Generating steady shaft speed on a wind turbine means constantly adjusting the pitch of each blade to accommodate wind variations. The blades connect to a huge hub mounted to a shaft that turns a gearbox and a generator. Best turbine efficiency calls for a continuous pitch control on the blades.
Blade pitch is powered by either an electric or hydraulic drive. Electric-pitch control uses slip rings to conduct power to motors rotating in the hub. Hydraulic systems, on the other hand, use a rotary union to deliver hydraulic power to the drive motor. The industry is split about 45% electric and 55% for hydraulic controls. The advantage of the hydraulic control is that its power density is higher than electrical equipment and it needs fewer components, making for a simpler system. There are other pluses.
Rotating unions, such as those from Deublin Co., are precision mechanical devices for transferring fluid from stationary sources to rotating machinery. Ball bearings in a typical union support the rotating components (attached to the machinery) against the stationary component (attached to the fluid supply) and a mechanic seal prevents leaks. While often found on wind turbines, rotating unions device work well in other applications such as air clutches, gearboxes, machine tool spindles, and more.