GB2572401A - Fastener - Google Patents

Abstract

A fastener 1 for holding together two or more structures comprises a tail 12 configured to engage with at least one of the structures such that relative movement of the fastener and the at least one structure along an axial direction of the fastener is prevented by the engagement; and a head 11. The head 11 comprises a first head part 111 configured for engagement with a first type of installation tool; and a second head part 112. The first head part 111 is detachable from the rest of the fastener 1.

Description

[0001] The present invention relates to fasteners for holding together two or more structures, to assemblies comprising a first structure and a second structure held together by a fastener, and to methods of joining a first structure and a second structure using a fastener.

BACKGROUND [0002] Most conventional aircraft wings have a structural wing box, formed by front and rear spars joined to upper and lower cover panels. It is this wing box which carries the main operational loads experience by the wing. Any structures mounted to such wings (e.g. landing gear, engine mounting pylons) are generally attached to the wing box. Moreover, at least some of the space within the wing box may be used as a fuel tank.

[0003] It may be desirable to form the wing box with few (or no) holes or openings to provide access to the interior space. As such, any fasteners used to close the wing box and to mount structures to the wing box must be suitable for one-sided (or “blind”) installation. Onesided installation means that the fastener is installed entirely from the exterior of the wing box, so that the installation process does not require any access to the interior of the wing box.

SUMMARY [0004] A first aspect of the present invention provides a fastener for holding together two or more structures. The fastener comprises a tail configured to engage with at least one of the structures such that relative movement of the fastener and the at least one structure along an axial direction of the fastener is prevented by the engagement; and a head. The head comprises a first head part configured for engagement with a first type of installation tool; and a second head part. The first head part is detachable from the rest of the fastener.

[0005] Optionally, the fastener is configured to be tightened by rotating the fastener about its axis.

[0006] Optionally, the first head part is detachably connected to the second head part.

[0007] Optionally, the fastener is configured such that the first head part detaches from the rest of the fastener when a predetermined amount of torque is applied to the first head part.

[0008] Optionally, the fastener is configured such that the first head part shears off from the rest of the fastener when the predetermined amount of torque is applied to the first head part.

[0009] Optionally, the fastener comprises a frangible section between the first head part and the second head part, wherein the frangible section is configured to break when the predetermined amount of torque is applied to the first head part.

[0010] Optionally, the predetermined amount of torque is at least 150 Nm.

[0011] Optionally, the first head part has an outer circumferential surface shaped to match an end of the first type of installation tool.

[0012] Optionally, the second head part is between the tail and the first head part.

[0013] Optionally, the second head part is configured for engagement with a second, different type of installation tool.

[0014] Optionally, the second head part comprises an axially-extending recess shaped to match an end of the second type of installation tool.

[0015] Optionally, the second head part has a truncated-conical form such that it is countersinkable into the surface of a structure.

[0016] Optionally, the first head part comprises an axial bore.

[0017] A second aspect of the present invention provides an assembly. The assembly comprises a first structure; a second structure; and a fastener extending through the first structure and the second structure. The fastener comprises a tail and a head. The tail is engaged with at least one of the first and second structures such that relative movement of the fastener and the at least one structure along an axial direction of the fastener is prevented by the engagement. The head is configured for engagement with an installation tool up to a given level of applied torque. The head is flush with a surface of one of the first and second structures and wherein the residual torque on the fastener is greater than the given level of applied torque.

[0018] Optionally, the fastener is a fastener according to the first aspect from which the first head part has been detached.

[0019] A third aspect of the invention provides an assembly comprising a first structure held to a second structure by a fastener according to the first aspect.

[0020] Optionally, each of the structures comprises an aircraft structure.

[0021] Optionally, an external surface of at least one of the first and second structures that is adjacent the head of the fastener is part of an aerodynamic surface of an aircraft.

[0022] Optionally, at least one of the first and second structures is a skin panel for an aircraft wing.

