Wind turbine with a superconductive generator having an improved thermally insulating structure

Abstract

The invention relates to a wind turbine with a generator and a method of assembling a generator thereof, wherein the generator comprises a rotor rotatably arranged relative to a stator. The rotor comprises a plurality of superconducting pole units arranged on a back iron which is spaced apart from a rotor structure by a number of thermally insulating plates or beams. Said plates or beams are located between either ends of the rotor and orientated relative to the rotational direction of the rotor. Each plate has a first end firmly connected to another first beam extending in an axial direction and a second end firmly connected to another second beam also extending the axial direction. The first beams are further firmly connected to the back iron while the second beams are further firmly connected to the rotor structure. The thermally insulating plates or beams provide a flexible and cheap support interface that is able to adapt to the tolerances of the individual components.

Claims

1. A wind turbine comprising: a wind turbine tower, a nacelle arranged on top of the wind turbine tower, a rotatable hub arranged relative to the nacelle, which hub is connected to at least two wind turbine blades, a generator rotatably connected to the hub, wherein the generator comprises a rotor arranged rotatably relative to a stator, the rotor comprises a back iron and a rotor structure, the rotor further comprises at least one pole unit arranged relative to the back iron, the at least one pole unit comprises at least one rotor coil made of a superconductive material, the stator comprises at least one pole unit with at least one stator coil, wherein the at least one rotor coil is configured to interact with the at least one stator coil via an electromagnetic field when the rotor is rotated relative to the stator, wherein the rotor further comprises at least one support element arranged between the back iron and the rotor structure, the at least one support element comprises a first end connected to the back iron and a second end connected to the rotor structure, wherein the at least one support element is made of a thermally insulating material, wherein the back iron comprises a side surface facing the rotor structure and the rotor structure comprises a corresponding side surface facing the back iron, wherein the first end is connected to the side surface and the second end is connected to the corresponding side surface, wherein the first and second ends extend in an axial direction defined by the rotor, wherein a plurality of support elements are arranged relative to each other along an axial direction defined by the rotor, wherein at least one mounting element is arranged at at least one of the first and second ends of each of the plurality of support elements, wherein the at least one mounting element is firmly connected to at least one of the back iron and the rotor structure such that the support elements are firmly connected to the back iron or to the rotor structure via the at least one mounting means, wherein the mounting means comprising bolts, nuts and/or screws.

2. A wind turbine according to claim 1, wherein the at least one support element is orientated relative to the rotational direction of the rotor, wherein the at least one support element from the first end towards the second end substantially extends in the same direction as a rotational direction of the rotor.

3. A wind turbine according to claim 1, wherein at least one beam shaped element is arranged at at least one of the first and second ends, wherein the at least one beam shaped element extends in the axial direction.

4. A wind turbine according to claim 3, wherein the at least one of the first and second ends and the at least one beam shaped element are firmly connected by mounting means or bonding means.

5. A wind turbine according to claim 4, wherein the at least one of the first and second ends and the at least one beam shaped element are firmly connected by a combination of mounting means and bonding means.

6. A wind turbine according to claim 3, wherein the at least one beam shaped element forms part of the at least one of the first and second ends.

7. A wind turbine according to claim 3, wherein the at least one beam shaped element comprises at least one relief element configured to reduce stresses in the at least one beam shaped element.

8. A wind turbine according to claim 1, wherein one of the at least one of the first and second ends and the at least one beam shaped element has a wedge shaped end facing the other of the at least one of the first and second ends and the at least one beam shaped element, wherein said other of the at least one of the first and second ends and the at least one beam shaped element has a corresponding end shaped to receive said wedge shaped end.

9. A wind turbine according to claim 1, wherein the at least one support element comprises at least one reinforcing element which extends between the first end and the second end.

10. A wind turbine according to claim 1, wherein the at least one support element is made of a fiber reinforced material, particularly fibre reinforced plastics.

11. A wind turbine according to claim 1, wherein the at least one support element is made of a first layer sandwiched between at least two second layers, wherein one of the first layer and the at least second layer has a greater structural strength than the other layer.

12. A wind turbine according to claim 1, wherein the plurality of support elements comprise at least one first support element and at least one second support element, wherein the at least one first support element from its first end towards its second end substantially extends in one direction relative to a rotational direction of the rotor, and the at least one second support element from its first end towards its second end substantially extends in an opposite direction.

