Abstract:
A barrel nut is provided having an internal female threaded portion adapted to engage a male threaded portion of a fastener. The barrel nut includes a bearing surface and a top surface opposed to the bearing surface. A sidewall surface is interposed substantially between the bearing surface and the top surface. The sidewall surface has a profile shaped so as to reduce the material used in fabricating the barrel nut.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an improved barrel nut and, especially, to an improved barrel nut for a rotor blade of a wind turbine. 
       BACKGROUND OF THE INVENTION 
       [0002]    Apart from the aerodynamic design of a wind turbine rotor blade, the quality and weight of the rotor blades are essentially determined by the design of the blade connection to the rotor hub, i.e. the blade root section. What makes the design of the blade connection to the rotor hub a difficult task is the load transfer from the fiber composite structure of the rotor blade into the metal structure of the rotor hub. Such a load transfer is difficult in principle due to the substantially different properties of the materials involved. A known technique for attaching the rotor blades to the hub is to form nut and bolt connection between the blade root and the hub. A barrel nut is typically used to secure the bolt. 
         [0003]    Typically, cylindrical shaped barrel nuts have been used to secure the bolt. These known barrel nuts are large and heavy, and contribute to the overall weight that must be supported by the bearings and tower of a wind turbine. In addition a large amount of material is utilized to fabricate the nuts, and material costs can add up to a substantial amount in large, utility grade wind turbines. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    According to an aspect of the invention, a barrel nut is provided having an internal female threaded portion adapted to engage a male threaded portion of a fastener. The barrel nut includes a bearing surface and a top surface opposed to the bearing surface. A sidewall surface is interposed substantially between the bearing surface and the top surface. The sidewall surface has a profile shaped so as to reduce the material used in fabricating the barrel nut. 
         [0005]    According to a further embodiment of the present invention, a wind turbine having at least one barrel nut is provided, and the barrel nut has an internal female threaded portion adapted to engage a male threaded portion of a fastener. The barrel nut includes a bearing surface and a top surface opposed to the bearing surface. A sidewall surface is interposed substantially between the bearing surface and the top surface. The sidewall surface has a profile shaped so as to reduce the material used in fabricating the barrel nut. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    A full and enabling disclosure of the present invention, including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures wherein: 
           [0007]      FIG. 1  is a schematic view of a wind turbine. 
           [0008]      FIG. 2  is a front view of a wind turbine rotor blade. 
           [0009]      FIG. 3  is a view of a flange section of a wind turbine rotor blade. 
           [0010]      FIG. 4  is a side view of a blade root portion according to an embodiment of the present invention. 
           [0011]      FIG. 5  is a cross-sectional view of a T-bolt connection in a flange section according to an embodiment of the present invention. 
           [0012]      FIG. 6  is a cross-sectional view of a T-bolt connection between the blade root and a flange portion of a rotor hub according to an embodiment of the present invention. 
           [0013]      FIG. 7  shows fastening bolts according to an embodiment of the present invention. 
           [0014]      FIG. 8  is a side view of a T-bolt connection between the blade root and a flange portion of a rotor hub according to an embodiment of the present invention. 
           [0015]      FIG. 9  is a cross-sectional view of a nut and bolt connection between the blade root and a flange portion of a rotor hub according to an embodiment of the present invention. 
           [0016]      FIG. 10  is a cross-sectional view of a nut and bolt connection between the blade root and a flange portion of a rotor hub according to another embodiment of the present invention. 
           [0017]      FIG. 11  is a cross-sectional view of a nut and bolt connection between the blade root and a flange portion of a rotor hub according to yet another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Reference will now be made in detail to the various embodiments of the invention, one or more examples of which are illustrated in the figures. Each example is provided by way of explanation of the invention, and is not meant as a limitation of the invention. For example, features illustrated or described as part of one embodiment can be used on or in conjunction with other embodiments to yield yet a further embodiment. It is intended that the present invention includes such modifications and variations. 
         [0019]      FIG. 1  is a schematic view of a conventional wind turbine  100 . The wind turbine  100  includes a tower  110  to which a machine nacelle  120  is mounted at its top end. A hub  130  bearing three rotor blades  140  is mounted to a lateral end of the machine nacelle  120 . 
