STATE OF THE ART An effort is currently underway to increase the capacity of wind power plants so as to extract a much electrical power as possible using as few power plants as possible. One way to increase the capacity of a wind power plant is to increase the dimensions.

The fastening of a wind power plant rotor blade to a hub is conventionally accomplished using flanges and bolt joints. Larger and heavier rotor blades subject the bolt fastenings to greater stresses. This entails that the number of bolt fastenings must be increased, and that the large rotor blades must be reinforced in the fastening zones so that they can withstand the loads that arise during operation.

US 4,834, 616 describes a system that securely locks a rotor blade to a hub. A securing element is arranged on the hub so that the element extends outwardly from the hub. The securing element is shaped like a truncated cone, with its narrow end mounted on the hub. In a fastening region of the rotor blade for fastening the hub to the rotor blade, the rotor blade is equipped with a woven composite layer that encloses the securing element of the hub and, owing to the shape of the securing element, holds said element in place.

DESCRIPTION OF THE INVENTION One object of the invention is to simplify the foregoing design. In one embodiment this has been accomplished by means of a device on a rotor blade for joining the rotor blade to a hub.

The hub and the rotor blade are, e. g. , intended for a wind power application, and the hub is intended to be connected to a drive shaft mounted inside the wind power plant, which shaft

drives a generator. A fastening part of the rotor blade comprises a composite material and is designed so as to enclose a connecting element on the hub. The device is characterized in that the composite material in the fastening part is a fiber composite comprising a first set of fibers that extends along the fastening part in a direction that lies roughly within a range of (10°- 80°) relative to the longitudinal direction of the fastening part, and a second set of fibers that extends along the fastening part in a direction within said interval, but with the fibers oriented at an angle in relation to the longitudinal direction of the fastening part, which angle has the opposite sign vis-a-vis the first set of fibers. The fastening part thereby exhibits transverse contraction in connection with tensile loads on the blade with a component in the longitudinal direction of the fastening part, which in turn imposes a clamping grip on the connecting element. The transverse contraction for angles within the range of-10° to +10° is very small, while the clamping forces decrease dramatically as the angles approach 90°. To achieve a good balance between the requirement of high transverse contraction and the requirement of heavy clamping forces, the range for a preferred embodiment is (30°- 60°), e. g. roughly 45°.

Each fastening part with associated connecting element must be designed to enable the introduction of the connecting element into the fastening part while, at the same time, in one mounting position, a force fit is achieved between the fastening part and the connecting element. Furthermore, in a preferred embodiment in said mounting position, the fastening part is arranged in close contact with the connecting element along essentially the entirety of the length of the fastening part that encloses the connecting element. In one simple embodiment the inside dimension of the fastening part and the outside dimension of the connecting element are essentially constant along the entire enclosed length.

The term"force fit"is used here to mean that the grip on the connecting element must be sufficiently strong to hold the rotor blade in position on the hub when no tensile forces are acting on the rotor blade, and that the grip will not release when the tensile loads begin to exert an effect, starting from the unloaded state. As the tensile loads increase as a result of the increasing tensile loads that arise as the rotor blade turns, the transverse contraction and compressive forces also begin to grow stronger. The connecting element/fastening part

connection thus holds the rotor blade in place, regardless of the rotational orientation of the rotor blade; the stronger the tensile forces, the greater the transverse contraction, and thus the stronger the grip on the connecting element.

To facilitate the introduction of the connecting element into the fastening part while at the same time achieving a satisfactory force fit in the mounted position, the connecting element according to one embodiment is made of a material that shrinks when cooled, and at least one channel for a coolant medium is simultaneously incorporated in the connecting element. The connecting element can thus be supplied with coolant during mounting, whereupon the coolant supply is discontinued once the connecting element is in its mounted position in the fastening part in order to achieve the necessary force fit.

The invention further comprises a device for connecting a rotor blade to a hub with one or several of the foregoing properties.

The invention also comprises a method for mounting a rotor blade on a connecting element of a hub. The method is characterized in that a fastening part on the rotor blade is designed so as to tightly enclose the connecting element of the hub, in that the fastening part is made of a fiber composite with a first set of fibers that extends along the fastening part in a direction lying roughly within a range of (10°-80°) in relation to the longitudinal direction of the blade, in that the fastening part is slipped over the connecting element to a connecting position, and in that tensile loads are applied to the blade that act outwardly from the hub, whereupon the fastening part exhibits transverse contraction so that a clamping grip is imposed on the connecting element.

The aforedescribed device and method makes it possible to avoid using bolts to fasten the rotor blade to the hub. Because the fastening part of the blade thus requires no bolt holes, which reduce its strength, the fastening part can be made thinner.

BRIEF DESCRIPTION OF FIGURES Figure 1 shows a side view of a wind power plant,

Figure 2 shows a front view of a hub in a wind power plant, Figure 3 schematically depicts the fastening of a rotor blade to a hub in a wind power plant, Figure 4 shows a cross-section of a fastening part on a rotor blade according to a first embodiment, Figure 5 shows a cross-section of a fastening part on a rotor blade according to a second embodiment, and Figure 6 shows a side view of the fastening part of the rotor blade.

