Abstract:
A wind turbine including a drive train. The drive train includes at least a rotor for transforming wind into rotation of a rotor hub, to provide a drive torque, a generator for transforming at least a part of the drive torque into electrical power, and at least one coupling for connecting a first drive train component to a second drive train component for transferring the drive torque between the components. The coupling includes a first coupling part with a first coupling area, the first coupling area being connected with a second coupling area of a second coupling part, whereby the drive torque is transferred from one of the areas to the other of the areas during operation of the coupling.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of pending International patent application PCT/DK2008/000371 filed on Oct. 22, 2008 which designates the United States and claims priority from Danish patent application PA 2007 01526 filed on Oct. 23, 2007, the content of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a wind turbine comprising a drive train with at least one coupling for connecting a first drive train component to a second drive train component, a method for coupling a first drive train component of the drive train of a wind turbine to a second drive train component of the drive train and use of a wind turbine. 
     BACKGROUND OF THE INVENTION 
     A wind turbine known in the art comprises a tapered wind turbine tower and a wind turbine nacelle positioned on top of the tower. A wind turbine rotor with a number of 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 . 
     As large modern wind turbines gets bigger and bigger both in size and in power output the challenge of transferring the torque through the drive train of the wind turbine i.e. from the rotor to the generator becomes more pronounced. 
     The drive train typically comprises several interconnected drive train components such as the rotor, a main shaft, one or more gearboxes, a brake system, an overload protection system and a generator where the different drive train components are interconnected by means of some sort of detachable or dismountable coupling enabling that the torque can be transferred in and between the components and at the same time enabling that the individual drive train components can be dismounted and/or replaced. 
     A known way of providing a coupling in the drive train is to use a so-called shrink-disk. The shrink-disc functions by converting locking screw clamp loads into radial contact pressures applied to the shafts of the mounted components, in effect “shrinking” it onto the shafts. Ideally this type of coupling will result in a zero-backlash mechanical interference fit, however wide friction joints (as required for large torque transfer) are sensitive to shaft bending and rotating loads, which may cause micro-slip and subsequently fretting or fretting corrosion. Furthermore when the torque exceeds the capacity of the joint, the coupling may slip and locally weld together. Once this happens, the coupling is very hard to remove again and removal will typically destroy one or both of the joined shafts. 
     From European patent application EP 1 445 484 A1 it is also known to provide a wind turbine with a friction coupling which by means of bolts creates a clamping force on the radial extending surfaces of one or more discs. However this type of coupling is complex and difficult to dismount. 
     An object of the invention is therefore to provide the drive train of a wind turbine with an advantageous torque transferring coupling technique. 
     SUMMARY OF THE INVENTION 
     The invention provides for a wind turbine comprising a drive train. The drive train includes at least a rotor for transforming wind into rotation of a rotor hub, to provide a drive torque, a generator for transforming at least a part of the drive torque into electrical power, and at least one coupling for connecting a first drive train component to a second drive train component for transferring said drive torque between the components. The coupling comprises a first coupling part with a first coupling area, the first coupling area being connected with a second coupling area of a second coupling part, whereby the drive torque is transferred from one of the areas to the other of the areas during operation of said coupling, wherein the first coupling area is provided with a first positive engaging structure engaging a corresponding second positive engaging structure of the second coupling area, and wherein both the first and second positive engaging structures are extending inwardly from positions near or at an outer periphery of the first and second coupling parts, respectively. 
     It would be virtually impossible or at least very complex and expensive to make the wind turbine so that all the drive train components are maintained perfectly and rigidly aligned at all times during operation of the wind turbine. Especially because the loads transferred through the drive train of a wind turbine is particularly varying e.g. due to constantly changing wind direction, wind speed, wind shear over the rotor, meteorological conditions and corresponding constant changing of operation parameters such as teeter angle, pitch angle of the blades, yaw angle and of course because the rotor constantly rotates making any variation in the load transferred from the individual blades act as a rotating moment or radial force on at least some of the drive train components. 
     It is therefore advantageous that the coupling areas are provided with positive engaging structures which are mutually engaging in that it hereby is ensured that the large torque load is transferred at all times substantially without wearing the coupling. 
     In an aspect of the invention said first and second areas comprises axial means for transferring axial loads through said coupling at least in one direction. 
