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TECHNICAL FIELD
The invention relates to the field of fasteners and, in particular, to
flush head fasteners which can be blended into the external contour of a
vehicle such as the aerodynamic surfaces of an aircraft.
BACKGROUND INFORMATION
Flush head fasteners are used where surfaces must be smooth and free of
protrusions, for example, the Aerodynamic surfaces of aircraft; where, in
order to reduce drag, flush head fasteners are almost exclusively used.
The fastener hole is countersunk at a total included angle of around 100
degrees for metals and 130 degrees for composites with the total underhead
included angle of the fastener made approximately equal thereto. In order
to achieve a smooth aerodynamic contour, the countersink overall diameter
is usually made slightly larger than the diameter of the fastener head
such that upon installation, the top surface of the fastener head will be
below the aerodynamic surface. This is also necessary because the fastener
at the junction of the upper and underhead surface of the head normally
includes a small cylindrical land to eliminate any sharp edges that would
otherwise exist at the junction thereof What is usually left upon
installation is a small countersunk ring about the fastener head Of
course, if the fastener is being installed on a curved surface, the
countersunk hole will be elliptical in shape and thus the gap around the
fastener head becomes uneven. These sort of gaps are commonly called
"eyebrows". Another surface discontinuity is the typical internal
wrenchinq recess in the fastener head, i.e., phillips, slot, tri-wing,
allen, etc. Thus, if it is necessary to have an absolutely smooth
aerodynamic surface or if no electrical discontinuities can be tolerated,
the recess and the exposed portion of the countersunk hole "eyebrows" must
be covered.
Typically, putties are used; conductive ones if electrical continuity is
required. Another method is to use covers bonded to the fastener head or
retained by means of a protrusion on the cap which is jammed into the
internal wrenching recess in the fastener head. Neither of these methods
provide any guarantee of a smooth external contour and, of course gaps and
discontinuities, while reduced in size, will always be present.
The internal wrenching recess can be eliminated if the fastener can be
torqued from the shank end. For example, the internal wrenching recess can
be provided at the threaded end of the fastener However, in many
applications, access to the opposite end of the fastener is restricted so
this solution is not often viable. Another approach is to use detachable
(frangible) external. torquing surfaces attached to the head of the
fastener designed to break off after the proper torque level is reached.
Such a fastener has several disadvantages: there is no way of removing it
except by means of an easy out (requiring drilling into the fastener head
itself) and a rough external head surface remains. Thus, such fasteners
are not used on external aerodynamic surfaces.
Another problem occurs when the aerodynamic surface is made of composite
materials, such as graphite fiber reinforced epoxy resins. When used in
the wings of aircraft which contain fuel, a significant safety problem can
exist; for while the structure of the wing may be made of composite
materials, the fasteners used are typically metal in order to obtain
sufficient structural strength. The difference in electrical conductivity
between the composite material and the fastener causes lightning to become
attached to the metallic fastener head and the fastener can conduct
current into the interior of the wing and cause internal arcing inside the
fuel tank. If fuel vapors are present in the tank, such arcing can cause
an explosion. Therefore, it is necessary to provide some sort of
protection for external metal fasteners used in composite structures to
prevent such lightning strikes attaching thereto.
Typically, a coat of paint is applied over the aerodynamic surfaces
However, the gap between the fastener head and its countersink hole can
cause the paint to thin and crack around the outer edge of the fastener
This thinning and cracking of the paint in turn causes a difference in
electrical conductivity, which causes electrical streamers to form around
the edges of the fastener heads. These streamers create an electrical
field and increase the chance of a lightning strike hitting a fastener
head. To eliminate this possibility various devices to cover the fastener
head have been proposed. For example, U.S. Pat. No. 4,630,168 "Lightning
Protection Fastener" by J. Hunt, discloses a fastener having a metallic
head and shaft and a dielectric cap covering a top portion of the head.
When the head and cap are installed into the countersunk hole in the skin,
the cap being resilient tends to fill the gap between the top portion of
the head and the inner side walls of the countersink. The cap forms with
the skin an aerodynamic surface which is sufficiently continuous to
receive and maintain a crack free coat of paint having a uniform
thickness.
