GB2516835A

GB2516835A - Cap to accommodate washers

Info

Publication number: GB2516835A
Application number: GB1313669.2A
Authority: GB
Inventors: Richard Dobbin; Anthony Dowson
Original assignee: Airbus Operations Ltd
Current assignee: Airbus Operations Ltd
Priority date: 2013-07-31
Filing date: 2013-07-31
Publication date: 2015-02-11
Anticipated expiration: 2033-07-31
Also published as: EP3027917A1; EP3027917B1; GB201313669D0; CN105408642A; US20160159493A1; GB2516835B; WO2015015153A1; US9764854B2

Abstract

A cap 140 includes a body 141, a cavity to enclose the end 120 of a fastener, and a cured sealing material 150, the fastener head 122 and the cap body 141 having interlocking features in the form of two or more first locking features 143, axially spaced apart by a pitch P, and one or more second locking features 123. In use the distance D between the fastener head 122 and an associated structure 110 is an integer multiple of the pitch P between the first locking features 143 such that the cap can be used with fasteners that may include or exclude one or more washers 130. The first and second locking features 143, 123 may comprise protrusions and recesses. There may be an interference fit between at least one of the protrusions and the recess in which it is received. A plurality of vanes 144 may protrude from the cap body and may include the first or second locking features 143, 123. The cap 140 may be formed from inner and outer cap members 141, 142 with the cured sealing material 150 injected therebetween via a nozzle connector 147 and sealing the cap 140 to the structure 110. The cap is suitable to suppress sparks or lightning especially in a joint 100 of a composite or metal aircraft panel. The fastener may be a nut and bolt, a rivet or a swaged fastener.

Description

CAP TO ACCOMMODATE WASHERS

FIELD OF THE INVENTION

The present invention relates to a joint comprising a cap forming a sealed cavity around an end of a fastener with one or more washers, and a kit of parts and method of installation for forming such a joint.

BACKGROUND OF THE iNVENTION

Figure 1 is a side view of part of a fastener assembly passing through a panel I, which may be a composite or metallic panel. The assembly comprises a fastener comprising an externally threaded bolt 2, an internally threaded nut 3, and a washer 4 (the fastener may alternatively comprise any other known fastener type, such as a rivet or swage fastener). In the event of a lightning strike hitting the panel I and attaching to thc fastener, sparking, plasma or out-gassing may occur at the locations indicated by reference 5 in Figure I. The panel I may provide a fuel tank boundary and the fastener may therefore be immersed in fuel. A lightning strike at the fastener may thus provide sparking and hot gas ignition sources which could cause ignition of the fuel.

A known method of providing spark suppression is described in EP-A-033401 1. A volume of gas is enclosed by a cap around the fastener. The gas provides spark suppression for arcing that may occur between the composite structure and the metal fastener during any lightning strike.

SUMMARY OF THE INVENTION

It is often necessary to install a spark and!or plasma out-gassing suppression cap on fastener heads both with and without washers. A single piece of structure, such as an aircraft panel, may include hundreds of fasteners, some with washers and some without, and it is usually impractical to install a different type or size of cap on those fastener heads with washers than on those without. In general terms, the invention proposes a joint in which a spark suppression cap and fastener head have locking features which enable them to be attached to one another, both when a washer is installed under the fastener head and when there is no washer.

A first aspect of the invention prevides a joint comprisin. a structure; a fastener passing thmugh the structure with an end of the fastener protruding from the structure, the end of the fastener comprising: an axially extending shaft, a fastener head carried by the shaft, and one or more washers located between the fastener head and the structure, the one or more washers spacing the fastener head and the structure apart by a distance; a cap having a cap body with a cavity which cncloscs the end of the fastener; and a cured sealing material which seals the cavity, wherein: one of the fastener head and the cap body is formed with two or more first locking features which are axially spaced apart by a pitch, and the other of the cap body and the fastener head is formed with one or more second locking features which are each capable of interlocking with one of the first locking features, at least one of the second locking features is interlocked with a respective one of the first locking features to secure the cap to the fastener head; and the distance between the fastener head and the structure is an integer multiple of the pitch between the first locking features.

