FIELD OF THE INVENTION

The present invention relates to a connector for connecting wind turbine blade portions to one another, and a method of connecting wind turbine blade portions to one another.

BACKGROUND OF THE INVENTION

A wind turbine converts kinetic energy of the wind into electrical energy. A generator converts the wind energy captured by a rotor, having one or more rotor blades, into electrical energy which is usually supplied to a utility grid. The generator is housed in a nacelle together with the various components required to operate and optimize the performance of the wind turbine. A tower supports the load presented by the nacelle and the rotor. In a horizontal axis wind turbine (HAWT) the rotor blades extend radially outwardly from a central hub which rotates about a longitudinal axis aligned generally horizontally. In operation, the blades are configured to interact with the passing air flow to produce lift that causes the rotor to rotate within a plane substantially perpendicular to the direction of the wind.

A conventional rotor blade is made from an outer shell and one or more inner spars in a hollow space bounded by the outer shell. The spar serves to transfer loads from the rotating blade to the hub of the wind turbine. Such loads include tensile and compressive loads directed along the length of the blade arising from the circular motion of the blade and loads arising from the wind which are directed along the thickness of the blade, i.e. from the windward side of the blade to the leeward side.

As wind turbine blade lengths increase, it has become desirable to manufacture and transport blades as separate portions and to construct the blades on a site closer to the wind turbine, or to construct the blades when a portion of the blade is fixed to a hub of a wind turbine. A mechanical connection is used to connect the blade sections to one another.

SUMMARY OF THE INVENTION A first aspect of the invention provides a wind turbine blade portion connector, configured to attach a first blade portion to a second blade portion of a wind turbine blade, comprising: a double-ended threaded stud bolt having two threaded portions with opposing threads configured to be threadedly engaged with respective threaded holes of the first blade portion and the second blade portion, a tool engagement feature fixed in rotation with and disposed between the two threaded portions for rotating the threaded stud bolt, and a bearing surface disposed between the two threaded portions; and a spring, configured to bear against the bearing surface and exert a tensile force on the first and/or second threaded portions of the stud bolt in a direction parallel or co-axial to a longitudinal direction of the stud bolt when disposed between the bearing surface and the first or second blade portion.

Provision of a stud bolt having two threaded portions with opposing threads provides a simple and reliable assembly method. Use of a spring configured to exert a tensile force on the first and/or second threaded portions of the stud bolt allows the stud bolt to be held in tension. This tensile pre-load narrows the overall range of tension to which the stud bolt is subjected in use, and as such provides improved fatigue properties of the connector, and a more reliable connection, while also providing a connection mechanism with improved fatigue properties. In addition, the use of a spring increases the distance over which the tension is applied (compared to a nut simply tightened up against the blade portion) and this increased distance reduces the torque on the stud bolt and also provides more sensitivity in applying pre-load

The bearing surface may be integral with the tool engagement feature.

The bearing surface may be comprised in a stop fixed in an axial direction with respect to the threaded stud bolt. The stop may be spaced axially from the tool engagement feature.

The tensile force exerted by the spring on the first and/or second threaded portions of the stud bolt may be in a direction aligned with a longitudinal direction of the stud bolt when disposed between the bearing surface and the first and/or second blade portion. The spring may be configured such that the first or second threaded portions of the stud bolt can extend through the spring.

The spring may comprise a plurality of springs.

At least one of the springs may be disposed on a first side of the tool engagement feature, and at least one of the springs may be disposed on a second side of the tool engagement feature.

The tool engagement feature may be moveable along the stud bolt, in the longitudinal direction of the stud bolt.

The plurality of springs may be substantially the same as one another in terms of one or more of: shape, size, material properties.

The or each spring may be a spring washer.

The or each spring may be one or more of: a Belleville washer, a conical washer, an annular washer.

The first blade portion may be disposed closer to a root end of the wind turbine blade, in use, than the second blade portion.

The first blade portion may be a root blade portion or an intermediate blade portion of the wind turbine blade.

In addition or alternatively, the second blade portion may be a tip blade portion or an intermediate blade portion of the wind turbine blade.

The blade portion connector may comprise a load indicating means, configured to indicate the load applied to the stud bolt.

