Abstract:
A wind turbine includes a rotor having at least one wind turbine blade connected to a rotor hub through at least one blade pitch bearing. The pitch bearing includes at least one outer ring, at least one center ring and at least one inner ring. The pitch bearing has a first rolling element arrangement between the outer ring and the center ring, and a second rolling element arrangement between the center ring and the inner ring, wherein at least one of the first and the second rolling element arrangements includes at least two rows of rolling elements. The at least two rows are at different axial positions in relation to a rotational axis. The play in the first row of roller elements is greater than the play in the second row of roller elements. A pitch bearing for a wind turbine is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(a) to DK Application No. PA 2010 00261, filed Mar. 29, 2010. This application also claims the benefit of U.S. Provisional Application Ser. No. 61/318,615, filed Mar. 29, 2010. Each of these applications is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The invention relates to a wind turbine comprising a rotor including at least one wind turbine blade connected to a rotor hub through at least one pitch bearing, wherein the at least one pitch bearing comprises at least one outer ring, at least one centre ring and at least one inner ring. The invention further relates to a pitch bearing for a wind turbine. 
     BACKGROUND 
     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, 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 pitching arrangement comprising a pitch bearing between the hub and the blade, and some sort of mechanism, most often a hydraulic cylinder or an electrical motor, to provide the force for pitching the blade and maintaining it in a given position. This pitching arrangement typically enables each blade to be turned at least 60° around their longitudinal axis. 
     As the size of the modern wind turbines increases, the load, on most of the different parts which a wind turbine consists of, also increases. Notably, the loads on the pitching arrangement are significantly increased due to increased blade size and overall power output of the wind turbine. 
     From European patent application EP 1 741 943 A2 it is therefore known to use a three ring bearing as a wind turbine bearing. However, the inner and outer rings of a three ring bearing have a tendency to deflect outwards and away from the centre ring when the bearing is subject to large axial loads. When the inner and outer rings is fixed, for example, against the rotor hub at a bottom end of the rings, the upper ends of the rings will deflect more than the bottom ends. Three ring bearings are usually provided with at least two rows of rolling elements between the centre ring and a first ring of the outer and the inner ring, and at least one row of rolling elements between the centre ring and the second ring of the outer and the inner ring. This ensures that the large axial loads are transferred between the rings by means of as many contact surfaces as possible given weight, financial and manufacturing considerations. But, if one end of the bearing rings deflects more than the other, the loads are not distributed evenly between the at least two rows of rolling elements. That is, the heavier the bearing is loaded, the more uneven the loads are distributed between the at least two rows of rolling elements. 
     In EP 1 741 943 A2, a solution to this problem is postulated and includes supporting the outer ring near its contact surface by means of a supporting part or by reinforcing the outer ring by increasing the thickness of the ring the further away from the contact surface it extends. However, supporting the outer ring requires a close fit with the supporting part thereby increasing the manufacturing cost of both the outer ring and the supporting part and such a solution makes it considerably more difficult to mount and dismount the bearing. Moreover, reinforcing the rings to compensate for the tendency to deflect increases both the weight and the cost of the ring considerably. 
     Accordingly, an aspect of the invention is to provide a wind turbine pitch bearing design with an improved load distribution. 
     SUMMARY 
     An embodiment of the invention provides for a wind turbine comprising a rotor including at least one wind turbine blade connected to a rotor hub through at least one blade pitch bearing. The at least one pitch bearing comprises at least one outer ring, at least one centre ring, and at least one inner ring. The pitch bearing further comprises a first rolling element arrangement arranged between the at least one outer ring and the at least one centre ring and a second rolling element arrangement arranged between the at least one centre ring and the at least one inner ring, wherein at least one of the first and the second rolling element arrangements comprises at least two rows of rolling elements. 
     The at least two rows of rolling elements are arranged at different axial positions in relation to an axis of rotation of the pitch bearing so that a first row of the at least two rows of rolling elements is arranged closer to the wind turbine part to which the inner ring and the outer ring are connected, and a second row of the at least two rows of rolling elements is arranged closer to the wind turbine part to which the centre ring is connected. The play of the rolling elements in the first row of rolling elements is greater than the play of the rolling elements in the second row of rolling elements. 