Thc joint can thus easily bc reconfigured to cxcludc thc onc or morc washers, without any modification of the cap or other fastener parts. That is, since the pitch of the axially spaced first locking features is an integer factor of the thickness of the one or more washers (i.e. the distance between the fastener head and the structure), by interlocking the at least one second locking feature with a different respective one of the first locking features the axial position of the cap relative to the fastener head can be altered by an amount corresponding to that washer thickness. In this way, the cap of the invention can be installed on fastener ends with and without washers.

This arrangement simplifies manufacture of an assembly comprising multiple fasteners since it is not necessary to select one type of cap for fastener ends with washers and a different type of cap for fastener ends without washers.

The interlocked first and second locking features primarily serve to secure the cap to the fastener head both during and after assembly. For example, in embodiments in whichthesealingmaterialnjectedintothecapafteritisinstalledonthefastener head the sealing material willtendto force the cap away fit,mthe fastener head, with a typical load of 5-30kg being required to prevent the cap becoming detached. The interlocked first and second locking features thus avoid thc installer needing to apply such a force to hold the cap in place during installation. They also may prevent any undesirable lifting of the cap away from the fastener head which could allow sealing material to enter the cavity, thus compromising the lightning strike performance, and keep the cap in place during curing of the sealing material, even when the cap is inverted or when it is accidentally knocked. Undesirable lifting of the cap away from the fastener head may be caused by hydraulicing of the sealing material, particularly where the sealing material is located between inner and outer cap members.

The scaling material may comprise a material with adhesive properties and may adhere the cap to the structure and/or the fastener end. The interlocking first and second locking features may thus provide a secondary means of attachment between the cap and the fastener head in the event that the sealing material bond fails.

The first and second locking katures may comprise any mechanical locking features which can interact to prevent relative axial movement between the cap and the fastcncr cnd. In a prcfcrrcd cmbodimcnt cach of either the first locking features or thc second locking features comprises a protrusion and each of the other of the first locking features and the second locking features comprises a recess, and at least one of the protrusions is received within a respective one of the recesses to secure the cap to the fastener head. Such protrusions and recesses are straightforward to manufacture.

There is preferably an interference fit between said at least one of the protrusions and the recess within which it is received. This arrangement provides a particularly robust interconnection between the cap and the fastener end.

In some embodiments the fastener head or cap body is formed with two or more second locking features, the other of the fastener head or cap body is finned with three or more first locking features, and at least two of the second locking features are interlocked with a respective two of the first locking features. Such dual interlocking provides a particularly stung interconnection between the cap and fastener end.

The cap may comprise a plurality of vanes protruding from the cap body into the cavity, the vanes each being formed with either the two or more first locking features or the one or more second locking features. The vanes thus serve to locate the cap on the fastener end and provide the means of interconnection between them.

A second aspect of the present invention provides a method of installing a cap on an end of a fastener protruding from a structure through which the fastener passes, wherein: the end of the fastener comprises an axially extending shaft, a fastener head carried by the shaft, and one or more washers located between the fastener head and the stnieture, the one or more washers spacing the fastener head and the stnicture apart by a distance; the cap has a cap body with a cavity; one of the fastener head and the cap body is formed with two or more first locking features which are axially spaced apart by a pitch, and the other of the cap body and the fastener head is formed with one or more second locking features which are each capable of interlocking with one of the first locking features; and the distance between the fastener head and the structure is an integer multiple of the pitch between the first locking features, the method including the steps of: installing the cap on the end of the fltstcner such that the cavity encloses the end of the fastener and at least one of the second locking features is interlocked with a respective one of the first locking features; and applying a sealing material to seal the cavity, and curing the sealing material.

A third aspect of the invention provides a kit of parts for forming ajoint, including: a fastener for passing through a structure, an end of the fastener comprising an axially extending shaft, a fastener head canied by the shaft, and one or more washers fbr locating between the fastener head and the structure, the one or more washers having a thickness configured to space the fastener head and the structure apart by a distance; a cap having a cap body with a cavity configured to enclose the end of the fItstener and a curable sealing material for sealing the cavity, wherein: one of the fastener head and the cap body is formed with two or more first locking features which are axially spaced apart by a pitch, the other of the cap body and the fastener head is formed with one or more second locking features which are each capable of interlocking with one of the first locking features; each of the one or more second locking features is configured to interlock with a respective one of the two or more first locking features to secure the cap to the fastener head; and the distance between the fastener head and the structure is an integer multiple of the pitch between the first locking features.