[0023] Optionally, one of the first and second structures comprises a captive nut, and the tail of the fastener is engaged with the captive nut.

[0024] A fourth aspect of the invention provides an aircraft wing comprising the assembly according to the second aspect or the third aspect.

[0025] A fifth aspect of the invention provides a method of joining a first structure and a second structure. The method comprises:

providing a first structure and a second structure which are to be joined by one or more fasteners;

providing a fastener comprising a tail part, a first head part configured to engage with a first type of installation tool, and a second head part disposed between the tail and the first head part;

installing the fastener on the first structure and the second structure by applying a torque to the first head part of the fastener using the first type of installation tool; and detaching the first head part of the fastener from the rest of the fastener when a particular torque is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS [0026] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0027] Figure la is a schematic side view of an example fastener according to the invention;

[0028] Figure lb is a schematic top view of the example fastener of Figure la;

[0029] Figure 2a is a schematic side view of a different example fastener according to the invention;

[0030] Figure 2b is a schematic top view of the example fastener of Figure 2a;

[0031] Figures 3a-d illustrate an example process of installing the example fastener of Figures 2a and 2b;

[0032] Figure 4 is a schematic view of an example aircraft comprising an example assembly according to the invention; and [0033] Figure 5 is a flow chart illustrating an example method of forming an assembly according to the invention.

DETAILED DESCRIPTION [0034] The examples described below relate to a fastener for holding together two or more structures. Each example fastener comprises a tail and a head. The tail is configured to engage with at least one of the structures such that relative movement of the fastener and the at least one structure along an axial direction of the fastener is prevented by the engagement. The head comprises a first head part configured for engagement with a first type of installation tool and a second head part. In each example fastener according to the invention the first head part is detachable from the rest of the fastener.

[0035] The term “installation” tool as used herein is intended to refer to any tool that is engageable with a fastener to install or remove that fastener from a structure, and/or to adjust the tightness of the fastener. For some applications of the invention, a certain type of instahation toot may be used onty to remove fasteners according to the invention (e.g. because a different type of installation tool is used to install the fasteners). A tool which is not used for installing a fastener, but instead is used to remove or adjust the tightness of the fastener is nonetheless considered to be an installation tool for the purposes of this specification.

[0036] Example fasteners according to the invention confer several advantages. Used in conjunction with a captive nut, they can be installed from the head end without requiring access to the tail end - that is, the example fasteners according to the invention are suitable for one-sided (blind) assembly processes. Moreover, as will be explained in more detail in the following description, example fasteners according to the invention can be especially suitable for high torque applications and/or for applications in which a smooth aerodynamic surface is required. Some examples may also be suitable for applications in which it is desirable for the fastener to be removable.

[0037] Figures la and lb show an example fastener 1 according to the invention. Figure la is a side view of the fastener 1 and Figure lb is a top view of the fastener 1 (with respect to the orientation shown in Figure la). The fastener 1 comprises a head 11 and a tail 12. For the purposes of this specification, a head or a head part should be understood as a part of a fastener which prevents movement along the axial direction of the fastener, of a structure on which the fastener is installed. Typically a fastener head may achieve this function by having a larger radius than a tail of the fastener. In the illustrated example the tail 12 is threaded for part of its length, such that it is able to engage with a threaded bore in a structure (e.g. a nut) to prevent relative movement of the fastener 1 and the structure along a direction parallel to the long axis X of the fastener 1. Other examples are possible in which the tail is configured to engage with the structure by any other suitable mechanism, e.g. an interference fit, swaging of the tail, or the like.