The problem here is that the cap encompasses a portion of the underhead
conical surface and when installed is wedged between the fastener head and
the countersunk hole. This presents a problem in heavily loaded structures
in that the resilient material between the fastener head and countersink
will tend to extrude under cyclic loading and the fastener may eventually
become loose fitting therein. Additionally, since the cap is installed
with a given thickness and tolerance it may not always provide an
absolutely smooth external contour. Another approach to providing
protection for metal fasteners in composite structures is found in U.S.
Pat. No. 4,681,497 "Encapsulated Fastener" by I. Berecz. Here the head and
shank are encapsulated in composite material and thus, it is subject to
the same deficiencies as the preceding example. Another approach can be
found in U.S. Pat. No. 4,502,092 "Integral Lightning Protection System for
Composite Aircraft Skins" by E.T. Bannink, Jr. et al. Here a plastic strip
is placed over the outer surface and a potting compound is used to fill
the space over the fastener. In U.S. Pat. No. 4,628,402 "Lightning
Protection of Fasteners in Composite Material" by J.H. Covey, one
embodiment uses a rubber plug over the fastener to fill the space above
the fastener caused by the layers of dielectric material None of the above
fastener systems designed for use with composite materials are suitable
for use with metal surfaces. Thus, there is no available fastener design
that can provide a smooth continuous aerodynamic surface in both composite
and metal structures and which does not require a paint be applied over
the surface, etc.
Thus, it is a primary object of the subject invention to provide a flush
head fastener that is capable of producing an aerodynamically smooth
external surface.
It is another primary object of the subject invention to provide a flush
head fastener that is capable of providing an aerodynamically smooth
external surface when the surface is curved.
It is another object of the subject invention to provide a flush head
fastener that produces no electrical discontinuities on the surface after
installation.
It is a further object of the subject invention to provide a flush head
fastener that can be used with both metal and composite materials.
DISCLOSURE OF THE INVENTION
The invention is an improved flush head fastener for use in countersunk
fastener holes. The fastener is designed for use in structures wherein the
fastener must be completely conformed with the surface and any gap between
the fastener head and countersink must be completely filled In detail, the
invention is a flush head fastener of the type having a head and shank
portion for joining two or more structural elements together. The
structural elements incorporate a fastener hole therein with a countersunk
end for receiving the head of the fastener The countersunk portion of the
hole is in communication with one external surface of the structural
elements such as an exterior aerodynamic surface of an aircraft. A layer
of material is joined to the top of the head of the fastener which is
readily deformable relative to the fastener shank and head and structural
elements; such that upon installation of the fastener in the hole, the
layer of material can be deformed fillinq any gap between the fastener and
the countersink and excess material above that needed to blend the layer
of material into conformity with the surface can be readily removed.
Preferably, the layer of material has a melting point substantially lower
than the melting point of the flush head fastener and the structural
elements such that after installation of the fastener, the layer of
material can be heated to a temperature wherein it is readily deformable
relative to the fastener and the structural elements.
If the external surface is made of titanium and a steel fastener is used
then the layer of material could be made of a soft non-heat treated
titanium alloy or the pure metal itself This layer can be friction welded
to the head of the fastener. Other means that can be used are brazing,
soldering, and bonding, etc. If the external surface is aluminum and a
titanium or steel fastener is used, then the layer of material can be pure
aluminum or a compatible solder. Whether or not the layer of material
would be heated or not would depend on the combination of materials
selected which will be obvious to those skilled in the art. However, if
the external surface is a composite material having an organic matrix,
then, ideally, the layer of material is selected from the large group of
presently available thermoplastic resins. Here the thermoplastic resin
layer can easily be heated to a point where it is deformable. In most
applications the heating of the layer of material need only raise the
temperature to a point wherein it is easily deformable. It must be
understood that the layer of material could be heated to a point wherein
it melts.
While the above description has been directed at a fastener having a metal
head and shank, it must be understood that they could be made of a plastic
material or an organic matrix composite material as long as the plastic or
matrix materials have a higher service temperature (higher melting point)
and/or are harder than the layer of material. Furthermore, the fastener
could be designed for use in a spherical countersink, dimpled, or
counterbored hole and the like.
