U.S. patent application number 15/621590 was filed with the patent office on 2017-09-28 for fastener systems that provide eme protection.
This patent application is currently assigned to The Boeing Company. The applicant listed for this patent is The Boeing Company. Invention is credited to Peter A. Coronado, Sean D. Morden, Randall A. Rogers, Richard P. Whitlock.
Application Number | 20170275019 15/621590 |
Document ID | / |
Family ID | 49223838 |
Filed Date | 2017-09-28 |
United States Patent
Application |
20170275019 |
Kind Code |
A1 |
Whitlock; Richard P. ; et
al. |
September 28, 2017 |
FASTENER SYSTEMS THAT PROVIDE EME PROTECTION
Abstract
Fasteners are inserted into a stack of members and terminated
with parts having at least one of a dry dielectric coating and a
dry dielectric seal at select locations to protect against
electromagnetic effects (EME).
Inventors: |
Whitlock; Richard P.;
(Seattle, WA) ; Rogers; Randall A.; (Auburn,
WA) ; Coronado; Peter A.; (Lynnwood, WA) ;
Morden; Sean D.; (Federal Way, WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Boeing Company |
Chicago |
IL |
US |
|
|
Assignee: |
The Boeing Company
Chicago
IL
|
Family ID: |
49223838 |
Appl. No.: |
15/621590 |
Filed: |
June 13, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13434835 |
Mar 29, 2012 |
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15621590 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B64D 45/02 20130101;
F16B 33/004 20130101; B21J 15/142 20130101; Y10T 29/49947 20150115;
F16B 43/001 20130101; B21J 15/147 20130101; F16B 33/06
20130101 |
International
Class: |
B64D 45/02 20060101
B64D045/02; F16B 33/06 20060101 F16B033/06 |
Claims
1. A method of fastening a stack of members, comprising: inserting
fasteners into the stack of members; and terminating the fasteners
with a part, the part including: a fastener termination body having
a central opening and a washer; a dry dielectric coating on the
fastener termination body; and a first dielectric seal secured to
the fastener termination body and partially covering the central
opening at one of a distal and proximal location; wherein each of
the dry dielectric coating and the first dielectric seal has a
thickness and composition for inhibiting lightning current.
2. The method of claim 1, wherein the first dielectric seal is
configured to protect against arcing, sparking and hot particle
ejection, and wherein the dry dielectric coating is configured to
protect against arcing and sparking.
3. The method of claim 1, wherein the stack of members comprises a
stack of aircraft members, and wherein at least one member of the
stack of aircraft members comprises a composite material.
4. The method of claim 1, wherein the stack of members includes an
aircraft skin and a metal member.
5. The method of claim 1, wherein inserting at least one of the
fasteners includes fitting a fastener shaft into a through-hole in
the stack, the method further comprising coating the fastener shaft
with a dry resin including electrically conductive particles.
6. The method of claim 1, wherein inserting at least one of the
fasteners includes fitting a fastener shaft into a through-hole in
the stack, the fastener including a head and a dielectric seal at a
proximal location on the head.
7. The method of claim 1, wherein inserting at least one of the
fasteners includes inserting a fastener head having an upper
surface with multiple mechanical interlocking sites that are
configured to retain a dielectric layer, the dielectric layer
covering the upper surface of the head, the dielectric layer
mechanically interlocked with the interlocking sites.
8. The method of claim 1, further comprising adding a conductive
layer to an upper surface of the stack prior to inserting fasteners
into the stack of members.
9. The method of claim 1, wherein the first dielectric seal is
positioned at an inner proximal location of the washer.
10. The method of claim 9, further comprising positioning a second
dielectric seal at the distal location, wherein the first and
second dielectric seals are separated by a gap.
11. The method of claim 1, further comprising coating the washer
with a dielectric coating.
12. A method of fastening a stack of members, comprising: inserting
fasteners into the stack of members; terminating the fasteners with
a part, the part including: a fastener termination body having a
central opening and including a washer; a dry dielectric coating on
the fastener termination body; and a first dielectric seal secured
to the fastener termination body and partially covering the central
opening at an inner proximal location of the washer; wherein each
of the dry dielectric coating and the first dielectric seal has a
thickness and composition for inhibiting lightning current; and
coating the washer with a dielectric coating.