     The outer and inner ring will deflect more the further away from the part to which they are attached the rings extend. When the bearing is loaded at nominal load, i.e., the load it is subject to during normal operation of the wind turbine. This means that the row closest to the second part will not transfer as much or any load in a traditional three ring pitch bearing. However, by providing the row closest to a first part of the blade and the hub to which the inner and the outer ring is attached with greater play than the row closest to a second part of the blade and the hub to which the centre ring is attached when the pitch bearing is not loaded at less than nominal load, it is possible to compensate for or balance the deflection in that the row closest to a first part, initially comprising the greatest play, will be more loaded the more the bearing is loaded, thereby allowing the load to be more evenly distributed between the two rows when the bearing reaches its nominal load. 
     It should be noted that the line “wherein at least one of the first and the second rolling element arrangements comprises at least two rows of rolling elements” in this context covers embodiments where the first rolling element arrangement comprises two or more rows of rolling elements, where the second rolling element arrangement comprises two or more rows of rolling elements, or where both the first and the second rolling element arrangements comprises two or more rows of rolling elements. But even if both the first and the second rolling element arrangements comprises two or more rows of rolling elements, only one has to include a first row with greater play than the second row of rolling elements. That is, a three ring pitch bearing comprising two rows of rolling elements in both the first and the second rolling element arrangements would also be covered by embodiments of the invention even if only the first or the second rolling element arrangement comprises rows of different play. 
     Even further, it should be noted that both the terms “the wind turbine part, to which the inner ring and the outer ring are connected” and “the wind turbine part to which the centre ring is connected” includes both that the bearing rings are fixed directly against the respective wind turbine parts, i.e., the hub or the blade, or that the bearing rings are fixed indirectly against the respective wind turbine parts through some intermediate additional part formed separate from both the bearing and the respective wind turbine part. 
     In an aspect of the invention, a first row rolling element race at the first row is of the same size as a second row rolling element race at the second row and wherein the greater play is enabled by forming the rolling elements of the first row smaller than the rolling elements of the second row. 
     Providing the greater initial play of the first row by making the rolling elements of the first row slightly smaller than the rolling elements of the second row, and maintaining the races of both the first and the second row at the same size, is advantageous in that it will only require that the rolling elements of the first row are made to a different tolerance than the rolling elements of the second row, which provides for a cost-efficient bearing. 
     In an aspect of the invention, the rolling elements of the first row are of the same size as the rolling elements of the second row and wherein the greater play is enabled by forming a first row rolling element race at the first row greater than a second row rolling element race at the second row. 
     Providing the greater initial play of the first row by making the rolling element race of the first row bigger smaller than the rolling element race of the second row, and maintaining the rolling elements of both the first and the second row at the same size, is advantageous in that if all the rolling elements are of the same size, the assembly of the pitch bearing becomes considerably easier thereby reducing the cost of the bearing. 
     In an aspect of the invention, the rolling elements of the first row are smaller than the rolling elements of the second row and wherein a first row rolling element race at the first row is greater than a second row rolling element race at the second row. 
     Thereby is achieved an advantageous embodiment of the invention. 
     In an aspect of the invention, the outer ring comprises an outer ring contact surface being fixed towards a corresponding outer ring hub contact surface of the hub and the inner ring comprises an inner ring contact surface being fixed towards a corresponding inner ring hub contact surface of the hub. 
     Thereby is achieved an advantageous embodiment of the invention. 
     In an aspect of the invention, the centre ring comprises a centre ring contact surface being fixed towards a corresponding centre ring blade contact surface of the blade. 
     Thereby is achieved an advantageous embodiment of the invention. 
     In an aspect of the invention, the first rolling element arrangement is arranged further away from the axis of rotation of the pitch bearing than the second rolling element arrangement. 
     Thereby is achieved an advantageous embodiment of the invention. 
     In an aspect of the invention, the at least one centre ring comprises a bottom surface and a bottom surface plane, wherein the bottom surface plane is perpendicular to an axis of rotation of the pitch bearing and extending through the bottom surface, wherein the first row of the at least two separate rows of rolling elements is arranged at a first row distance from the bottom surface plane, and wherein the inner ring and/or the outer ring further comprises a support part including a support face substantially facing the centre ring and including a back face substantially facing away from the centre ring, wherein the at least two separate rows of rolling elements are supported against the support face and wherein a contact surface part protrudes from the back face, wherein the contact surface part includes a contact face substantially facing in a general axial direction towards the wind turbine part to which the inner ring and the outer ring are connected, wherein the contact face is fixed towards a corresponding hub contact surface or a corresponding blade contact surface, and wherein the contact face is arranged at a contact face distance from the bottom surface plane and wherein the first row distance is smaller than the contact face distance. 