The blade portion connector may be provided as a part of a wind turbine blade. The wind turbine blade may comprise: a first blade portion including a first blade portion end surface at one end of the first blade portion and having a first blade portion threaded hole open at the first blade portion end surface; a second blade portion including a second blade portion end surface at one end of the second blade portion and having a second blade portion threaded hole open at the second blade portion end surface; and at least one blade portion connector, wherein the first threaded portion of the stud bolt is threadedly engaged with the first blade portion threaded hole and the second threaded portion of the stud bolt is threadedly engaged with the second blade portion threaded hole such that the first blade portion is connected to the second blade portion by the blade portion connector; and wherein the spring bears against the bearing surface so as to exert a tensile force on the first and/or second threaded portions of the stud bolt in a direction parallel or co-axial to a longitudinal direction of the threaded stud bolt.

The first blade portion may further comprise a first insert embedded in the first blade portion and having the first blade portion threaded hole.

In addition or alternatively, the second blade portion may further comprise a second insert embedded in the second blade portion and having the second blade portion threaded hole.

The first insert and/or the second insert may have an undulating outer surface. The first insert and/or the second insert may include a bushing.

At least one of the inserts may be metallic, comprise a metal or alloy, or consist of a metal or alloy.

The first insert and/or the second insert may be secured within respective elongate fibre composite bodies.

The respective inserts may be embedded at one end of the respective elongate fibre composite bodies. The first insert and/or the second insert may be integrated within an end face of a spar cap of the respective first and/or second blade portions.

The first and the second blade portion end surfaces may be spaced apart.

The connector may extend between the first and the second blade portion end surfaces.

The or each of the first blade portion and the second blade portion may have a shell that defines a suction side, a pressure side, a leading edge, and a trailing edge of the blade.

The wind turbine blade may comprise a plurality of the blade portion connectors, the first blade portion having a plurality of the first blade portion threaded holes, the second blade portion having a plurality of the second blade portion threaded holes, and a plurality of the springs, wherein each double-ended stud bolt is threadedly engaged in respective first and second blade portion threaded holes, and the springs bear against the respective tool engagement features so as to exert a tensile force on the first and/or second threaded portions of the stud bolts in a direction parallel or co-axial to a longitudinal direction of the respective threaded stud bolts.

A second aspect of the invention provides a method of assembling a wind turbine blade, comprising: providing: a first blade portion; a second blade portion; and a blade portion connector, the blade portion connector comprising: a double-ended threaded stud bolt having two threaded portions with opposing threads configured to be threadedly engaged with respective threaded holes of the first blade portion and the second blade portion, a tool engagement feature fixed in rotation with and disposed between the two threaded portions for rotating the threaded stud bolt, and a bearing surface disposed between the two threaded portions; and a spring disposed between the bearing surface and the first or second blade portion; rotating the tool engagement feature so as to insert: the first threaded portion of the stud bolt into the first threaded hole of the first blade portion; and/or the second threaded portion of the stud bolt into the second threaded hole of the second blade portion, wherein rotating the tool engagement feature is carried out at least until the spring bears against the bearing surface and exerts a tensile force on the first and/or second threaded portions of the stud bolt in a direction parallel or co-axial to a longitudinal direction of the stud bolt by compressing the spring.

The first blade portion may include a first blade portion end surface at one end of the first blade portion and having the first blade portion threaded hole open at the first blade portion end surface.

The second blade portion may include a second blade portion end surface at one end of the second blade portion and having the second blade portion threaded hole open at the second blade portion end surface.

Rotating the tool engagement feature may be stopped when the tensile force reaches a desired load and the first and second blade portion end surfaces remain spaced apart at the desired load.

Rotating the tool engagement feature may be stopped when the spring is at least partly compressed between the tool engagement feature and the first or second blade portion.

Rotating the tool engagement feature may be stopped when the spring is at the point of being wholly compressed between the tool engagement feature and the first or second blade portion.

The method may further comprise determining the optimal tensile load to be exerted on the stud bolt.

The method may further comprise selecting one or more springs having properties that provide the determined optimal tensile load to be exerted on the stud bolt when the one or more springs are elastically deformed before yield.