Any of the optional, or desirable, features discussed above or below in relation to any of the aspects of the invention may be applied to any other aspect, either individually or in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a side view of a prior art fastener assembly; Figure 2 is a partial cross-sectional view of a joint according to a first embodiment of the invention, takcn at the plane indicated by the line A-A in Figure 4; Figure 3 shows the partial cross-sectional view of the joint of Figure 2, but with the washer removed; Figure 4 is a transverse cross-sectional view of the joint of Figure 2; Figure 5 is a partial cross-sectional view of a joint according to a second embodiment of the invention; and Figure 5 shows the partial cross-sectional view of the joint of Figure 5, but with the washer removed.

DETAILED DESCRIPTION OF EMBODIMENT(S)

A joint 100 shown in Figure 2 comprises a structure 110 such as an aircraft skin panel, which in this embodiment is a composite aircraft structural component, but may be a metallic component or a hybrid composite-metallic component. An end 120 of a fastener protrudes from the structure. The fastener comprises an axially extending shaft 121 which passes through the structure 110, and a fastener head 122. In this case the fastener head 122 is a nut screwed onto the shaft 121, although it may alternatively be a bolt head carried by the shaft. A washer 130 is located between the fastener head 122 and the structure 110 and spaces the fastener head and the structure apart by a distance D. Figure 3 shows the joint 100 without the washer 130 fitted.

A cap 140 encloses the end of the fastener. The cap comprises an inner cap member 141 and an outer cap member 142. A cured sealing material 150 fills the void between the inner and outer cap members and adheres the cap 140 to the structure 110. The sealing material 150 also serves to bond the inner cap member 141 and outer cap member 142 together, and adds structural rigidity to the cap 140. The inner and outer cap members 141, 142 are injection moulded from a thermoplastic material such as glass-filled polyetherimide (PEI). A suitable glass-filled PEI is Ultem'4 2400, which includes 40% glass fibres by volume. The inner and outer cap members may alternatively be made by moulding, by an additive manufacturing process, or by any other suitable process.

The inner cap member 141 is a generally thin-walled dome-shaped member, with a correspondingly dome-shaped inner air cavity which encloses the fastener end 120.

The inner cap member 141 is generally made up of a base cylindrical portion and an upper dome portion. Six identical vanes 144 protrude into the cavity from the base cylindrical portion, the vanes 144 being generally elongate and extending substantially in the axial direction of the fastener end 120. The vanes 144 abut the fastener head 122 to ensure the inner cap member 141 is correctly located coaxial with the fastener end 120.

Each vanc 144 is formcd with three rcccsscs 143 which arc axially spaccd apart by a pitch P. The fastener head is formed with two protrusions 123, each being received within a respective one of the recesses 143 to secure the cap to the fastener head and prevent relative axial movement therebctween. This arrangement interconnects the cap 140 and fastener end 120 to prevent the cap 140 from becoming detached from the fastener head during application of the sealing material 150, as discussed below.

The recesses 143 and protrusions 123 have a depth of from 0.1mm to 0.5mm, typically 0.3mm. Such a size has been found to be sufficiently large to secure the cap to the fastener head, while also being small enough to permit ejection of the cap from a plastics injection mould. The recesses 143 and protrusions 123 are sized such that there is an interference fit between them. That is, the largest diameter of the protrusions 123 (major diameter) is marginally larger than the smallest diameter of the recesses 143 (minor diameter), as illustrated in Figures 2 and 3. An appropriate interference fit is approximately 0.025 or 0.050 mm.

The pitch P between the recesses is chosen such that the desired distance D between the fastener head 122 and the structure 110 to accommodate the installation of one or more washers therein is an integer multiple of that pitch P. For example, if the desigu is to accommodate the installation of a washer which is 1.6mm thick, a washer which is 0.8mm thick, or a combination of both such washers (i.e. a washer thickness of 2.4mm), the pitch P must be 0.8mm, since 0.8mm is an integer factor of all of those washer thicknesses.