13. The method of claim 12, wherein the first dielectric seal is
configured to protect against arcing, sparking and hot particle
ejection, and wherein the dry dielectric coating is configured to
protect against arcing and sparking.
14. The method of claim 12, wherein the stack of members comprises
a stack of aircraft members, and wherein at least one member of the
stack of aircraft members comprises a composite material.
15. The method of claim 12, wherein the stack of members includes
an aircraft skin and a metal member.
16. The method of claim 12, further comprising adding a conductive
layer to an upper surface of the stack prior to inserting fasteners
into the stack of members.
17. A method of fastening a stack of members, comprising: inserting
fasteners into the stack of members; terminating the fasteners with
a part, the part including: a fastener termination body having a
central opening and including a washer; a dry dielectric coating on
the fastener termination body; a first dielectric seal secured to
the fastener termination body and partially covering the central
opening at an inner proximal location of the washer; and a second
dielectric seal secured to the fastener termination body at a
distal location, the first and second dielectric seals being
separated by a gap; wherein each of the dry dielectric coating, the
first dielectric seal, and the second dielectric seal has a
thickness and composition for inhibiting lightning current; and
coating the washer with a dielectric coating.
18. The method of claim 17, wherein the first dielectric seal is
configured to protect against arcing, sparking and hot particle
ejection, and wherein the dry dielectric coating is configured to
protect against arcing and sparking.
19. The method of claim 17, wherein the stack of members comprises
a stack of aircraft members, and wherein at least one member of the
stack of aircraft members comprises a composite material.
20. The method of claim 17, wherein the stack of members includes
an aircraft skin and a metal member.
Description
BACKGROUND
[0001] Lightweight composite materials hold promise for the
aircraft industry. Fiber composites provide a significant
improvement in specific strength and stiffness over conventional
metal alloys. Better specific strength and stiffness translates
into weight savings, which translates into fuel savings and lower
operating costs. Additionally, composites do not corrode like
aluminum, and they are more resistant to fatigue.
[0002] Composite structures in these aircraft do not readily
conduct away the extreme electrical currents and electromagnetic
forces generated by lightning strikes.
[0003] Aircraft with composite structures may be equipped with
protection against electromagnetic effects (EME) from lightning
strikes. For example, conductive media may be provided on a surface
to divert lightning current away from underlying metal fastener
systems. In addition, gaps between fastener parts and gaps between
fastener parts and structural members may be filled with dielectric
sealant. Even if some current is not diverted, the sealant prevents
arcing and sparking across the gaps.
[0004] The sealant provides EME protection. However, the process of
injecting the sealant is time consuming as it involves multiple
steps, including surface cleaning, sealant curing and overfill
removal. Moreover, the sealant adds weight to an aircraft. The
weight added to a single fastener system might seem insignificant.
However, applying the sealant to tens of thousands of fasteners in
a single aircraft can add hundreds of pounds.
SUMMARY
[0005] According to an embodiment herein, a method comprises
inserting fasteners into a stack of members and terminating the
fasteners with parts having at least one of a dry dielectric
coating and a dry dielectric seal at select locations to protect
against electromagnetic effects (EME).
[0006] According to another embodiment herein, a fastener comprises
a fastener head having an upper surface with multiple mechanical
interlocking sites that are configured to retain a dielectric
layer; and a dry dielectric layer covering the upper surface of the
head. The dielectric layer is mechanically interlocked with the
interlocking sites.
[0007] According to another embodiment herein, a part for
terminating a fastener comprises a fastener termination body having
a central opening, and at least one feature for protecting against
EME effects. The at least one feature includes at least one of a
dielectric coating and a dielectric seal partially covering the
central opening at a select location. The seal is secured to the
body. The coating and the seal have a thickness and composition for
inhibiting lightning current. The select location includes at least
one of a distal location and a proximal location.