13. A wind turbine according to claim 1, wherein the plurality of support elements comprise at least a first set of support elements and at least a second set of support elements, wherein at least one of the support elements of the first set intersects at least one of the support elements of the at least second set.

14. A wind turbine according to claim 1, wherein the plurality of support elements comprise a first set of support elements and at least a second set of support elements, wherein the at least one mounting element of the first set and the at least one mounting element of the at least second set are aligned along a common axial line.

15. A wind turbine according to claim 1, wherein the at least one mounting element is firmly connected to at least one of the back iron and the rotor structure by mounting means or bonding means or a combination thereof.

16. A wind turbine according to claim 1, wherein the at least one mounting element is firmly connected to the at least one support element by at least one pin connection.

17. A wind turbine according to claim 16, wherein the at least one support element has a thickness between 80 millimetres and 120 millimetres.

18. A method of assembling a generator of a wind turbine according to claim 1, wherein the method comprises the steps of: providing a rotor of a generator, wherein the rotor at least comprises a rotor structure, arranging a back iron of the rotor relative to the rotor structure, positioning at least one support element relative to the rotor structure and the back iron, mounting a first end of said at least one support element to the back iron, and further mounting a second end of said at least one support element to the rotor structure, wherein the step of positioning the at least one support element comprises arranging a plurality of support elements along an axial direction defined by the rotor, wherein the at least one mounting element is firmly connected to said at least one of the first and second ends such that the support elements are firmly connected to the back iron or to the rotor structure via the at least one mounting means, wherein the mounting means comprising bolts, nuts and/or screws.

19. A method according to claim 18, wherein the at least one support element is arranged between a side surface of the back iron and a corresponding side surface of the rotor structure, wherein the at least one support element is angled relative to a tangential direction of at least one of the side surface and the corresponding side surface.

20. A method according to claim 18, wherein the first or second end is mounted to the back iron or to the rotor structure before arranging the back iron relative to the rotor structure.

21. A method according to claim 18, wherein the method further comprises the step of: arranging at least one beam shaped element on at least one of the side surface and the corresponding side surface, and positioning at least one of the first and second ends relative to said at least one beam shaped element.

22. A method according to claim 21, wherein the at least one beam shaped element and the at least one of the first and second ends is firmly connected by using mounting means or bonding means or a combination thereof.

23. A method according to claim 18, wherein the method further comprises the step of: arranging at least one mounting element at at least one of the first and second ends of each of the plurality of support elements.

24. A method according to claim 18, wherein the method further comprises steps of: positioning at least one first support element relative to a rotational direction of the rotor so that it substantially extends in one direction, and positioning at least one second support element relative to the at least one first support element so that it substantially extends in an opposite direction.

25. A method according to claim 18, wherein at least one of said plurality of support elements intersects at least one other support element.

26. A method according to claim 18, wherein at least one of the at least one support element, the at least one beam shaped element and the at least one mounting element are manufactured by pultrusion or extrusion.

Description

DESCRIPTION OF THE DRAWING

(1) The invention is described by example only and with reference to the drawings, wherein:

(2) FIG. 1 shows an exemplary embodiment of a wind turbine;

(3) FIG. 2 shows an exemplary embodiment of a generator in the wind turbine;

(4) FIG. 3 shows a first embodiment of the rotor of the generator shown in FIG. 2;

(5) FIG. 4 shows a sectional view of the back iron and the rotor structure;

(6) FIG. 5 shows a first embodiment of the first beam;

(7) FIG. 6 shows a first embodiment of the second beam;

(8) FIG. 7 shows a second embodiment of a respective beam;

(9) FIG. 8 shows a third embodiment of the respective beam;

(10) FIG. 9 shows a fourth embodiment of the respective beam;

(11) FIG. 10 shows three embodiments of an overlapping joint between the respective beam and the plate;

(12) FIG. 11 shows three other embodiments of the overlapping joint shown in FIG. 10;

(13) FIG. 12 shows a second embodiment of the rotor of the generator;

(14) FIG. 13 shows the plate and mounting element shown in FIG. 12;

(15) FIG. 14 shows a sectional view of the rotor shown in FIG. 12;

(16) FIG. 15 shows the rotor shown in FIG. 12 seen in the axial direction;

(17) FIG. 16 shows a first embodiment of assembling the rotor of the generator;

(18) FIG. 17 shows a second embodiment of assembling the rotor of the generator;

(19) FIG. 18 shows a third embodiment of the plates and the mounting elements of the rotor shown in FIGS. 16-17; and

(20) FIG. 19 shows a fourth embodiment of the rotor of the generator.

