Abstract:
The invention relates to a wind turbine including at least two pitch controlled wind turbine blades. Each blade has pitch bearings including two or more bearing rings, and pitch controlling means for pitching the blades by means of the bearings. The blades are mounted on a hub via the pitch bearings and the pitch bearings include separate flexibility enhancing means for controlling loads in the bearings.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The invention relates to a wind turbine and use hereof. 
     DESCRIPTION OF THE RELATED ART 
     A wind turbine known in the art comprises a wind turbine tower and a wind turbine nacelle positioned on top of the tower. A wind turbine rotor with three wind turbine blades is connected to the nacelle through a low speed shaft, which extends out of the nacelle front as illustrated on  FIG. 1 . 
     Modern wind turbines control the load on the rotor by pitching the blades in and out of the incoming wind. The blades are pitched to optimize the output or to protect the wind turbine from damaging overloads. 
     To perform the pitch each blade is provided with a pitch bearing between the hub and the blade, and some sort of mechanism, most often a hydraulic cylinder, to provide the force for pitching the blade and maintaining it in a given position. This pitching arrangement enables each blade to be turned approximately 90° around their longitudinal axis. 
     The wind load on the blades get bigger as modern wind turbines get bigger, and as both blades and hub get bigger the more relatively soft and flexible they get. These facts make the pitch bearings very crucial components of the rotor, in that they have to be able to transfer the moment produced by the wind load to the hub and at the same time enable that the blades can rotate freely and accurately. 
     To ensure this the obvious solution would be simply to make the pitch bearings bigger, but the bearing balls have already almost reached the limit of what is industrially available, and bigger balls would therefore be economically disadvantageous. Furthermore, the ring size would increase with increased ball size and thereby increase the bearing cost and weight significantly. 
     Another solution would be to provide the blade with more than one bearing spaced apart as shown in e.g. DE 3415428 A1 and U.S. Pat. No. 4,668,109. This is advantageous in that it enables relatively small bearings to transfer the moment. But this design is space consuming and requires a high degree of rigidity and thereby increased cost and weight of both the hub and the blades. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides a large modern wind turbine pitch bearing without the mentioned disadvantages. 
     The invention particularly provides for a cost and weight efficient wind turbine pitch bearing which can transfer the loads on and from the blades to the hub without reducing the bearings durability or functionality. 
     That is, the invention provides for a wind turbine comprising at least two pitch controlled wind turbine blades. Each blade comprises one or more pitch bearings including two or more bearing rings, and pitch controlling means for pitching the blades by means of the bearings. The blades being mounted on a hub via the pitch bearings characterized in that, the one or more pitch bearings comprise separate flexibility enhancing means for controlling loads in the bearings. 
     It is advantageous to provide the pitch bearings with separate flexibility enhancing means, in that the flexibility enhancing means ensures the durability or functionality of the bearing even though the bearing rings are distorted by the load on and from the blades. Hereby the bearings can transfer a bigger moment or load without the weight or the cost of the bearing being raised much. 
     It should be emphasised that the term “hub” is to be understood as the part of the wind turbine to which the blades are attached. The term “hub” therefore also covers the teetering device to which the blades are attached on teeter wind turbines. 
     In an aspect of the invention, said bearings comprise two rows of rolling elements. 
     The more rows of rolling elements the more sensitive the bearings life and functionality are to a distortion of the bearing rings. It is therefore advantageous to provide a bearing comprising two rows of rolling elements with separate flexibility enhancing means. 
     It should be emphasised that the term “rolling elements” is to be understood as any form of rolling parts of a bearing such as balls, rollers, needles or other. 
     In an aspect of the invention, said bearings comprise three or more rows of rolling elements. 
     Hereby is achieved an advantageous embodiment of the invention. 
     In an aspect of the invention, said bearings comprise one or more first and one or more second separate flexibility enhancing means. 
     By providing the bearings with both a first and a second separate flexibility enhancing means the bearing becomes more flexible. This is advantageous in that it ensures the durability or functionality of the bearing even though the bearing rings are distorted. 
     In an aspect of the invention, said separate flexibility enhancing means are displaced from the load transferring surfaces of said bearing rings. 
     It is advantageous to position the separate flexibility enhancing means away from the surfaces of the bearing rings on which the rolling elements roll in that it enables a more simple design of the separate flexibility enhancing means and it enables the flexibility enhancing means to provide the bearing with more flexibility. 
     In an aspect of the invention, at least one of said one or more bearing rings comprise through holes for blade attachment means such as screws, bolts, studs or rivets. 
     It is advantageous that one or more of the bearing rings comprise through holes in that it enables a simple way of attaching the bearing. 