     By providing the coupling areas with axial load means for transferring axial loads through the coupling at least in one direction, it is ensured that if axial loads are to be transferred through the coupling, the areas are only pulled slightly apart or pressed more firmly together or both at different areas of the coupling areas. This is advantageous in that even though this design theoretically allows for axial movement in the coupling, the coupling can handle this substantially without wearing and because the coupling areas comprise positive engaging structures engaging with each other, the torque transferring quality of the coupling is substantially not affected of the axial loads even if this is accompanied by a rotating radial load. 
     In an aspect of the invention said axial means is provided by establishing said areas so that they are non-parallel with the axis of rotation of said coupling. 
     If the coupling areas are parallel with the axis of rotation of the coupling, one of the areas will have to fit into the other, like e.g. a traditional key and slot shaft connection, a six-spline socket connection or a shrink-disc coupling. But these coupling or connection types are relatively poor at transferring axial loads and they are particularly prone to wear when exposed to rotating loads from shaft bending or imbalanced loads through the bearing. 
     As soon as the coupling areas are not parallel with the axis of rotation of the coupling, the coupling is provided with axial means capable of transferring axial loads at least in one direction and it is hereby enabled that if the coupling is affected by a rotating load, the imbalanced loads can be transferred trough the coupling substantially without wearing it. 
     In an aspect of the invention said areas are established in a face angle of between 45° and 135°, preferably between 80° and 100° such as 90° in relation to the axis of rotation of said coupling to establish said axial means. 
     If the face angle gets below 45° or above 135° the coupling areas will have a tendency to wedge when exposed to an axial load e.g. from the torque transferring parts or from fixation means maintaining the coupling areas in close contact, which in worst case could make it virtually impossible to disassemble the coupling. However as long as the coupling areas are established within the present face angle range the coupling parts will not wedge and in this face angle range the coupling will furthermore enhance its self-aligning ability. 
     Positioning the areas in a face angle of substantially 90° in relation to the axis of rotation of the coupling is advantageous in that, it enables a much more simple manufacturing procedure hereby reducing cost and in that it reduces the need for axial displacement when assembling or dissembling the coupling, hereby simplifying the assembly procedure. 
     In an aspect of the invention said first and second positive engaging structure of said first and second coupling areas respectively are formed by alternating bulges and recesses. 
     Bulges—suitable for engaging a corresponding recess—e.g. in the form of teeth, pins, knobs or any other kind of protuberance of the face of the coupling areas will increase the coupling areas ability to transfer torque—particularly in relation to friction coupling or shrink couplings. When the bulges and recesses engage the risk of the coupling slipping is highly reduced whereby the durability of the coupling is increased without it affecting the couplings torque transferring quality or its ability to be dismantled. 
     In an aspect of the invention said positive engaging structure of said first and second coupling areas are formed as substantially radial extending teeth. 
     To transfer torque efficiently it is important that the positive engaging structure is established substantially transverse to the direction of rotation and it is therefore advantageous that the teeth extends radial. 
     Furthermore teeth is an efficient way of utilizing the coupling area in that teeth will ensure that the torque is transferred over a relatively large area hereby reducing local stress and the risk of micro slip. 
     Even further, teeth extending radially from the centre of the coupling in substantially all directions will further provide the coupling with the ability of handling radial loads in that at least some of the teeth will always lock the coupling towards radial movement no matter the direction of the radial load. 
     In an aspect of the invention said positive engaging structure of said first coupling area and/or said second coupling area is formed integrally in said area. 
     The positive engaging structure has to be able to transfer large torque loads to the coupling areas and by forming the positive engaging structure integrally in the areas any risk of slip and wear is eliminated. 
     In an aspect of the invention said first coupling part or said second coupling part is formed integrally in said first drive train component and said second drive train component. 
     Likewise, coupling parts has to be able to transfer large torque loads to the drive train components and by forming the coupling parts integrally in the drive train components any risk of slip and wear is eliminated. Furthermore, expensive and complex connections or couplings between the coupling parts and the drive train components can be avoided. 
     In an aspect of the invention at least one of said first drive train component and said second drive train component is selected from a group comprising: a drive train shaft, a main shaft, a main bearing, a coupling, a brake, a gearbox, a rotor hub and a generator. 
