Attachment system

Abstract

A bolt anchor (110) is arranged for use in a first structure (104) and to secure a bolt (134) which engages a second structure (106), to thereby secure the first structure (104) and the second structu

Description

TECHNICAL FIELD The present disclosure relates to attachment systems. Aspects of the invention relate to a bolt anchor, an attachment system, an assembly and a vehicle. BACKGROUND Conventional attachment systems of a bolt and anchor type for attaching structures together can suffer from drawbacks. As the bolt is tightened into the anchor, an excess of torque force can be generated, which may damage the attachment system and / or surrounding structures. Additionally, where for a given application, multiple attachment systems are required, and yet at least some of the attachment systems used are tailored to specific attachment points, cost and / orthe potential for installation errors (e.g. using the incorrect attachment system for a given attachment point) may be increased. Such tailoring may for instance be introduced in order to address a requirement for different bolt lengths at different attachment points. Addressing such difficulties may be complicated where the attachment system is required to have particular characteristics e.g. blind anchoring and / or a sealing capability. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a bolt anchor, an attachment system, an assembly and a vehicle as claimed in the appended claims. According to an aspect of the present invention there is provided a bolt anchor arranged for use in a first structure and to secure a bolt which engages a second structure, to thereby secure the first structure and the second structure to each other, the bolt anchor comprising an axially extending internal bore defined by a side wall, a first end and a second end and arranged to receive a bolt threaded portion of the bolt, where the second end is enclosed and where the internal bore comprises: an engagement portion; and an axially elongated overrun portion, where the overrun portion is substantially coaxial with the engagement portion and is positioned further than the engagement portion from the first end of the internal bore, the first end being that into which the bolt threaded portion is received, and the side wall of the internal bore has an engagement portion thread in the engagement portion. According to another aspect of the present invention there is provided a bolt anchor arranged for use in a first structure and to secure a bolt which engages a second structure, to thereby secure the first structure and the second structure to each other, the bolt anchor comprising an axially extending internal bore defined by a side wall, a first end and a second end and arranged to receive a bolt threaded portion of the bolt, the internal bore comprising: an engagement portion; and an axially elongated overrun portion, where the overrun portion is substantially coaxial with the engagement portion and is positioned further than the engagement portion from the first end of the internal bore, the first end being that into which the bolt threaded portion is received, and the side wall of the internal bore has an engagement portion thread in the engagement portion, and the side wall of the internal bore in the overrun portion is of a cross-sectional diameter greater than a major thread diameter of the engagement portion thread. Consequently, whilst the engagement portion thread may in use engage with a bolt thread of the bolt threaded portion to secure the bolt in the bolt anchor, the cross-sectional diameter of the side wall of the internal bore in the overrun portion may be sufficient so that where any of the bolt threaded portion enters the overrun portion, there is substantially no engagement of the bolt threaded portion with the side wall in the overrun portion. The bolt anchor may accommodate various penetration extents of the bolt threaded portion into the overrun portion without affecting torque / frictional forces as the bolt is screwed for engagement. This may be achieved by the bolt engaging with the full thread length of the engagement portion thread. Increased torque / frictional forces with greater thread engagement might otherwise cause excessive heating, risking damage to surrounding structures and / or risking damage to the engagement portion thread and / or the bolt thread, potentially compromising the joint provided by the bolt and bolt anchor. The characteristics of the bolt anchor may facilitate its use despite bolt threaded portion penetration extent being unclear and / or in different locations where different bolt threaded portion penetration extents are expected. This may simplify manufacture, inventory management and use as well as potentially accelerate installation and / or reduce the likelihood of installation errors. The enclosing of the second end of the internal bore, opposite to its first