanufacturing such a gearbox, it is best to use a case carburized, inner-race bearing with coated rollers. In addition, altering clearance across the individual rows improves load sharing, thereby providing 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 the high speed and intermediate positions. These simple upgrades can greatly enhance the overall performance of the gearbox, leading to significant life extension.
In main-shaft bearings, modular wind-turbine designs commonly use spherical roller bearings (SRB) to support and carry main-shaft loads. 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. One is high thrust load on a radial SRB bearing. 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 factor is inadequate lube film generation. Generally, operating conditions for the main shaft’s bearing are not ideal for lubricant-film generation. With a max operating speed of ~20 rpm, the bearing surface speed and lube-film generation may be insufficient to separate the roller-to-race asperities.
After several years of analyses, experimentation, and design work, upgrades are now available in the market for existing turbines, and more sophisticated engineering designs for newer turbine platforms.
A direct interchange to existing fleets is an ideal solution. At least one company offers a wear-resistant SRB that uses engineered surface technology in combination with enhanced surface finishes. Wear resistant bearings increase raceway protection against micropitting by reducing shear stresses and asperity interactions. The engineered surface is a durable, proprietary tungsten carbide, amorphous-hydrocarbon coating. Generally, such coatings are moderately harder than HRc60 steel, 1 to 2-µm thick, and have low friction coefficients when sliding against steel. The engineered surface on the rollers polish and repair raceways damaged during operation. With enhanced surface finishes, the lubricant film increases thickness, helping reduce asperity contacts. The engineered surface also reduces asperity interactions and surface-shear stresses that cause wear. The benefits lead to an increased calculated bearing life and a reduction in rolling torque.
A tapered roller bearing (TRB) main-shaft design with preload characteristics 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.
For one, a widespread single tapered roller bearing offers an economical 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 of radial and 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. WPE