Abstract:
A wind turbine blade bearing is disclosed. The bearing includes a first ring being connectable to a wind turbine rotor blade; a second ring being connectable to a hub; and a number of elements for transferring forces between the first ring and the second ring. The second ring includes a number of shear pins for additionally connecting the second ring to the hub.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European Patent Office application No. 11151133.3 EP filed Jan. 17, 2011. All of the applications are incorporated by reference herein in their entirety. 
       FIELD OF INVENTION 
       [0002]    The present invention relates to a wind turbine blade bearing, a wind turbine hub and a wind turbine. 
       BACKGROUND OF INVENTION 
       [0003]    The blade bearings on wind turbines consist of an outer ring, an inner ring and balls to transfer forces from the inner ring to the outer ring. During loading of the blade, the balls generate an axial and a radial force component. The axial component transfers the bending moment from inner ring to outer ring. The radial component is undesirable, but unavoidable as the balls contact the rings in a certain angle. The radial component will push sideways on the outer ring, and cause it to slide on the hub. To avoid sliding pre-stressed bolts are used, and the surfaces are rough. This has until recently been sufficient to avoid sliding, but as rotor diameters increase, it is no longer a feasible solution to solve the problem with pre-stressed bolts. 
       SUMMARY OF INVENTION 
       [0004]    Therefore, it is a first objective of the present invention to provide an advantageous wind turbine blade bearing. It is a second objective to provide an advantageous wind turbine hub. A third objective of the present invention is to provide an advantageous wind turbine. 
         [0005]    The embodiments of the objectives are provided by the independent claims. The depending claims define further developments of the invention. 
         [0006]    The inventive wind turbine blade bearing comprises a first ring and a second ring. The first ring is connectable to a wind turbine rotor blade. The second ring is connectable to a hub. The inventive wind turbine blade bearing further comprises a number of elements for transferring forces between the first ring and the second ring. The second ring comprises a number a shear pins for additionally connecting the second ring to the hub. The second ring may further comprise a number of bolts for connecting the second ring to the hub. 
         [0007]    By the introduction of a number of shear pins in the most critical area of the surface, the sliding can be completely avoided. The use of pre-stressed bolts may still be required as the joint is also loaded in tension. The shear pins may be located in between the boltholes, and may be manufactured to very tight tolerances. 
         [0008]    The elements for transferring forces between the first ring and the second ring may, for example, be balls. Moreover, the first ring can be at least partly located inside of the second ring. In this case the first ring is an inner ring and the second ring is an outer ring. 
         [0009]    Preferably, the second ring may be pre-stressed to the hub, for example by means of the bolts. Moreover, the second ring may comprise a number of bolt holes. The bolt holes can be located along the circumference of the second ring. Each shear pin can be located in between two bolt holes. 
         [0010]    Advantageously, the inventive wind turbine blade bearing comprises a combination of pre-stressed bolts and a limited number of shear pins located at the most critical area. Due to production tolerances, it is currently not practically possible to use shear pins on the entire circumference. Hence, the shear pins can be located at a portion of the circumference of the second ring which covers up to one third of the circumference of the second ring. Advantageously, the shear pins can be located at a portion of the circumference of the second ring which covers up to one fourth of the circumference of the second ring. Preferably, the shear pins are located at a portion of the circumference of the second ring which covers up to one fifth of the circumference of the second ring. 
         [0011]    The inventive wind turbine hub comprises a wind turbine blade bearing as previously described. The inventive wind turbine comprises an inventive hub or an inventive wind turbine blade bearing as previously described. 
         [0012]    Generally, the present invention has the advantage, that it provides a blade bearing which effectively avoids sliding between the outer ring and the hub and which can be applied for high rotor diameters. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Further features, properties and advantages of the present invention will become clear from the following description of an embodiment in conjunction with the accompanying drawings. 
           [0014]      FIG. 1  schematically shows a wind turbine. 
           [0015]      FIG. 2  schematically shows part of a hub and part of a rotor blade in a perspective view. 
           [0016]      FIG. 3  schematically shows an inventive wind turbine blade bearing in a sectional view. 