end, may be a consequence of the internal bore being a blind bore, or as a result of an end cap being fitted to the second end of the internal bore. The latter may be advantageous as providing a sealing solution where the overrun portion has been created by drilling into the bolt anchor shank from an end of the bolt anchor shank opposite to its interface with the bolt anchor head. Where provided, the end cap may be a push-fit / interference fit in the internal bore. In some embodiments the end cap may be arranged to have a radial diameter substantially no larger than that of the bolt anchor shank. This may be facilitated by the end cap being of an internal push-fit / interference fit type. Providing enclosure without creating a local area of increased cross-sectional diameter beyond that of the bolt anchor shank, may allow for the bolt anchor shank to be inserted and secured in the cavity of the first structure. The enclosing may seal the bolt threaded portion inside the internal bore of the bolt anchor. This may be advantageous to better preserve the bolt and in preserving sealing of the first structure (where indeed it is sealed). The enclosed nature of the second end may be advantageous in preventing the ingress of fluid from an interior of the fist structure into the internal bore and potentially egress of that fluid beyond the bolt anchor (e.g. into one or more surrounding structures and / or an external environment). It may for instance be that at least under certain circumstances, fluid inside of the first structure (which may be a sealed structure) is under pressure, and may therefore be more likely to escape via the bolt anchor if the second end were not enclosed. The bolt anchor may advantageously define and / or maintain and / or clear a path for at least part of the bolt threaded portion and / or enclose the end of the bolt. The axial extent of the overrun portion may be sufficient to fully accommodate the bolt threaded portion under all penetration extents envisaged for a given application. The axial extent of the overrun portion may be at least substantially one quarter or one third or one half the axial extent of the engagement portion thread. In this way the extent of possible penetration variations beyond the engagement portion may be increased. The overrun portion may be unthreaded / smooth and / or of too great a diameter to be tapped by the bolt. The overrun portion may have a substantially consistent cross-sectional diameter. The bolt anchor may be arranged for use in securing a battery / battery pack / battery assembly to an electric or hybrid vehicle. In some embodiments the bolt anchor comprises a bolt anchor shank and a bolt anchor head, where the internal bore passes through both the bolt anchor shank and the bolt anchor head and where the bolt anchor head forms a radially extending ledge for locating the bolt anchor. The radially extending ledge may, in use, overlap and contact a surface of the first structure, thereby at least in part locating the bolt anchor. The first structure may be arranged to receive the bolt anchor shank in a cavity of the first structure through the surface of the first structure. The bolt anchor shank may be axially aligned with respect to the radially extending ledge. The radially extending ledge and / or bolt anchor shank may serve to properly locate the bolt anchor in the first structure. In some embodiments the bolt anchor shank comprises an external shank thread. The external shank thread may, in use, engage a cavity thread in the cavity of the first structure to secure the bolt anchor to the first structure. The cooperating external shank thread and cavity thread may serve to properly secure the bolt anchor in the first structure. The bolt anchor shank thread may be longer than and / or extend axially beyond the engagement portion thread. In some embodiments the bolt anchor head is elongated in the axial direction to form a spacer. The spacer may be arranged to occupy a void or gap between the first structure and the second structure when they are fully secured to each other. The spacer may therefore serve to stabilise an attachment / joint between the first and second structures made by the bolt anchor and bolt. Additionally or alternatively, the spacer may serve to sheath and / or protect the bolt threaded portion in the region of the void or gap. The spacer may be of substantially consistent cross-sectional shape and / or size (e.g. diameter). According to a still another aspect of the present invention there is provided an attachment system comprising the bolt anchor as described previously and the bolt, where the bolt threaded portion is longer than the engagement portion thread. In this way, the bolt threaded portion can reach and engage with the engagement threaded portion with an increased variety of different penetration extents of the bolt threaded portion into the bolt anchor. This may allow securing of the