[0008] According to another embodiment herein, a fastening system
comprises a fastener, at least one fastener termination part, and
EME protection features for the fastener and each fastener
termination part. The EME protection features are taken from the
group consisting of a conductive resin coating on a shaft of the
fastener, an interlocked dielectric cover on a head of the
fastener, a dielectric seal on a proximal location of the fastener
head, a dielectric seal on a distal location of the termination
part, a dielectric seal on an proximal location of the termination
part, a dielectric coating on a distal location of the termination
part, and a dielectric coating on a proximal location of the
termination part.
[0009] These features and functions may be achieved independently
in various embodiments or may be combined in other embodiments.
Further details of the embodiments can be seen with reference to
the following description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is an illustration of a method of fastening a stack
of members.
[0011] FIG. 2 is an illustration of a commercial aircraft.
[0012] FIGS. 3A and 3B are illustrations of a first embodiment of
an EME-protective fastener.
[0013] FIGS. 4A and 4B are illustration of a second embodiment of
an EME-protective fastener.
[0014] FIGS. 5 and 6 are illustrations of other embodiments of
EME-protective fasteners.
[0015] FIG. 7 is an illustration of a stack including a composite
member and a conductive layer.
[0016] FIG. 8 is an illustration of select EME protection locations
on a nut.
[0017] FIGS. 9-12 are illustrations of different embodiments of
washers and nuts having EME protection features and different
combinations of the washers and nuts.
[0018] FIG. 13 is an illustration of an aircraft structure.
DETAILED DESCRIPTION
[0019] Reference is made to FIG. 1, which illustrates a method of
fastening a stack of members. The stack has two or more members. In
some embodiments, all of the members in the stack are made of metal
(e.g., aluminum, titanium). In other embodiments, at least one of
the members in the stack is made of composite material. For
instance, an outer member is made of composite material and an
inner member is made of metal. In other embodiments, all of the
members in the stack may be made of composite material.
[0020] The method includes inserting fasteners into the stack
(block 110). In some embodiments, the fasteners are inserted in
pre-formed through-holes in the stack. In other embodiments, the
fasteners are inserted without drilling holes. Fasteners may be
countersunk into the outer members whose outer surfaces require
aero smoothness (e.g., aircraft skin). The fasteners may have EME
protective features including dry dielectric layers interlocked to
upper surfaces of fasteners heads, and dry dielectric seals on
lower surfaces of the fasteners heads.
[0021] The method further includes terminating the fasteners with
parts having EME protective features. The EME protective features
for the termination parts include dry dielectric coatings and dry
dielectric seals (e.g., dielectric sleeves) at select locations
(block 120).
[0022] Without these EME protection features, arcing and sparking
may occur. Arcing occurs when two metallic parts are in near
contact, and dielectric properties of the medium between break down
due to high currents flowing between the parts. Sparking occurs
when a high current ionizes air in a gap between two parts, the air
breaks down, and current flows across the gap. Hot particle
ejection occurs when a high current in a composite part causes
composite material to chemically break down and generate hot
expanding gas.
[0023] Both the dielectric coating and the dielectric seal prevent
arcing and sparking by blocking the electrical bonding path between
parts. The dielectric coating and the dielectric seal, alone and in
combination, inhibit arcing and sparking across gaps between
fastener parts (e.g., gap between fastener and nut, and gap between
washer and nut) and gaps between fastener parts and structure
(e.g., gaps between fastener and structure, nut and structure, and
washer and structure). The dielectric seal performs the additional
function of preventing hot gas from being ejected across gaps and
keeping the gas within the fastening system.
[0024] In addition to or in the alternative, some embodiments of
the fasteners may reduce arcing, sparking and hot particle ejection
by improving the electrical bonding path. In these embodiments,
shafts of the fasteners may be coated with a conductive coating
(e.g., a conductive resin) to improve the electrical bond between
the fastener and stack and thereby reduce contact resistance. In
some embodiments, the fasteners may be clearance fitted into the
holes. In other embodiments, the fasteners may be interference
fitted into the holes. The interference fit improves electrical
bonding between the fastener and stack (by increasing surface
contact area), which reduces current densities and lowers overall
path resistance.
[0025] The use of dry dielectric coating on parts of a fasteners
system offers several advantages over the use of sealant to fill
the gaps. The advantages include ease and speed of installation.
