Type of hub
The hub of the wind turbine is that component that connects the blades to the main shaft and ultimately to the rest of the drive train. The hub transmits and must withstand all the loads generated by the blades. Hubs are generally made of steel, either welded or cast.
Details in hubs differ considerably depending on the overall design philosophy of the turbine, but in general there are three basic types:
1. Rigid hubs
2. Teetering hubs
3. Hubs for hinged blades
Some common type hubs are illustrated in the figure in the side.
A rigid hub is designed to keep all major parts in a fixed position relative to the main shaft. The term rigid hub doesn't exclude hubs in which the blade pitch can de varied, but every other blade motion is not allowed. A rigid hub must be strong enough to withstand all the loads that can arise from any aerodynamic loads on the blades, as well as dynamically induced loads, such as those due to rotation and yawing.
Teetering hubs are used on nearly all two-bladed wind turbines. This is because a teetering hub can reduce loads due to aerodynamic imbalances or loads due to dynamic effects from rotation of the rotor or yawing of the turbine. Teetering hubs are considerably more complex than are rigid hubs. In the page Number of Blades the relative motion of the blades is shown. During normal operation a teetering hub will move only a few degrees forward and backwards. During high winds, starts and stops, or high yaw rates, greater teeter excursions can occur. To prevent impact damage under these conditions, teeter dampers and compliant stops are provided.
An example of a turbine with a teetering hub is shown in the next figure. It is a Boeing MOD 2, 2.5 MW machine with 94m rotor diameter and 61m tall tower.
A hinged hub is in some ways a cross between a rigid hub and a teetering hub. It is basically a rigid hub with ‘hinges’ for the blades. The hinge assembly adds some complexity, however. Teetering hubs have the advantage that the two blades tend to balance each other, so lack of centrifugal stiffening during low rpm operation is not a major problem. There is no such counterbalancing on a hinge blade, however, so some mechanism must be provided to keep the blades from flopping over during low rotational speed.
Some nice examples of (dutch) turbines with hinged hubs are shown below. The first one is Lagerwey 18/80 and the second is Lagerwey 30/250.
- Rotor hub configurations: four topologies