     In an aspect of the invention, said bearings comprise three bearing rings. 
     Bearings comprising three bearing rings always comprise at least two rows of rolling elements. The more rows of rolling elements the more sensitive the bearings durability and functionality are to a distortion of the bearing rings. It is therefore advantageous to provide a bearing comprising three bearing rings with separate flexibility enhancing means. 
     In an aspect of the invention, said separate flexibility enhancing means comprise at least one angle compensating mean. 
     Angle compensating means enables the rolling elements of the bearing to transfer the loads from one bearing ring to another even though one or more of the bearing rings are distorted and therefore not perfectly aligned. Angle compensating means are therefore advantageous in that they increase the bearings flexibility. 
     In an aspect of the invention, said at least one angle compensating mean is a separate 360° ring or more than one ring parts together forming a full 360° ring. 
     It is advantageous that the angle compensating means is a 360° ring or more than one ring parts together forming a full 360° ring in that the bearings form a 360° ring. 
     In an aspect of the invention, said at least one angle compensating mean is positioned so that at least one of said rows of rolling elements rolls on one first surface of said angle compensating mean and one second opposing surface of said angle compensating mean is in contact with the middle section of said bearing. 
     By placing the angle compensating mean between the rolling elements and its opposing contact surface on the middle section of a bearing ring, it enables the angle compensating mean to absorb or distribute any misalignment or angle differences between the rolling elements and their opposing contact surface on the middle section, whether that being a temporary, a permanent, a local or an overall misalignment or angle difference. 
     In an aspect of the invention, said first surface is a plane roller surface and said second opposing surface is a semicircular or substantially semicircular contact surface. 
     It is advantageous to provide the angle compensating mean with a plane roller surface for the rolling elements to roll on and an opposing semicircular contact surface facing the contact surface of the bearing, in that it enables the angle compensating mean to twist or distort locally or overall to absorb or distribute any misalignment or angle differences between the rolling elements and their opposing contact surface. 
     In an aspect of the invention, said separate flexibility enhancing means include one or more plates attached to one or more of said bearing rings by means of e.g. screws, bolts, studs, rivets, adhesive means or welding. 
     By providing the bearing rings with one or more plates it is possible to control the bearing rings rigidity and thereby ensure that even though the rings are distorted the bearings still have their full functionality and no part of the bearing is damaged or gets excessively strained. 
     In an aspect of the invention, at least one of said one or more plates is a strengthening plate providing additional non-uniform or substantially non-uniform rigidity to said bearing ring on which it is attached. 
     By making the plates so that they provide non-uniform rigidity to the bearing ring on which they are attached, the plates can provide the bearings with rigidity and/or flexibility where it is needed, without increasing the bearings weight much. 
     In an aspect of the invention, said additional non-uniform or substantially non-uniform rigidity is provided by means of one or more holes in said strengthening plates. 
     It is advantageous to provide the plates with one or more holes, in that it is a simple and efficient way of controlling the plates rigidity and/or flexibility 
     In an aspect of the invention, said separate flexibility enhancing means comprises two or more radially separated rolling element cages. 
     It is advantageous to provide the bearings with several radially separate rolling element cases, in that it enables the possibility of varying distances between the cages and thereby increases the bearings flexibility. 
     In an aspect of the invention, said rolling element cages are separated by compression zones. 
     By separating the rolling element cages by means of compression zones the bearing becomes more flexible. 
     In an aspect of the invention, said compression zones are formed integrally in one or both longitudinal ends of said rolling element cages. 
     Forming the compression zones integrally in the ends of the rolling element cases is advantageous, in that it provides for a simple and cost efficient way of providing a bearing with compression zones. 
     In an aspect of the invention, said integrally formed compression zones are formed as transverse slits in the rolling element cages. 
     Transverse slits in the rolling element cages are a simple and cost efficient way of providing a bearing with compression zones. 
     In an aspect of the invention, facing transverse slits of juxtaposed rolling element cages are slit transversely from opposite sides. 
     By slitting the rolling element cages from opposite sides the facing slits together form a simple and efficient compression zone. 
     In an aspect of the invention, said compression zones are compression parts separate from said rolling element cages. 
     By using compression parts separate from said rolling element cages it is possible to use rubber blocks, springs or other as the compression zones. This is advantageous, in that it provides for a simple and cost efficient way of providing a bearing with compression zones. 
     In an aspect of the invention, said separate flexibility enhancing means includes hollow rolling elements such as hollow rollers. 
     By making the rolling elements hollow they become more flexible and therefore their ability to handle distortion and misalignment of the bearing rings is improved. 