The dry dielectric may be coated onto the fastener parts prior to
installation so that during fastening, the fasteners need only be
installed. Extra steps, such as masking parts of the stack,
injecting sealant, and curing the sealant, are eliminated.
[0026] Another advantage is a reduction in weight. The dry
dielectric coatings and seals may be thinner than layers of
sealant. In some embodiments, the thickness of the dielectric
coating or seal may be 30 mils .+-.10 mils. Moreover, less surface
area is covered, since an entire surface of a fastener part is not
covered with a dry dielectric coating or dry dielectric seal.
[0027] Another advantage is redundant EME protection. Different
embodiments of fasteners may be used in combination with different
embodiments of termination parts (e.g., nuts and washers) to
provide multiple levels of protection against EME.
[0028] These advantages are substantial with respect to a
commercial aircraft, such as the commercial aircraft 200
illustrated in FIG. 2. The aircraft 200 includes a fuselage 210,
wing assemblies 220, empennage 230, and landing gear assemblies
240. In this commercial aircraft 200, one million fastener systems
might be used to fastener together the various components 210-240
and their subcomponents.
[0029] In certain locations of the aircraft 200, EME protection is
desirable. For instance, EME protection is desirable for wing
assemblies and other structures. To provide EME protection, the
wing assemblies 220 may be assembled with as many as 20,000
fasteners according to a method herein. The advantages realized by
weight savings and time and ease of installing those 20,000
fasteners are quite substantial.
[0030] In the following paragraphs, different types of
EME-protective fastener parts will be described. These fasteners
parts include fasteners and parts that terminate the fasteners
(e.g., nuts and washers). These fastener parts may have dry
dielectric coatings and/or dry dielectric seals at select locations
to protect against EME.
[0031] First, the EME-protective fasteners will be described.
Examples of fasteners include, but are not limited to bolts and
rivets.
[0032] Reference is now made to FIGS. 3A and 3B, which illustrates
a first embodiment of an EME protective fastener 310. FIG. 3B is a
sectional view taken along lines 3B-3B in FIG. 3A. The fastener 310
includes a head 320 and a shaft 330. An upper surface 322 of the
fastener head 320 has multiple mechanical interlocking sites that
are configured to retain a dielectric layer 340. Each interlocking
site includes a protrusion 324 extending upwards from the upper
surface 322. The protrusion 324 has an undercut portion 326
proximate the upper surface 322. In the embodiment of FIGS. 3A and
3B, the protrusions 324 may be spokes 324 that extend radially
outward and have a cross section of an inverted triangle.
[0033] The fastener 310 further includes a dielectric layer 340
covering the fastener head 320. The dielectric layer 340 is not
limited to any particular dielectric material. Examples include,
but are not limited to, thermosets, thermoplastics, rubber,
ceramics, and elastomers.
[0034] The dielectric layer 340 fills the undercut portions and all
other space between the protrusions 324. The dielectric 340 is also
in intimate contact with the upper surface 322 and the protrusions
324.
[0035] In some embodiments, the dielectric layer 340 may be formed
by flowing a dielectric material over the fastener head 320. When
the dielectric material flows over the surface 322, it fills the
undercuts 326 and all space between the protrusions 324. Once the
dielectric material hardens, it is interlocked with the protrusions
324 and thereby secured to the fastener head 320. In other
embodiments, the dielectric layer 340 may be formed by powder
coating, painting, plating, spraying, sputtering, deposition, or
any other process that allows the dielectric material fills the
undercut portions 326.
[0036] In this manner, the dielectric layer 340 is mechanically
interlocked with the interlocking sites prior to fastener
installation. The fastener 310 with the dielectric layer 340 is
preinstalled to the head 320.
[0037] The interlocking of the dielectric layer 340 to the fastener
head 320 is especially advantageous for lightning strikes. The
interlocking ensures that the dielectric layer 340 maintains
intimate contact with the fastener head 320 during and after such
events.
[0038] The dielectric layer 340 may be retained by additional
means. As a first example, the surface 322 may have roughness to
enhance the retention to the dielectric layer 340. As a second
example, the dielectric layer 340 may be made of a material (e.g.,
adhesive sealant) that adheres to the surface 322.