The method may further comprise measuring the tensile load exerted on the stud bolt. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 shows a wind turbine;

Figure 2 shows a wind turbine blade comprising separable wind turbine blade portions; Figure 3 shows a cross section of the wind turbine blade;

Figure 4 shows a detail view of the spar cap in the blade shell of Figure 3 at location A;

Figure 5 shows part of a blade portion connector and blade portions;

Figure 6 shows a blade portion connector;

Figure 7 shows a part of an assembly process for a blade;

Figure 8 shows a continuation of the assembly process of Figure 7;

Figure 9 shows a spring; and

Figure 10 shows part of a blade portion connector and blade portions.

DETAILED DESCRIPTION OF EMBODIMENT(S)

In this specification, terms such as leading edge, trailing edge, pressure side, suction side, thickness and spar cap are used. While these terms are well known and understood to a person skilled in the art, definitions are given below for the avoidance of doubt. The term leading edge is used to refer to an edge of the blade which will be at the front of the blade as the blade rotates in the normal rotation direction of the wind turbine rotor.

The term trailing edge is used to refer to an edge of a wind turbine blade which will be at the back of the blade as the blade rotates in the normal rotation direction of the wind turbine rotor.

A pressure side (or windward surface) of a wind turbine blade is a surface between the leading edge and the trailing edge, which, when in use, has a higher pressure than a suction side of the blade.

A suction side (or leeward surface) of a wind turbine blade is a surface between the leading edge and the trailing edge, which will have a lower pressure acting upon it than that of a pressure side, when in use.

The thickness of a wind turbine blade is measured perpendicularly to the chord of the blade and is the greatest distance between the pressure surface and the suction surface in a given cross section perpendicular to the blade spanwise direction.

The term spar cap is used to refer to a longitudinal, generally spanwise extending, reinforcing member of the blade. The spar cap may be embedded in the blade shell, or may be attached to the blade shell. The spar caps of the windward and leeward sides of the blade may be joined by one or more shear webs extending through the interior hollow space of the blade. The blade may have more than one spar cap on each of the windward and leeward sides of the blade. The spar cap may form part of a longitudinal reinforcing spar or support member of the blade. In particular, a first and a second spar cap may form part of the load bearing structure extending in the longitudinal direction that carries the flap-wise bending loads of the blade.

The term shear web is used to refer to a longitudinal, generally spanwise extending, reinforcing member of the blade that can transfer load from one of the windward and leeward sides of the blade to the other of the windward and leeward sides of the blade. Figure 1 shows a wind turbine 10 including a tower 12 and a nacelle 14 disposed at the apex of the tower 12.

A rotor 16 is operatively coupled via a gearbox to a generator (not shown) housed inside the nacelle 14. The rotor 16 includes a central hub 18 and a plurality of rotor blades 20, which project radially outwardly from the central hub 18. While the example shown in Figure 1 has three blades, it will be realised by the skilled person that other numbers of blades are possible.

When wind blows against the wind turbine 10, the blades 20 generate a lift force which causes the rotor 16 to rotate, which in turn causes the generator within the nacelle 14 to generate electrical energy.

Figure 2 is a view of one of the blades 20. The blade 20 extends from a generally circular root 28 to a tip 40 in a longitudinal spanwise direction, and between a leading edge 36a, 36b and a trailing edge 32a, 32b, in a transverse chordwise direction. The blade 20 comprises a suction surface 30a, 30b, and a pressure surface 34a, 34b. A thickness dimension of the blade extends between the suction surface 30a, 30b and the pressure surface 34a, 34b.

The blade 20 is a split wind turbine blade formed of a first portion 22 and a second portion 24, which may be connected at a connection joint indicated by connection line 66. The connection line 66 between the first and second blade portions 22, 24 may be a spanwise split, with the connection line 66 being chordwise.

Each wind turbine blade portion 22, 24 may be manufactured separately. Alternatively the blade portions 22, 24 may be manufactured together in the same mould and then split. Each blade portion is then transported to a site for erection of the wind turbine. The blade portions are then joined together at the erection site to form the completed blade 20 before each blade is mounted to the hub 18 to form the rotor 16 of the wind turbine. The blades 20 may also be coupled while a portion of the blade 20 is connected to the hub 18. This can reduce the load which is needed to be lifted by a crane and can also allow replacement of only a portion of a blade, for example if a tip portion has been damaged. The first blade portion 22 has a pressure surface 34a, a leading edge 36a, a suction surface 30a and a trailing edge 32a. The first blade portion 22 extends from the blade root 28 to the connection line 66. The blade 20 also comprises a second blade portion 24, extending from the blade connection line 66 to the blade tip 40 and the second blade portion 24 has a pressure surface 34b, a leading edge 36b, a suction surface 30b and a trailing edge 32b. The connection joint may be covered by a fairing, which may provide a surface over the joint with minimal worsening of the aerodynamic profile of the two blade portions.