     In an aspect of the invention, the hole in said hollow rollers has a larger diameter at the ends than in the middle. 
     Distortion and misalignment of the bearing rings results in, that one end of e.g. a roller has to transfer a bigger load than the rest of the roller. It is therefore advantageous to make the hole of a larger diameter at the ends and make the hole of a smaller diameter at the middle, to make the rolling element more rigid towards evenly distributed loads. 
     In an aspect of the invention, the hole in said hollow rollers is a straight through hole. 
     Providing the rollers with straight through holes is a simple and cost efficient way of providing the roller with enhanced flexible qualities. 
     In an aspect of the invention, the hole in said hollow rollers is a blind hole in one or both ends of said hollow rollers. 
     Providing the roller with blind hole in the ends, is a simple and cost efficient way of providing the rollers with enhanced flexible qualities and maintaining the rollers rigidity towards evenly distributed loads. 
     In an aspect of the invention, said separate flexibility enhancing means include rollers with varying diameter. 
     If the rollers are of varying diameter their ability to handle distortion and misalignment of the bearing rings is improved. This is advantageous in that it provides for a more flexible bearing. 
     In an aspect of the invention, said rollers are rounded in a longitudinal direction. 
     Rounding the rollers in a longitudinal direction is a simple and cost efficient way of providing the rollers with the ability to handle distortion and misalignment of the bearing rings. 
     In an aspect of the invention, said rollers bulge at the middle. 
     By making the rollers bulge at the middle, they are able to handle distortion and misalignment of the bearing rings more efficiently. 
     The invention further provides for a wind turbine comprising at least two pitch controlled wind turbine blades, each blade comprising one or more pitch bearings including two or more bearing rings, and pitch controlling means for pitching said blades by means of said bearings, said blades being mounted on a hub via said pitch bearings, characterized in that said pitch bearings comprise at least three rows of rolling elements, said rows having a common diameter. 
     Hereby is achieved an advantageous embodiment of the invention. 
     In an aspect of the invention, said pitch bearings comprise at least three further rows of rolling elements, said further rows having another common diameter. 
     Hereby is achieved an advantageous embodiment of the invention. 
     The invention further provides for use of a wind turbine where said wind turbine is a variable speed pitch wind turbine. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention will be described in the following with reference to the figures in which 
         FIG. 1 . illustrates a large modern wind turbine as seen from the front, 
         FIG. 2  illustrates a cross section of a wind turbine hub connected to a hub through a pitch bearing comprising plates, 
         FIG. 3  illustrates the same embodiment of a pitch bearing as illustrated in  FIG. 2  as seen from the top, 
         FIG. 4  illustrates a part of a cross section of a pitch bearing comprising three rows of rolling elements, 
         FIG. 5  illustrates a part of a cross section of a pitch bearing comprising six rows of rolling elements, 
         FIG. 6  illustrates a part of a cross section of a pitch bearing comprising angle compensating means, 
         FIG. 7  illustrates the same embodiment of a pitch bearing as illustrated in  FIG. 6  as seen from the top, 
         FIG. 8  illustrates an embodiment of a hollow roller as seen from the front and the side, 
         FIG. 9  illustrates an embodiment of a rounded roller as seen from the front and the side, 
         FIG. 10  illustrates a part of a cross section of a pitch bearing comprising rollers, 
         FIG. 11  illustrates a part of a cross section of another pitch bearing comprising rollers, 
         FIG. 12  illustrates an embodiment of rolling element cages and separate compression parts, 
         FIG. 13  illustrates an embodiment of rolling element cages comprising compression zones, 
         FIG. 14  illustrates the same embodiment of rolling element cages as illustrated in  FIG. 13  in a compressed state, and 
         FIG. 15  illustrates a part of a cross section of a pitch bearing comprising rolling element cages. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a wind turbine  1 , comprising a tower  2  and a wind turbine nacelle  3  positioned on top of the tower  2 . The wind turbine rotor  4 , comprising three wind turbine blades  5 , is connected to the nacelle  3  through the low speed shaft which extends out of the nacelle  3  front. 
       FIG. 2  illustrates a cross section of a wind turbine blade  5  connected to a hub  7  through an embodiment of a pitch bearing  9 . In this embodiment the pitch bearing  9  is a triple row  27  ball bearing, but it could also be a double or four rowed  27  bearing. 
     The pitch bearing has to transfer forces mainly from three different sources. The blade  5  (and the bearings  9  themselves off cause) is under constant influence of the force of gravitation. The direction of the gravitational force varies depending on the blades  5  position, inducing different loads on the pitch bearings  9 . When the blade is in motion the bearing  9  is also under influence of a centrifugal force, which mainly produces an axial pull in the bearing  9 . Finally the bearings  9  are under influence of the wind load on the blades  5 . This force is by far the greatest load on the bearings  9  and it produces a massive moment, which the bearings  9  have to stand. 