     Providing the support part with a contact surface part is advantageous in that it increases the rigidity of the bearing ring locally, thereby allowing the local deflection rate of the bearing ring to be better controlled. 
     Furthermore, positioning the contact surface part on the support part so that the contact face of the contact surface part is further away from the bottom surface plane than the first row of rolling elements is advantageous in that this bearing ring design ensures that the support part is more flexible at the first row of rolling elements than the second row of rolling elements, thereby allowing that the bearing ring to be more easily deflected at the first row of rolling elements so that the deflection explained previously in relation to prior art pitch bearings, is better balanced thereby ensuring a better load distribution between the two rows of rolling elements. 
     Even further, providing the bearing ring fixation in the contact surface part protruding from the back side of the support is also advantageous in that it allows the bearing ring fixation to be moved further away from the rows of rolling elements so that the bearing fixation has less of an influence on the bearing deflection, and it is easier to control the deflection of the bearing ring by means of its rigidity. 
     In an aspect of the invention, the support part extends into a depression of the hub or the blade. 
     Making the support part extend into a depression is advantageous in that it is thereby possible to allow for a more free and controlled deflection of the support part at the first row of rolling elements. 
     The invention further provides for a pitch bearing for a wind turbine, wherein the pitch bearing comprises at least one outer ring, at least one centre ring including a bottom surface and a bottom surface plane, wherein the bottom surface plane is perpendicular to an axis of rotation of the pitch bearing and extends through the bottom surface, and at least one inner ring. 
     The pitch bearing further comprising a first rolling element arrangement arranged between the at least one outer ring and the at least one centre ring and a second rolling element arrangement arranged between the at least one centre ring and the at least one inner ring, wherein at least one of the first and the second rolling element arrangements comprises at least two rows of rolling elements. 
     The at least two rows of rolling elements are arranged at different axial positions in relation to an axis of rotation of the pitch bearing so that a first row of the at least two rows of rolling elements is arranged closer to the bottom surface plane than a second row of the at least two rows of rolling elements, and wherein the play of the rolling elements in the first row of rolling elements is greater than the play of the rolling elements in the second row of rolling elements. 
     The more the outer ring and the inner ring deflect outwards, the more the load will be transferred by only the first row in a traditional pitch bearing, which is disadvantageous in that the more the bearing is loaded, the more unevenly the load is distributed between the two or more rows. Therefore, by providing the bearing with an initial greater play at the first row, the load will become better distributed the more the bearing is loaded, which is advantageous in that the rolling elements can handle an uneven load distribution better at a small load than at a heavy load. 
     In an aspect of the invention, the centre ring comprises a centre ring contact surface adapted for being fixed against a corresponding first wind turbine part and wherein the centre ring contact surface is arranged at the end of the centre ring in the general axial direction opposite the bottom surface plane, and wherein the inner ring comprises an inner ring contact surface adapted for being fixed against a corresponding second wind turbine part and wherein the outer ring comprises an outer ring contact surface also adapted for being fixed against the corresponding second wind turbine part and wherein the inner ring contact surface and the outer ring contact surface are arranged at the end of the rings in the general axial direction of the bottom surface plane. 
     Thereby is achieved an advantageous embodiment of the invention. 
     In an aspect of the invention, the first row of the at least two separate rows of rolling elements is arranged at a first row distance from the bottom surface plane, and wherein the inner ring and/or the outer ring further comprises a support part including a support face substantially facing the centre ring and including a back face substantially facing away from the centre ring, wherein the at least two separate rows of rolling elements are supported against the support face and wherein a contact surface part protrudes from said back face, wherein the contact surface part includes a contact face substantially facing in a general axial direction towards bottom surface plane, wherein the contact face is arranged at a contact face distance from the bottom surface plane and wherein the first row distance is smaller than the contact face distance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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 wind turbine hub comprising three blades as seen from the front; 
         FIG. 3  illustrates a simplified cross section of a nacelle, as seen from the side; 
         FIG. 4  illustrates a cross section of one side of a first embodiment of a pitch bearing comprising three rows of rolling elements; 
         FIG. 5  illustrates a cross section of one side of a second embodiment of a pitch bearing comprising four rows of rolling elements; and 
         FIG. 6  illustrates a cross section of one side of a third embodiment of a pitch bearing comprising four rows of rolling elements and support parts arranged in a hub depression. 