While the example shown in Figure 2 has two blade portions, it will be understood that a blade may have three or more blade portions with a connection joint between adjacent blade portions.

While the first wind turbine blade portion 22 is shown as being nearer the root of the blade 20 and second wind turbine blade portion 24 is shown as being nearer the tip of the blade 20, the labels "first" and "second" are not intended to be limiting and any specific property disclosed as being associated with the first or second wind turbine blade portion may be applied to the other wind turbine blade portion.

As shown in Figure 3, the second wind turbine blade portion 24 may include an outer blade shell 42 defining a hollow interior space 44 with a shear web 46 extending internally between upper and lower parts of the blade shell 42. The blade shell 42 may comprise two half-shells 42a, 42b which are separately moulded before being joined together (at the leading edge 36b and the trailing edge 32b) to form the blade portion 22. It will be appreciated that the blade shell 42 need not be formed as two half-shells which are subsequently joined together but may be formed as a unitary shell structure, together with the shear web 46, in a "one shot" single shell process. The blade shell 42 may include a laminate composite material such as glass fibre and/or carbon fibre for example. The first wind turbine blade portion 22 may be constructed similarly.

Figure 4 shows a detail view of the region where the shear web 46 meets the blade shell 42. A spar cap 48 may be incorporated into the outer shell 42, as shown in Figure 4, or may be attached to the outer shell 42. The spar cap 48 is an elongate reinforcing structure extending substantially along the full length of the blade portion 22, 24. The spar cap 48 may include composite material, such as carbon fibres. For example, the spar cap may include pultruded fibrous strips of material such as pultruded carbon fibre composite material or other carbon fibre reinforced plastic material. The pultruded fibres may be oriented in a longitudinal direction of the spar cap 48 which may be aligned with the longitudinal or spanwise direction of the blade 20.

One or more layers 52 of glass fibre reinforced plastic may be provided over the outside of the spar cap 48. The layers 52 collectively form an outer skin of the blade shell 42. One or more further layers of glass fibre reinforced plastic may provide an inner skin 54 of the blade shell 42 with a core material 50 between the outer skin 52 and the inner skin 54. The core material may be a light structural foam, though other core materials such as wood, particularly balsa wood may alternatively be used to provide a lightweight core material. It will be appreciated that a near identical connection may be made between the shear web 46 and the other side of the blade shell 42.

As shown in figure 5, a connection between blade portions, such as between a first blade portion 22 and second blade portion 24, is provided by the wind turbine blade portion connector 100. The first blade portion 22 may be as described above, or may be a section or component part of the first blade portion 22 described above. Equally, the second blade portion 24 may be as described above, or may be a section or component part of the first blade portion 24 described above.

The wind turbine blade portion connector 100 comprises a double-ended threaded stud bolt 300 having two threaded portions 310, 320 with opposing threads, and a tool engagement feature 330, and a bearing surface 330, 340, 350 disposed between the two threaded portions. The bearing surface may extend radially outwardly of the stud bolt, i.e. in a plane perpendicular to a longitudinal axis of the stud bolt.

The opposing threads of the threaded portions 310, 320 are configured to be threadedly engaged with respective threaded holes 211 , 221 of the first blade portion 22 and the second blade portion 24. The two threaded portions 310, 320 may be substantially the same shape, size and/or material as one another. Alternatively they may be differently sized. The two threaded portions 310, 320 may comprise or consist of a metal and/or alloy. The two threaded portions 310, 320 may only differ from one another in that they have oppositely extending threaded directions to one another. The or each threaded portion 310, 320 may have a threaded surface portion that extends over most of, or all of, its length.

