Why do you need a gearbox in a wind turbine?
The short answer is that you don’t need one – if you are using a direct drive WTG. But even if the solution without gearbox is used by several manufacturers (e.g. the Goldwind 2.5 PMDD, Enercon models, etc.) the majority of makers decided to include this technology.
Purpose of the gearbox is to increase the rpm (revolutions per minute). The blades rotate very, VERY slowly. It is also important to mention that the longest the blade, the lower is the tip speed of the blade: you do not want to increase it to avoid generating noise and to lower the loads on the blade itself.
In order to reach the correct rotational speed and generate power at the frequency needed by the grid you will need to use a gearbox between the main shaft (connected to the blades) and the secondary, “high speed” shaft linked to the rotor in the generator. The conversion ratio depend on the WTG model, but can be around 1:100.
The gearbox must survive over 20 years with very high, cyclical loads. Torque can be extreme during emergency shutdown, and is usually high during start ups. The failure of a gear box is a very big problem, as you will have a long production downtime and you will need a crane to disassembly the broken component and install the new one.
Additionally, gearboxes should be as silent as possible, have low vibrations and dissipate quickly the heat produced by the internal mechanisms. Therefore lubrication systems and vibration absorber mechanism are crucial in their design.
Gearboxes are usually built using planetary gearing system, and are equipped with several auxiliary system. For instance, it is possible to analyse the density of particles dissolved in the lubricant oil and the way the gearbox vibrate to detect problems and predict possible failures.