Abstract:
A root bushing for a blade root of a wind turbine rotor blade is provided, wherein the root bushing has a bushing body and a connecting body for connecting the wind turbine rotor blade to a hub of a wind turbine and wherein the connecting body is releasably connected to the bushing body. A blade root, a wind turbine rotor blade, and a wind turbine are also provided.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of European Application No. EP14171377 filed 5 Jun. 2014, incorporated by reference herein in its entirety. 
       FIELD OF INVENTION 
       [0002]    The present invention relates to a root bushing for a blade root of a wind turbine rotor blade, to a blade root, to a wind turbine rotor blade and to a wind turbine. 
       BACKGROUND OF INVENTION 
       [0003]    Modern wind turbine rotor blades are built from fiber-reinforced plastics. A rotor blade typically comprises an airfoil having a rounded leading edge and a sharp trailing edge. The rotor blade is connected with its blade root to a hub of the wind turbine. The blade root comprises a plurality of root bushings. The root bushings can be provided with an internal thread. Bolts are engaged with theses threads to connect the blade root to the hub. EP 1 486 415 A1 describes such a root bushing. 
       SUMMARY OF INVENTION 
       [0004]    It is one object of the present invention to provide an improved root bushing for a blade root of a wind turbine rotor blade. 
         [0005]    Accordingly, a root bushing for a blade root of a wind turbine rotor blade is provided, wherein the root bushing comprises a bushing body and a connecting body for connecting the wind turbine rotor blade to a hub of a wind turbine, wherein the connecting body is releasably connected to the bushing body. 
         [0006]    In particular, the connecting body is separable from the bushing body. The root bushing is advantageous in that when the connecting body is damaged or worn out it can be replaced without replacing the whole root bushing. This means, the connecting body can be replaced by a new one with only a minimum repair to the blade composite material afterwards. In contrast to that, rotor blades with known root bushings comprising a thread being directly cut into the material of the root bushing have to be replaced completely when the thread of the root bushing is damaged. 
         [0007]    According to a further embodiment, the connecting body is interchangeable. 
         [0008]    According to a further embodiment, the connecting body is positive-fitted to the bushing body. This prevents a relative movement between the connecting body and the bushing body. Advantageously, a little loose tolerance on a spherical interface between the connecting body and the bushing body during assembly of the parts makes the connection between the parts self-aligning in the beginning of a pre-load of the connection. 
         [0009]    According to a further embodiment, the bushing body comprises a receiving section for receiving the connecting body. The receiving section may be a bore. 
         [0010]    According to a further embodiment, the connecting body has a curved front face and wherein the receiving section is shaped corresponding to the curved front face of the connecting body. Advantageously, a surface between the front face and the receiving section has a spheric curvature. Due to the spheric curvature, the demands of perpendicularity between the receiving section and the connecting body can be lowered which reduces the costs to produce the parts. Alternatively the front face and the corresponding receiving section can have a flat, a triangular, a sinusoidal or any other geometry. 
         [0011]    According to a further embodiment, the connecting body has a cylindrical shape with a centered thread. The thread is cut into a central bore provided in the connecting body. Advantageously, the connecting body has a circular cross-section. Advantageously, the cylindrical shape of the connecting body has at least one straight side in its interface to the bushing body together with a spherical front face of the connecting body. This ensures that the connecting body will not rotate together with a connection bolt during assembly or disassembly. 
         [0012]    According to a further embodiment, the bushing body comprises a cylindrical basic portion and a slanted extension portion, wherein the cylindrical basic portion and the slanted extension portion are made of one piece. Advantageously, the bushing body is made of a metal alloy. In particular, the extension portion has a gradually reduced cross-section to a pointed or nearly pointed end. Thus, the extension portion has a gradually increased flexibility. 
         [0013]    According to a further embodiment, connecting body is receivable in the cylindrical basic portion. In particular, the basic portion comprises the receiving section. 
         [0014]    According to a further embodiment, the bushing body and the connecting body are made of different materials. With known root bushings, the choice of material is predicted by the required strength of the threaded part of the root bushing. By providing the connecting body as a separate part, a weaker and thus cheaper material may be used for the bushing body. 
         [0015]    According to a further embodiment, the material which the connecting body is made of has a higher strength than the material which the bushing body is made of. In particular, the material which the connecting body is made of has a higher grade than the material which the bushing body is made of. 
         [0016]    Further, a blade root of a wind turbine rotor blade comprising such a root bushing is provided. 
         [0017]    Further, a wind turbine rotor blade comprising such a root bushing and/or such a blade root is comprised. 
         [0018]    Further, a wind turbine comprising such a root bushing, such a blade root and/or such a wind turbine rotor blade is provided. 
         [0019]    “Wind turbine” presently refers to an apparatus converting the wind&#39;s kinetic energy into rotational energy, which may again be converted to electrical energy by the apparatus. 
         [0020]    Further possible implementations or alternative solutions of the invention also encompass combinations—that are not explicitly mentioned herein—of features described above or below with regard to the embodiments. The person skilled in the art may also add individual or isolated aspects and features to the most basic form of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Further embodiments, features and advantages of the present invention will become apparent from the subsequent description and dependent claims, taken in conjunction with the accompanying drawings, in which: 
           [0022]      FIG. 1  is a perspective view of a wind turbine according to one embodiment; 
           [0023]      FIG. 2  is a perspective view of a wind turbine rotor blade according to one embodiment; 
           [0024]      FIG. 3  is an end view of the wind turbine rotor blade according to  FIG. 2 ; 
           [0025]      FIG. 4  is a sectional view of a root bushing for a root of the wind turbine blade according to  FIG. 2 ; and 
           [0026]      FIG. 5  is another sectional view of the root bushing according to  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0027]    In the Figures, like reference numerals designate like or functionally equivalent elements, unless otherwise indicated. 
