History
When the windmills of the 18th century included the feature of rotor orientation via the rotation of the nacelle, an actuation mechanism able to provide that turning moment was necessary. Initially the windmills used ropes or chains extending from the nacelle to the ground in order to allow the rotation of the nacelle by means of human or animal power.
Another historical innovation was the fantail. This device was actually an auxiliary rotor equipped with plurality of blades and located downwind of the main rotor, behind the nacelle in a 90° (approximately) orientation to the main rotor sweep plane. In the event of change in wind direction the fantail would rotate thus transmitting its mechanical power through a gearbox (and via a gear-rim-to-pinion mesh) to the tower of the windmill. The effect of the aforementioned transmission was the rotation of the nacelle towards the direction of the wind, where the fantail would not face the wind thus stop turning (i.e. the nacelle would stop to its new position).
The modern yaw drives, even though electronically controlled and equipped with large electric motors and planetary gearboxes have great similarities to the old windmill concept. They still use a means of mechanical energy “production” (i.e. electric motor), a method to increase the torque (i.e. gearbox) and a gear-rim mounted on the fixed portion of the wind turbine and in constant mesh with the output gear of the said gearbox.
Read more about this topic: Yaw Drive
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