     
    
    
     DETAILED DESCRIPTION 
       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 wind turbine rotor  4  comprising a hub  7  and three blades  5  as seen from the front. 
     The pitch bearings  9  have to transfer forces mainly from three different sources. The blade  5  (and the bearings  9  themselves) is under the constant influence of the force of gravitation. The direction of the gravitational force varies depending on the blades position, inducing different loads on the pitch bearings  9 . When the blade is in motion, the bearings  9  are also under the influence of a centrifugal force, which mainly produces an axial pull in the bearings  9 . Finally, the bearings  9  are under the 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. 
     The load on and from all the pitch bearings  9  has to be transferred to the hub  7  and further into the rest of the wind turbine  1 . This fact makes the load transferring between the pitch bearings  9  and the hub  7  very crucial, especially when the loads get higher due to larger blades and increased power output. 
     In this embodiment of the invention, the rotor  4  comprises three blades  5  but in another embodiment, the rotor  4  could comprise one, two, four or more blades  5 . Each blade  5  is connected to the hub  7  through a pitch bearing  9  enabling the blade  5  to turn around its longitudinal axis. 
     At the centre of the hub  7  is illustrated the rotors axis of rotation  6  around which the entire rotor  4  rotates. 
       FIG. 3  illustrates a simplified cross section of a nacelle  3  of a prior art wind turbine  1 , as seen from the side. Nacelles  3  exists in a multitude of variations and configurations, but in most cases the drive train in the nacelle  3  almost always comprise one or more of the following components: a gearbox  15 , a coupling (not shown), some sort of breaking system  16  and a generator  17 . A nacelle  3  of a modern wind turbine  1  can also include a converter  18  (also called an inverter) and additional peripheral equipment such as further power handling equipment, control cabinets, hydraulic systems, cooling systems and more. 
     The weight of the entire nacelle  3  including the nacelle components  15 ,  16 ,  17 ,  18  is carried by a nacelle structure  19 . The components  15 ,  16 ,  17 ,  18  are usually placed on and/or connected to this common load carrying nacelle structure  19 . In this simplified embodiment, the load carrying nacelle structure  19  only extends along the bottom of the nacelle  3 , for example, in form of a bed frame to which some or all the components  15 ,  16 ,  17 ,  18  are connected. In another embodiment, the load carrying structure  19  could comprise a gear bell, which through a main bearing (not shown) could transfer the load of the rotor  4  to the tower  2 . Alternatively, the load carrying structure  19  could comprise several interconnected parts such as latticework. 
     In this embodiment, the blades  5  of the wind turbine rotor  4  are connected to the hub  6  through pitch bearings  9 , thereby allowing that the blades  5  to rotate around their longitudinal axis. 
     The pitch angle of the blades  5  could then, for example, be controlled by linear actuators, stepper motors or other means for rotating the blades  5  (not shown) connected to the hub  6  and the respective blade  5 . 
       FIG. 4  illustrates a cross section of one side of a first embodiment of a pitch bearing  9  comprising three rows of rolling elements  13 . 
     In this embodiment, the outer ring  24  and the inner ring  26  are both connected directly to the hub  7  in that an outer ring contact surface  27  is fixed directly against a hub contact surface  33  and that an inner ring contact surface  34  is also fixed directly against the hub contact surface  33  by means of connection means  52 . The centre ring is connected to the blade  5  in that a centre ring contact surface  23  is fixed directly against a blade contact surface  54 . However, in another embodiment of the invention, the pitch bearing  9  could be flipped 180° so that the inner ring  26  and the outer ring  24  are connected to the blade  5  either directly or through a connection member (not shown), and the centre ring  25  is connected to the hub  7  either directly or through a connection member (not shown). 
     In this embodiment, all the contact faces  23 ,  27 ,  33 ,  34 ,  54  are even and level surfaces. But in another embodiment, one, two, three, four or more of the surfaces  23 ,  27 ,  33 ,  34 ,  54  could be sloped, curved or, for example, comprise matching protrusions and grooves, for example, to guide the bearing  9  or to centre the bearing  7  in relation to the hub  7  and/or the blade  5 . 
