Wind Turbine Bearings
There are many rotating parts in wind turbine, all of which use bearings. The most important of these are main shaft bearings, yaw bearing and pitch bearings. The structure of these bearings is different from ordinary bearings. This courseware will introduce its structure and characteristics.
Main shaft bearing

Generally, there are two bearings on the front and back of the main shaft bearing. The force that the main shaft bearing bears mainly includes the weight of the rotor and the hub, and the force of the wind acting on the main shaft through the wind rotor. Therefore, the main shaft bearings are mainly subjected to radial forces and also to axial forces due to wind, due to the change of wind, the turbulence of the wind, the main shaft will bend and oscillate, and the main shaft can be rotated normally when the shaft is bent, that is, the bearing must have good self-aligning properties.

This courseware introduces the wind turbine main shaft front bearing. Its installation position is behind the rotor hub. The main force of the bearing is radial force and overturning moment. Spherical roller bearings have good self-aligning properties, allowing slight deflection and misalignment between the inner and outer rings when the bearings are in operation. Therefore, spherical roller bearings are widely used as main shaft bearings for wind turbine. Figure 1 is a structural view of a spherical roller bearing.

Spherical Roller Bearing
Figure 1 - Spherical Roller Bearing

Like ordinary bearings, spherical roller bearings consist of an outer ring, an inner ring, a roller and a cage. The main feature is that the roller is different from the raceway.

The inner circumference of the outer ring of the spherical roller bearing is a spherical surface, and the center of the ball is at the center of the bearing. This spherical surface is the raceway. The bearing has two columns of rollers, sharing the spherical raceway of the outer ring, the roller is drum-shaped, and the drum surface is matched with the outer spherical raceway, called a spherical roller. Corresponding to the two columns of rollers, there are two raceways in the inner ring of the bearing, which are inclined at an angle to the bearing axis, as shown in the left figure of figure 2. The two columns of rollers can not only rotate around the bearing axis, but also rotate at the center of the bearing. The right figure of figure 2 is the state when the inner ring axis is inclined at 5 degrees to the outer ring axis, and the inner ring can still rotate freely around its own axis. Therefore, spherical roller bearings have self-aligning properties, also known as spherical roller bearings. Since the contact between the roller and the outer ring of the inner ring is that the wire is not a point, it can withstand the radial heavy load and the impact load.

Spherical Roller Bearing
Figure 2 - Spherical Roller Bearing
The spherical roller bearing is installed in the bearing pedestal, and the two sides are clamped by the bearing end-shield, and the bearing end-shield leaves a space where the inner ring and the roller are inclined (0.5 degree is sufficient), and figure 3 is a schematic diagram of the main shaft bearing structure of the wind turbine.
Wind turbine spindle bearing
Figure 3 - Wind turbine spindle bearing 
Yaw bearing and pitch bearing

The nacelle is mounted on the tower by the yaw bearing, and the blades are mounted on the hub via pitch bearings. These bearings are subjected to axial and radial forces and are subject to large overturning moments.

Figure 4 is a cross-sectional view of a yaw bearing. The bearing consists of an outer ring, an inner ring, a roller and a cage (the cage is not shown), and the roller uses a spherical roller. The difference from the ordinary bearing is that the raceway is different.

External tooth four-point contact ball bearing
Figure 4 - External tooth four-point contact ball bearing
The raceway of the inner and outer rings of the bearing adopts a peach-shaped cross section, as shown in figure 5 (to strengthen the difference, the ball is green), when there is no load or pure radial load, the steel ball and the inner and outer ring raceways are presented as four-point contact (red dot in the figure indicates the contact point position), so it is also called four-point contact ball bearing. When the bearing is subjected to radial force, the steel ball is subjected to force at four points, but when the axial load or the overturning moment acts, the steel ball and the inner and outer ring raceways become two-point contact.
The raceway of a four-point contact ball bearing
Figure 5 - The raceway of a four-point contact ball bearing

Four-point contact ball bearings are widely used in yaw bearings and pitch bearings of wind turbines when the axial load is large and the overturning moment is large. The bearing has a large diameter and a relatively small thickness relative to the diameter, which is also called a slewing bearing.

As the yaw bearing or pitch bearing of the wind turbine, the driving gear is usually made directly on the bearing ring. Figure 4 is the external gear four-point contact ball bearing, and the teeth are made outside the bearing outer ring; Figure 6 is the internal tooth four-point contact ball bearings, the teeth are made inside the inner ring of the bearing.

Internal tooth four-point contact ball bearing
Figure 6 - Internal tooth four-point contact ball bearing
The above slewing bearing has only one row of rollers, called single-row four-point contact ball bearing, some heavy-duty equipment should use double-row four-point contact ball slewing bearing. Figure 7 is a schematic diagram of the structure of a double-row four-point contact ball slewing bearing retransmitted from the network.
Double-row four-point contact ball slewing bearing 
Figure 7--Double-row four-point contact ball slewing bearing 
The spherical roller is point contact, the bearing capacity is limited, the cylindrical roller is the line contact, and the bearing capacity is strong. In order to bear the multi-directional load at the same time, the cylindrical roller axis is arranged at 45 degrees with the bearing axis, and the adjacent rollers are arranged in a cross arrangement. (Perpendicular to each other), see figure 8 (the orange is the roller in the figure). Figure 9 is also a picture of the cross-cylindrical roller slewing bearing structure (pictures are from the network). With crossed cylindrical rollers, you can withstand larger loads.
Cross cylindrical roller slewing bearing
Figure 8 - Cross cylindrical roller slewing bearing
Cross cylindrical roller slewing bearing picture
Figure 9--Cross cylindrical roller slewing bearing picture
For slewing bearings used in applications where the axial load is particularly heavy and the radial load is light, the rollers are combined with different rollers, the cylindrical rollers are used to bear the axial load, and the spherical rollers are used to bear the radial load, called the ball column combined slewing bearings, see figure 10 (pictures are from the network). The ball column combined slewing bearing mainly bears axial load and has poor ability to withstand overturning moment.
Ball column combined slewing bearing
Figure 10 - Ball column combined slewing bearing
In addition to the yaw bearing and pitch bearing of the wind turbine, the slewing bearing is also used for heavy equipment such as cranes, tower cranes, metallurgical mining machinery, and heavy industry. There are other structural forms of the slewing bearing, which will not be introduced here.
 
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