[0038] The head 11 comprises a first head part 111 and a second head part 112. The second head part 112 is between the first head part 111 and the tail. The first head part 111 is hatched in the Figures, whilst the second head part 112 is not hatched. In the illustrated example the first head part 111, the second head part 112 and the tail 12 are integrally formed from a single piece of material (e.g. a metallic material), but this need not be the case in all examples. The first head part 111 is detachable from the rest of the fastener 1. Since in the illustrated example the second head part 112 is between the first head part 111 and the tail, in this example the first head part 111 is detachable from the second head part 112. The example fastener 1 is configured to be tightened by rotating the fastener 1 about the axis X. Such rotation may be caused by applying a torque to a head part of the fastener 1 by means of an installation tool. The fastener 1 may be configured such that the first head part 111 detaches from the rest of the fastener 1 when a predetermined amount of torque is applied to the first head part 111. In the illustrated example the fastener 1 is configured such that the first head part 111 shears off from the second head part 112 when a predetermined amount of torque is applied to the first head part 111, as will be explained in more detail below.

[0039] The first head part 111 is configured for engagement with a first type of installation tool (not shown). The first head part may, for example, have an outer circumferential surface shaped to match an end of the first type of installation tool. In the illustrated example, the cross-section of the first head part 111 has a hex shape. In the illustrated example the first head part 111 is solid, and has a smooth top surface, although neither of these features need necessarily be present in other examples. A first type of installation tool has a correspondingly-shaped recess or bore, configured to snugly receive the first head part, such that relative rotational movement between the first head part 111 and the tool (or at least the part of the tool comprising the recess) is substantially prevented. The first type of tool may be, for example, a socket wrench, or any suitable installation tool known in the art. The particular configuration of the first head part 111 may vary from the example of Figure 1. For example, the cross-section of the first head part 111 may have a star shape. It may be desirable for the configuration of the first head part 111 to be the same as or similar to a conventional fastener head configuration, to enable a conventional installation tool to be engaged with the first head part 111. The first head part Illis configured to enable a relatively high level of torque to be applied to the fastener 1 by an installation tool engaged with the first head part 111.

[0040] The second head part 112 of the particular example fastener 1 has a truncatedconical form, as is conventional for countersinkable fasteners. The second head part 112 may thereby be countersunk into the surface of a structure, such that a top surface (that is, the radially-extending surface distal from the tail end of the second head part 112) of the second head part 112 can be flush with the surface of the structure after the fastener 1 has been installed. The second head part 112 is formed integrally with the tail 12.

[0041] Between the first head part 111 and the second head part 112 there is a relatively weak “frangible” section 13 of the fastener 1. The size of the frangible section 113 is exaggerated in Figure la so that it can clearly be seen. The frangible section 13 may be formed integrally with the rest of the fastener 1 and may be weakened, for example, by reducing the amount of material in the frangible section 113 as compared with the immediately adjacent parts of the fastener 1. In the illustrated example, the amount of material is reduced by as a result of the diameter of the fastener being smaller in the frangible section 113 (in other words, a circumferential recess has been formed in the fastener between the first head part 111 and the second head part 112. In the illustrated example, the circumferential recess has the form of a substantially square channel. In other examples, the circumferential recess may comprise a channel of any other shape. In some examples the circumferential recess is formed by the frangible section 13 comprising a tapering section at the base of the first head part, such that an upper end (that is, the end adjacent the first head part 111) of the frangible section 13 has a greater diameter than a lower end (that is, the end adjacent the second head part 112) of the frangible section 13. In some examples the frangible section 13 may comprise radial holes or recesses, a porous material, or a material which is different to and inherently weaker than a material from which the rest of the fastener 1 is formed (e.g. a bonding agent, or the like). Various techniques for forming conventional frangible components (e.g. frangible nuts) are known and may be employed to create the frangible section of the fastener 1.