     In this embodiment of the invention, all the bearing contact surfaces  23 ,  27 ,  34  are fixed directly against hub contact surface  33  or the blade contact surface  54 . But in another embodiment, one or more washers, a distance piece, vibrations dampers or other could be provided between one or more of the bearing contact surfaces  23 ,  27 ,  34  and the hub contact surface  33  or the blade contact surface  54 . In any event, the connection means  52  would ensure that the bearing contact surfaces  23 ,  27 ,  34  are fixed towards the hub contact surface  33  and/or the blade contact surface  54 . 
     In this embodiment of the invention, the connection means  52  are all stud bolts. But in another embodiment of the invention, however, the connection means  52  could be screws, rivets, bolts or any other kind of connectors suited for connecting pitch bearing rings  24 ,  25 ,  26  to wind turbine parts  5 ,  7 . 
     The pitch bearings  9  allows the centre ring  25  to rotate around the pitch bearings axis of rotation  10  in relation the inner  26  and outer ring  24  so that the blades  5  are also turned. 
     In this embodiment of the invention, the pitch bearing  9  comprises a first rolling element arrangement  11  arranged between the centre ring  25  and the outer ring  24  and comprises two separate rows  21 ,  22  of rolling elements  13 . But in another embodiment, the first rolling element arrangement  11  could comprise three, four or more rows of rolling elements  13 . 
     The pitch bearing  9  is also provided with a second rolling element arrangement  12  comprising only one row of rolling elements  13 . But in another embodiment, the second rolling element arrangement  12  could include two, three, four or more rows of rolling elements  13 . 
     In this embodiment of the invention, the first row of rolling elements  21  of the first rolling element arrangement  11  are made with greater play  8  than the second row of rolling elements  22  of the first rolling element arrangement  11 . In this embodiment, this is done by forming the races of the first and second row substantially identically, and by making the balls of the first row  21  at a slighter smaller diameter than the balls of the second row  22 . 
     In another embodiment of the invention, greater play  55  could also be achieved by forming the rolling elements identically and then making the race  14  of the first row  21  slightly larger than the race  20  of the second row  22 . 
     In this embodiment of the invention, a bottom surface plane  53  is perpendicular to the pitch bearing axis of rotation  10  and extends through the bottom surface  47  of the centre ring  25 . In this embodiment, the entire bottom surface  47  also extends perpendicular to the pitch bearing axis of rotation  10 , making the bottom surface plane  53  extend through the entire bottom surface  47 . However, in another embodiment of the invention, the bottom surface  47  could be slanting, rounded, curved or in another way be other than completely parallel with the bottom surface plane  53 . In such cases, the axial location of the bottom surface plane  53  is defined by the extreme point of the bottom surface  47 , i.e., the point of the bottom surface  47  which extends furthest in the given axial direction. 
     In this embodiment, the first row  21  of the first rolling element arrangement  11  is arranged at a first row distance  48  from the bottom surface plane  53 , and the second row  22  of rolling elements  13  is arranged at a second row distance  27  from the bottom surface plane  53  so that the first row  21  is closer to the bottom surface plane  53  than the second row  22 . 
     It should be noted that the pitch bearing axis of rotation  10  illustrated in  FIGS. 5 and 6  is not shown in a realistic distance from the pitch bearing  9 . It is very likely that the pitch bearing axis of rotation  10  would be considerably further away from the pitch bearing  9 . 
       FIG. 5  illustrates a cross section of one side of a second embodiment of a pitch bearing  9  comprising four rows of rolling elements  13 . 
     In this embodiment of the invention, the pitch bearing  9  comprises a first rolling element arrangement  11  arranged between the centre ring  25  and the outer ring  24  and comprising two separate rows  21 ,  22  of rolling elements  13 , and a second rolling element arrangement  12  also comprising two rows of rolling elements  13 . 
     In this embodiment of the invention, the first row of rolling elements  21  of the first rolling element arrangement  11  and the second rolling element arrangement  12  are made with greater play  8  than the second row of rolling elements  22  of the first rolling element arrangement  11  and the second rolling element arrangement  12 . 
     In this embodiment of the invention, all the rolling elements  13  in all the rows  21 ,  22  are balls. But in another embodiment of the invention, some or all the rolling elements  13  could be rollers, needles or other members suitable for transferring loads substantially frictionless, or at least with very little friction, between the rings  24 ,  25 ,  26  of a pitch bearing  9 . 