As the skilled person will appreciate, the terms “opposing threads” and “oppositely extending threaded portion” refer to an arrangement in which one threaded portion extends in a clockwise orientation and the other threaded portion extends in a counter clockwise orientation. The opposing threads ensure that as the stud bolt is rotated in a first direction of rotation with the threaded ends engaged in their respective threaded holes of the first and second blade portions, the first and second blade portions are drawn towards each other. Rotating the stud both in a second direction of rotation opposite the first direction causes the first and second blade portions to move away from each other.

The tool engagement feature 330 is fixed in rotation with, and disposed between, the two threaded portions 310, 320 for rotating the threaded stud bolt 300.

As shown in figure 5, the bearing surface may be integral with the tool engagement feature 330. The tool engagement feature 330 may extend radially outwardly of the connector 100. This has the advantage of allowing the whole of the bolt 300 to be held in tension and/or compression in use.

The connector 100 may define a longitudinal axis X and the tool engagement feature 330 may extend radially outwardly from the longitudinal axis X. The tool engagement feature 330 may be configured to be received in any conventional tool suitable for mechanical rotation of a part, such as a manual or automated torque wrench. The tool engagement feature 330 may be a nut. The tool engagement feature may be a protrusion, a recess, a facet, etc. The tool engagement feature 330 may define a polygon, such as an octagonal or hexagonal shape in cross section, or, the tool engagement feature 330 may define a cog shape to engage with a chain that can turn multiple tool engagement features in unison.

As shown in Figure 6, the connector 100 also comprises a spring 400, which is configured to bear against the bearing surface and exert a tensile force on the first and/or second threaded portions 310, 320 of the stud bolt 300 in a direction parallel or co-axial to the longitudinal direction X of the stud bolt when disposed between the bearing surface and the first or second blade portions 22, 24. The bearing surface reacts the spring force of the compression spring into a tensile force in the stud bolt.

The spring 400 may be configured such that the first and/or second threaded portions 310, 320 can extend through the spring 400. The spring 400 may comprise, or be provided as, a plurality of springs 400, such as the springs 401 to 406, and 411 to 416, shown in Figure 6, or the springs 401 to 408, and 411 to 416 shown in Figure 7. The spring 400 may be configured to elastically deform so as to generate a force of at least 500 kN, preferably at least 600 kN, although the amount of force required will depend on the size of the stud bolt. The inventors have discovered that this mechanical property is particularly advantageous in a wind turbine blade portion connector 100.

The spring 400 may be elastically compressible. The spring 400 may be configured such that in use it is only subjected to elastic and no, or minimal, plastic deformation. At least one spring 401 to 406 may be disposed on a first side 331 of the tool engagement feature 330. At least one spring 410 to 416 may be disposed on a second side 332 of the tool engagement feature 330. Each spring 400 in the plurality of springs may be substantially the same as one another in terms of one or more of: shape, size and material properties. The or each spring 400 may be configured such that it can easily be placed onto the first or second threaded portion 310, 320 of the double-ended threaded stud bolt 300, for example by comprising an internal aperture. The or each spring 400 may be a spring washer, and/or may be one or more of: a Belleville washer, a conical washer, an annular washer. An example of a Belleville washer is shown in Figure 9.

The tool engagement feature 330 may be fused to the two threaded portions 310, or may be provided, cast, or machined as a single unitary piece with the threaded portions 310, 320. However, advantageously the tool engagement feature 330 may be movable in the longitudinal direction of the stud bolt. If the tool engagement feature 330 can slide on the stud bolt (yet still be in fixed rotation with the stud bolt) the loads on the stud bolt on the opposing sides of the tool engagement feature 330 will be equalised when the tool engagement feature 330 is turned.

The connector 100 may additionally comprise one or more load indicating components, such as a load indicating washer. The or each load indicating washer may be a direct tension indicator. The or each load indicating washer may comprise one or more raised protrusions, which may collapse to a specified gap as washer is compressed. A feeler gauge may be used to measure a specified gap, which may be indicative of a predetermined load. As noted above, figure 5 shows a bearing surface which is integral with the tool engagement feature 330. The bearing surface may alternatively be comprised in one or more stop features 340, 350. The stop feature may be spaced from the tool engagement feature 330, for example as shown in figure 10. The stop feature may abut the tool engagement feature. The stop feature may be freely movable along the axially length of the stud bolt until abutment with the tool engagement feature. Alternatively the stop feature may be fixed axially with respect to the stud bolt. The stop feature may be differently shaped to the tool engagement feature. The stop feature may be freely rotatable with respect to the tool engagement feature. Alternatively the stop feature may be fixed in rotation with respect to the stud bolt. The or each stop feature 340, 350 may be proximate the tool engagement feature 330. The or each stop feature 340, 350 may be fixed to the tool engagement feature 330.