[0042] The frangible section 13 is configured to break when a predetermined amount of torque is applied to the first head part 111. The predetermined amount of torque may be equal to an amount of torque necessary to install the fastener 1 at a given tightness. The predetermined amount of torque may be determined based on the requirements of a particular application of the fastener 1. In the particular illustrated example, the fastener 1 is configured for high-load applications, and a relatively high level of torque is required to be applied in order to install it at a sufficient tightness. For this particular example, the diameter of the fastener may be at least 12 mm and the predetermined amount of torque may be at least 150 Nm. In other examples the diameter of the fastener may be in the range 2-18 mm and the predetermined amount of torque may be in the range 2-500 Nm. The level of applied torque required to break the frangible section 13 may be tailored by, for example, varying the amount of material in the frangible section 13, and/or varying the type of the material of the frangible section 13. In the illustrated example, the level of applied torque required to break the frangible section 13 is tailored by varying the depth of the circumferential recess.

[0043] The frangible section 13 is configured to break in such a manner that substantially all of the material comprised in the frangible section 13 is removed from the second head part 112. Preferably the frangible section 13 is configured to break in such a manner that the top surface of the second head part 112 (i.e. the surface in which the recess 113 is formed) is substantially smooth. For example, the frangible section 13 may be configured to shear off from the second head part 112. It is advantageous for the detaching of the first head part 111 (by breaking of the frangible section 13) to leave a substantially smooth top surface on the second head part 112 because it means that a continuous smooth surface is formed by the top surface of the second head part 112 and the surrounding surface of a structure on which the fastener 1 is installed. Such a continuous smooth surface is particularly desirable where the structure is an aerodynamic structure, such as an aircraft wing skin panel. [0044] Figures 2a and 2b show a second example fastener 2 according to the invention. Figure 2a is a side view of the fastener 2 and Figure 2b is a top view of the fastener 2 (with respect to the orientation shown in Figure 2a). The fastener 2 comprises a head 21 and a tail

22. The head 21 comprises a first head part 211 and a second head part 212. The parts of the fastener 2 are the same as the equivalent parts of the fastener 1 of Figures la and lb, except that the second head part 212 is configured for engagement with a second type of installation tool (not shown), which is different to the first type.

[0045] In particular, the example second head part 212 shown in Figures 2a and 2b comprises an axially-extending recess 213. The recess 213 is shaped to match an end of the second type of installation tool. In the illustrated example, the recess 213 is cross-shaped as viewed from the head end of the fastener, and has a V-shaped profile in the axial direction. The matching end of the second type of installation tool is configured to be snugly received within the recess 213, such that relative rotational movement between the second head part 212 and the tool (or at least the part of the tool comprising the matching end) is substantially prevented. The second type of tool may be, for example, a screwdriver, or any suitable installation tool known in the art. The particular configuration of the second head part 212 may vary from the example of Figures 2a and 2b. It may be desirable for the configuration of the second head part 212 to be the same as or similar to a conventional fastener head configuration, to enable a conventional installation tool to be engaged with the second head part 212. In some examples the second head part 212 is configured to enable a relatively low level of torque (that is, lower than the level of torque which is able to be applied to the fastener by an installation tool engaged with the first head part 211) to be applied to the fastener 2 by an installation tool engaged with the second head part 212.

[0046] The axially-extending recess 213 becomes exposed, and thereby accessible to a second type of installation tool, by the detachment of the first head part 211 (which occurs in the same manner as detachment of the first head part 111 of the example fastener 1). To ensure that the detachment of the first head part 211 exposes the recess 213, the first head part 211 comprises an axial bore 214. The diameter of the axial bore 214 is at least equal to a largest radial dimension of the recess 213 in the second head part. In the illustrated example, the axial bore 214 extends for the full axial length of the first head part 211, meaning that the recess 213 is visible when the fastener 2 is viewed from the top as in Figure 2b. A bore which extends for the full axial length of the first head part 211 may simplify manufacture of the fastener 2. However; alternative examples in which the axial bore 214 does not extend right to the top (that is, the end distal from the rest of the fastener 2) of the first head part 211 are also within the scope of the invention.

