Switching to a particular double row, tapered-roller bearing is one way to reduce total turbine weight while increasing component service life. The design, called Nautilus from SKF Bearings, carries all rotor loads on a single bearing rather than on a traditional two-bearing arrangement, one that separately carries radial and axial loads. By doing the work of two bearings, say SKF engineers, the Nautilus lets designers build it directly between the machine frame and hub, eliminating the main shaft. As a result, only rotor torque transmits to the drivetrain.
In the Nautilus bearing, two rows of tapered rollers, arranged back-to-back, lock the unit axially and provide an extremely high stiffness. The company adds that the large number of rolling elements allows extreme precision between raceways and provides extremely high load carrying capacity. Capable of being preloaded, the bearing has a large diameter that makes it strong enough to withstand yaw and tilt moments. Only torsional forces reach the generator or gearbox.
Benefits include significant reduction to nacelle weight and dimensions along with friction and energy losses. The company says component reduction and the bearing’s rugged design combine to lessen drivetrain loads and improve reliability.
This single bearing carries all rotor loads, forces, and movements, and allows transmitting only rotor torque to the drivetrain. This results in longer drivetrain service life and higher turbine reliability.
Without a main shaft to contend with, engineers have more room to design other innovative design features. The company says Nautilus bearings are compatible with a range of geared and gearless turbine designs. Oil lubrication works well when the bearing is mounted inside a gearbox. Units mounted outside the gearbox can come in a sealed, grease-lubricated arrangement that provides high performance under the most extreme offshore or cold climates.
In addition, the Nautilus cage, made from an polyetherehterketone polymer (PEEK), is segmented to make it more flexible, and minimize contact forces and friction. The segmented cage also means there are no additional cage forces to add to normal internal bearing forces. Hence, it can operate in a flexible environment with pre-load, lending stiffness to the drive train and the surrounding structure. Segmentation also improves roller guiding, providing optimized load distribution among the rollers. In addition, the PEEK cage has extremely low friction and makes the bearing less sensitive to inadequate lubrication conditions, resulting in less wear. Therefore either grease or oil can lubricate the Nautilus bearing. SKF adds that the bearing is operating in multi-megawatt models worldwide and it will meet future needs of multi-megawatt size turbines, onshore or offshore, with or without gearboxes.
SKF Bearings Inc.