The or each stop feature 340, 350 may extend radially outwardly from the longitudinal axis X. The or each stop feature 340, 350 may have a bearing surface which is substantially planar, and/or disc-shaped. The or each stop feature 340, 350 may be, or have the shape of, a flat washer.

There may be provided two stop features 340, 350. The tool engagement feature 330 may be disposed between two stop features 340, 350. Each stop feature 340, 350 may be spaced apart from the tool engagement feature 330. Each stop feature may be spaced apart from the tool engagement feature by the same axial distance as one another, or by a different distance. A first stop feature 340 may be provided adjacent the first threaded portion 310 and/or a second stop feature 350 may be provided adjacent the second threaded portion 320. The or each stop feature 340, 350 may be provided as a separate piece to the first and second threaded portions 310, 320, attached to the stud bolt 300 by any suitable means, such as welding, or may be integrally formed with the first and second threaded portions 310, 320.

In a further arrangement, the tool engagement feature may define a first bearing surface adjacent one of the threaded portions, and a stop feature having a second bearing surface may be adjacent the other of the threaded portions of the stud bolt. The connector 100 may be used in a method of assembly of a blade 20, to provide an assembled wind turbine blade 20. The method comprises: providing: a first blade portion 22; a second blade portion 24; and a blade portion connector 100, the blade portion connector 100 comprising: a double-ended threaded stud bolt 300 having two threaded portions 310, 320 with opposing threads configured to be threadedly engaged with respective threaded holes of the first blade portion 22 and the second blade portion 24, a tool engagement feature 330 fixed in rotation with and disposed between the two threaded portions 310, 320 for rotating the threaded stud bolt 300, a bearing surface 330, 340, 350 extending away from the two threaded portions 310, 320; and a spring 400 disposed between the bearing surface 330 and the first or second blade portion 22, 24; rotating the tool engagement feature 330 so as to insert: the first threaded portion 310 of the stud bolt 300 into the first threaded hole 211 of the first blade portion 22; and/or the second threaded portion 320 of the stud bolt 300 into the second threaded hole 221 of the second blade portion 24, wherein rotating the tool engagement feature 330 is carried out at least until the spring 400 bears against the bearing surface 330, 340, 350 and exerts a tensile force on the first and/or second threaded portions 310, 320 of the stud bolt 300 in a direction parallel or co-axial to a longitudinal direction X of the stud bolt 300 by compressing the spring 400.

As demonstrated in Figures 7 and 8, the connector 100 may be used to assemble a first blade portion 22 to a second blade portion 24, to provide an assembled wind turbine blade 20 according to at least the following steps. The wind turbine blade 20 may be assembled by providing the double-ended threaded stud bolt 300 and placing a plurality of springs 401 to 408 on the first threaded portion 310 and a second plurality of springs 410 to 416 on the second threaded portion 320. The connector 100 may be aligned with a first threaded hole 211 of the first blade portion 22. The tool engagement feature 330 may then be rotated so as to rotate the bolt 300, and insert the first threaded portion 310 into the first threaded hole 211 of the first blade portion 22. Rotation of the tool engagement feature 330 may also insert the second threaded portion 320 of the stud bolt 300 into the second threaded hole 221 of the second blade portion 24. This rotation is carried out at least until the spring 400 or at least one of the plurality of springs 401 to 408, 410 to 416, bears against the bearing surface, which may be the tool engagement feature 330, so as to exert a tensile force on the first and/or the second threaded portion 310, 320 of the stud bolt 300 in a direction parallel or co-axial to a longitudinal direction X of the stud bolt 300 by compressing the spring 400 or at least one of the plurality of springs 401 to 408, 410 to 416.

As demonstrated in Figure 7, the method may include the additional step of rotating the tool engagement feature 330 so that the first threaded portion 310 is inserted into the first blade portion 22, and then continuing to rotate the tool engagement feature 330 so as to simultaneously insert the first threaded portion 310 into the first blade portion 22 and the second threaded portion 24 into the second blade portion 24.

