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There are various types of wind turbine bearings, with a large range of sizes, complex load conditions, and harsh working environments. They mainly include yaw/pitch bearings, mainshaft bearings, gearbox bearings, and generator bearings.
Yaw/pitch bearings are installed as shown in Figure 2. The yaw bearing is a slewing bearing installed in the wind turbine nacelle base, which supports the entire nacelle and ensures the alignment of the nacelle with the direction of the wind force through the yaw drive mechanism, and withstands the axial force, radial force, overturning moment generated by the wind sweeping the blades, and the weight of the nacelle; the pitch bearing is installed in the root of the wind turbine blade, which adjusts the angle of the blades relative to different wind speeds through the pitch mechanism, and withstands the axial force, radial force, and overturning moment generated by the blades and the wind sweeping the blades.
Yaw/pitch bearings are large in size, high in load capacity, and have prominent eccentric load characteristics. They not only bear the weight of several tens or even hundreds of tons of the nacelle, but also are affected by extreme environments such as lightning, sandstorms, strong winds, and salt spray. Disassembly and maintenance are difficult, and high reliability is required, which needs to meet a service life requirement of 20 years. In addition, the rotational speed of yaw/pitch bearings is low (similar to a structural member, more stopping than rotating), the load is complex (axial force, radial force, overturning moment, and impact), the structural transformation (the pitch bearing has been changed from a four-point contact ball bearing to a three-row cylindrical roller bearing), and the application verification time is short. The structural design, failure mechanism, and test methods are significantly different from those of general-purpose bearings, and technical research needs to be carried out on the basis of fully considering the structural and working characteristics of yaw/pitch bearings.
Mainshaft bearings are the most important rotating support component in the main drive train. Variations in operating conditions such as start-up and shutdown can lead to changes in the friction pair and a decrease in lubrication performance, and impact between the rolling elements and cages can cause atypical fatigue failure. Therefore, in the design process of mainshaft bearings, complex operating condition adaptability design and reliability design need to be carried out to meet its long life and high reliability requirements. Currently, major wind turbine manufacturers use SKF, Schaeffler, and other world-renowned brands to ensure the reliability of wind turbine operation.
The main types of support configuration of megawatt wind turbine mainshaft bearings are three-point support, two-point support, and single-point support [2].
The three-point support has a simple layout structure, low assembly requirements, and adopts a set of spherical roller bearings for mainshaft bearings, which, together with the elastic supports on both sides of the gearbox, bear the weight of the main drive train and the bending moment generated by the external wind load. The disadvantages of the three-point support are poor dynamic characteristics of the mainshaft and poor shaft system stiffness. The mainshaft will transmit a part of the load to the gearbox, which requires high reliability of the gearbox. Because of its low installation cost, most doubly-fed wind turbines use three-point support.
The two-point support has the following combination forms: 1) Use two sets of spherical roller bearings together in pair, with the axial floating of the spherical roller bearings on the hub side and only bearing radial force, and the spherical roller bearings on the gearbox side bearing both axial and radial forces. This layout form is easy to install, and the good self-aligning performance of the spherical roller bearings can offset certain installation errors and tilting caused by the mainshaft deflection. It can also compensate for the axial size changes caused by temperature changes. It is widely used in small megawatt wind turbines. 2) Use cylindrical roller bearings and double-row tapered roller bearings together, with the cylindrical roller bearings able to float axially and the double-row tapered roller bearings axially fixed, mainly used in low-speed permanent magnet (direct-drive) wind turbines. 3) Use two sets of tapered roller bearings in pair, with good shaft system stiffness, compact structure, and high installation requirements. Currently, this structure is used in some foreign 5 MW or above high-speed permanent magnet (doubly-fed) wind turbines and domestic medium-speed permanent magnet (semi-direct-drive) wind turbines.
Single-point support mostly uses large-diameter double-row tapered roller bearings, which bear all the weight of the main drive train and external wind loads through the mainshaft bearings. This layout form can bear large radial loads, axial loads, and overturning moments but is expensive.
The gearbox is used to transmit the low-speed rotation of the mainshaft to the high-speed rotation of the generator. Gearbox bearings have a large range of sizes and speed, mainly including cylindrical roller bearings, tapered roller bearings, etc., as shown in Figure 3.
Research on wind turbine gearbox bearings started relatively late in China, with weak technology. The high thermal effects and wear problems of domestic gearbox bearings lead to high failure rates. The high-speed end bearings of the gearbox for large-scale wind turbines rely on imports. The failure mechanisms and causes of damage of gearbox bearings are still under research. It has been found that white etching cracks (Figure 4) are the main cause of early failure of gearbox bearings, and blackening technology can be used to improve this failure phenomenon [3]. Therefore, research needs to be carried out on key technologies such as gearbox bearing design methods and manufacturing processes to meet the long life and high reliability requirements of high-power, high-speed gearbox bearings.
Generator bearings are used to support the rotor system of the generator, usually using cylindrical roller bearings and ball bearings. Complex electromagnetic induction and other non-steady-state working conditions greatly affect the service life of generator bearings. Their main failure forms are fatigue spalling and electric erosion (Figure 5) [4]. At present, insulation sleeves are mainly used for insulation treatment of generator bearings. Wind turbine generator insulation bearing technology is still in the research and development stage in China. The National Key R&D Program specialized in “Insulation Bearing Technology” established by the Ministry of Science and Technology in 2020 aims to overcome the technologies such as insulation bearing design, manufacturing, and testing, and the developed insulation bearings for wind turbines are shown in Figure 6. At present, foreign countries have developed generator bearings above 10 MW, while China has just mastered the technology of 5 MW generator bearings. High-power generator bearings are basically imported.
