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Shape-changing solution

WORLDWIDE: Most modern wind turbines have pitch control, so the blades can turn on their axis. Cyclic pitching, used less frequently, is an advanced system where the blade angles are adjusted continuously during each rotation.

This is known to reduce loads, but has the drawback of increased pitch system usage and can cause accelerated wear and more downtime. During operation, pitch bearings are exposed to tilting moments, aerodynamic thrust and gravity loads that feature two load-direction changes per revolution.

Design drivers

When it comes to large rotors, it is aerodynamic tilting moments that have become the design driver, says IMO Energy managing director Werner Schröppel.

Conventional, four-point contact, ball-type blade bearings consist of two axially interspaced rows of balls, running into bearing races or channels integrated into inner and outer rings. Tilting moments transferred to the bearing introduce axial and additional radial forces, the latter which cause separation of the rings. Standard lifetime calculations assume that the blade root and hub will retain rigidity, with only the blade bearing deforming under load. This has proved adequate for turbines up to around 3MW, but as the rotor diameter increases, two negative affects with the increasing rotor diameters of larger turbines, two negative results combine.

"First, the ratio between the tilting moment and the blade root diameter worsens because the moment load increases cannot be compensated by similar increments in blade root diameter through costs and feasibility considerations," says Schröppel. "Second, the built-in stiffness of adjacent structures, especially hub and blade root, cannot match these moment load increments."

Weight gain

These combined effects threaten the operational life of larger four-point bearings. Suppliers counteract the first effect — blade root and hub dynamic deformation — with structural stiffening reinforcements or stronger, more expensive, pitch drives. "In some 3-4MW class turbines total stiffening mass already exceeds 2,000 kilogrammes per pitch axis, and many stiffener design patents mean intellectual-property issues can arise, says Schröppel.

Another effect is cyclic deformation of the bearing raceway geometry, with variations in the gap width between inner and outer race. The balls no longer roll along an ideal spherical pathway but at a raceway that opens elliptically towards the edge. This gap width variation is predicted to increase sixfold between 2MW turbines and 6MW-plus turbines, under normal conditions, according to IMO, with a potentially huge negative impact on bearing life.

Schröppel cites a request for 7MW class four-point ball bearings with high pitch activity. IMO calculations indicated a very large blade root diameter, while required hub reinforcements would mean major increases in mass, cost and logistical effort. Limited availability of the required large ball diameters was another constraint.

Double axis

IMO's T-Solid principle is based on double-axial ball bearings, which have long been used for heavy-duty machinery. The key difference is that the decisive moment load is fully carried by axial forces without causing the rings to separate, says Schröppel. "To absorb the relatively minor radial loads from blade mass and aerodynamic thrust, we incorporated a radial-roller raceway."

IMO's test rig results show that large-diameter four-point bearings cannot achieve their theoretical life requirement in high-load applications without massive stiffening. T-Solid was shown to exceed these same requirements without suffering from shape-changing and with no stiffening needed. "Design-to-cost analysis showed T-Solid offers the most efficient approach to cut cost of energy, even though pure bearing cost is higher," says Schröppel. The product is currently being assesed by certification body GL Renewables for the T-Solid application in wind turbines.

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