first and second structures despite application specific variations in the distance between a bolting surface of the second structure interfacing with a bolt head of the bolt and the bolt anchor. Further, where the bolt threaded portion penetrates into the overrun portion in one or more of those applications, the engagement area of the engagement portion thread and bolt threaded portion is limited in accordance with the length of the engagement portion (and in particular the length of the engagement portion thread). This in turn limits the torque / frictional forces associated with engaging the bolt with the bolt anchor. The attachment system may be arranged for use in securing a battery / battery pack / battery assembly to an electric or hybrid vehicle. In some embodiments the attachment system comprises a joint stabiliser defining an axially extending bolt passage arranged to receive the bolt threaded portion to pass therethrough, the joint stabiliser having radial outwardly extending bracing flanges at its ends and the bolt threaded portion being of sufficient axial length to at least pass through the bolt passage, enter the internal bore and engage the engagement portion thread. Advantageously, the joint stabiliser may stabilise the joint where there is a cavity in the second structure through which the bolt passes. It may be for instance that in use, the radial outwardly extending bracing flanges abut opposing surfaces of the second structure, where the opposing surfaces define a cavity in the second structure. The second structure may for instance be a vehicle body in white and the joint stabiliser may for instance fill a cavity in the vehicle body in white. The bolt threaded portion may be of sufficient axial length to engage a pre-determined proportion of the axial length of the engagement portion thread and optionally to extend beyond the engagement portion thread into the overrun portion. Respective walls of the second structure having the opposing surfaces may have through bores therethrough in order to permit the passage of the bolt threaded portion. A bolt head of the bolt may be arranged to engage with a bolting surface of the second structure to provide a clamping force to aid in securing the first and second structures to each other. The bolting surface may be a surface of the one ofthe walls having the through bores that is furthest from the first structure. Further, the bolting surface may face in substantially the opposite direction to the one ofthe opposing surfaces that forms part ofthe same wall as the bolting surface. In some embodiments the attachment system comprises an end cap arranged to be fitted to and seal the second end ofthe internal bore ofthe bolt anchor. The end cap may seal the bolt threaded portion inside the internal bore of the bolt anchor. This may be advantageous in better preserving the bolt and in preserving sealing ofthe first structure (where indeed it is sealed). According to a further aspect ofthe present invention there is provided an assembly comprising the bolt anchor as described previously, the first structure and the second structure. In some embodiments the assembly comprises multiple substantially identical instances ofthe bolt anchor, each provided in the first structure at respective attachment points, where a first ofthe attachment points is associated with a first distance between a first bolting surface ofthe second structure and the respective bolt anchor when the first and second structures are fully secured to each other; and a second of the attachment points is associated with a second distance between a second bolting surface ofthe second structure and the respective bolt anchor when the first and second structures are fully secured to each other, and where the first and second distances are different. Consequently, the assembly may be arranged for use in an application where the penetration extents ofthe bolt threaded portions into at least some ofthe respective bolt anchors will vary in view of distance variation between a bolting surface ofthe relevant part ofthe second structure engaging with the bolt head ofthe bolt and the respective bolt anchor. The design ofthe bolt anchor may facilitate commonality in its form for use in such an application despite this variation in bolt threaded portion penetration extent in use. The commonality may simplify manufacture, inventory management and use as well as potentially accelerate installation and / or reduce the likelihood of installation errors. As will be appreciated, the first and second bolting surfaces may in use be engaged by respective bolt heads of respective bolts to provide clamping force. The first and second structures may be fully secured to each other at least in part in accordance with respective bolts being secured in and by the respective bolt anchors and via the clamping force generated by the bolt heads at the first and second bolting surfaces. The