The method may include the additional step of determining the optimal tensile load to be exerted on the stud bolt 300. The method may also comprise the additional step of selecting one or more springs 400 having properties, such as mechanical or material properties, that provide the determined optimal tensile load to be exerted on the stud bolt 300 when the one or more springs 400 is elastically deformed before yield. The mechanical and/or material properties may comprise one or more of: Young’s modulus, shear modulus, toughness, yield strength, and fatigue response. The method may also comprise the additional step of selecting a determined number of springs 400 having a determined thickness and/or shape.

The method may comprise arranging the spring 400 or springs in a determined orientation. For example, where Belleville washers are used as the springs 400, the washers may be tessellated or arranged at 180° rotation relative to one another. The method may include the additional step of measuring the load to which the connector 100, specifically the bolt 300, is subjected. The method may include the step of rotating the tool engagement feature 300 until a predetermined gap defined by one or more load indicating components is achieved. This may be measured using a distance measuring tool and/or a feeler gauge. The method may include the additional step of fixing or locking the tool engagement feature 330 in place when a pre-determined load, position or gap has been achieved. This may be achieved by wire locking.

An assembled wind turbine blade 20, assembled by means of the method described herein or by any other appropriate method, comprises: a first blade portion 22, a second blade portion 24, and at least one blade portion connector 100 as described in any of the examples described or shown herein.

The first blade portion 22 includes a first blade portion end surface at one end, and has a first blade portion threaded hole 211 open at the first blade portion end surface. The second blade portion 24 includes a second blade portion end surface at one end, and has a second blade portion threaded hole 221 open at the second blade portion end surface.

The first threaded portion 310 of the stud bolt 300 is threadedly engaged with the first blade portion threaded hole 211 and the second threaded portion 320 of the stud bolt 300 is threadedly engaged with the second blade portion threaded hole 221 such that the first blade portion 22 is connected to the second blade portion 24 by the blade portion connector 100. The spring 400 bears against the bearing surface, which may be the tool engagement feature 330 of the connector 100 so as to exert a tensile force on the first and/or second threaded portions 310, 320, of the stud bolt 300 in a direction parallel or co-axial to a longitudinal direction X of the threaded stud bolt 300.

The first blade portion 22 may further comprise a first insert 212 embedded in the first blade portion 22. The first insert 212 may comprise the first blade portion threaded hole 211. Equally, the second blade potion 24 may further comprise a second insert 222 embedded in the second blade portion 24. The second insert 222 may comprise the second blade portion threaded hole 221.

The first insert 212 and/or the second insert 222 may be secured within respective elongate fibre composite bodies. The respective inserts 212, 222 may be embedded at one end of the respective elongate fibre composite bodies, which may in turn be embedded in a respective blade portion 22, 24. The first insert 212 and/or the second insert 222 may be integrated within an end face of a spar cap 48 of the respective first and/or second blade portions 22, 24.

One or more of: the first insert 212, the second insert 212, 222, the spring 400, one of the plurality of springs 400, may be configured such that the spring 400, or a spring of the plurality of springs 400, can bear against the respective first or second insert 212, 222. One or more of: the first insert 212, the second insert 212, 222, the spring 400, one of the plurality of springs 400, may be configured such that the spring 400, or a spring of the plurality of springs 400, does not bear against any component of the blade in which the insert 212, 222 is embedded.

The first and/or second insert 212, 222, may comprise a tapered portion and/or a cuboidal portion. By providing a tapered portion, there may be provided a greater surface area for adhering the composite body to the first blade portion 22. The first insert 212 and/or the second insert 222 may have an undulating outer surface and/or include a bushing. The undulating outer surface of the bushing may be wrapped in composite material, in which the fibres of the composite material may lie within grooves in the undulating outer surface and thereby the bushing may be securely fixed within the composite body. This can increase the strength of the interface between the bushing and the composite material. In additional, the first and/or second insert 212, 222 may be glued or cured in position.

The first insert 212 and the second insert 222 may have different lengths to one another. Specifically, a bushing of one of the first and second insert 212, 222 may be shorter than a bushing of the other of the first and second insert 212, 222.

The first and second blade portion end surfaces may be spaced apart and the connector 100 may extend between the first and the second blade portion end surfaces.