         [0020]    The basic configuration of a rotor blade  140  is shown in  FIG. 2 . Therein, the rotor blade  140  includes a root section  142  which serves for mounting rotor blade  140  to hub  130 . Opposite to root section  142 , the tip end  146  of rotor blade  140  is disposed. A body section  144  of the rotor blade  140  extends between the root section  142  and the tip end  146 . 
         [0021]    Next, the design of root section  142  and, especially, a flange portion of the root section is explained with reference to  FIGS. 3 to 6 . 
         [0022]      FIG. 3  shows the blade flange  310  of a rotor blade when seen from below, i.e. from the root section  142  to the tip  146  of the rotor blade. The flange section  310  has an essentially circular ring-shaped cross section. The blade flange  310  further includes a number of openings  320  corresponding to longitudinal bores (not shown). The openings  320  have a width WL and are evenly spaced by a distance DH along a circumferential direction of the flange section  310 . When the rotor blade  140  is mounted to a rotor hub  130 , fastening bolts are inserted into the longitudinal bores to form, as one example, a bolt connection as will be explained in more detail below. 
         [0023]      FIG. 4  shows a side view of the blade root  142 . Therein, it is shown that radial bores  442 ,  444  are provided in the root section of the blade. These radial bores  442 ,  444  are configured to receive the cross-bolts of the T-bolt connection. Longitudinal bores  432 ,  434  (shown in broken lines) extend from the radial bores  442 ,  444  to the hub end  310  of the blade. At the hub end, the longitudinal bores  432 ,  434  provide openings  422 ,  424  through which fastening bolts can be inserted. As shown in  FIG. 4 , the radial bores are arranged along the circumference of the blade root  142 . However, the radial bores can be divided in a first group  442  of bores located proximate the hub end of the blade and a second group of bores  444  located distant from that end. In other words, the first group of radial bores  442  is spaced a small longitudinal distance S from the hub end  310  of the blade and the second group of radial bores  444  is spaced a long longitudinal distance L from the hub end  310 . Accordingly, also the longitudinal bores are divided in a first group  432  extending only the small longitudinal distance S from the hub end  310  of the blade and a second group  434  extending the long longitudinal distance L from the hub end  310 . Thus, the radial bores  442  of the first group are longitudinally spaced from the radial bores  444  of the second group by the difference L-S between the long and short longitudinal distances. Typically, this distance L-S is in the range of 0.5 to 10 times, preferably in the range of 1 to 3 times, of the diameter WR of the radial bores. Alternatively or in addition, the distance L-S may be in the range of 1 to 60%, preferably 5 to 40%, even more preferably 10 to 30%, of the long longitudinal length L. As it is shown in  FIG. 4 , the radial bores are alternately selected from the first group  442  and the second group  444 . Thus, a distance D 1  with a sufficient amount of root laminate is provided between adjacent radial bores  442 ,  444  of the first and second groups. As a result, the circumferential spacing DH (see  FIG. 3 ) can be reduced so that more bolt connections can be provided for a given root diameter. 
         [0024]    In the above described embodiment, the diameters WR, WL of the radial and longitudinal bores were selected to be identical, respectively. Also, only a single small longitudinal distance S from the hub end  310  and only a single long longitudinal distance L from the hub end  310  are provided. However, it should be understood that the present invention may be also realized with varying values of the diameters WR, WL of the radial or longitudinal bores. Likewise, there may be several small and long longitudinal distances provided as long as it is obeyed that two neighboring radial bores are sufficiently spaced in accordance with the root laminate margins. 
         [0025]      FIG. 5  is a longitudinal cross-sectional view of the flange section of blade root  142  along line A-A′ in  FIG. 4 . The longitudinal bore  432  is located essentially in the middle between the outer sidewall surface and the inner sidewall surface of the rotor blade wall. Furthermore, a radial bore  442  is provided in the sidewall at one end of the longitudinal bore  432 . At the opposite end, longitudinal bores  432  and  434  have opening  422  and  424  in the blade flange, respectively. When the rotor blade  140  is mounted to the rotor hub  130 , barrel nuts are inserted into the radial bores  442 ,  444  to form a connection with fastening bolts inserted into the longitudinal bore  432  as will be explained in more detail below with reference to  FIG. 6 . 