[0047] An example process of installing a fastener according to the invention will now be described with reference to Figures 3a-d and Figure 5. In block 501 of Figure 5, a first structure and a second structure are provided, which are to be joined by one or more fasteners according to the invention. Figure 3a shows a first structure 31 and a second structure 32 which are to be held together (in the arrangement shown in Figure 3a) by one or more example fasteners 2 of the type shown in Figures 2a and 2b. In some examples the first structure 31 and the second structure 32 may be aircraft structures. At least one of the first and second structures 31, 32, may be a skin panel for an aircraft wing. In the illustrated example, the second structure 32 is a skin panel for an aircraft wing and a lower surface of the second structure (with respect to the orientation shown in Figure 2a) is configured to form part of an aerodynamic surface of an aircraft.

[0048] The provided first and second structures 31, 32 comprise a fastener hole 33, which extends through both the first structure 31 and the second structure 32. The fastener hole 33 is configured to receive a fastener 2 of the type shown in Figures 2a and 2b, such that the top surface (with respect to the orientation shown in Figure 2a) of the second head part 212 of a fastener 2 installed in the hole 33 is flush with a lower surface (with respect to the orientation shown in Figure 3a) of the second structure 32. The first structure 31 comprises a captive nut 34. The captive nut 34 is configured to be engageable with the tail 22 of a fastener 2. In the illustrated example, an inner circumferential surface of the captive nut 34 is threaded. The captive nut 34 may be fixedly attached to the first structure 31 by any suitable mechanism (e.g. rivets, bolts, welding, a bonding agent), or may be formed integrally with the first structure 31. The nature of the attachment of the captive nut 34 to the first structure 31 is such that relative rotation of the captive nut 34 and the first structure 31 is substantially or entirely prevented.

[0049] In block 502, a fastener is provided which comprises a tail part, a first head part configured to engage with a first type of installation tool, and a second head part. The second head part is disposed between the tail and the first head part. The provided fastener may be a fastener of the same type as either of the example fasteners 1, 2 described above.

[0050] In block 502 the provided fastener is installed on the first structure and the second structure by applying a torque to the first head part of the fastener using the first type of installation tool. Figure 3b illustrates the situation just before the torque is applied to an example fastener 2. In Figure 3b the tail 22 of the fastener 2 is partly inserted into the fastener hole 33. The insertion movement of the fastener 2 into the hole 33 is represented by the block arrow, and may be driven by an installation tool or manually. Further movement of the fastener 2 in the direction of the block arrow results in the end of the tail 22 contacting the threads on the inner surface of the captive nut 34. At this point, continued progress of the fastener 2 into the fastener hole 33 is achieved by rotating the fastener 3 as indicated by the block arrow in Figure 3c. Rotation of the fastener 2 is achieved by engaging a first type of installation tool (not shown) with the first head part 211 of the fastener 2 as described above. The engaged part of the first installation tool is then rotated in the direction of the block arrow. Rotation of the first installation tool (which may be driven manually, or by any suitable power source such as an electric motor) applies a torque to the first head part 211 and thereby causes the fastener 2 to rotate in the direction of the block arrow. Engagement between the thread on the fastener tail 22 and the thread on the inner surface of the captive nut cap 34 causes the fastener 2 to move further into the fastener hole 33 as it is rotated, until the top surface of the second head part 212 is substantially flush with the lower surface of the second structure 32, as shown in Figure 3c.

[0051] One or both of the axial length of the fastener 2 and the axial length of the recess in the captive nut cap 34 is selected in dependence on the total thickness of the first and second structures when arranged as shown in Figure 3a. In particular, these lengths are selected such that the second head part 212 contacts the lower surface of the second structure 32 (in particular, a recessed part of the lower surface) before the tail end of the fastener tail 22 contacts the base of the recess in the captive nut 34. This allows the fastener 2 to be further rotated once the second head part 212 has contacted the lower surface of the second structure 32, in order to exert a compressive force (or preload) on the first and second structures 31, 32. The magnitude of this compressive force depends on the tightness at which the fastener 2 is installed, which in turn depends on the level of applied torque at which rotation of the fastener 2 is stopped.