first and second bolting surfaces may be different parts of the same surface. The bolt anchors may in particular have substantially the same dimensions e.g. at least one, some or all of the following in any combination: o the same axial length of the engagement portion thread; o the same axial length of the spacer; o the same axial length of the overrun portion; o the same bolt anchor shank length; o the same internal bore length; and o the same external shank thread length. It may be that a specific number of bolts are required for a given application (e.g. designed securing / installation of the first structure to the second structure). Consequently, a corresponding number of bolt anchors may be provided. In some embodiments the assembly comprises a corresponding number of the bolts to the number of bolt anchors and the bolts are substantially identical. The bolts may in particular have substantially the same dimensions e.g. at least one, some or all of the following in any combination: o the same axial length of the bolt threaded portion; o the same axial length of the bolt head; and o the same overall length of the bolt. The assembly may be arranged for use in an application where the penetration extents of the bolt threaded portions into the respective bolt anchors vary in view of distance variation between a bolting surface of the relevant part of the second structure engaging with the bolt head of the bolt and the respective bolt anchor. In some embodiments the assembly comprises multiple joint stabilisers, each arranged to define an axially extending bolt passage for the bolt threaded portion of a given instance of the bolts to pass therethrough and having radially outward extending bracing flanges at its ends arranged to abut opposing surfaces ofthe second structure, where the opposing surfaces define a cavity in the second structure and where further at least two ofthe joint stabilisers have different axial lengths between their respective bracing flanges. Each joint stabiliser may be intended for use at a specific attachment point between the first and second structures to which it is suited i.e. given the distance between the opposed surfaces ofthe second structure and / or given the distance between a bolting surface ofthe relevant part ofthe second structure engaging with the bolt head ofthe bolt and the respective bolt anchor at the specific attachment point. In some embodiments the assembly may comprise end caps in number corresponding to the number of bolt anchors. According to a still further aspect of the present invention there is provided a vehicle comprising the attachment system as previously described, a first structure and a second structure wherein the second structure is a body of the vehicle and the first structure is attached to the body of the vehicle at least in part by the attachment system. The attachment system may be suited to a vehicle application given that it can simplify manufacture, inventory management and use as well as potentially accelerate installation and / or reduce the likelihood of installation errors. In some embodiments the vehicle comprises at least one additional instance of the attachment system previously described, each used at respective additional attachment points by which the first structure and the body of the vehicle are further secured to each other, where the first attachment point is associated with a first distance between a first bolting surface of the body of the vehicle engaged by a bolt head of the bolt to provide a clamping force and the bolt anchor when the first structure and body of the vehicle are fully secured to each other, and where at least one of the additional attachment points is associated with a second distance between a second bolting surface of the body of the vehicle engaged by a bolt head of the respective bolt to provide a clamping force and the respective bolt anchor when the first structure and body of the vehicle are fully secured to each other and where the first and second distances are different. The first and second distances may be determined by the design / geometry of the body of the vehicle and / or the first structure at the relevant attachment point. Despite the difference between the first and second distances, the nature of the attachment system may mean that substantially identical bolt anchors and bolts may be used and may be used for all attachment points. The commonality may simplify manufacture, inventory management and use as well as potentially accelerate installation and / or reduce the likelihood of installation errors. As will be appreciated, there may be three or more different distances between a respective bolting surface of the body of the vehicle engaged by a bolt head of the respective bolt to provide a clamping force and the respective bolt anchor (and substantially identical bolt anchors and bolts used for all). As will be appreciated, the first and second