Having thus described the fastener it now necessary to describe the method
of installing. The steps are basically as follows:
A countersunk fastener hole is formed in the structural elements It is
necessary that the countersink be located on the external surface of one
of the structural elements. The fastener, with the layer of material
joined thereto is installed in the fastener hole. Thereafter, the layer of
material is deformed filling any gap between the fastener and the
countersink end of the hole and the excess material above that needed to
blend the layer of material into conformity with the surface is removed.
This is accomplished by a) deforming the material by use of impact, for
example that provided by a rivet gun, until the material flows into and
fills any gap between the countersunk and fastener. Thereafter, any excess
material is removed by sanding, chiseling, shaving or the like. b) the
layer of material is heated until it is readily deformable and can be
impacted or faired to fill any gaps between the fastener head and
countersink Thereafter, a sharp edged instrument can be used to remove any
excess material.
In one preferred embodiment, the head of the fastener incorporates an
internal wrenching recess and the layer of material includes a wrenching
tool access hole therethrough in communication with the internal wrenching
recess in the fastener head. A plug is provided, made of the same material
as the layer of material or other compatible materials, which can be
inserted into the hole and/or recess after torquing the fastener in place
and which will become part of the fastener after the step of deforming.
In another preferred embodiment the layer of material incorporates
frangible external wrenching surfaces joined to the layer of material
which can be broken off when the proper torque level is reached.
In still another preferred embodiment of the invention the underhead
surface of the fastener is conical in shape having a total included angle
substantially equal to the total included angle of the countersink hole.
The layer of material extends out from the head of the fastener having a
conical shaped underside contiguous with the conical underside of the head
and having a total included angle substantially equal thereto. Thus, when
this fastener is installed into the countersunk fastener hole. no gap will
exist between the material layer and countersink, and, little or no
deformation of the layer of material need take place, although some
deformation may be required to insure line-to-line contact. Thereafter,
the excess material can be removed in the manner previously described.
The novel features which are believed to be characteristic of the
invention, both as to its organization and method of operation, together
with further objects and advantages thereof, will be better understood
from the following description in connection with the accompanying
drawings in which presently preferred embodiments of the invention are
illustrated by way of examples. It is to be expressly understood, however,
that the drawings are for purposes of illustration and description only
and are not intended as a definition of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Illustrated in Figure 1 is a partial cross-sectional view of the fastener
installed in structural elements; the right side of the figure showing the
fastener as provided while on the left side the fastener is shown after
installation is complete.
Illustrated in FIG. 2 is an enlarged partial cross-sectional view of the
fastener shown in FIG. 1
Illustrated in FIG. 3 is a partial cross-sectional view of a second
embodiment of the fastener installed in structural elements; with the
right side of the figure showing the fastener as provided while on the
left side the fastener is shown after installation is completed.
Illustrated in FIG. 4, is a partial cross-sectional view of a third
embodiment of the fastener installed in structural elements; with the
right side of the figure showing the fastener in the as provided and the
left side of the figure the fastener is shown after installation is
completed.
Illustrated in FIGS. 5, 6, 7, and 8 are partial cross-sectional views of
the fastener illustrating the process by which the fastener is installed
in the structural elements.
BEST MODE FOR CARRYING OUT THE INVENTION
Illustrated in FIG. 1 is a partial cross-sectional view of two structural
elements, indicated by numerals 10 and 12, respectively, joined together
by the improved flush head fastener, generally indicated by numeral 14.
The fastener 14 is primarily designed for use on aircraft, and, in
particular, to the fastening of external skins or other external
structural elements to interior structures. Thus, for purposes of
illustration, structural element 10 can be considered the outer skin of an
aircraft having an exterior surface 16 exposed to aerodynamic forces and
lightning strikes as well as illumination by threat radars. The fastener
14 is shown installed as provided on the right side of the figure and
after installation is completed on the left.