[0052] In block 504, the first head part of the fastener is detached from the rest of the fastener when a particular torque is achieved. In some examples (such as the example illustrated by Figures 3a-d) detaching the first head part of the fastener exposes a feature of the second head part of the fastener which enables the second head part to engage with the second type of installation tool. In other examples detaching the first head part of the fastener exposes a smooth surface of the second head part. In the installation process illustrated by Figures 3a-d, the first head part 211 is detached by using the first type of installation tool to apply an increasing level of torque to the first head part 211 until the predetermined amount of torque to break the frangible section 23 of the fastener 2 is reached. At this point the first head part 211 detaches from the rest of the fastener, as shown in Figure 3d. It can be seen from Figure 3d that the detachment of the first head part 211 (and the frangible section 23) exposes the top surface of the second head part 212, and thereby enables access to the recess 213 (e.g. by an installation tool of the second type).

[0053] Although the process 500 has been described mainly with reference to an example fastener of the type shown in Figures 2a and 2b, it may equally be applied, without modification, to an example fastener of the type shown in Figures la and lb.

[0054] The result of the installation process 500 as illustrated by Figures 3a-d is an assembly 3 (shown in Figure 3d), comprising the first structure 31, the second structure 32; and a fastener 2’ extending through the first structure 31 and the second structure 32. The fastener 2’ comprises a tail 22 engaged with at least one of the first and second structures 31, 32 such that relative movement of the fastener 2’ and the at least one structure along an axial direction of the fastener 2’ is prevented by the engagement; and a head 212 configured for engagement with an installation tool. The fastener 2’ is a part of the fastener 2. The tail of the fastener 2’ is the same as the tail of the fastener 2 and the head of the fastener 2’is the second head part 212 of the fastener 2.

[0055] In the particular example, the head of the fastener 2’ is configured to engage with the second type of installation tool (although in other examples, e.g. relating to the example fastener 1, the head of the fastener comprised in the assembly 3 may have a substantially smooth upper surface). The head of the fastener 2’ is flush with the lower surface of the second structure 32. The head of the fastener 2’ is configured for engagement with the second type of installation tool up to a given level of applied torque, where the given level is set by the configuration and material properties of the head of the fastener 2’. If a torque above the given level is applied to the head of the fastener 2’, damage to the head of the fastener 2’ (e.g. deformation of the recess 213) is very likely to occur.

[0056] In examples where the head of the fastener 2’ is configured for engagement with a second type of installation tool, the residual torque on the fastener 2’ may be greater than a residual torque value that could be achieved by applying a torque to the head of the fastener 2’ using the second type of installation tool. This is possible because the residual torque on the fastener 2’ was achieved by applying torque to the first head part 211 of the fastener 2 using the first type of installation tool, and not by applying torque to the head of the fastener 2’ using the second type of installation tool. It is difficult to apply a sufficiently high torque to a countersinkable fastener which is configured to create a flush surface when installed, because of the limitations on the fastener head design this requirement creates. Applying a torque above a certain level to a conventional countersinkable fastener may typically result in damage to the fastener head. Fasteners according to the invention address this issue by enabling both high-torque installation and a flush surface when installed. As such, fasteners according to the invention may be particularly advantageous for certain aerospace applications, as will be described in more detail below. For the particular example of Figure 3d, the diameter of the fastener 2’ may be at least 12 mm and the residual torque on the fastener 2’ may be at least 150 Nm. In other examples the diameter of the fastener may be in the range 2-18 mm and the residual torque may be in the range 2-500 Nm.