bolting surfaces may be different parts of the same surface. In some embodiments the first structure is a sealed structure. The bolt anchor may be particularly suited to applications where the first structure is sealed, because it may provide a mechanism for securing the first structure and the vehicle body to each other without a need to be able to access a nut for tightening onto the bolt. Additionally or alternatively, when sealed, the bolt anchor may provide a mechanism to preserve the sealing of the first structure by retaining the bolt threaded portion (regardless of its penetration extent beyond the engagement portion thread) in the internal bore. The cavity / bolt anchor may penetrate the sealed structure. In some embodiments the first structure is a traction battery assembly of the vehicle. The vehicle may for instance be an electric or hybrid vehicle and may be powered at least in part by the battery assembly. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in anyway and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a perspective view of a vehicle according to an embodiment of the invention; Figure 2 shows perspective views of a bolt anchor according to an embodiment of the invention; Figure 3 shows a cross-sectional view of an attachment system according to an embodiment of the invention; and Figure 4 shows a cross-sectional view of multiple attachment systems according to an embodiment of the invention. DETAILED DESCRIPTION A vehicle (in this case an electric vehicle) in accordance with an embodiment of the present invention is described herein with reference to the accompanying Figures. The vehicle is shown generally at 100 and uses multiple instances of substantially identical attachment systems 102 to secure a first structure (in this case a traction battery assembly 104) of the vehicle 100 to a second structure (in this case a vehicle body 106 of the vehicle 100). Each attachment system 102 is provided at a corresponding specific attachment point 108 between the traction battery assembly 104 and the vehicle body 106 (see Figures 3 and 4). Considering a single example instance of the attachment system 102, reference is made to Figures 2 and 3. The attachment system 102 has a bolt anchor 110 (in the example shown, a spacer bolt). The bolt anchor 110 has an axially extending bolt anchor shank 112, a bolt anchor head 114 and an axially extending internal bore 116 passing through the shank 112 and head 114. The shank 112 has an external shank thread 118 used to secure the bolt anchor 110 into the battery assembly 104. This is achieved through engagement of the external shank thread 118 with a complementary cavity thread 120 in a cavity 122 of the battery assembly 104. The cavity 122 extends through a top surface 124 of, and into, the battery assembly 104, at a location corresponding to a specific one of the attachment points 108. The bolt anchor head 114 is provided at one end of the shank 112. The bolt anchor head 114 has a radially extending ledge 126 arranged to clamp the top surface 124 of the battery assembly 104 when the bolt anchor 110 is fully engaged (screwed into and torqued) with the cavity thread 120, thereby locating the bolt anchor 110 with respect to the battery assembly 104. Prior to engaging the bolt anchor 110 with the cavity thread 120, a sealant may be applied between the radially extending ledge 126 and the top surface 124 to provide a seal therebetween. The bolt anchor head 114 is elongated in the axial direction away from the radially extending ledge 126 and step 112 to form a spacer 128 that in this case is cylindrical in shape. In use, the spacer 128 occupies (in a region of the relevant attachment point 108) a gap 130 that exists between the traction battery assembly 104 and the vehicle body 106 when they are fully secured to each other. The bolt anchor head 114 has a drive face 131a (top of the spacer 128) formed with a drive feature, in this case a hexagonal drive feature 131b. In use, the hexagonal drive feature 131b is used to rotate (screw) the bolt anchor 110 for engagement and torquing of the shank thread 118 with the cavity thread 120. As will be appreciated, alternatives are possible (e.g. the bolt anchor head 114 being arranged to serve as a bolt head having, for instance, a hexagonal shape). The axially extending internal bore 116 passes through the bolt anchor 110 in the axial direction, consequently passing through both the bolt anchor head 114 and the shank 112. A first end 132 of the internal bore 116 (through which a bolt 134 enters when securing the traction battery assembly 104 to the vehicle body 106) defines an opening into the bolt anchor 110 via the bolt anchor head 114. A second end 136 of the internal bore 116 defines an opening into bolt anchor 110 via the shank 112. The internal bore 116 also has a side wall 138, which extends between the first 132 and second 136 ends. The internal bore 116 consists of two