The fastener 14 is mounted in a countersunk fastener hole 18 with the
countersunk portion 20 in communication with the surface 16 and having a
total included angle indicated by numeral 22 (typically 100 degrees for
metals and 130 degrees for composites). Fastener 14 includes a head
portion 30 having a top surface 32 and a conical shaped underhead surface
34 having a total included angle substantially equal to the angle 22 of
the countersink portion 20. The fastener further includes a small
cylindrical land 36 at the junction of the top and underhead surfaces 32
and 34, respectively. This land 36 is machined to eliminate any sharp
edges that would normally exist at the junction of the surfaces 32 and 34.
However, this land 36 is extremely small and is shown out of proportion
for illustration purposes only. Thus, a volume indicated by numeral 38 is
created between the fastener head and countersink. Also the countersink
portion 20 is, typically, larger than the overall diameter of the land 36,
thus, the volume 38 is ring shaped. On a contoured surface the volume 38
takes on a more crescent shape.
The fastener 14 further includes a shank portion 40 having a threaded end
42 with an internal wrenching recess 43 and is held in place by means of a
nut and washer, indicated by numerals 44 and 46, respectively. Thus, a
fastener can be locked in place by use of tools applied from the shank end
of the fastener.
Still referring to FIG. 1 and additionally to FIG. 2, which is an enlarged
view of a portion of the left side of FIG. 1, a layer of material 50
having a thickness 51 (right side of FIG. 1) is joined to the top surface
32 of the fastener 14. If the structural element 10 is steel, or aluminum,
the fastener is typically high strength heat treated steel or titanium.
The layer 50 would be made of a soft aluminum or titanium material or a
solder such as those covered by Federal Standard QQ-S-571, Tin Alloy,
Tin-lead Alloy and Lead Alloy Solders. If solder is used for the layer of
material it can be heated to a very soft, readily deformable state (having
a melting point well below that of the aluminum or titanium structural
element 10 or the head and shank of the fastener) such that the structural
element or fastener are not effected at this temperature. By a process to
be subsequently discussed, the layer of material 50 is deformed so as to
fill the volume 38 about the head portion 30 (between the land 36 and
countersink portion 20) and, thereafter, the excess material, indicated by
dotted lines and numeral 52 on the left side of Figure 1, is removed The
remaining material layer 54 and surface 16 are blended into one smooth and
continuous surface. The overall thickness 51 of the layer of material 50
is therefore selected to provide sufficient material to fill the volume 38
and the remaining unfilled portion of the countersink 20 above the surface
32 of the head portion 30. This thickness will depend upon the allowable
(selected) tolerances of the various dimensions of the countersink portion
20 and the head portion 30. The end result is that no discontinuities will
exist that could reflect incoming radar signals, the surface is
aerodynamically smooth and there are no electrical discontinuities.
If the structural element 10 is a composite material having an organic
matrix, such as a thermoplastic or thermosetting resin matrix with
fiberglass or Kevlar filamentary reinforcements, then, the layer 50 could
be a thermoplastic material such as a polycarbonate; one that melts upon
heating and does not cure (with a melting point well below the melting
point of the matrix-material). Another suitable material is Nylon 6
manufactured by the Dupont Corporation, Wilmington, Del. In either case
the metal fastener is protected from lightning strike attachment If the
structural element 10 were made of graphite reinforced PEEK
(polyetheretherkeytone) manufactured by ICI, Americas, Inc., Wilmington,
Delaware then the layer of material could be a carbon fiber reinforced
Nylon 6 to eliminate electrical discontinuities. As can be seen, whether
the layer of material is a metal, dielectric material or a dielectric
material loaded with conductive fillers will depend upon the particular
application.
Illustrated in FIG. 3 is a partial cross-sectional view of a second
embodiment of the fastener indicated by numeral 14A. The fastener 14A
differs only in having an internal wrenching recess 60 in the head 30A and
the elimination of the internal wrenching recess 43 (see FIG. 1) from the
shank 40A. All of the other previously mentioned dimensions of the
fastener are identical, and thus, have the same identifying number. The
layer of material, indicated by numeral 50A, having a thickness 51A,
incorporates external wrenching surfaces 62 connected by means of a necked
down frangible portion 64. Thus, the nut 44 can be prevented from rotation
as the fastener is torqued. However, in most applications the fastener
would be engaging a plate nut (not shown). When the proper torque level is
reached the wrenching surfaces 62 can be "torqued off" separating at the
necked down portion 64. Thereafter, the same procedure is used, which
again will be subsequently discussed, to produce the completely installed
fastener with excess material 52A removed leaving remaining layer 54A, as
illustrated on the left side of FIG. 3.