     Hereby is achieved an advantageous embodiment of the invention. 
     In an aspect of the invention said first drive train component or said second drive train component is a rotating shaft of said wind turbine gearbox such as a sun gear shaft or an input shaft of said gearbox. 
     The gearbox is properly the most complex mechanical unit in the wind turbine and it is therefore important that the gearbox or at least essential parts of the gearbox can be dismantled either in case of maintenance, repairs, replacement or simply to enable access. It is therefore particularly advantageous that the coupling according to the present invention is used for coupling torque transferring parts of the wind turbine gearbox. 
     Furthermore, the planet gears of the gearbox each only has to transfer parts of the total torque load and all gears or shafts after the sun gear will operate at increased rotational speed whereby the torque is reduced. The input shaft and the sun gear shaft is therefore the most strained torque transferring parts of the gearbox and it is therefore particularly advantageous to use a coupling according to the present invention for coupling a torque transferring part to the sun gear or the input shaft of the gearbox. 
     In an aspect of the invention said first or second coupling area of said rotating shaft defines an outer periphery of said drive train component. 
     Making the coupling area define the outer periphery of the drive train component is advantageous in that it enables a simpler mounting and dismantling of the drive train component and in that a large diameter coupling area is stronger towards torque. 
     In an aspect of the invention said first coupling area and said second coupling area comprises fixation means for substantially maintaining at least the axial position of said first coupling area in relation to said second coupling area at least during operation of said coupling. 
     At least to some degree a coupling of a wind turbine drive train will have to be able to handle axial loads either by absorbing them or by transferring them further down the drive train. By maintaining the mutual position of the coupling areas by means of fixation means it is ensured that the axial loads are transferred through the coupling substantially without affecting any part of the coupling particularly regarding wear. 
     In an aspect of the invention said fixation means are mechanical fixation means such as screws, bolts or rivets. 
     Mechanical fixation means are inexpensive and they are simple to mount and dismount and it is therefore advantageous to use these for fixating the coupling areas in relation to each other. 
     In an aspect of the invention at least one of said first drive train component and/or said second drive train component is hollow. 
     Thick-walled hollow shafts and solid shafts of the same outer diameter substantially have the same strength toward torque but hollow shafts can be made more flexible radial and they weigh less or a hollow shaft can be made almost equally stiff at significantly lower weight (the essence is a much better stiffness/weight ratio). A coupling according to the present invention allows for transferring of both axial loads and torque substantially without loads travelling through the centre of the coupling and since material is substantially not needed to transfer loads, it is advantageous to make the torque transferring parts hollow to save weight of the coupling and to ensure flexibility. 
     In an aspect of the invention said coupling further comprise radial load transferring means for transferring at least a part of a radial load between said first coupling part and said second coupling part and/or for mutually aligning said first coupling part and said second coupling part. 
     For certain coupling configurations the positive engaging structures does not enable that the coupling is self-centering and self-aligning and it is therefore advantageous to provide the coupling with radial load transferring means for transferring radial loads through the coupling substantially without straining the positive engaging structures. 
     In an aspect of the invention said coupling is self-centering and/or self-aligning because said first and second positive engaging structures are extending inwardly from positions near or at an outer periphery of the first and second coupling parts, respectively. 
     Because the positive engaging structures engage each other it is enabled that the coupling is self-centering/self-aligning, which means a tremendous advantage in the drive train of a wind turbine where the loads are constantly changing in size, direction and in number. 
     In an aspect of the invention said second coupling area of said second drive train component comprises a centre aperture through which said first drive train component can be mounted and dismantled. 
     Providing the second coupling area with a centre aperture is advantageous in that the first drive train component can be mounted and dismantled from the first drive train component side which would provide more free space to perform the process. 
     The invention further provides for a method for coupling a first drive train component of the drive train of a wind turbine to a second drive train component of the drive train. The method comprises the steps of
         providing the first drive train component with a first coupling part including a first coupling area and providing the second drive train component with a second coupling part including a second coupling area, and   joining the first coupling part with the second coupling part by engaging a positive engaging structure of the first coupling area with a corresponding positive engaging structure of the second coupling area and so that the first coupling area and the second coupling area are capable of transferring axial loads through the coupling at least in one direction.       