     Furthermore, in this embodiment of the invention, the rolling elements  13  in all the rows  21 ,  22  in the first rolling element arrangement  11  and the second rolling element arrangement  12  are of the same type. But in another embodiment, the rolling elements  13  in the first rows  21  could be different from the type of the rolling elements  13  in the second rows  22  or the first rows  21  and/or the second rows  22  could each comprise rolling elements  13  of different types. Or the rolling elements  13  in the rows  21 ,  22  in the first rolling element arrangement  11  could be different from type of rolling elements  13  in the rows  21 ,  22  of the second rolling element arrangement  12 . Likewise, the size of the rolling elements  13  in the two first rows  21  could be different from each other and the size of the rolling elements  13  in the two second rows  22  could be different from each other. 
     In this embodiment of the invention, the increased play in the first row  21  compared to the second row  22  is achieved by making the balls of the first row  21  by another ISO tolerance than the balls of the second row  22 . That is, if the races  14 ,  22  of both the first and the second row  21 ,  22  are the same nominal size and made with the same ISO tolerance, the balls could also be made at the same nominal size, but the balls of the first row  21  could be made to, for example, a f6 ISO tolerance, and the balls of the second row  22  could be made to, for example, a h6 ISO tolerance thereby allowing the first row  21  to have greater play than the second row  22 . And visa-versa if the rolling elements  13  of both the rows  21 ,  22  were made at the same size and tolerance and only the tolerance of either the upper or the lower race  14 ,  22  was changed. 
     This principle also applies if the rolling elements  13  and the race  14  of the first row  21  were of a different nominal size than the rolling elements  13  and the race  22  of the second row  21 . Then, a change in ISO tolerance class for at least one of the four elements (two rows of rolling element  13  and two races  14 ,  22 ) would entail a greater play in one of the rows  21 ,  22  compared to the other row  21 ,  22 . 
     However, in another embodiment of the invention, the difference in play could be achieved by simply making the nominal size of the rolling elements  13  of the second row  22  greater than the nominal size of the rolling elements  13  of the first row  21  while forming their races  14 ,  22  substantially identically, or visa-versa. 
       FIG. 6  illustrates a cross section of one side of a third embodiment of a pitch bearing  9  comprising four rows of rolling elements  13  and support parts  28  arranged in a hub depression  35 . 
     The outer ring  24  does in this embodiment of the invention comprise a support part  28  including a support face  29  which faces the centre ring  25 . In this embodiment, the support face  29  is substantially parallel with the opposite and corresponding face of the centre ring  25 . But in another embodiment, one or both faces could be sloped, curved or in another way provided with features making them none-parallel. 
     On the other side of the support part  28 , the support part  28  is provided with a back face  30  facing away from the centre ring  25 . In this embodiment of the invention, the support face  29  and the back face  30  are parallel besides for the two races in the support face  29 , in which the rolling elements  13  of the first row  21  and the second row  22  runs, thereby providing the support part  28  with a substantial constant thickness in its entire axial extent. However, in another embodiment, the support face  29  or the back face  30  or both could be sloped, curved or in another way provided with features making the faces  29 ,  30  non-parallel. 
     In this embodiment, a contact surface part  31  protrudes from the back face  30  of the support part  28  in a direction away from the centre ring  25 . An underside of the contact surface part  31 , facing in a general axial direction towards the bottom surface plane  53 , forms a contact face  32  which in this embodiment is fixed directly against a corresponding hub contact surface  33  by means of connection means  52 . In this embodiment, both the contact face  32  and the hub contact surface  33  are even and level surfaces. In another embodiment, however, one or both of the surfaces  32 ,  33  could be sloped, curved or, for example, comprise matching protrusions and grooves, for example, to guide the bearing  9  or to centre the bearing  7  in relation to the hub  7  or blade  5 . 
     In this embodiment of the invention, the contact face  32  is fixed directly against the hub contact surface  33 , but in another embodiment, one or more washers, a distance piece, vibrations dampers or other could be provided between the contact face  32  and the hub contact surface  33 . In any event, the connection means  52  would ensure that the contact face  32  is fixed towards the hub contact surface  33 . 