PREFERRED EMBODIMENTS Reference number 1 in Figure 1 generally designates a wind power plant consisting of a tower 2, a housing 3 arranged on top of the tower, and a wind turbine arranged in the housing 3 and comprising a hub 4 and a rotor blade 5 that is arranged on the hub. The tower 2 is secured to the underlying surface, either on land or at sea.

In Figure 2 the hub 4 comprises connecting elements 6 for the rotor blades 5. The connecting elements 6 protrude from the hub 4 and are either realized in one piece with the rest of the hub or else constitute separate parts mounted on the hub. The connecting elements 6 in the embodiment shown have a constant cross-sectional surface along their entire length. Each connecting element 6 has a bevel 7 at its other end. The radius of the non-beveled end of the connecting element is characteristically roughly 1-3 mm larger than the radius at the other end. In a case where the connecting elements are made of a material that shrinks in cold, such as steel, copper or some other metal, one or a plurality of channels 8 for a coolant are incorporated in each connecting element. In the example shown in the figure, a channel passes via the hub 4 through all the connecting elements 6 from a starting point 9, where the coolant is pumped into the channel, to an end point 10, where the coolant is drained from the channel.

Reference number 12 in Figure 3 designates a fastening part for the rotor blade 5. The fastening part 12 in the example shown exhibits a constant cross-sectional area along its entire length, and comprises a recess surrounded by a wall 13 designed so as to complement the surface of the connecting element 6 to permit the introduction of one of the connecting elements into the fastening part. Each connecting element 6 is designed to be force-fit into its

associated fastening part 12. In accordance herewith, the beveled end of the cross-sectional surface of the connecting element coincides in one exemplary embodiment with the cross- sectional surface of the fastening part. To facilitate the mounting process, the metal material in the connecting element 12 is cooled via the cooling channel 8 during the mounting of each fastening part 12 to the connecting element of the hub, whereupon the grip between the connecting element 6 and the fastening part 12 arises once the coolant is discontinued and the connecting element resumes the ambient temperature. In one exemplary embodiment (not shown), the rotor blade 5 comprises a tubular inner portion and an aerodynamically advantageously designed outer portion, whereupon one end of the tubular inner portion intended to face toward the hub constitutes the rotor blade connection 12. For example, both the inner portion and the outer portion of the rotor blade are made of a fiber composite material. The choice of material for the connection 12 will be described in detail below.

In the example in Figure 4, the fastening part 12 has a circular cross-section both externally and internally. The wall 13 is characteristically 5-100 mm thick, and the cylinder diameter is characteristically 0.5-3 m. In the example in Figure 5, the fastening part 12 has an oval cross- section both externally and internally.

The fastening part 12 is made of a fiber composite material, which can be composed of a plastic base reinforced with fiber threads. The plastic is, e. g. an epoxy plastic, vinylester plastic or polyester plastic. Carbon fiber is the currently existing fiber material that has the best properties for reinforcing the plastic, i. e. it is rigid, light and highly durable. Other fiber materials may of course be used; for instance, a composition is conceivable in which the carbon fibers are mixed with fiberglass or aramid fibers.

In Figure 6 a first set of fibers 14 is embedded in the plastic so that the fibers extend into the wall 13 of the shaft 12 in a direction of between 10° and 80° relative to the longitudinal direction of the fastening part. The fibers should be embedded in the plastic in a plurality of layers along the entire thickness of the wall. There is also a second set of fibers 15 embedded in the plastic so that they extend into the wall of the shaft in a direction of between-80° and-

10° relative to the longitudinal direction of the fastening part. The fact that the first set is disposed at a positive angle in relation to the fastening part 12 while the second set is disposed at a negative angle indicates that the angles of the two sets of fibers must have different signs in order to obtain a plaited structure in which fibers from the two sets of fibers cross one another.

This fiber composite structure gives the fastening part 12 the property that it is stretched out somewhat in its longitudinal direction when subjected to tensile forces along its longitudinal direction, whereupon transverse contraction occurs that securely pinches the connecting element inserted into the fastening part 12. The tensile forces characteristically arise as a result of the dead weight of the rotor blade and its rotation about the hub (centrifugal forces). The higher the tensile force, the greater the transverse contraction, and thus the harder the fastening part 12 pinches the connecting element 6.

In one exemplary embodiment, both sets of fibers extend in a direction of (30°-60°) relative to the longitudinal direction. With the fibers oriented at 45'relative to the longitudinal direction, the ratio between the longitudinal expansion and the transverse contraction of the fiber composite wall 13 will be 1: 1. The way in which the fiber material is to be embedded in the plastic is not the object of this patent application, but rather constitutes an optimization problem in which the requirement of transverse contraction is set against the requirement of clamping force. The variables that are controllable are the fiber angles, the amounts of fiber in the two sets of fibers, and the way in which the fibers are arranged in the wall 13. For example, an embodiment is conceivable in which the angle relative to the longitudinal direction of the fastening part differs for the first and the second set of fibers. The fiber composite could also contain a third and a fourth set of fibers in which the fiber directions differ from those of the first and second sets of fibers, but still fall within the range of (10°- 80°). With respect to the fiber contents of the two sets of fiber, both sets of fibers in one exemplary embodiment contain equal amounts of fiber, while in an alternative exemplary embodiment the sets of fibers contain different amounts of fiber.