         [0026]      FIG. 6  is a cross-sectional view of a nut and bolt connection formed between the root section  142  and a flange  610  of the rotor hub  130 . A barrel nut  620  is inserted into the radial bore  442 . The barrel nut  620  comprises a central opening with a female thread. The opening is aligned with the position of the longitudinal bore  432 . Furthermore, the flange  610  of the rotor hub  130  abuts against the lower face of the flange section  310 . The hub flange  610  has a through-hole  612  dimensioned to fit with the longitudinal bore  432  of the blade flange  310 . The through-hole  612  and the longitudinal bore  432  are aligned with each other so that a fastening bolt  630  can be inserted into the through-hole  612  and the longitudinal bore  432 . The fastening bolt  630  includes a male thread  632  fitting with the female thread of the barrel nut  620 . Fastening bolt  630  is fixed to barrel nut  620  via screwed fastening so that a nut-bolt connection is established. A barrel nut  640  may also be used at the opposite end of bolt  630 . Thus, the rotor blade  140  can be fixed to the rotor hub  130 . 
         [0027]    Although the configuration of the nut-bolt connection between the rotor blade and the hub was described with reference to a short longitudinal bore  432 , it should be understood that the configuration for a long longitudinal bore  434  is basically identical except for the longitudinal extension of the longitudinal bore. Especially, according to one embodiment of the present invention the configuration of the nut-bolt connection differs only in the longitudinal length of the longitudinal bores and corresponding fastening bolts for short and long bores, respectively. 
         [0028]    In the above described bolt-connection arrangement, the flange  310  of the rotor hub, the barrel nut  620  and the fastening bolt  630  are typically made of steel. Typically, the blade flange  310  is made of a fiber reinforced matrix. For example, the fiber reinforced matrix of the blade flange is a material including glass fibers and/or carbon fibers embedded in a matrix material. For example, the matrix material is selected from the group of an epoxy resin, an epoxy novolac, a thermosetting resin like epoxy resin, epoxy novolacs, polyesters, polyimides, both condensation and addition types, phenolic resins, and bismaleimides. Any of those resins may be selected according to the specific technical purpose the matrix material is applied to. Particularly, the resin system should be selected with respect to a particular fiber reinforcement for producing a finished hybrid fiber reinforced part with the desired mechanical and environmental properties. The resin is usually degassed under vacuum after mixing of a hardener/catalyst in the resin, to eliminate or remove all entrapped air from the liquid resin. The resin should therefore be capable of proceeding through a vacuum pressure cycle environment of heat and time without formation of gas bubbles or voids. In such a matrix material, carbon and/or glass fibers are embedded and are typically provided in the form of fiber mats. However, the fibers may also be provided in the form of a non-woven or roving fabric. 
         [0029]      FIG. 7  shows fastening bolts according to an embodiment of the present invention. On the left-hand side of  FIG. 7 , a fastening bolt  630  of a short type is shown. The short fastening bolt  630  is adapted for being inserted into a short longitudinal bore  432 . The short fastening bolt  630  includes a head section  634 , a short shaft  636 , and a male-threaded end  632  opposite to the head section. The male-threaded end portion  632  is adapted to be screwed into a female thread of a barrel nut  620 . The shaft portion  636  has a short length LS which is dimensioned to extend over the short length S of the longitudinal bore  432  and a through-hole  612  of the hub flange. In alternative embodiments, head section  634  can comprise a barrel nut having a female thread, which can be fastened over a threaded end portion of shaft  636 . Alternatively, the head section  634  may be formed integral with the shaft  636 . In this case, the head section  634  may be formed as a hexnut. 
         [0030]    On the right-hand side of  FIG. 7 , a fastening bolt  730  of the long type is shown. Its basic configuration is the same as for the short-type bolt  630 . The long fastening bolt  730  includes a head section  734 , a short shaft  736 , and a male-threaded end  732  opposite to the head section. As with the short bolt  630 , a barrel nut can be used on the male-threaded end  732  and/or on the head section  734 . 
