6 Wind turbine bearings 6.1 Overview of the development of my country's wind power industry Since 2006, my country's wind power industry has developed in a 'blowout' manner. As of 2020, the country (except Hong Kong, Macao and Taiwan) has installed more than 15x104 units and the cumulative installed capacity has exceeded 2.9x109kW. Due to the influence of policies and absorptive capacity, my country's wind power industry experienced two ups and downs during this period, and has now entered a stage of rapid development (Table 9). From the perspective of industrial layout: my country's wind power industry is developing from centralized development to decentralized development; from 'three north' (northwest, north, northeast) to central, east, and south; to 'two seas' (offshore, overseas) . From the perspective of product structure: the average unit capacity is increasing year by year, from below 1 000 kW in 2010 to 2100 kW in 2017, and 2 668 kW in 2020, which is still increasing; the mainstream models are land 2.5~3.0 MW on the sea, 4~5 MW offshore; the double-fed type and the direct drive type coexist, and the proportion of the direct drive type gradually increases. Tab.9 New and accumulated installed capacity of wind turbines in China from 2008 to 2018 After the rush to install due to policy factors in 2020, the demand for wind power bearings will not drop a lot , but tend to be stable, mainly due to the gradual increase in the demand for offshore wind turbines, and the previous wind farm has reached a lifespan of 20 years and needs to be replaced. According to the '30·60 Dual-Carbon Goal' set by the state for carbon peaking in 2030 and carbon neutrality in 2060, non-fossil energy is required to account for 25% of the total energy in 2030, and the newly installed capacity of photovoltaic and wind power must not be increased. Less than 10x109kW, the average annual new installed capacity is about 1 × 109kW, of which the average annual new installed capacity of wind power is about 0.5 × 109kW, which is more than double the average of previous normal years. 6.2 Types of wind turbine bearings Wind turbine bearings mainly include yaw bearings, pitch bearings, main shaft bearings, speed increaser bearings, and generator bearings (Figure 15). Among them, the yaw bearing, pitch bearing and main shaft bearing below 2.5 MW have been localized. Gearbox bearings, generator bearings of various powers, and main shaft bearings above 2.5 MW are mainly imported and need to be autonomous. Fig.15 Diagram of wind turbine bearings6.2.1 The yaw and pitch bearings are shown in Figure 16. The yaw bearing is installed at the top of the tower and at the bottom of the engine room, bearing the full weight of the main drive system of the wind turbine. It is used to adjust the windward direction of the fan accurately and timely. The pitch bearing combines the blade with the hub, and adjusts the windward angle of the blade according to the wind direction to achieve the best condition. Yaw and pitch bearings must have sufficient strength and the ability to withstand the combined action of axial force, radial force and overturning moment, requiring smooth operation, small starting torque, and good lubrication, anti-corrosion and sealing performance. Figure 16. Structure diagram of yaw and blade bearings for windturbine The yaw and blade bearings for windturbine are extra-large slewing bearings with an outer diameter of about 0.5~4.5 m and a weight of 0.5~6.0 t. The structure of yaw bearing mainly includes single row four point contact ball slewing bearing (no gear, internal gear, external gear) and double row four point contact ball slewing bearing (no gear, internal gear, external gear) 2 types; the pitch bearings mostly use double-row, same-diameter, four-point contact ball slewing bearings (no gear type, internal gear type). In recent years, with the continuous increase of the single-unit capacity of wind turbines, the problems of insufficient rigidity and bearing capacity of the pitch system including bearings have been exposed. It is necessary to improve the design of the overall structure of the pitch system in order to improve the rigidity and bearing capacity. Among them, one of the proven effective measures of the pitch bearing is to replace the ball bearing with a roller bearing. At present, the foreign replacement rate has reached 30%, and the domestic replacement rate is about 15%. 6.2.2 Main shaft bearing The main shaft of the wind turbine plays the role of supporting the hub and blades and transmitting torque to the speed increaser. The main shaft bearing (Fig. 17) is the key component of the main drive chain system of the wind turbine, not only to bear the wind load, but also to It bears the gravity load of the main shaft and the speed increaser, and the working conditions are complicated. According to the single unit power, overall structure, working conditions, manufacturing cost, installation process and other factors of the wind turbine, the spindle bearings need to adopt different configurations: Fig.17 Structure diagram of spindle bearings for wind turbine1) Three-point support , A self-aligning roller bearing is used to form a three-point support with the elastic supports on both sides of the speed increaser, which is used in low-megawatt wind turbines.