portions, an engagement portion 140, which extends from and is nearer to the first end 132 and an axially elongated overrun portion 142, which extends from and is nearer to the second end 136. The engagement portion 140 has an engagement portion thread 144 in the side wall 138. In use, the engagement portion thread 144 engages a bolt threaded portion 146 of the bolt 134. In this manner, the bolt 134 is retained by the bolt anchor 110. The overrun portion 142 is coaxial with the engagement portion 140, but is unthreaded. Additionally, the side wall 138 of the internal bore 116 in the overrun portion 142 is of a cross-sectional diameter greater than a major thread diameter 148 of the engagement portion thread 144. Consequently, any of the bolt threaded portion 146 which, whilst being screwed into the bolt anchor 110, extends beyond the engagement portion thread 144 into the axially elongated overrun portion 142, does not engage with the side wall 138 in the axially elongated overrun portion 142. The battery assembly 104 is a sealed structure. That is, it comprises a sealed outer casing forming a chamber in which battery cells of the battery assembly 104 are contained. In the present embodiment however, the cavity 122 penetrates the battery assembly 104 (that is, it penetrates the sealed outer casing) at the relevant attachment point 108. Further, the shank 112 of the relevant bolt anchor 110 also penetrates the battery assembly 104. This has the potential to compromise the sealing of the battery assembly 104, which might in principle allow the escape of material from the battery assembly 104 (e.g. cell venting gasses). However, the second end 136 of the internal bore 116 is sealed by an end cap 147 (stopper) as shown in Figure 2, thereby assisting in preserving the sealed nature of the battery assembly 104. The end cap 147 is a push-fit into the second end 136. In addition, sealant is provided at and / or around at least part of the interface of the end cap 147 and the second end 136 to thereby contribute to preserving the sealed nature of the battery assembly 104. The sealant between the radially extending ledge 126 and the top surface 124 may also assist in preserving the sealed nature of the battery assembly 104. The vehicle body 106 in this example, has a double skin wall 152 of first 154 and second 156 walls at the attachment point 108. The first 154 and second 156 walls are separated by a cavity 158 at the attachment point 108. Through bores 160 are provided through the first 154 and second 156 walls in order to permit the passage of the bolt threaded portion 146 through the double skin wall 152 for engagement with the bolt anchor 110. The first 154 and second 156 walls run, at the attachment point 108, in respective planes that are substantially normal to the intended penetration direction of the bolt threaded portion 146 when it passes through the through bores 160 and is engaged in the bolt anchor 110 via the engagement portion thread 144. An upper surface of the first wall 154 (facing away from the battery assembly 104 when it is secured to the vehicle body 106), provides a bolting surface 162 which may be engaged by a bolt head 164 of the bolt 134 (or as in this case via an intermediate washer 165) to provide a clamping force securing the battery assembly 104 to the vehicle body 106, A lower surface of the second wall 156 (facing the battery assembly 104 when it is secured to the vehicle body 106), provides an abutment face 166 towards which the drive face 131a of the bolt anchor 110 is clamped when the bolt 134 is fully engaged. In some embodiments as shown, a clip-on gasket 169 is provided between the abutment face 166 and drive face 131a for sealing. As will be appreciated, the gasket 169 might in other embodiments be omitted e.g. such that the abutment face 166 and drive face 131a directly contact. More generally, gaskets and / or washers may optionally be used at various suitable locations as part of the attachment system 102. The cavity 158 is defined between opposing surfaces 170 of the first 154 and second 156 walls. Located within the cavity 158 in this example is a joint stabiliser 172. The joint stabiliser 172 has a substantially cylindrical main body 174 having an axially extending bolt passage 176 passing therethrough. At the ends of the main body are radially outwardly extending bracing flanges 178, each of which is aligned with and abuts, in use, a respective one of the opposing surfaces 170. In addition, the joint stabiliser 172 is positioned so that its axially extending bolt passage 176 is aligned with the through bores 160. The joint stabiliser 172 is sized (e.g. the length of its cylindrical main body 174) to suit the spacing between the opposing surfaces 170 at the attachment point 108. In use, the joint stabiliser 172 serves to limit or prevent deflection of the first 154 and / or second 156 walls as a result of force exerted by the engaged bolt 