         [0031]      FIG. 8  is a side view of a nut-bolt connection between the blade root  142  and a flange portion  610  of the rotor hub according to an embodiment of the present invention. In such a nut-bolt connection, the barrel nuts  620  can be inserted into the radial bores  442 ,  444  with their female-threaded opening facing the longitudinal bores  432 ,  434 . In addition, the barrel nuts  620  can also be used on the head section of bolts  630  and  730 . Short and long longitudinal fastening bolts  630  and  730  are inserted into the short and long longitudinal bores  432 ,  434  and corresponding through-holes  612  of the hub flange  610 , respectively. In one embodiment of the present invention, barrel nuts fasten the fastening bolts  630  and  730 . In some cases, the barrel nuts  620  and the fastening bolts  630 ,  730  can be preinstalled in the rotor blade during manufacturing. In this case, the fastening bolts  630 ,  730  have a male-threaded portion on both end sides. Then, the rotor blade is mounted on the hub flange on the construction site, i.e. the ends of the fastening bolts  630 ,  730  extending from the openings  420  are inserted into the through-holes  612  of the hub flange  610 . Subsequently, the nut-bolt connection can be finished from inside the hub by fastening barrel nuts onto the male-threaded end portions of the fastening bolts. 
         [0032]    The barrel nuts  620  may be used on either or both ends of the bolts  630 ,  730 . The barrel nuts can comprise a large amount of material, and in one example, may use about seven pounds of steel for each nut. Each blade requires many barrel nuts, and a utility grade wind turbine usually has two or three blades per turbine. This results in a large number of barrel nuts being used in each wind turbine. It would be advantageous to reduce the weight of each barrel nut, both for cost reduction (less material used) and for stress reduction (less weight). 
         [0033]      FIG. 9  illustrates a cross-sectional view of a nut and bolt connection formed between the root section  142  and a flange  610  of the rotor hub  130 , according to an aspect of the present invention. A barrel nut  920  is inserted into the radial bore  442 . The barrel nut  920  comprises a central opening with a female thread. Furthermore, the flange  610  of the rotor hub  130  abuts against the lower face of the flange section  310 . The fastening bolt  630  includes a male thread fitting with the female thread of the barrel nut  920 . Fastening bolt  630  is fixed to barrel nut  920  via screwed fastening so that a nut-bolt connection is established. A barrel nut  940  may also be used at the opposite end of bolt  630 . Thus, the rotor blade  140  can be fixed to the rotor hub  130 . 
         [0034]    Barrel nuts  920  and  940  have had a portion of their material removed, as is indicated by the dotted lines in  FIG. 9 . For example, an according to one embodiment of the present invention, about one third or more of the material comprising each barrel nut can be removed. A straight, curved, stepped or compound shape can be cut away from a conventional cylindrical barrel nut to obtain an improved barrel nut having leas material. By using less material, the cost can be reduced. There are many barrel nuts used in a wind turbine, so the cost savings could be substantial. In addition, the improved barrel nuts  920 ,  940  have reduced weight, which reduces the stresses experienced by the wind turbine components. A benefit of reduced weight is that the lifetime of individual wind turbine components can be increased. 
         [0035]      FIG. 10  illustrates a cross-sectional view of a nut and bolt connection formed between the root section  142  and a flange  610  of the rotor hub  130 , according to another aspect of the present invention. The barrel nuts  1020  and  1040  have a stepped profile.  FIG. 11  illustrates a cross-sectional view of a nut and bolt connection, according to yet another aspect of the present invention. The barrel nuts  1120  and  1140  have a curved or curvilinear profile portion. All of the embodiments herein described provide a barrel nut having reduced material. 
         [0036]    The angled, curved or stepped cut can be designed to not cause any substantial change in the joint stiffness ratio for bolt fatigue, as the bearing area is still about 98+% of the contact area. The area eliminated from the barrel nut has a lower-than-average pressure. The barrel nut will not have any substantially reduced strength, as the critical point is a section through the threaded hole. The bending moment also drops sharply away from the edge of the hole, so there is expected to be a minimal change in stiffness. The stepped cut option almost entirely eliminates the laminate shear stress concentration because 180 degrees of the barrel nut bearing area is always maintained. 
         [0037]    A number of different barrel nut shapes have been illustrated, however, it is to be understood that reduced material sections of the barrel nuts may extend around the entire nut or only a portion of the nut. For example, the angled cut in  FIG. 9 , may extend 360 degrees around the nut, or it may be placed in apportion on one or two sides of the nut. Barrel nuts of multiple configurations may be used concurrently (e.g., an angled cut nut  920  as shown in  FIG. 9  could be used in the same wind turbine as a stepped nut  1040  as shown in  FIG. 10 ). 
         [0038]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims of they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.