     By arranging the coupling areas so that they can transfer axial loads at least in one direction, it is ensured that while the axial loads are transferred through the coupling the couplings ability to transfer torque is maintained or even improved due to the engaging positive engaging structures. This is advantageous in that it improves the couplings ability to transfer rotating loads, varying load and varying imbalanced loads, in that this method reduces the risk of slip and thereby wear. 
     In an aspect of the invention said method is a method for coupling drive train components in a wind turbine according to any of the above mentioned wind turbines. 
     Even further the invention provide for use of a wind turbine according to any of the above relating to wind turbines, wherein said wind turbine is a Megawatt wind turbine. 
     The larger the wind turbine is in power output the large the wind turbine or at least some of the wind turbine parts also has to be in physical size. Wind turbines with a rated power output above one Megawatt have to transfer so large torque loads through the drive train that traditional couplings becomes less effective or durable. Furthermore, with so large wind turbines an imbalanced load—or worse—a varying imbalanced load will have a great affect on the coupling because of the more flexible nature of the large drive train construction and it is therefore particularly advantageous to use a coupling according to the invention in a Megawatt wind turbine. 
    
    
     
       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 known in the art, as seen from the front, 
         FIG. 2  illustrates a simplified cross section of a nacelle, as seen from the side, 
         FIG. 3  illustrates a cross section of an embodiment of a coupling comprising external fixation means, as seen from the side, 
         FIG. 4  illustrates a cross section of an embodiment of a coupling comprising internal fixation means, as seen from the side, 
         FIG. 5  illustrates an embodiment of a first coupling part, as seen in from the front, 
         FIG. 6  illustrates a cross section of a gear wheel comprising a first coupling part, as seen from the side, 
         FIG. 7  illustrates a partial cross section of a sun gear, as seen from the side, 
         FIG. 8  illustrates a cross section of a coupling comprising angled coupling areas, as seen from the side, and 
         FIGS. 9A ,  9 B and  9 C illustrates a cross section of three different embodiments of corresponding positive engaging structures, as seen from the side. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3  illustrates a cross section of an embodiment of a coupling  12  comprising external fixation means  24 , as seen from the side. 
     In this embodiment of the invention the coupling  12  comprise a first coupling part  16  which is formed integrally in a first drive train component  14  which in this embodiment is the a shaft  26  for transferring torque from the rotor hub  11  to the input shaft of a wind turbine gearbox  6 . 
     Likewise, the coupling  12  comprise a second coupling part  17  which is formed integrally in a second drive train component  15  which in this embodiment is the input shaft of a wind turbine gearbox  6 . 
     As previously explained, in some wind turbine embodiments the rotor hub  11  is substantially directly connected to a ring gear, a planet carrier or other of a input shaft-less gearbox  6  and the first drive train component  14  could then be the output shaft of the gearbox  6  and the second drive train component  15  could be the generator  8  or in a further embodiment of the invention the first drive train component  14  or the second drive train component  15  could be a generator input shaft, an input or output shaft of a braking system  7 , the main shaft  26 , a output shaft of the hub  11  or any other torque transferring part of the wind turbine drive train  13 . 
     It is also feasible that the present invention are used for arranging an intermediate flange between e.g. the turbine main shaft  26  and the gearbox input shaft, essentially disc that engages with identical toothings on both mates, for example a brake disc, a encoder flange or a rotor lock device. There may be only one bolt connection to hold all three members together. 
     In another embodiment of the invention one or both of the coupling parts  16 ,  17  could also be formed separately from the torque transferring parts  14 ,  15  e.g. if the coupling parts  16 ,  17  was attached to the torque transferring parts  14 ,  15  through a shrink joint, a bolted joint, a key joint, a pin joint or other or any combination hereof. 
     In this embodiment of the invention the first coupling part  16  comprise a first coupling area  18  formed integrally in the coupling part  16  and likewise the second coupling part  17  comprise a second coupling area  19  formed integrally in the coupling part  17 . 
     In another embodiment of the invention the coupling areas  18 ,  19  could also be formed separately from the coupling parts  16 ,  17  e.g. if the coupling areas  18 ,  19  was attached to the coupling parts  16 ,  17  through a shrink joint, a bolted joint, a key joint, a pin joint or other or any combination hereof. 