[0039] In other embodiments of EME-protective fasteners, the
interlocking sites may have other cross-sections and they may be
arranged in other patterns. Examples of other cross-sections
include, but are not limited to square and circular cross-sections.
Cross sections having areas of high stress concentration (e.g.,
corners) are not preferred. Rounded cross sections provide better
mechanical properties.
[0040] An EME-protective fastener herein is not limited to any
particular number of interlocking sites. However, it has been found
that a range of five to nine interlocking sites provides a good
balance between interlocking strength (with the dielectric layer)
and fastener clamp-up strength.
[0041] Reference is now made to FIGS. 4A and 4B, which illustrates
a second embodiment of an EME-protective fastener 410. The EME-
protective fastener 410 includes interlocking sites 420 arranged in
a grid. Each interlocking site 420 includes a post 422 extending
upward from an upper surface 414 of the fastener head 412. The post
422 terminates in a knob 424. The post 422 and the surfaces between
the knob 424 and fastener head 412 define an undercut portion
426.
[0042] Reference is now made to FIGS. 5 and 6, which illustrate
embodiments of fasteners 510 and 610 including heads 512 and 612
and dielectric seals 520 and 620. The seals 520 and 620 may be
mechanically attached to the fastener heads 512 and 612. The seals
520 and 620 be made of a polymeric material such as
polytetrafluoroethylene (PTFE), or thermoplastic polymers such as
polyether ether ketone (PEEK) and polyamide-imides. When the
fastener 510 or 610 is inserted into a stack and terminated, its
seal 520 or 620 makes compressive contact with the stack.
[0043] In the fastener 510 of FIG. 5, the seal 520 fits around a
periphery of the fastener head 512. The seal 520 may have a ridge
522 that fits into a groove in the fastener head 512. The seal 520
inhibits arcing and sparking at the edge of the fastener head 520,
and it impedes hot expanding gas underneath the head 512 from
escaping.
[0044] In the fastener 610 of FIG. 6, the seal 620 has the shape of
an O-ring and fits in a groove in an underside of the fastener head
612. The seal 620 inhibits arcing and sparking, and it impedes
escape of any hot expanding gas between the stack and the
fastener's shaft 614. The upper surface and side of the fastener
head 612 may be covered with a dielectric coating 630 to prevent
arcing and sparking at the edge of the fastener head 612.
[0045] Reference is now made to FIG. 7, which illustrates stack 700
including a composite member 710 and a conductive layer 720 (e.g.,
metal strips, expanding foil) that covers the composite member 710.
An EME-protective fastener 750 is inserted into a through-hole 730
in a composite member 710 and countersunk into the conductive layer
720. The dielectric layer 752 on the fastener's head 754 is flush
with the conductive layer 720.
[0046] In some embodiments, the entire fastener shaft 230 may be
coated with a dry conductive resin coating 756 (shaded in gray)
having electrically conductive particles dispersed in a polymer
matrix). Examples of the conductive particles include, but are not
limited to, metal particles, and nanotubes. It has been found that
the resin, in combination with an interference fit, improves the
electrical bond path between the fastener 750 and the composite
member 710. The resin fills voids between the fastener shaft 758
and the composite member 710. By doing so, the resin increases the
effective contact area with fibers of the composite member 710.
[0047] As the fastener 750 is being terminated (e.g., as a nut is
being wrenched onto the fastener 750), it should not rotate within
the through-hole 730. To prevent rotation without damaging the
dielectric layer 752, an anti-rotation measure may be used.
Examples of anti-rotation measures include, but are not limited to,
creating an interference (or friction) fit between the fastener
shaft 758 and the member 710; providing the shaft 758 with a
cross-sectional shape that prevents rotation (e.g., a hexagonal
shape for the shaft and through-hole), and keying the shaft 758 to
the member 710.
[0048] Fasteners herein may be anywhere in an aircraft. As
examples, fasteners herein may be used in door surrounds,
electrical equipment housing, and parts that are subject to ground
fault currents. Fasteners herein may be terminated with standard
parts (e.g., standard nuts) or they may be terminated with
EME-protective parts herein.