(21) In the following text, the figures will be described one by one, and the different parts and positions seen in the figures will be numbered with the same numbers in the different figures. Not all parts and positions indicated in a specific figure will necessarily be discussed together with that figure.

POSITION NUMBER LIST

(22) 1. Wind turbine 2. Tower 3. Foundation 4. Nacelle 5. Hub 6. Wind turbine blades 7. Generator 8. Stator 9. Rotor 10. Pole units with superconductive coils 11. Back iron 12. Rotor structure 13. Side surface of rotor structure 14. Side surface of back iron 15. Support elements, plates 16. First end of plate 17. Second end of plate 18. Rotational direction 19. First beam 20. Second beam 21. First end of beam 22. Second end of beam 23. Side surfaces of beam 24. Mounting means 25. Groove in beam 26. Bonding means 27. Pins 28. Bushings 29. Projecting elements 30. Grooves for receiving projecting elements 31. Support elements, plates 32. First mounting element 33. Second mounting element 34. Through holes of first mounting element 35. Holes of second mounting element 36. Protrusion of back iron 37. Protrusion of rotor structure 38. First plate 39. Second plate 40. Projecting elements, plate members 41. Through hole 42. Pin 43. Fingers 44. First beam, support element 45. Second beam, support element

DETAILED DESCRIPTION OF THE INVENTION

(23) FIG. 1 shows an exemplary embodiment of a wind turbine 1. The wind turbine 1 comprises a wind turbine tower 2 provided on a foundation 3. A nacelle 4 is arranged on top of the wind turbine tower 2 and configured to yaw relative to the wind turbine tower 2 via a yaw system (not shown). A hub 5 is rotatably arranged relative to the nacelle 4, wherein at least two wind turbine blades 6 are mounted to the hub 5, here three wind turbine blades are shown. The hub 5 is connected to a rotary machine in the form of a generator (shown in FIG. 2) arranged in the nacelle 4 via a drive shaft for producing a power output.

(24) FIG. 2 shows an exemplary embodiment of the generator 7 connected to the hub 5. Here, only a central rotational axis (indicated by dotted line) of the drive shaft is shown for illustrative purposes. The generator 7 comprises a stator 8 and a rotor 9 rotatably arranged relative to the stator 8. The stator 8 comprises a plurality of pole units (indicated by dotted lines) having stator coils configured to interact with rotor coils located in a plurality of pole units 10.

(25) At least the rotor coils are made of a superconductive material which is operated below its critical temperature. Thus, at least the pole units 10 act as superconducting pole units. The stator coils are made of a conductive material, such as cupper, operated at an ambient temperature.

(26) FIG. 3 shows a first embodiment of the rotor 9 where the pole units 10 are arranged on a back iron 11 facing the stator 8, e.g. on an outer side surface (shown in FIG. 3). A cooling system (not shown) is used to cool the pole units 10 down to a cryogenic operating temperature between 10 K and 70 K.

(27) The rotor 9 further comprises a rotor structure 12 having an inner support part facing the drive shaft and a yoke facing the back iron 11. The inner support part is here shaped as a disc having one or more cut-cuts as shown in FIG. 2. The inner support part is mounted to the drive shaft using mounting means. The yoke is here shaped as a ring or tubular element having a side surface 13 facing the back iron 11. The back iron 11 is further shaped as a ring or tubular element having a side surface 14 facing the rotor structure 12.

(28) The back iron 11 is spaced apart from the rotor structure 12 by a number of support elements 15 arranged between the side surfaces 13, 14. The support elements are shaped as plates 15 which are made of a thermally insulating material, e.g. fibre reinforced plastics (FRP), so that the back iron 11 is thermally insulated from the rotor structure 12. The rotor structure 12 is operated at an ambient temperature between 250 K and 350 K. Each plate 15 has a first end 16 facing the back iron 11 and a second end 17 facing the rotor structure 12.