     In this embodiment the first coupling area  18  comprise a first positive engaging structure  20  in the form of alternating bulges  22  and recesses  23  covering the entire first coupling area  18  and the second coupling area  18  comprise a corresponding second positive engaging structure  32  also in the form of alternating bulges  22  and recesses  23  covering the entire second coupling area  19 . 
     In this embodiment of the invention both the first and second positive engaging structures  20 ,  32  are extending inwardly from positions at an outer periphery  33  of the first and second coupling parts  16 , 17 , respectively. 
     In this embodiment of the invention the coupling  12  comprise axial means in form of the couplings areas  18 ,  19  being established substantially perpendicular to the axis of rotation  31  of the coupling  12  in that the face angle A of both couplings areas  18 ,  19  are 90° in relation to said axis of rotation  31 . Hereby the coupling  12  is able to transfer axial forces from one coupling area  18 ,  19  to the other  18 ,  19 . 
     In this embodiment of the invention the coupling  12  further comprise fixation means for maintaining the coupling areas  18 ,  19  engaged with each other at all times—hereby ensuring the torque transferring quality of the coupling  12 —and to enable that the coupling  12  is able to transfer a axial load which would attempt to pull the coupling areas  18 ,  19  apart. 
     In this embodiment the fixation means  24  are bolts extending through fixation apertures  27  of the first coupling part  16  and engaging threaded fixation apertures  27  in the second coupling part  15  but in another embodiment the fixation means  24  could be clamps, screws, rivets or other and both parts  16 ,  17  could be threaded or non of the parts  16 ,  17  could comprise thread and the bolts or screws would then comprise nuts. 
     In this embodiment the fixation means  24  are a number of bolts arranged on a circle in the coupling areas  18 ,  19  but in another embodiment of the invention the fixation means  24  could be arranged on a circle outside the coupling areas  18 ,  19 , centrally inside the coupling areas  18 ,  19 , asymmetrically inside or outside the coupling areas  18 ,  19 , the fixation means  24  may be one centrally arranged bolt/nut or any combination hereof. However it is preferred that the fixation means  24  is arranged at the neutral plane of bending. 
     In this embodiment of the invention the coupling  12  further comprise radial load transferring means  25  at least for assisting in aligning the two coupling parts  16 ,  17  and transferring radial loads through the coupling  12 . 
     In this embodiment of the invention the radial load transferring means  25  is formed as a separate spigot engaging and guiding both parts  16 ,  17  but in another embodiment the radial load transferring means  25  could be formed integrally in one or the coupling parts  16 ,  17  and then engage the other part  16 ,  17  much like how the coupling areas  18 ,  19  engage. 
       FIG. 4  illustrates a cross section of an embodiment of coupling  12  comprising internal fixation means  24 , as seen from the side. 
     In this embodiment the bulges  22  and recesses  23  are constituted by substantially identical radial extending teeth formed in the surface of both the first coupling area  18  and the second coupling area  19 . By displacing the top of the teeth of one of the coupling areas  18 ,  19  angularly, during joining, so that the top of the teeth on one area  18 ,  19  is placed between the tops of the teeth on the other area  18 ,  19  the teeth  20 ,  32  of the two areas  18 ,  19  will engage when brought together and as long as the areas  18 ,  19  are maintained in close contact the coupling  12  is capable of transferring large torque loads substantially backlash-free, which is highly advantageous for applications presenting a varying and reversing load such as wind turbines  1 . 
     Since the forces applied by the fixation means for maintaining the alignment of the parts  16 ,  17  substantially are all in the axial direction of the coupling  12  and since substantially all other forces acting on the coupling  12  are torque or axial forces this coupling design enables that the coupling parts  16 ,  17  and the torque transferring parts  14 ,  15  can be hollow. This is a big advantage in that it hereby is possible to guide wires, conductors, hoses, pipes, rods or other through the centre or even to allow for personnel access through the centre of the coupling  12 . Furthermore, the weight of the torque transferring parts  14 ,  15  can be reduced substantially without reducing their ability to transfer torque, and with relatively little effect on stiffness. 
     In this embodiment the fixation means  24  are arranged on the inside of the coupling  12  e.g. enabling that neighboring external bearings or other could be dismounted if the coupling was disengaged. In that 
     In this embodiment of the invention the coupling  12  does not comprise separate radial load transferring means  25  in that the positive engaging structures  20 ,  32  of the coupling areas  18 ,  19  are formed to also act as radial load transferring means  25  as the positive engaging structures  20 ,  32  are designed to lock the two areas  18 ,  19  toward radial movement in all radial directions. 