In the illustrated embodiment of Figures 2 and 3 the distance D corresponds to the thickness of the washer 130 and D is equal to P. The effect of this is that the inner cap 141 can be secured to the fastener head 122 in one of two axial positions: one in which the washer 130 can be located between the structure 110 and the fastener head (Figure 2); and onc in which there is no washcr (Figure 3). In thc configuration shown in Figure 2 the two protrusions 123 of the fastener head 122 are received within the two recesses 143 of the inner cap 141 which are furthest from the structure 110, so that there is a gap (D) between the fastener head and the structure within which the washer 130 is located. In the configuration shown in Figure 3 the two protrusions 123 are instead received within the two recesses 143 which are closest to the structure 110 so that there is no gap between the fastener head and the structure.

In other embodiments there may be no vanes 144, and the recesses 143 may be formed as continuous or discontinuous annular grooves around the circumference of the cylindrical portion of the inner cap member 141. In a further variation, the recesses 143 may be formed in the fastener head 122 and the protrusions 123 may be formed in the inner cap 141. In another variation, the recesses may be replaced by protrusions, for example in the form of ratchet teeth or similar; the protrusions of the cap being forced past the protrusions of the fastener head during installation of the cap onto the fastener head, and the two sets of protrusions thereafter abutting one another to prevent unwanted removal of the cap.

The outer cap member 142 is also a generally thin-walled dome shaped member, and is shaped to fit over the inner cap member 141 so that there is a continuous sealing volume between them which is filled with the cured sealing material 150. The outer cap member 142 is generally made up of an upper dome portion which generally corresponds in shape to the dome portion of the inner cap member 141, and an annular flared skirt portion 145 which terminates at a lip 146.

Thc outcr cap mcmbcr 142 has a central opcning with a cylindrical nozzlc conncctor 147 which extends from the its upper dome portion, the nozzle connector 147 being sized to interconnect with the nozzle (not shown) of a sealing material injector gun (not shown). The injector gun delivers a continuous flow of sealing material 150 into the scaling volume via the opening of the nozzle connector 147. In the present embodiment a two-part polysulphide based sealant such as MC-238 Class A/B, produced by NaftosealTM is preferred. A suitable alternative material is a. two-part epoxy based structural adhesive such as Scotch-WeldTM 7256 B/A, produced by 3M1.

The flared skirt portion 145 of the outer cap member 142 and thc lower portion of the inner cap member 141 together define an annular sealing volume within which a fillet of sealing material 150 is formed. The annular sealing volume has a generally flared shape such that its cross-sectional area increases with distance from the nozzle opening 147 of the outer cap member. The annular sealing volume is open at its lower facc such that thc scaling material 150 can flow outwardly from thc scaling volume between the inner and outer cap members and into contact with the structural element 110. The flared shape of the annular sealing volume provides a large adhesion area for the scaling material 150, which, once cured, acts to seal the cap 140 to the structural element 110. Moreover, this large adhesion area is achieved within a relatively small footprint on the structural element.

The lip 146 of the outer cap member 142 is axially offset from the base edge of the inner cap member 141 in the embodiment of Figures 2 and 3. This arrangement permits a small amount of sealing material to protrude from the base of the outer cap member 142 so that the user can visually detect whether the seal is complete. It also ensures that the outer cap member 142 does not clash with features of the structural element, such as ramps or radii (fillets), in the event that the fastener is located very close to such features. Such a potential clash is not uncommon in aircraft structures, where it is possible for the outer edge of a fastener to be located only 1.6mm from the edge of a fiHet with a 5mm radius. Tt also ensures that the seal is able to accommodate small surface feature deviations in the structural element, and is sufficiently thick to have a degree of flexibility once cured.

In some embodiments the sealing volume between the inner and outer cap members 141, 142 may include features to facilitate the even distribution of sealing material andior to rcduce resistance to flow of sealing material. For example, the scaling volume may include a reservoir (not shown) directly beneath the nozzle opening 147 to receive sealing material directly therefrom. Such a reservoir may reduce flow resistance and promote a uniform flow around the circumference of the sealing volume. Other such features may include a flow restriction region 39 (not shown) comprising a region of decreased cross-sectional area in the sealing volume to increase resistance to flow of sealing material 150 in that region. This provides an even distribution of sealing material 150 around the annular sealing volume because sealing material 150 becomes backed up' above the flow restriction region until the whole sealing volume is filled with sealing material.