(29) FIG. 4 shows a sectional view of the rotor structure 12 and the back iron 11. Here, the pole units 10 are omitted for illustrative purposes. The plates 15 are positioned so that the first and second ends 16, 17 extend in the axial direction as illustrated in FIGS. 2 and 3. The plates 15 are further orientated so that they extend in a combined radial and tangential direction and, thus, substantially extend in the same direction as the rotational direction 18 of the rotor 9 as indicated in FIG. 4.

(30) A first beam 19 is arranged at the first end 16 of the plate 15 and a second beam 20 is arranged at the second end 17 of the plate 15. The first and second beams 19, 20 extend along the side surfaces 13, 14 in the axial direction as shown in FIGS. 3 and 4. The first beam 19 is firmly connected to the back iron 11 and the first end 16 of the plate 15. The first beam 20 is firmly connected to the rotor structure 12 and the second end 17 of the plate 15.

(31) FIG. 5 shows a first embodiment of the first beam 19 having a first end 21 facing the plate 15 and a second end 22 facing in the opposite direction. The first beam 19 further has two opposite facing side surfaces 23 in the radial direction wherein one of which further acts as a contact surface for contacting the side surface 14, e.g. in a predetermined area thereof. Here, the first end 21 is placed in a first angled position, e.g. between 20 degrees and 80 degrees, relative to the tangential direction of the side surface 14.

(32) The first beam 19 comprises a first set of through holes for receiving mounting means 24 in the form of bolts and nuts for firmly connecting the first beam 19 to the back iron 11. The back iron 11 comprises a corresponding set of through holes for the mounting means 24 as indicated in FIG. 4.

(33) FIG. 6 shows a first embodiment of the second beam 20 having a first end 21′ facing the plate 15 and a second end 22 facing in the opposite direction. The second beam 20 further has two opposite facing side surfaces 23 in the radial direction wherein one of which further acts as a contact surface for contacting the side surface 13, e.g. in a predetermined area thereof. Here, the first end 21′ is placed in a second angled position, e.g. parallel to the tangential direction of the side surface 13.

(34) The second beam 20 comprises a first set of through holes for receiving mounting means 24 in the form of bolts for firmly connecting the second beam 20 to the rotor structure 12. The rotor structure 12 comprises a corresponding set of through holes for the mounting means 24 as indicated in FIG. 4.

(35) At least one groove 25 is formed in the first ends 21, 21′ of the first and second beams 19, 20 for receiving the first and second ends 16, 17 of the plate 15 as shown in FIGS. 5 and 6. One or more of the inner surfaces of the respective groove 25 may act as contact surfaces for contacting one or more corresponding surfaces on the respective end 16, 17 of the plate 15. Bonding means 26 in the form of glue is applied to these contact surfaces for firmly connecting the plate 15 to the first and second beams 19, 20. Alternatively, the respective beam 19, 20 comprises a second set of through holes for receiving mounting means 24 in the form of bolts and nuts, or the first set of through holes of the respective beam 19, 20 is further used for firmly connecting the plate 15 to the respective beam 19, 20 as shown in FIG. 4.

(36) FIG. 7 shows a second embodiment of a respective beam 19, 20 wherein the second set of through holes is configured for receiving other mounting means 27 in the form of pins. The pins are pushed through the web laminate of the respective end 16, 17 of the plate 15 so that the fibres in the laminate are pushed aside without breaking. This, in turn, increases the structural strength of the plate 15 around the trough holes formed by the pins.

(37) FIG. 8 shows a third embodiment of a respective beam 19, 20 wherein the beam forms part of the plate 15′. In this configuration, the respective end 16′, 17′ that defines the beam has an increased thickness compared to the rest of the plate 15′. Bushings 28, e.g. metal bushings, are placed in the first set of through holes for added structural strength of the plate 15′ around these through holes. The plate 15′ is then firmly connected to the back iron 11 or rotor structure 12 via mounting means 24.

(38) FIG. 9 shows a fourth embodiment of a respective beam 19, 20 wherein the beam forms part of the plate 15′. This configuration differs from the embodiment of FIG. 7 by the respective end 16′, 17′ having the same thickness as the rest of the plate 15′. Mounting means 24 can then be inserted through these bushings 28′ for firmly connecting the plate 15′ to the back iron 11 or rotor structure 12.

(39) FIG. 10 shows three embodiments of an overlapping joint between the respective beam 19, 20 and the respective end 16, 17 of the plate 15.

(40) In FIG. 10A, the respective beam 19, 20 has a rectangular cross-sectional profile seen in the tangential direction. The respective end 16, 17 of the plate 15 and, thus, the groove 25 further have a rectangular profile.