           [0017]      FIG. 4  schematically shows the blade bearing in a top view. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0018]    An embodiment of the present invention will now be described with reference to  FIGS. 1 to 4 . 
         [0019]      FIG. 1  schematically shows a wind turbine  1 . The wind turbine  1  comprises a tower  2 , a nacelle  3  and a hub  4 . The nacelle  3  is located on top of the tower  2 . The hub  4  comprises a number of wind turbine blades  5 . The hub  4  is mounted to the nacelle  3 . Moreover, the hub  4  is pivot-mounted such that it is able to rotate about a rotation axis  9 . A generator  6  is located inside the nacelle  3 . The wind turbine  1  is a direct drive wind turbine. 
         [0020]      FIG. 2  schematically shows part of the hub  4  and part of a rotor blade  5  in a perspective view. The rotor blade  5  comprises a root portion  7 . The root portion  7  comprises a flange  8 . The flange  8  comprises a number of bolt holes  12 . The rotor blade  5  can be connected to the hub  4  by means of the flange  8  and by means of the blade bearing  20 . 
         [0021]    The inventive wind turbine blade bearing  20  comprises an inner ring  10  and an outer ring  11 . The outer ring  11  comprises a number of bolt holes  14 . The outer ring  11  is connected to the hub  4 , for example by means of the bolt holes  14 . For example, the outer ring  11  can be bolted or screwed to the hub  4 . 
         [0022]    The inner ring  10  comprises a number of bolt holes  13 . The rotor blade  5  can be connected, for example bolted or screwed, to the inner ring  10  by means of the bolt holes  13  and corresponding bolt holes  12  of the flange  8 . The centre line of the root portion  7  and of the blade bearing  20  is designated by reference numeral  16 . The inner ring  10  is located radially inside of the outer ring  11  referring to the centre line  16 . 
         [0023]    To allow for a rotation of the rotor blade  5  about the centre line  16  the inventive blade bearing is constructed as ball bearing. The balls are located between the inner ring  10  and the outer ring  11  and are not shown in  FIG. 2 . The radial forces acting on the outer ring  11  due to forces transferred from the rotor blade  5  via the inner ring  10  and the balls are designated by reference numeral  15 . The radial forces  15  may cause the outer ring  11  to slide on a surface of the hub  4 . To avoid this, the inventive blade bearing  20  comprises a number of shear pins  18  which are located between the outer ring  11  and the hub  4 . 
         [0024]      FIG. 3  schematically shows part of the inventive blade bearing  20  in a sectional view. The balls  17  are located between the inner ring  10  and the outer ring  11 . The outer ring  11  comprises a number of openings  21 . Preferably, each opening  21  is located between two bolt holes  14 . The hub  4  comprises a number of openings  22  corresponding to the openings  21  of the outer ring  11 . The openings  22  of the hub and the openings  21  of the outer ring  11  are facing each other. 
         [0025]    A number of shear pins  18  are located in the openings  21  of the outer ring  11  and in the openings  22  of the hub. When the radial force  15  exceeds a predefined value, the shear pin  18  breaks and prevents further damages. 
         [0026]    Preferably, the outer ring  11  is pre-stressed to the hub  4 , for example by means of bolts or screws. 
         [0027]      FIG. 4  schematically shows the blade bearing  20  in a top view. Preferably, the shear pins  18  are only located at the most critical portion. Therefore, in  FIG. 4  the portion which is equipped with shear pins  18  covers only one fourth of the whole circumference of the outer ring  11 . Alternatively, the portion of the outer ring  11  which is equipped with shear pins  18  may cover only one fifth of the whole circumference or may cover up to one third of the whole circumference of the outer ring  11 . 
         [0028]    While the invention has been described in terms of a certain preferred embodiment and suggested possible modifications thereto, other embodiments and modifications apparent to those of ordinary skill in the art are also within the scope of this invention without departure from the spirit and scope of this invention. Thus, the scope of the invention should be determined based upon the appended claims and their legal equivalents, rather than the specific embodiments described above.