Yaw bearing

The yaw bearing is the most crucial and cost intensive component of a yaw system found on modern horizontal axis wind turbines. The yaw bearing must cope with enormous static and dynamic loads and moments during the wind turbine operation, and provide smooth rotation characteristics for the orientation of the nacelle under all weather conditions. It has also to be corrosion and wear resistant and extremely long lasting. It should last for the service life of the wind turbine) while being cost effective.

History

Windmills of the 18th century began implementing rotatable nacelles to capture wind coming from different directions. The yaw systems of these "primitive" windmills were surprisingly similar to the ones on modern wind turbines. The nacelles rotated by means of wind driven yaw drives known as fantails, or by animal power, and were mounted on the windmill towers by means of an axial gliding bearing.

These gliding bearings consisted of multiple gliding blocks fixed on the windmill tower structure. These blocks maintained sliding contact with a gliding ring on the nacelle. The gliding blocks were wooden cube-like pieces with convex gliding surface covered with animal fat, or even lined with copper (or brass) sheet as a friction reduction means. These wooden blocks were fixed in wooden slots, carved in the wooden bearing substructure, by means of nails or wedges and were carefully leveled to create a flat surface where the nacelle gliding ring could glide. The gliding blocks, despite the lubrication would wear quite often and would have to be exchanged. This operation was relatively simple due to the wedge-based connection between substructure and gliding blocks. The gliding blocks were further locked via movable locking devices which, in a different form, remain as a technical solution in modern gliding yaw bearings.