     Incorporating the radial load transferring means  25  in the positive engaging structures  20 ,  32  of the coupling areas  18 ,  19  is advantageous in that no additional radial load transferring means surfaces or parts need be machined. Additional centering means may only be needed for very high radial loads, particularly for controlling the amount if displacement and relative motion of the toothed areas with respect to wear, fretting corrosion and adhesive wear. 
       FIG. 5  illustrates an embodiment of a first coupling part  16 , as seen in from the front. 
     In this embodiment of the invention the first positive engaging structure  20  is formed by radial extending teeth formed in the surface of the coupling area  18 . The contour lines of the teeth all cross the centre of rotation  31  making the teeth wider at the outer periphery  33  of the coupling area  18  that at the inner periphery of the coupling area  18 . 
     The illustrated coupling part  16  would therefore be able to transfer radial loads when engaged with a corresponding second coupling part  17  in that the teeth&#39;s shape also would lock the coupling parts  16 ,  17  for mutual radial movement. 
     In this embodiment of the invention the coupling area  18  is provided with eight evenly spaced fixation apertures  27  for accommodating fixation means  24  which will force the coupling areas  18 ,  19  together. However, in another embodiment the coupling area  18  could be provided with another number of fixation apertures  27 —either more or fewer—, the fixation apertures  27  could be located elsewhere on the coupling part  16  or the coupling part  16  could comprise no fixation apertures e.g. if the coupling areas  18 ,  19  was held together by means of clamps or the like. 
       FIG. 6  illustrates a cross section of a gear wheel comprising a first coupling part  16 , as seen from the side. 
     In this embodiment the gear wheel is a planet gear of an epicyclical wind turbine gearbox  6  but the gear wheel could just as well be a sun gear of a epicyclical wind turbine gearbox  6 , a gear wheel of a wind turbine gear transmission stage or another type of gear wheel suitable for transferring large torque loads in the wind turbine gearbox  6 . 
     In this embodiment the gear wheel is formed integrally with a long hollow shaft which ends up in a flange forming a first coupling part  16 . On the side of the flange facing the gear wheel, the coupling part  16  is provided with a coupling area  18  provided with teeth formed transversely to the direction of rotation for engaging with the second coupling area  19  of another coupling part  17 . 
     By making the inner diameter of the coupling area  18  bigger than the outer diameter of the gear wheel it is enabled that the gear wheel can be dismounted through the second coupling part  17  as illustrated in  FIG. 7 . By providing the gear wheel with a relatively long and thin-walled shaft it is ensured that the gear wheel can flex a little radial in relation to the coupling area  18  both transversely and regarding parallelism with the axis of rotation  31  of the coupling area  18 . This is advantageous in that it hereby is possible to compensate for any temporary or permanent misalignment in the gearbox substantially without affecting the gear wheel and couplings  12  ability to transfer torque. 
       FIG. 7  illustrates a partial cross section of a sun gear  28 , as seen from the side. 
     In this embodiment of the invention the gear wheel illustrated in  FIG. 6  is used as a sun wheel  28  in the planetary stage of a wind turbine gearbox  6 . Through the gear wheel shaft and first coupling area  18  the torque generated by the planet gears (not shown) meshing with the sun gear  28  is transferred to a second coupling area  19  of a second coupling part  19  on a second drive train component  15  in the form of a hub of a gear  30  of a gear transmission stage of the wind turbine gearbox  6 . 
     Two bearings  29  substantially ensures the alignment of the second drive train component  15  and the coupling  12  while the long sun gear shaft ensures some flexibility of the sun gear  28  radial. 
     In this embodiment the coupling areas  18 ,  19  is fixated in a engaging position by means of a large number of bolts  24  fitted from the back side of the coupling  12  in relation to the sun gear  28 , hereby allowing for simple dismantling of the coupling  12  and the sun gear  28 . 
     Particularly when the sun gear  28  or planet gear are a helical gear the thrust is proportional to torque and if the helix angle is chosen such that it “pulls” the faces of the coupling areas  18 ,  19  into engagement, then this thrust increases the axial pre-load proportional to the torque transfer needs. 