     In this embodiment of the invention, the connection means  52 , connecting the inner ring  26  and the outer ring  24  to the hub  7  and the centre ring  25  to the blade  5 , are bolts. But in another embodiment of the invention the connection means  52  could be screws, rivets, stud bolts or any other kind of connectors suited for connecting pitch bearing rings  24 ,  25 ,  26  to wind turbine parts  5 ,  7 . 
     The radial extent of the contact surface part  31  is so large that the entire outer ring  24  can be carried by the contact surface part  31  and all the connection means  52  can be arranged to penetrate only the contact surface part  31 . 
     The contact surface parts  31  axial location on the back face  30  of the support part  28  may be pertinent in that the part of the support part  31  from which the contact surface part  31  protrudes will be considerably more rigid than the part of the support part  28  which is not supported by a support part  31 . Thus, in the present embodiment, the upper part (i.e., the part furthest away from the rotor axis of rotation  6  and the bottom surface plane  53 ) of the support part  28  is considerably more rigid than the lower part. 
     The contact surface parts  31  is therefore positioned on the support part  28  so that the contact face  32  is further away from the rotor axis of rotation  6  and the bottom surface plane  53  than the first row of rolling elements  21 . In this embodiment of the invention, the contact face  32  of the outer ring  24  is arranged almost level with the second row of rolling elements  22 . However, the contact face  32  would only have to be arranged further away from the rotor axis of rotation  6  and the bottom surface plane  53  than the first row of rolling elements  21  to achieve aspects of the invention. However, preferably the contact face  32  should be arranged further away from the rotor axis of rotation  6  and the bottom surface plane  53  than a middle point between the first row of rolling elements  21  and the second row of rolling elements  22 . But, it should also be noted that the further away from the first row of rolling elements  21  the contact face  32  is arranged, the more flexible the lower part of the support part  28  becomes and if this part becomes too flexible, the first row of rolling elements  21  will not be able to transfer as great a load as the second row of rolling elements  22 . Accordingly, the load transferred through the bearing  9  will therefore be less advantageously distributed between the row of rolling elements  21 ,  22 . 
     In this embodiment of the invention, the pitch bearing  9  is arranged so that the “free” end of the support parts  28  of the inner ring  26  and the outer ring  24  extends into a depression  35  in the hub  7 . In this embodiment, the depression  35  is formed as a groove formed integrally with the hub  7 , but in another embodiment, the inner wall  45  and the outer wall  41  of the depression could be formed by one or more separate parts. 
     In this embodiment, the outer diameter  38  of the support part  28  of the outer ring  24  is smaller than the outer diameter  39  of the depression  35 , thereby forming an outer gap  40  between the support part  28  of the outer ring  24  and the outer wall  41  of the depression  35  so that the “free” end of the support part  28  of the outer ring  24  may deflect more freely in the depression  35 . 
     Likewise, the inner diameter  43  of the depression  35  is smaller than the inner diameter  42  of the support part  28  of the inner ring  26 , thereby forming an inner gap  44  between the support part  28  of the inner ring  26  and the inner wall  45  of the depression  35  so that the “free” end of the support part  28  of the inner ring  26  also may deflect more freely in the depression  35 . 
     To even further ensure that the support parts  28  of the inner ring  26  and the outer ring  24  may deflect more freely in the depression  35 , the depression  35  and the support parts  28  are so formed that deepest depth of the depression  35  is deeper than the greatest extent of the support parts  28  so a bottom gap  36  is formed between the bottom of the depression  35  and the support parts  28 . 
     In this embodiment of the invention, the inner ring  26  is formed differently than the outer ring  24  in that the contact face  32  of the inner ring  26  is considerably closer to the rotor axis of rotation  6  and the bottom surface plane  53  than the contact face  32  of the outer ring  24 . All things being equal, this would reduce the flexibility of the “free” end of the support part  28  considerably and to compensate for this reduced flexibility, the radial extent of the “free” end of the support part  28  is reduced considerably in relation to support part  28  of the outer bearing ring  24 . Thus, to achieve an optimal load distribution between the two rows of rolling elements  21 ,  22 , it is important that the axial location for contact face  32  in relation to the radial thickness of the support part  28  matches the given nominal load that the bearing  9  is subject to. 
     The invention has been exemplified above with reference to specific examples of designs and embodiments of wind turbines  1 , wind turbine hubs  7  and pitch bearings  9 . 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.