(41) In FIG. 10B, the respective beam 19′, 20′ has a wedge shaped cross-sectional profile seen in the tangential direction. The thickness measured between the side surfaces 23′ tapers from the second 22′ end towards the first end 21′. The respective end 16, 17 of the plate 15 and, thus, the groove 25 have a rectangular profile.

(42) In FIG. 10C, the respective end 16″, 17″ of the plate 15″ and, thus, the groove 25′ have a wedge shaped cross-sectional profile seen in the tangential direction, and the groove 25′ has a corresponding inverted wedge shaped cross-sectional profile. The thickness measured between the side surfaces of the plate 15″ tapers towards the edge of the end 16″, 17″ as indicated in FIG. 10C.

(43) FIG. 11 shows three other embodiments of the overlapping joint between the respective beam 19, 20 and the respective end 16, 17 of the plate 15. In these configurations, the respective beam 19, 20 and the plate 15 each comprise at least one projecting element 29 and at least one groove 30 configured to receive an opposite projecting element 29.

(44) In FIG. 11A, the projecting elements 29 and the grooves 30 have a rectangular profile seen in the tangential direction. Likewise, the respective beam 19, 20 has a rectangular profile.

(45) In FIG. 11B, at least one of the projecting elements 29′ of the respective beam 19″, 20″ has a wedge shaped profile and at least one of the corresponding grooves 30′ has an inverted wedge shaped profile. The thickness of this wedge shaped projecting element 29′ tapers towards the edge of that element as indicated in FIG. 11B. The respective beam 19″, 20″ has a wedge shaped cross-sectional profile as shown in FIG. 10B.

(46) FIG. 12 shows a second embodiment of the rotor 9′ of the generator 7 wherein a plurality of support elements 31 are arranged along the axial direction. The support elements 31 are shaped as plates 31 made of a thermally insulating material, e.g. fibre reinforced plastics (FRP). Each plate 31 is firmly connected to the back iron 11′ via a first mounting element 32. Each plate 31 is further firmly connected to the rotor structure 12′ via a second mounting element 33. Here, five plates 31 in the axial direction are shown. Only the yoke of the rotor structure 12′ is shown here for illustrative purposes.

(47) FIG. 13 shows the plate 31 and the mounting elements 32, 33 thereof. The plates 31 are made of a thermally insulating material, e.g. fibre reinforced plastics (FRP), so that the back iron 11′ is thermally insulated from the rotor structure 12′.

(48) The mounting elements 32, 33 have a first end facing the plate 31 and a second end facing in the opposite direction. The mounting elements 32, 33 further have two opposite facing side surfaces wherein one of which further acts as a contact surface for contacting the side surface of the back iron 11′ or the rotor structure 12′.

(49) The first and second mounting elements 32, 33 are configured to firmly connect the plate 31 to the back iron 11′ and the rotor structure 12′. The first mounting element 32 comprises a set of through holes 34 for receiving mounting means in the form of bolts for firmly connecting the first mounting element 32 to the back iron 11′. The back iron 11′ comprises a corresponding set of holes (not shown) for receiving the mounting means. The second mounting element 33 comprises a set of holes 35 for receiving mounting means in the form of bolts for firmly connecting the second mounting element 33 to the rotor structure 12′. The rotor structure 12′ comprises a corresponding set of through holes (shown in FIG. 14) for receiving the mounting means.

(50) FIG. 14 shows a sectional view of the rotor 9′ wherein the mounting elements 32, 33 are positioned relative to protrusions 36, 37 located on the side surfaces 13′, 14′ of the back iron 11′ and the rotor structure 12′.

(51) Each of the first mounting elements 32 of the plates 31 in the axial direction is firmly connected to the protrusion 36 on the side surfaces 14′ of the back iron 11′ as indicated in FIGS. 12 and 14. The mounting means of the first mounting element 32 can be accessed from a substantially radial direction as indicated in FIGS. 12 and 14.

(52) Each of the second mounting elements 33 of the plates 31 in the axial direction is firmly connected to the protrusion 37 on the side surfaces 13′ of the rotor structure 11′ as indicated in FIGS. 12 and 14. The mounting means of the second mounting element 32 can be accessed from a substantially tangential direction as indicated in FIGS. 12 and 14.