[0049] In the following paragraphs, different types of
EME-protective fastener termination parts will be described. These
parts include nuts and washers.
[0050] Reference is now made to FIG. 8, which illustrates a
fastener 830 extending through a member of a stack 800, and a
washer 810 and nut 820 that terminate the fastener 830. FIG. 8 also
illustrates various locations on the washer 810 and nut 820 that
may be covered with a dry dielectric coating dielectric seal to
protect against EME. The locations on the nut 820 include an inner
distal location 840, an outer proximal location 850, and an inner
proximal location 860. The locations on the washer 810 include an
inner proximal location 870 and an inner distal location 880. The
terms "proximal" and "distal" refer to the distance from the stack
800. For instance, the proximal location of the nut 820 is closer
to the stack 800 than the distal location. The terms "inner" and
"outer" refer to the distance from the fastener shaft.
[0051] Instead of covering the entire washer 810 and nut 820 with
sealant to protect against EME, only one or more of these locations
840-880 may be coated or sealed with dry dielectric material.
[0052] If a seal is used, the seal is secured to a central opening
in the body and protrudes axially from the body. In some
embodiments, the seal protrudes at least 7 mils from the body. The
seal may be made of a polymeric material such as
polytetrafluoroethylene (PTFE), or thermoplastic polymers such as
polyether ether ketone (PEEK) and polyamide-imides. The seal may be
mechanically attached by using protrusions and undercuts that allow
polymeric material to flow into the undercuts.
[0053] In some embodiments, an exterior surface of the body is
covered completely by a dielectric coating. Examples of
compositions for the dielectric coating include, but are not
limited to, thermosets, thermoplastics, ceramics, glass, and
elastomers. The dielectric coating may be applied by a process such
as anodic conversion coating (e.g., plasma electrolyte oxidation),
which is an electrochemical surface conversion treatment. Other
processes for applying the dielectric include, but are not limited
to, powder coating, spraying, plating, sputtering, and
deposition.
[0054] Reference is made to FIG. 9, which illustrates a combination
including a washer 910 having a seal 920 at the inner proximal
location. The distal location is not covered. The outer surface of
the washer 910 is coated with a dielectric material.
[0055] The combination also includes a nut 950 having a seal 960 at
the outer proximal location, a seal 970 at the inner proximal
location, and a dielectric coating 980 at the distal location.
There is no dielectric coating on the exterior surface of the nut
950.
[0056] When the nut 950 and the washer 910 are mated with the
stack, the inner proximal seal 920 of the washer 910 is pressed
against the stack, and the inner proximal seal 970 of the nut 950
is received in the central opening of the washer 910. As the nut
950 is tightened, the washer seal 920 folds back and is crushed
against the stack, thereby filling any voids between the washer 910
and the stack.
[0057] Reference is made to FIG. 10, which illustrates a
combination including a washer 1010 having a seal 1020 at the inner
proximal location; and a nut 1050 having a seal 1060 at the inner
proximal location, a dielectric coating 1070 at the distal
location, and a dielectric coating 1080 on the outer surface
(instead of a seal at the outer proximal location).
[0058] Reference is made to FIG. 11, which illustrates a
combination including a washer 1110 having a seal 1120 at the inner
proximal location, and a concentric ridge 1130 formed at a
periphery of the washer 1110. The combination also includes a nut
1150 having a seal 1160 at the inner proximal location, a
dielectric coating 1170 on the outer surface (instead of a seal at
the outer seal location), and a seal 1180 at the inner distal
location.
[0059] When the nut 1150 and the washer 1110 are mated with the
stack, the inner proximal seal 1120 and the ridge 1130 of the
washer 1110 are pressed against the stack, and the inner proximal
sleeve 1160 of the nut 1150 is received in the central opening of
the washer 1110. As the nut 1150 is tightened, the inner proximal
seal 1120 of the washer 1110 folds back and is crushed against the
stack. The concentric ridge 1130 also maintains a seal between the
washer 1110 and the stack, even if the face of the washer 1110 is
not parallel with the bottom face of the stack.