134. The joint stabiliser 172 also in use stabilises the attachment in the region of the cavity 158, countering any shear force which may occur. At one or more of the other attachment points and / or in other embodiments, it will be appreciated that there may be a single skin wall or an alternative such as a box section (rather than the double skin wall 152). Optionally, the joint stabiliser 172 may be omitted (e.g. in the case of a single skin wall). In use, the bolt anchor 110, with its end cap 147 in place, is secured into the battery assembly 104 at the attachment point 108. The is achieved via rotating (screwing) the bolt anchor 110 such that the external shank thread 118 engages and progressively screws into the cavity thread 120. This is continued until the radially extending ledge 126 engages the top surface 124 of the battery assembly 104 and is torqued to a predetermined rating. The bolt threaded portion 146 is passed through the through bores 160 and bolt passage 176 before entering the internal bore 116 of the bolt anchor 110 where it meets the engagement portion thread 144. The bolt is rotated (screwed) such that the bolt threaded portion 146 engages and progressively screws into the engagement portion thread 144. This is continued until the bolt head 164 of the bolt 134 engages the bolting surface 162 and is torqued to a pre-determined rating. Any of the bolt threaded portion 146 that extends beyond the engagement portion thread 144 of the engagement portion 140, enters the overrun portion 142 and so, engaging nothing there, does not increase the torque experienced by the bolt 134 as it is rotated. This means that the attachment system 102 can accommodate various penetration extents of the bolt threaded portion 146 into the overrun portion 142 without affecting torque / frictional forces as the bolt 134 is screwed for engagement. Additionally, securing the battery assembly 104 to the vehicle body 106 can be achieved without access to a nut for the securing of a bolt 134. This may be desirable / necessary where for instance it is desired to provide for an attachment point inboard of an edge of a sealed structure e.g. the battery assembly 104. Referring now to Figure 4, the above description is representative of each attachment system 102 contributing to securing the battery assembly 104 to the vehicle body 106 at each attachment point 108. Indeed, it is envisaged that the bolt anchor 110 and bolt 134 of each of the attachment systems 102 are substantially identical. However, the circumstances presented by the vehicle body 106 at different of the attachment points 108 are different, which the bolt anchor 110 and bolt 134 are able to accommodate as explained further below. At different of the attachment points 108, the distance between the opposing surfaces 170 varies. This is due to the design of the double skin wall 152 giving rise to local variations in the separation and / or thicknesses of the first 154 and second 156 walls. At different of the attachment points 108, respective instances of the joint stabiliser 172 are therefore sized (axial length between its bracing flanges 178) to suit the size of the cavity 158 created by the fist 154 and second 156 walls. Additionally, the different distances between the opposing surfaces 170 gives rise to different distances between the bolting surface 162 and the respective bolt anchor 110. With the use of bolts 134 having consistent bolt threaded portion 146 lengths (as here), this means that the penetration extents of the bolt threaded portions 146 into their respective bolt anchors 110, will be different. Given the nature of the attachment system 102, this however is accommodated without impacting on the torque imparted during the process of engaging the bolt 134 with the respective bolt anchor 110. Specifically, given that any ‘over penetration’ of a given bolt threaded portion 146 will simply be accommodated in the relevant overrun portion 142, torque is not increased by this ‘over penetration’. In order to satisfy the requirements of attaching the particular battery assembly 104 to the particular vehicle body 106, the axial length of the overrun portion 142 is selected to be at least the 5 length of the variation in bolt 134 penetration extent that will occur across all of the attachment points 108 given the differences in distances between the opposing surfaces 170. Further, in order to ensure securing of the battery assembly 104 to the vehicle body 106 at each attachment point 108, the axial extent of the engagement portion thread 144 may be selected so as to be substantially sufficient such that a pre-determined proportion of its thread is engaged by the engagement portion thread 144 of the bolt that has a minimal (e.g. 10 the minimum) penetration extent into its respective bolt anchor 110, given the differences in distances between the opposing surfaces 170. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. 15