(53) FIG. 15 shows the rotor 9′ seen in the axial direction wherein a first plate 38 is positioned so that it substantially extends in the same direction as the rotational direction 18 of the rotor 9′. A second plate 39 is positioned so that it substantially extends in the opposite direction of the rotational direction 18 of the rotor 9′. The first and second plates 38, 39 are offset relative to each other in the axial direction as shown in FIG. 14. The second mounting elements 33 of the first and second plates 38, 39 are further offset relative to each other in the radial direction as shown in FIGS. 14 and 15.

(54) The adjacent first and second plates 38, 39, seen in the axial direction, form one set located on the circumference of the rotor structure 12′. Here, six sets of first and second plates 38, 39 are shown along the circumference of the rotor structure 12. The first plate 38 of this one set is positioned during assembly so that it intersects a second plate 39 of an adjacent set as shown in FIG. 15. Likewise, the second plate 39 of this one set is positioned during assembly so that it intersects a first plate 38 of another adjacent set as shown in FIG. 15.

(55) FIG. 16 shows a first embodiment of a method of assembling the rotor 9″ according to the invention. Initially, a rotor structure 12 is provided. A number of second mounting elements 33′ are arranged on the side surface 13 and firmly connected to the rotor structure 12. A number of first and second plates 38′, 39′ are then positioned relative to the second mounting elements 33′ and firmly connected via first pin connections (shown in FIG. 18). A number of first mounting elements 32′ are positioned relative to the first and second plates 38′, 39′ and firmly connected via second pin connections (shown in FIG. 18). The back iron 11 is aligned relative to the rotor structure 12 and moved into position. The first mounting elements 32′ are then firmly connected to the back iron 11.

(56) The rotor coils are arranged on the back iron 11 before or after moving the back iron 11 into position.

(57) FIG. 17 shows a second embodiment of the method of assembling the rotor 9″. In this embodiment, the second mounting elements 33′ are arranged on the side surface 13 and firmly connected to the rotor structure 12. The back iron 11 is then moved into position relative to the rotor structure 12. The first and second plates 38′, 39′ are firmly connected to the respective first mounting elements 32′ separated from the rotor 9′ as shown in FIG. 17. The first and second plates 38′, 39′ with the first mounting elements 32′ are then arranged on the side surface 14 and firmly connected to the back iron 11 and the second mounting elements 33′ respectively. Here only one predetermined area of the back iron 11 is shown for receiving the first and second plates 38′, 39′ with the first mounting elements 32′.

(58) FIG. 18 shows a third embodiment of the rotor 9″ wherein the first and second mounting elements 32′, 33′ differ from the first and second mounting elements 32, 33. Here, only a sectional view of the rotor 9″ is shown. In this embodiment, the mounting elements 32′, 33′ have at least two projecting elements 40 extending radially outwards from a bottom part. The bottom part is configured for mounting and/or bonding to the back iron 11 or rotor structure 12. The projecting elements 40 each has a through hole 41 extending in the axial direction for receiving and holding a removable pin 42.

(59) Another through hole 41 is arranged in the first and second ends of the respective plate 38′, 39′. This through hole 41 also extends in the axial direction and is configured for receiving and holding the pin 42. The pin 42 is connected to both the first and second plates 38′ as shown in FIGS. 16 and 17.

(60) One or both of the mounting element 32′, 33′ optionally have a number of fingers 43 extending in the tangential direction as shown in FIG. 18. Here, the fingers 43 are only shown on the first mounting elements 32′. The fingers 43 are configured to be mounted and/or bonded to the back iron 11 or rotor structure 12 for optimal transfer of loads.

(61) FIG. 19 shows a fourth embodiment of the rotor 9′″ wherein the support element differs from the plate 15 and the plate 31. In this embodiment, the support elements are shaped as beams made of a thermally insulating material, e.g. fibre reinforced plastics (FRP). Each of the beams has a constant thickness along its length.

(62) A first support element or beam 44 and a second support element or beam 45 comprise a number of plate members or knuckles, e.g. one, two, or more, distributed along the width of the respective first and second ends as indicated in FIG. 19. Each plate member or knuckle has a trough hole 41 for receiving and holding the pin 42. The first and second mounting element 32″, 33″ may further comprise at least one additional projecting element, e.g. a plate member or knuckle, for added support. The pin 42 further extends through this additional projecting element.