The composite bodies within which the or each insert 212, 222 may be embedded, may be embedded within a spar cap 48. The spar cap 48 may comprise a composite slab such as a carbon slab or a fibre glass slab, adhered to or integrated with a blade shell. The slab may taper so as to have a width (in a chordwise direction which reduces in a direction moving away from the joint), and may be fixed to a carbon or fibreglass spar. By embedding the composite bodies within a spar cap 48, the load transferred across the joint can be directly transferred into a load bearing structure of the blade.

By using a composite slab or a spar cap 48 which diverges or increases in width toward a blade portion end surface, a greater number of connectors 100 may be used at a joint between two blade portions 22, 24. The wind turbine blade 20 may comprise a plurality of the blade portion connectors 100 and a plurality of springs 400. The first blade portion 22 may have a plurality of the first blade portion threaded holes 211. The second blade portion 24 may have a plurality of the second blade portion threaded holes 221. Each double ended stud bolt 300 may be threadedly engaged in respective first and second blade portion threaded holes 211 , 221 , and the springs 400 may bear against the respective tool engagement features 330 so as to exert a tensile force on the first and/or second threaded portions 310, 320 of the stud bolts 300 in a direction parallel or co-axial to a longitudinal direction X of the respective threaded stud bolts 300.

The blade 20 may comprise main spar caps 48 and/or rear spar caps. The main spar caps 48 may be located closer to a leading edge 36a, 36b of the blade than the rear spar caps 48. The rear spar caps may be located closer to a trailing edge 32a, 32b of the blade than the main spar caps 48.

The plurality of the first blade portion threaded holes 211 may be arranged in two groups: a first cluster and a second cluster. The first cluster may be provided on or proximate the main spar caps. The second cluster may be provided on or proximate the rear spar caps. The first cluster may include more first blade portion threaded holes 211 than the second cluster. The first cluster may include between 5 and 10 first blade portion threaded holes 211. The second cluster may include between 2 and 5 first blade portion threaded holes 211. As a skilled person will appreciate, the second blade portion threaded holes 221 may be arranged in corresponding clusters, at corresponding locations to the first blade portion threaded holes 211 so as to accommodate the connectors 100 therebetween.

Although a connection including a spar cap, composite slab, insert 212, 222 and connector 100 has been described, the connector 100 may be used without the composite slab, and the insert 212, 222 may be connected directly to the spar cap 48.

Although the tool engagement feature 330 is described as optionally having an octagonal shape, any appropriate shape may be used, such as triangular, square, pentagonal, hexagonal or any other polygonal shape, or a cog shape. Although an example of the connector 100 having 12 springs 400 is given shown in Figure 6, and an example of the connector 100 having 14 springs 400 is shown in Figures 7 and 8, any appropriate number of springs 400 may be provided. For example, one spring or two springs 400 may be provided. The number of springs 400 on the first threaded portion 310 may be equal to a number of springs on the second threaded portion 320. A tailored or optimal number of springs may be provided.

Although an example in which the spring 400 is a Belleville washer has been described, any appropriate spring washer or spring component may be used. For example, the spring 400 may be a helical spring, or a split spring washer. The spring 400 may be a helical spring disposed between two flat washers.

Although an example comprising main spar caps and rear spar caps is provided, it will be apparent that the blade 20 may have only a single spar, two spars, or more than two spars, or the connectors 100 may be positioned away from the spar locations. Instead of the rear spar, a trailing edge stringer or stringers may instead be provided. The trailing edge stringers may be incorporated into the blade portions 22, 24 so as to form part of the outer shell of the blade portions 22, 24. The trailing edge stringers may be formed from pre-cured, solid pultrusion strips for reinforcement or strengthening purposes in similar material and construction to the spar caps described above. The main difference between a stringer and a spar is that the stringer does not have a shear web extending between the shell halves through the hollow interior 44 of the blade. It should be recognised that in alternative examples, the trailing edge stringers may not form part of the outer shell of the blade portions 22, 24, but may engage with an inner surface of the outer shell.

Although examples in which the bearing surface is either integrally formed with the tool engagement feature 330 (as demonstrated by figure 5), or comprised in a stop 340, 350 (as demonstrated in figure 10) are described, any bearing surface configured to react the compression spring force into a tensile force in the bolt may be used.

Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.