Claims

1. A bolt anchor arranged for use in a first structure and to secure a bolt which engages a secondstructure, to thereby secure the first structure and the second structure to each other, the bolt anchor comprising an axially extending internal bore defined by a side wall, a first end and a second end and arranged to receive a bolt threaded portion of the bolt, where the second end is enclosed and where the internal bore comprises:an engagement portion; andan axially elongated overrun portion,where the overrun portion is substantially coaxial with the engagement portion and is positioned further than the engagement portion from the first end of the internal bore, the first end being that into which the bolt threaded portion is received, andthe side wall of the internal bore has an engagement portion thread in the engagement portion, andthe side wall of the internal bore in the overrun portion is of a cross-sectional diameter greater than a major thread diameter of the engagement portion thread.

2. The bolt anchor according to claim 1 comprising a bolt anchor shank and a bolt anchor head,where the internal bore passes through both the bolt anchor shank and the bolt anchor head and where the bolt anchor head forms a radially extending ledge for locating the bolt anchor.

3. The bolt anchor according to claim 2 where the bolt anchor shank comprises an.

4. The bolt anchor according to claim 2 or claim 3 where the bolt anchor head is elongated in theaxial direction to form a spacer.

5. An attachment system comprising the bolt anchor according to any of claims 1 to 4 and thebolt, where the bolt threaded portion is longer than the engagement portion thread.

6. The attachment system of claim 5 comprising a joint stabiliserdefining an axially extending bolt passage arranged to receive the bolt threaded portion to pass therethrough, the joint stabiliser having radial outwardly extending bracing flanges at its ends andthe bolt threaded portion being of sufficient axial length to at least pass through the bolt passage, enter the internal bore and engage the engagement portion thread.

7. The attachment system of claim 5 or claim 6 comprising an end cap arranged to be fitted toand seal the second end of the internal bore of the bolt anchor.

8. An assembly comprising the bolt anchor according to any of claims 1 to 4, the first structureand the second structure.

9. The assembly of claim 8 comprising multiple substantially identical instances of the boltanchor, each provided in the first structure at respective attachment points, wherea first of the attachment points is associated with a first distance between a first bolting surface of the second structure and the respective bolt anchor when the first and second structures are fully secured to each other; anda second of the attachment points is associated with a second distance between a second bolting surface of the second structure and the respective bolt anchor when the first and second structures are fully secured to each other, andwhere the first and second distances are different.

10. The assembly of claim 9 comprising a corresponding number of the bolts to the number of boltanchors and the bolts are substantially identical.

11. The assembly of claim 9 or claim 10 comprising multiple joint stabilisers, each arranged todefine an axially extending bolt passage for the bolt threaded portion of a given instance of the bolts to pass therethrough and having radially outward extending bracing flanges at its ends arranged to abut opposing surfaces of the second structure, where the opposing surfaces define a cavity in the second structure and where further at least two of the joint stabilisers have different axial lengths between their respective bracing flanges.

12. A vehicle comprising the attachment system of any of claims 5 to 7, a first structure and asecond structure wherein the second structure is a body of the vehicle and the first structure is attached to the body of the vehicle at least in part by the attachment system.

13. The vehicle according to claim 12 comprising at least one additional instance of the attachmentsystem of any of claims 6 to 8 each used at respective additional attachment points by which the first structure and the body of the vehicle are further secured to each other,where the first attachment point is associated with a first distance between a first bolting surface of the body of the vehicle engaged by a bolt head of the bolt to provide a clamping force and the bolt anchor when the first structure and body of the vehicle are fully secured to each other, andwhere at least one of the additional attachment points is associated with a second distance between a second bolting surface of the body of the vehicle engaged by a bolt head of the respective bolt to provide a clamping force and the respective bolt anchor when the first structure and body of the vehicle are fully secured to each other andwhere the first and second distances are different.

14. The vehicle according to any of claim 12 or claim 13 where the first structure is a tractionbattery assembly of the vehicle.15

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