The embodiment shown in Figures 5 and 6 is very similar to the embodiment of Figures 2 and 3, and like parts are identified with the same reference numerals to aid clarity. The only structural difference in this embodiment is the absence of the nozzle connector 147 and associated opening. Instead, the uncured sealing material 150 is first introduced into the concave dome portion of the outer cap member 142, the inner cap member 141 is installed onto the fastener end 120, and the outer cap member 142 is subsequently installed onto the inner cap member so that the uneured sealing material 150 flows between the inner and outer cap members and forms a fillet in the annular scaling volume, as shown in the figures. The sealing material 150 is then cured to adhere the inner cap to the outer cap, and to adhere the cap 140 to the structure 110.

In alternative embodiments it may be possible to have a one part cap. For example, such a cap may have the features of the inner cap member 141 of Figures 2-6 and an additional flared skirt extending radially outwardly from an upper region of its cylindrical base portion. A fillet of scaling material may then be formed within the annular sealing region formed beneath that flared skirt.

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.

Claims

Claims I. Ajoint comprising: a structure; a fastener passing through the stnicture with an end of the fastener protruding from the structure, the end of the fastener comprising: an axially extending shaft, a fastener head carried by the shaft and one or more washers located between the fastener head and the structure, the one or more washers spacing the fastener head and the structure apart by a distance; a cap having a cap body with a cavity which encloses thc end of thc fastcner and a cured sealing material which seals the cavity, wherein: one of the fastener head and the cap body is formed with two or more first locking features which are axially spaced apart by a pitch, and thc other of thc cap body and the fastener head is formed with one or more second locking features which are each capable of interlocking with one of the first locking features; at least one of the second locking features is interlocked with a respective one of the first locking features to secure the cap to the fastener head; and the distance between the fastener head and the structure is an integer multiple of the pitch between the first locking features.
2. A joint according to claim I, wherein each of either thc first locking features or the second locking features comprises a protrusion and each of the other of the first locking features and the second locking features comprises a recess, and at least one of the protrusions is received within a respective one of the recesses to secure the cap to the fastener head.
3. A joint according to claim 2, wherein there is an interference fit between said at least one of the protrusions and the recess within which it is received.
4. A joint according to any preceding claim, wherein the fastener head or cap body is formcd with two or more second locking features, thc othcr of thc fastener head or cap body is formed with three or more first locking features, and at least two of the second locking features are interlocked with a respective two of the first locking features.
5. A joint according to any preceding claim, wherein the cap comprises a plurality of vanes protruding from thc cap body into thc cavity, thc vanes each being formed with either the two or more first locking features or the one or more second locking features.
6. A method of installing a cap on an end of a fastener protruding from a structure through which thc fastener passes, wherein: the cnd of the fastener comprises an axially extending shaft, a fastener head carried by the shaft, and one or more washers located between the fastener head and the structure, the one or more washers spacing the fastener head and the structure apart by a distance; the cap has a cap body with a cavity; onc of thc fastcncr hcad and thc cap body is formed with two or more first locking features which are axially spaced apart by a pitch, and the other of the cap body and the fastener head is formed with one or more second locking features which are each capable of interlocking with one of the first locking features; and the distance between the fastener head and the structure is an integer multiple of the pitch betwecn the first locking fcatures, the method including the steps of: installing the cap on the end of the fastener such that the cavity encloses the end of the fastener and at least one of the second locking features is interlocked with a respective one of the first locking features; and applying a scaling matcrial to scal the cavity, and curing thc scaling matcrial.
7. A kit of parts for forming a joint, including: a fastener for passing through a structure, an end of the fastener comprising an axially extending shaft, a fastener head carried by the shaft, and one or more washers for locating between the fastener head and the structure, the one or more washers having a thickness configured to space the fastener head and the structure apart by a distance; a cap having a cap body with a cavity configured to enclose the end of the fastener; and a curable sealing material for sealing the cavity, wherein: one of the fastener head and the cap body is formed with two or more first locking features which are axially spaced apart by a pitch, the other of the cap body and thc fastcncr hcad is formed with onc or morc second locking features which are each capable of interlocking with one of the first locking features; each of the one or more second locking features is configured to interlock with a respective one of the two or more first locking features to secure the cap to the fastener head; and the distance between the fastener head and the structure is an integer multiple of the pitch between the first locking features.