     Load in axial direction is decisive for load carrying capacity in respect to torque and radial loads. Axial load may be achieved by either operating loads, or static pre-load, e.g. bolts. It may be advantageous for the load carrying capacity of the coupling  12  to apply higher than normal axial loads during the assembly for settling roughness peaks, and increase load sharing between the teeth, essentially by local yielding of some protruding teeth. Very high static pre-load is further advantageous to reduce the impact of tooth accuracy variation, especially tooth spacing tolerances or other. 
     Furthermore, one of both of the first and second coupling area  18 ,  19  surfaces may be hardened by tempering or the surfaces may in other ways be treated or coated to avoid/reduce the risk of fretting corrosion, tribo-corrosion and other. 
       FIG. 8  illustrates a cross section of a coupling  12  comprising angled coupling areas  18 ,  19 , as seen from the side. 
     In this embodiment of the invention the coupling  12  comprise a first coupling part  16  which is formed integrally in a first drive train component  14  which in this embodiment is the output shaft of a brake system  7  and the coupling  12  comprise a second coupling part  17  which is formed integrally in a second drive train component  15  which in this embodiment is the input shaft of a wind turbine generator  8 . 
     In this embodiment of the invention the coupling  12  comprise axial means  21  in that the coupling areas  18 ,  19  are established in a face angle A of 45° hereby enabling that the coupling areas  18 ,  19  can transfer axial loads through the coupling  12 . 
     Furthermore, by arranging the coupling areas  18 ,  19  in a face angle A of 45° it is furthermore ensured that coupling parts  16 ,  17  becomes even further self-aligning due to the coning shape of the areas  18 ,  19 . 
     In this embodiment the fixation means  24  are disposed perpendicular to the coupling areas  18 ,  19  by in another embodiment the fixation means  24  could be established parallel with the axis of rotation  31  of the coupling  12  or in another direction depending on e.g. the specific coupling design or other. 
       FIGS. 9A ,  9 B and  9 C illustrates a cross section of three different embodiments of corresponding positive engaging structure  20 ,  32 , as seen from the side. 
     It is evident for the skilled person that the first positive engaging structure  20  of the first coupling area  18  and the corresponding second positive engaging structure  32  of the second coupling area  19  can be formed in a multitude of ways. 
       FIGS. 9A ,  9 B and  9 C therefore only illustrates three embodiments of teeth extending radially inwards from the outer periphery  33  towards the centre of the coupling  12  making the size of the teeth increase as the diameter increases but in another embodiment the shape of the teeth could be uniform throughout the coupling areas  18 ,  19 . 
     The embodiment illustrated in  FIG. 9A  illustrates that the teeth  20 , 32  are formed substantially as ordinary gear teeth. These teeth are formed so there is no tip/root interference enabling that the torque is transferred through the flanks of the teeth. This teeth  20 ,  32  configuration enables flank centering and is the preferred embodiment in most applications with reversing torque i.e. in wind turbines  1 . 
     Furthermore the embodiment illustrated in  FIG. 9A  enables a very wide root of the individual teeth which is advantageous in that the shape of tooth root is decisive for load carrying capacity of the teeth. 
       FIG. 9B  illustrates an embodiment where the teeth  20 ,  32  are formed as formed by alternating square bulges  22  and square recesses  23 . To enable assembly of this coupling configuration there would have to be some play between the teeth for which reason this teeth design is not particularly advantageous for varying and reversing loads as presented in a wind turbine  1 . 
     The teeth configuration presented in  FIGS. 9A and 9B  entails that the first positive engaging structure  20  of the first coupling area  18  is a displaced replica of the corresponding second positive engaging structure  32  of the second coupling area  19 . However it is also feasible that the positive engaging structure  20 ,  32  of the two coupling areas  18 ,  19  corresponds without the two being alike. 
     In  FIG. 9C  is illustrated an embodiment where positive engaging structures  20 ,  32  of the two coupling areas  18 ,  19  are different. The corresponding convex and concave flanks could e.g. be advantageous for specific self-centering, self-aligning or torque transferring purposes. 
     The invention has been exemplified above with reference to specific examples of torque transferring parts  14 ,  15 , couplings  12 , positive engaging structures  20 ,  32  and other. 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.