     Since the pitch mechanism on traditional pitch wind turbine  1  usually can pitch the blade  5  a little over 90°, the load on the pitch bearings  9  is far from uniform under normal operation. The wind load on the blade  5  will make the blade  5  pull in the part of the inner ring  26  of the bearing  9  facing the wind and push on the part of the inner ring  26  facing away from the wind. When the inner ring  26  is pulled forcefully by the blade  5  the balls are pushed outwards and upwards in an angle of approximately 45° as indicated by the arrows. This force will result in an axial pull in the outer ring  24  and a radial push on the outer ring  24 . Since the bottom of the outer ring  24  is fixed against the hub  7 , the top of the ring  24  will have a tendency to deflect. Likewise will the inner ring  26  have a tendency to deflect at the bottom, since it is fixed against the blade at the top. If this deflection becomes too big, one or more of the rows  27  will be unable to transfer the loads, which could lead to a damaging load on the remaining balls. Because of the direction of the forces this problem is most pronounced at the part of the bearing facing the wind. This deflection could be reduced by making the rings thicker and therefore more rigid, but this would increase the cost and the weight of the bearing significantly. 
     Multi-rowed ball bearings are relatively cheap due to the well proven and relatively simple design, but they have the disadvantage of being relatively high in relation to the width, making them relatively poor regarding the transfer of large moment forces. 
       FIG. 2  therefore illustrates that the free end of the outer ring  24  is provided with an outer plate  10 . The outer plate  10  is fixed to the outer ring  24  and thereby enables the possibility of providing the outer ring  24  with rigidity where it is needed. Likewise, the inner ring  26  is provided with an inner plate  11 . To ensure flexibility of the inner plate  11  in the right places, the plate  11  is provided with a hole  8 , allowing for the inner ring  26  to deflect a little at the bottom to compensate for a little deflection at the top of the outer ring  24 , and thereby ensuring a constant distance between the two rings  24 ,  26  and a substantially identical load on all the rows  27  of balls  13 . 
       FIG. 3  illustrates the same embodiment of a pitch bearing as illustrated in  FIG. 2  as seen from the top. In this embodiment of the invention the outer plate  10  is shaped as an annular semicircular ring with increased width towards the middle of the semicircle. This design ensures most rigidity where the deflection of the outer ring  24  is biggest. In this embodiment of the invention the outer plate  10  covers approximately 180° of the outer ring  24  but in another embodiment the outer plate could cover more or less of the outer ring  24  or it could be formed as a full 360° annular ring. In another embodiment of the invention the outer plate  10  could also be of constant width. 
       FIG. 3  also illustrates that the inner plate  11  is provided with a plate hole  8  shaped as an ellipse. The hole  8  is also placed near the part of the bearing  9  where the load on the outer ring  24  is biggest. In another embodiment of the invention the hole  8  could have another shape such as circular, polygonal or the flexibility could be provided by means of a number of strategically placed holes  8 . 
       FIG. 4  illustrates a part of a cross section of a pitch bearing  9  comprising three rows  27  of rolling elements  13 . In this embodiment of the invention the pitch bearing  9  comprise an outer bearing ring  24  and an inner bearing ring  26 . The figure further illustrates that the bearing  9  comprise three rows  27  of balls  13 . The three rows of rolling elements  13  have the same diameter as illustrated by d 1 . 
       FIG. 5  illustrates a part of a cross section of a pitch bearing  9  comprising two columns of each three rows  27  of rolling elements  13 . Between the outer bearing ring  24  and the centre bearing ring  25  is positioned three rows  27  of balls  13  on the same diameter d 2 . Between the centre bearing ring  25  and the inner bearing ring  26  is positioned three other rows  27  of rolling elements  13  on a common diameter d 1 , which is different from d 2 . 
       FIG. 6  illustrates a part of a cross section of a pitch bearing  9  comprising angle compensating means  14 . On an ordinary bearing  9  of this type the top and/or the bottom row  27  of rollers  13  would press against the middle section during normal operation. To ensure long life of the rollers  13  and the bearing  9 , the transferred load has to be distributed evenly over the entire roller surface. This could be ensured if the rings were made very strong and rigid, but this would also mean a significant increase in cost and weight. 
