A wind turbine generally comprises a nacelle mounted on top of a tower, and enclosing a generator. One type of generator is mounted to a drive shaft, which is turned by a rotor comprising a number of blades mounted to a hub at the upwind end of the nacelle. The rotor turns in response to an airflow over the blades. The drive shaft must be securely held in some way to ensure efficient transfer of the rotation to the generator. To this end, a drive shaft is usually held in place by a housing, with one or more main bearings between drive shaft and housing to allow a smooth motion of the drive shaft. Usually, the main bearing is constructed using a roller bearing such as a ball bearing, roller bearing or tapered roller bearing.
However, the balls or rollers of such bearings must be machined to a very high degree of precision, and are generally very expensive. Wear and tear can result in damage to one or more bearings and the raceways. To replace a damaged part (roller or ball), it is generally necessary to dismantle the entire cage or race containing the moving parts, so that repair and replacement procedures are generally expensive.
As an alternative to a ball bearing or roller bearing, it is possible to use a fluid bearing or sliding bearing instead. In a bearing such as a fluid bearing, a thin film of fluid between a bearing pad and the supported component allows the component to slide easily over the bearing pad. However, it is problematic to distribute and design the pads of the fluid or sliding bearings in such a way that wear and maintenance costs are minimized. Generally, multiple pads would be evenly spaced—i.e. arranged in a uniform distribution—around the circumference at each end of the drive shaft, since any bearing design generally aims to distribute the load evenly over the bearing parts. However, in the case of a fluid bearing or a sliding bearing, such a design involves a relatively large number of bearing pads, resulting in higher costs. A large number of pads also leads to increased power loss, a greater “out of tolerance” risk that can lead to an overloaded pad that is slightly too thick, a greater risk of pad failure, etc.