[0057] The level of applied torque required to remove a fastener installed to have a given residual torque is generally less than the level of applied torque required to install that fastener to the given residual torque. As such, it is possible to remove the fastener Γ from the first and second structures 21, 22 by applying a torque to the fastener 1’ with the second type of installation tool, engaged with the recess 113 in the head of the fastener 1 ’. The fastener 1 ’ may thereby be removed from the assembly 2 without causing damage to the first structure 21 or the second structure 22. This feature is particularly advantageous for applications in which it may be desired to disassemble the assembly 2, e.g. to repair the assembly 2 or to provide access to a nearby component which requires inspection or repair. This feature may be particularly advantageous for certain aerospace applications, where one or both of the structures 21, 22 may need to be inspected and/or repaired multiple times during the operational life of an aircraft in which the structures are comprised.

[0058] Fasteners according to the invention may advantageously be employed in the aerospace industry, e.g. to facilitate aircraft manufacture. Figure 3 shows an example aircraft 300 which comprises one or more such fasteners. In particular, the aircraft 300 comprises a wing 301, which has a folding wing tip. The wing 301 is configured to fold by rotating about the axis Y. Folding of the wing 301 causes particularly high loads to be transmitted into the wing structure in the region immediately inboard of the folding joint. Consequently, fasteners used to join components of the wing box in this region must be able to withstand significantly higher loads than fasteners elsewhere on the wing box, or on a conventional non-folding wing. At the same time, the heads of these fasteners lie against the aerodynamic surface of the wing 301, so it is desirably for them to be flush with the wing surface. The wing box structure of the wing 301 therefore comprises a plurality of fasteners 1 according to the invention, since such fasteners may be large, may be installed with a high level of applied torque, and create a flush surface after installation. In particular, in the aircraft 300 a plurality of fasteners 1 is used to join a lower cover of the wing 301 to an inboard fitting of the wing-folding mechanism. The lower cover, fitting, and fasteners comprise an assembly, which may have the features of the assembly 2 described above in relation to Figure 2d. The aircraft 300 also includes a further wing, which has the same features and construction as the wing 301.

[0059] In a first block 601, a first component and a second component are provided. The first component and the second component are to be joined by one or more primary fasteners, to form an assembly. The primary fasteners may be configured to handle loads expected to be experienced by the assembly during operation of the assembly or of a device (such as an aircraft) in which the assembly is comprised. The primary fasteners may have the features of any of the example further fasteners described above. The first component and the second component may have the features of any of the example first and second components described above. The first and second components may be aircraft components.

[0060] In block 602, a first bond is formed between the first component and the second component. The first bond is formed by providing a hole through the first component and the second component, inserting a first part of a hollow secondary fastener into a first end of the hole, inserting a second part of the hollow secondary fastener into a second end of the hole, and connecting the first and second parts so as to resist axial separation. The holes may be provided in any suitable manner, such as drilling. The hollow secondary fastener may have the features of any of the example fasteners according to the invention described above. As such, the first pail and second parts may have the features of any of the example first and second bushes described above. Inserting the first and second parts may be performed in any of the manners described above in relation to example first and second bushes. Connecting the first and second parts may be performed in any of the manners described above in relation to the example first and second bushes.

[0061] In an optional block 603, a third component is provided. The third component is to be joined to the first and second components. Also in block 603, a fastener hole is created through the third component coaxial with the hole through the first and second components. The third component may, for example, be a component which is desired to be joined to the assembly formed by the first and second components at a later time. In some examples the first and second components may be components of an aircraft wing box, and the third component may be part of an engine mounting pylon. Providing the third component may comprise arranging the third component adjacent to the first and second components in a selected position, e.g. using jigs and/or clamps. The fastener hole may be formed in any suitable manner. The fastener hole may be formed in the same manner as the holes provided in the first and second components in block 602.

[0062] In block 604, a second bond is formed between the first component and the second component (and, if present, the third component) by installing a primary fastener through the hollow secondary fastener (and, if present, through the hole in the third component). The primary fastener may be installed in any of the manners described above in relation to example further fasteners. The second bond may be relatively stronger than the first bond.

[0063] It may be intended to join the first and second components by multiple primary fasteners. In such cases the method 600 may be performed in respect of each of the multiple secondary and primary fasteners. The method 600 may be performed sequentially, simultaneously, or a combination of both sequentially and simultaneously, until all primary fasteners intended to be used to join the first and second components have been installed.