       FIG. 6  therefore illustrates that the bearing  9  is provided with angle compensating means  14  in the form of two separate rings provided with a plan surface  16  facing the rollers  13  and a semicircular surface  15  facing the middle section  29 . In another embodiment of the invention the plane surface  16  could be provided with a groove, in which the rollers  13  would roll and/or the semicircular surface  15  could be formed as a plane surface with more or less rounded corners. 
     The angle compensating means  14  could be made of hardened steel, but it would still be so flexible, that it can twist a little to compensate for any angle differences between the roller surface and the opposing surface on the middle section  29 . 
       FIG. 7  illustrates the same embodiment of a pitch bearing as illustrated in  FIG. 6  as seen from the top. The angle compensating means  14  is illustrated in dotted lines as one full 360° ring. In another embodiment of the invention the angle compensating means  14  could be made of several individual or joined ring parts e.g. to lower the production costs. 
       FIG. 8  illustrates an embodiment of a hollow roller  12  as seen from the front and the side. The rollers  13  in roller bearings  9  as the ones illustrated in  FIGS. 4 ,  8  and  9  are all very sensitive to angle differences between the different rings  24 ,  25 ,  26 . If one of the rings  24 ,  25 ,  26  is under the influence of a heavy load, the design of the bearings  9  could result in such an angle difference, if the rings  24 ,  25 ,  26  are not strong and rigid enough. To ensure that the rollers  13  are not damaged by such an angle difference the rollers  13  could be provided with a hole  22  in the middle, providing it with that much flexibility that the roller  12  is not damaged even though the load on it is not evenly distributed. 
     In this embodiment of the invention the hole  22  is bigger near the ends of the roller  12 , to ensure most flexibility where it is most needed, but in another embodiment of the invention the hole could be straight or the flexibility could be provided by a blind hole in one or each end of the roller  12 . 
       FIG. 9  illustrates an embodiment of a rounded roller  28  as seen from the front and the side. As explained above angle differences between the bearing rings  24 ,  25 ,  26  can damage the rollers  13  or reduce their life. To provide the bearings  9  with the flexibility enabling it to handle distortion of the bearing rings  24 ,  25 ,  26  the rollers  13  could also be provided with a curved roller surface either by making it bulge or by rounding  17  the two edges. 
       FIG. 10  and  FIG. 11  illustrates parts of cross sections of two different embodiments of pitch bearings  9 . In these bearings  9  no angle compensating means are provided, so if the bearing rings  24 ,  25 ,  26  are distorted by the loads on and/or from the blades  5 , the rollers or their respective opposing contact surfaces could be damaged. In these types of bearings flexibility enhancing means such as angle compensating means  14 , inner and/or outer plates  10 ,  11 , hollow rollers  12 , rounded rollers  28  or any combination thereof would enable the bearings  9  to transfer a much greater load than otherwise possible without significantly increasing the cost or the weight of the bearings. 
       FIG. 12  illustrates an embodiment of rolling element cages  19 . To ensure that the rolling elements  13  of the pitch bearing  9  are separated and stay in place, the rolling elements  13  are provided with rolling element cages  19 . This could be a single 360° cage ensuring a constant distance between all the rolling elements  13 , but if one or more bearing rings  24 ,  25 ,  26  are distorted, a force could arise forcing one or more rolling elements  13  apart. If the rolling elements  13  mutual distance is fixed, this force could potentially damage the rolling elements  13  or their corresponding contact surfaces on the bearing rings  24 ,  25 ,  26 . As illustrated in  FIG. 12  the rolling element cages  19  could be divided into a number of separate cages  19  kept apart by compression parts  6 . The compression parts  6  can be compressed and thereby absorbing some of the separating forces. The compression parts  6  could e.g. be rubber blocks, metal springs or other elastical devices or materials. 
     In this embodiment of the invention each cage  19  contains 4 balls but in another embodiment of the invention the cage  19  could contain two, three, five, six or more balls or rollers  13 . 
       FIG. 13  illustrates another embodiment of rolling element cages  19 . In this embodiment of the invention each cage  19  comprise a compression zone  20 , in form of a transverse slit  21  formed integrally in the cage  19 . The cages  19  could be made from some sort of steel plate which then could be flame or laser cut into the desired shape. 
       FIG. 14  illustrates the same embodiment of rolling element cages  19  as illustrated in  FIG. 13  in a compressed state. 
       FIG. 15  illustrates a part of a cross section of an embodiment of a pitch bearing  9  comprising rolling element cages  19 . 
     The invention has been exemplified above with reference to specific examples of separate flexibility enhancing means in pitch bearings  9  for a wind turbine  1 . However, it should be understood that the invention is not limited to the particular examples described above but may be designed and altered in a multitude of varieties within the scope of the invention as specified in the claims.