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The pitch bearing, also named blade bearing, is a component of modern
The pitch bearing, also named blade bearing, is a component of modern wind turbine
A wind turbine is a device that converts the kinetic energy of wind into electrical energy. Hundreds of thousands of large turbines, in installations known as wind farms, now generate over 650 gigawatts of power, with 60 GW added each year. ...
s which connect the rotor hub and the rotor blade. The bearing allows the required oscillation to control the loads and power of the wind turbine. The pitch system brings the blade to the desired position by adapting the aerodynamic angle of attack
In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a reference line on a body (often the chord line of an airfoil) and the vector representing the relative motion between the body and the fluid through which it is m ...
. The pitch system is also used for emergency breaks of the turbine system.
Design
Mostly largerolling element bearing
In mechanical engineering, a rolling-element bearing, also known as a rolling bearing, is a bearing which carries a load by placing rolling elements (such as balls or rollers) between two concentric, grooved rings called races. The relative m ...
are used as pitch bearings. The bearing is subjected to high bending moments, radial and axial loads in both directions. Therefore, the rolling elements for state of the art
The state of the art (sometimes cutting edge or leading edge) refers to the highest level of general development, as of a device, technique, or scientific field achieved at a particular time. However, in some contexts it can also refer to a level ...
wind turbines are ball bearings, which are used in a double rowed four-point contact. This means each raceway carries on two points, and in sum four points are carrying. Other possible options are different arrangements of the rolling elements or multirow cylindrical roller bearings. Pitch bearing of modern wind turbines can reach diameters of more than 4 meters.
Change of the lubricants can be carried out only with great time and cost expenditure. Furthermore, due to the constant rotation of the hub, the used lubricant must remain in place. Therefore, the pitch bearings in wind turbines are usually lubricated with grease. The bearing experiences a wide range of operating conditions during operation. Therefore, the operating conditions are very difficult for greases over the entire turbine time. The industrial greases that have been used so far have very different compositions and do not always lead to the desired result of preventing wear.
Load situation
The load and operating situation of pitch bearings are for rolling element bearings comparatively unfavorable. The bearings are exposed to high loads and small reciprocating movements created by the pitch system orvibration
Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic function, periodic, such as the motion of a pendulum ...
s from the wind profile. The small reciprocating movements between rolling elements and raceway can lead to wear phenomena like false brinelling
False brinelling is a bearing damage caused by fretting, with or without corrosion, that causes imprints that look similar to brinelling, but are caused by a different mechanism. False brinelling may occur in bearings which act under small oscill ...
and fretting corrosion. Furthermore, the high loads can lead to truncation
In mathematics and computer science, truncation is limiting the number of digits right of the decimal point.
Truncation and floor function
Truncation of positive real numbers can be done using the floor function. Given a number x \in \mathbb ...
of the contact ellipse. Due to the small reciprocating movements calculation methods to estimate the bearing service life and the friction torque are not usable for pitch bearings. Newer controlling concepts of pitch control, like individual pitch control, will lead to a different operating behavior which in worst could favor false brinelling and fretting corrosion or in best case reduce such wear.
References
{{Wind power Bearings (mechanical) Wind turbines