[0064] Although the invention has been described above with reference to one or more preferred examples or 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.

[0065] Where the term “or” has been used in the preceding description, this term should be understood to mean “and/or”, except where explicitly stated otherwise.

Claims (22)

CLAIMS:

1. A fastener for holding together two or more structures, the fastener comprising:

a tail configured to engage with at least one of the structures such that relative movement of the fastener and the at least one structure along an axial direction of the fastener is prevented by the engagement; and a head comprising:

a first head part configured for engagement with a first type of installation tool;

and a second head part;

wherein the first head part is detachable from the rest of the fastener.

2. A fastener according to claim 1, wherein the fastener is configured to be tightened by rotating the fastener about its axis.

3. A fastener according to claim 1 or claim 2, wherein the first head part is detachably connected to the second head part.

4. A fastener according to any preceding claim, wherein the fastener is configured such that the first head part detaches from the rest of the fastener when a predetermined amount of torque is applied to the first head part.

5. A fastener according to claim 4, wherein the fastener is configured such that the first head part shears off from the rest of the fastener when the predetermined amount of torque is applied to the first head part.

6. A fastener according to claim 5, comprising a frangible section between the first head part and the second head part, wherein the frangible section is configured to break when the predetermined amount of torque is applied to the first head part.

7. A fastener according to any of claims 4 to 6, wherein the predetermined amount of torque is at least 150 Nm.

8. A fastener according to any preceding claim, wherein the first head part has an outer circumferential surface shaped to match an end of the first type of installation tool.

9. A fastener according to any preceding claim, wherein the second head part is between the tail and the first head part.

10. A fastener according to any preceding claim, wherein the second head part is configured for engagement with a second, different type of installation tool.

11. A fastener according to any preceding claim, wherein the second head part comprises an axially-extending recess shaped to match an end of the second type of installation tool.

12. A fastener according to any preceding claim, wherein the second head part has a truncated-conical form such that it is countersinkable into the surface of a structure.

13. A fastener according to any preceding claim, wherein the first head part comprises an axial bore.

14. An assembly comprising:

a first structure;

a second structure; and a fastener extending through the first structure and the second structure;

wherein the fastener comprises:

a tail engaged with at least one of the first and second structures such that relative movement of the fastener and the at least one structure along an axial direction of the fastener is prevented by the engagement; and a head configured for engagement with an installation tool up to a given level of applied torque;

wherein the head is flush with a surface of one of the first and second structures and wherein the residual torque on the fastener is greater than the given level of applied torque.

15. An assembly according to claim 14, wherein the fastener is a fastener according to any of claims 1 to 13 from which the first head part has been detached.

16. An assembly comprising a first structure held to a second structure by a fastener according to any of claims 1 to 13.

17. An assembly according to any of claims 14 to 16, wherein each of the structures comprises an aircraft structure.

18. An assembly according to claim 17, wherein an external surface of at least one of the first and second structures that is adjacent the head of the fastener is part of an aerodynamic surface of an aircraft.

19. An assembly according to claim 17 or claim 18, wherein at least one of the first and second structures is a skin panel for an aircraft wing.

20. An assembly according to any of claims 14 to 19, wherein one of the first and second structures comprises a captive nut, and the tail of the fastener is engaged with the captive nut.

21. An aircraft wing comprising the assembly according to any of claims 14 to 20.

22. A method of joining a first structure and a second structure, the method comprising: providing a first structure and a second structure which are to be joined by one or more fasteners;

providing a fastener comprising a tail part, a first head part configured to engage with a first type of installation tool, and a second head part disposed between the tail and the first head part;

installing the fastener on the first structure and the second structure by applying a torque to the first head part of the fastener using the first type of installation tool; and detaching the first head part of the fastener from the rest of the fastener when a particular torque is achieved.