         [0028]      FIG. 1  shows a wind turbine  1  according to an embodiment. 
         [0029]    The wind turbine  1  comprises a rotor  2  connected to a generator (not shown) arranged inside a nacelle  3 . The nacelle  3  is arranged at the upper end of a tower  4  of the wind turbine  1 . 
         [0030]    The rotor  2  comprises three blades  5 . The blades  5  are connected to a hub  6  of the wind turbine  1 . Rotors  2  of this kind may have diameters ranging from, for example, 30 to 160 meters. The blades  5  are subjected to high wind loads. At the same time, the blades  5  need to be lightweight. For these reasons, blades  5  in modern wind turbines  1  are manufactured from fiber-reinforced composite materials. Therein, glass fibers are generally advantageous over carbon fibers for cost reasons. Oftentimes, glass fibers in the form of unidirectional fiber mats are used. 
         [0031]      FIG. 2  shows a blade  5  according to one embodiment. 
         [0032]    The blade  5  comprises an aerodynamically designed portion  7 , which is shaped for optimum exploitation of the wind energy and a blade root  8  for connecting the blade  5  to the hub  6 . The blade  5  may be fixed to the hub  6  by means of bolts. 
         [0033]      FIG. 3  shows an end view of the blade root  8 . 
         [0034]    The blade root  8  comprises a plurality of root bushings  9  for a releasable connection of the blade  5  to the hub  6 . The root bushings  9  are embedded in the blade root  8  so that bolts (not shown) can be screwed into an internal thread of the root bushings  9  for a firm but releasable engagement therewith. The number of root bushings  9  is arbitrarily. In  FIG. 3  only three root bushings  9  are shown. 
         [0035]      FIGS. 4 and 5  are longitudinal sections of a root bushing  9 . 
         [0036]    In the following,  FIGS. 4 and 5  are referred to at the same time. The root bushing  9  comprises a basic portion  10  and an extension portion  11 . The basic portion  10  is cylindrical or tube shaped and has a central bore  12 . The extension portion  11  is slanted and has a gradually reduced cross-section to a pointed or nearly pointed end  13 . Thus, the extension portion  11  has a gradually increased flexibility. Advantageously, the basic portion  10  and the extension portion  11  are made of one piece. The basic portion  10  and the extension portion  11  together constitute a bushing body  14  of the root bushing  9 . The bushing body  14  may be made of a metal alloy. 
         [0037]    The root bushing  9  further comprises a connecting body  15  for connecting the blade  5  to the hub  6  of the wind turbine. The connecting body  15  has a cylindrical geometry with a circular cross-section. The cylindrical geometry has at least one straight side which prevents the connecting body  15  from rotating during assembly or disassembly of the blade  5 . Alternatively, the connecting body  15  may have an oval, a rectangular, a hexagonal, a star-shaped geometry or the like. The connecting body  15  has a centered thread  16 . The connecting body  15  is releasable connected to the bushing body  14 . In particular, the connecting body  15  is inserted into the bushing body  14 . The connecting body  15  is made of a metal alloy. The connecting body  15  is interchangeable so that when the connecting body  15  is damaged it can easily be replaced by a new one with only a minimum of repair to the blade composite material afterwards. To replace the connecting body  15 , a small hole is cut into a glass fiber skin of the blade  5  to get access to the connecting body  15 . Blades that comprise known root bushings with threads being cut directly into the material of the root bushings must be replaced completely when the thread is damaged. 
         [0038]    The bushing body  14  comprises a receiving section  17  for receiving the connecting body  15 . The receiving section  17  may be part of the cylindrical basic portion  10  so that the connecting body  15  is receivable in the basic portion  10 . The receiving section  17  is formed correspondingly to an outer geometry of the connecting body  15 . As shown in  FIGS. 4 and 5 , the connecting body  15  has a curved front face  18 . The receiving section  17  is shaped corresponding to the curved front face  18 . The curved front face  18  has a spheric curvature. Due to the spheric surface between the front face  18  and the receiving section  17 , the demands of perpendicularity between the connecting body  15  and the bushing body  14  can be lowered. Due to the spheric surface, the connecting body  15  is self-aligning towards the bushing body  14 . Alternatively, the front face  18  and the receiving section  17  may be flat, triangular or sinusoidal or may have any other curvature. The connecting body  15  is positive-fitted to the bushing body  14 . 
         [0039]    Advantageously, the bushing body  14  and the connecting body  15  are made of different materials. The material which the connecting  15  is made of has a higher grade or strength than the material which the bushing body  14  is made of. In particular, the material the bushing body  14  is made of is cheaper. 
         [0040]    In  FIG. 4  the connecting body  15  is shown being inserted into the receiving section  17  of the bushing body  14  and in  FIG. 5  the connecting body  15  is shown being taken out of the receiving section  17 . 
         [0041]    Although the present invention has been described in accordance with preferred embodiments, it is obvious for the person skilled in the art that modifications are possible in all embodiments. 
       REFERENCE NUMERALS: 
       [0000]    
       
           1  wind turbine 
           2  rotor 
           3  nacelle 
           4  tower 
           5  blade 
           6  hub 
           7  portion 
           8  blade root 
           9  root bushing 
           10  basic portion 
           11  extension portion 
           12  bore 
           13  end 
           14  bushing body 
           15  connecting body 
           16  thread 
           17  receiving section 
           18  front face