[0060] Reference is now made to FIG. 12, which illustrates a
combination including a washer 1210 having seals 1220 and 1230 at
the inner proximal and distal locations. There is a gap between
these seals 1220 and 1230. The combination further includes a nut
1250 with a dielectric coating 1270 over the entire exterior
surface, and a coating 1260 at the inner proximal location.
[0061] When the nut 1250 and the washer 1210 are mated with the
stack, the proximal seal 1220 of the washer 1210 is pressed against
the stack, and the distal seal 1230 of the washer 1210 is received
in the inner proximal location of the nut 1250. As the nut 1250 is
tightened, the proximal seal 1220 of the washer 1210 folds back and
is crushed against the stack, and the distal seal 1230 of the
washer 1210 folds back and is crushed against the nut 1250.
[0062] A washer herein is not limited to a concentric ridge at the
periphery. Other embodiments of washers may have concentric ridges
at intermediate and inner portions of the washer's upper face. Some
embodiments may have concentric ridges on both faces.
[0063] In some embodiments, the concentric ridge may be formed by
stamping a concentric groove in the washer, and filling the groove
with a dielectric ring. In other embodiments, a metal ridge may be
stamped in the washer.
[0064] Although FIGS. 9-12 illustrate nuts 950, 1050, 1150, 1250
that are open at both ends, a nut herein is not so limited. For
instance, one end of a nut herein may be closed to form a cap. The
cap may be made of a dielectric material, or the cap may be made of
metal with its inner surface coated with a dielectric material.
Such a cap would provide additional protection against distal
arcing and sparking.
[0065] A fastening system herein is not limited to the combinations
of fasteners, nuts and washers described above. A variety of EME
protection features may be mixed and matched. Multiple parts having
different features may be used. For example, the EME protection
features may be taken from the group consisting of a conductive
resin coating on a shaft of the fastener, an interlocked dielectric
cover on a head of the fastener, a dielectric seal on a proximal
location of the fastener head, a dielectric seal on a distal
location of the termination part, a dielectric seal on an proximal
location of the termination part, a dielectric coating on a distal
location of the termination part, and a dielectric coating on a
proximal location of the termination part.
[0066] Reference is now made to FIG. 13, which illustrates a
portion of an aircraft wing 1310. The wing 1310 includes skin 1320
made of a composite material such as carbon fiber reinforced
plastic (CFRP). The skin 1320 may be fastened to a substructure
(represented by the bracket) 1330 by metal fastener parts 1340,
1380 and 1390. The skin 1320 defines a lightning susceptible area
1350.
[0067] Each fastener 1340 has a dielectric layer 1342 that is
interlocked with a fastener head 1344. Each fastener 1340 may have
a conductive resin coating on its shaft 1346.
[0068] Each fastener 1340 is countersunk into the skin 1320. The
dielectric layer 1342 is flush with the upper surface of the skin
1320. A metal foil 1360 covers the skin 1320 to dissipate EME
current. A surfacing film (e.g., a glass-epoxy layer) 1365 is
applied over the skin 1320, and the surfacing film 1365 is covered
with paint 1370.
[0069] Each fastener 1340 is terminated with a washer 1380 and a
nut 1390. The washer 1380 and the nut 1390 have various EME
protection features.
[0070] There might be tens of thousands of fasteners 1340, washers
1380, and nuts 1390 in the wing 1310. By using EME protection
features herein instead of injecting sealant, the savings in
installation time is substantial. The reduction in weight is also
substantial.
[0071] FIG. 13 illustrates the surface of the washer 1380 and
bracket 1330 being slightly skewed. If the washer 1380 is provided
with an upper sleeve or a concentric ridge on the upper surface,
any gap between the washer 1380 and bracket 1370 will be filled
with dielectric material and sealed.
[0072] Similarly, if the nut 1390 is slightly skewed with respect
to the washer 1380, or if a washer is not used and the nut is
slightly skewed with respect to the stack, any gap between the nut
and washer or nut and stack will be filled with dielectric material
and sealed.
[0073] Thus, the EME protection features provide the added
advantage of making the structure less dependent on geometry. Even
if mating surfaces of the termination parts aren't perfectly flush,
EME protection is still provided.
* * * * *