Illustrated in FIG. 4 is a third embodiment of the fastener. The fastener
14B includes a wrenching recess 60B in the head 30B and the layer of
material 50B includes an aperture' 70 in communication with the recess
60B. Thus, as in the embodiment illustrated in FIG. 3, the fastener 14B
can be prevented from rotation when the nut 44 is torqued (or vice versa).
A plug 71, preferably made of the same material as the layer of material
50B or another suitable material is provided which can be inserted into
the recess 60B after the fastener 14B has been locked in place (left side
of FIG. 4). The layer 50B, having a thickness indicated by 51B, includes
an underhead surface 72 with an included angle 74 equal to angle 22 of the
countersink hole and extends upward and outward so as to almost completely
fill the countersink (leaving only the small volume 38' between the land
36 and countersink 20). This makes the filling of the countersink by the
process to be subsequently discussed much easier. Once the fastener is
installed, the insert or plug is installed (driven in place by a hammer or
the like) prior to removing excess layer material 52B, leaving remaining
layer 54B. If the fastener needs to be removed, the plug can be "dug out",
preferably after heating to soften it.
Having thus described three embodiments of the improved fastener, it is
readily apparent that the features of each can be interchanged. For
example, the concept of a conically shaped underhead of the layer of
materials 50B in the embodiment illustrated in FIG. 4, can be used in the
embodiments illustrated in FIGS. 1 and 3. Since the discussion of the
physical embodiments of the fasteners prior to installation and after
final installation has been completed, it is necessary to now discuss the
procedure for installing fastener.
Referring to FIG. 5, it can be seen that the fastener which was illustrated
in FIG. 4 is used as an example. After the fastener has been locked in
place, in curved structural elements 10A and 12A, the plug 71 is driven
into place as illustrated. Thereafter, as illustrated in FIG. 6, heat is
applied to the material layer 50B and plug 71 until, the materials are
soft and easily deformable (as illustrated, a heatlamp 75 is used).
Illustrated in FIG. 7, the layer 50B and plug 71 are tapped down to firmly
drive the layer of material 50B into contact with the countersink 20 and
filling volume 38'. This step typically deforms the layer of materials 50B
so that it has an irregular surface, indicated by numeral 76. This can
easily be accomplished by use of a rivet gun, partially illustrated and
indicated by numeral 77. In many cases, little or no impact will be
required and the step can be eliminated. However, with the embodiments
illustrated in 1 and 3, considerable impact may be necessary to achieve
filling of the volume 38'. It must also be noted that if the layer of the
material and plug is soft at room temperature heat may not be necessary.
Furthermore, if heat is applied, it may be possible to soften the layer of
material 50B sufficiently so that little tapping or driving is necessary.
Regardless, of whether the layer of material 50B requires heating and
driving, heating alone or driving alone, the next step as illustrated in
FIG. 8 is to remove the excess material 52B' leaving remaining layer 54B'.
This is easily accomplished by use of a chisel 80 having a knife edge 82
conforming to the contour of the curved surface 16'. It is important to
note that if the structural element 10 is a composite material with an
organic matrix, the layer of the material 50B and plug 71, ideally would
be a thermoplastic material, which could require heating to make it
deformable.
In some applications it may be desirable to heat the layer of material to a
temperature wherein it melts and is easily flowable provided the
structural elements can withstand the temperature. This would eliminate
the need for impact. The main advantage of melting is the elimination of
any gaps between the fastener head portion and the countersink improving
electrical continuity, etc. However, some difficulty may be encountered in
removing excess material and providing a smooth surface. Melting could
easily be accomplished when using a solder, thermoplastic or even
thermosetting materials such as an epoxy.
Thus, the main advantages of the fastener are readily apparent (1) it can
provide a smooth aerodynamic surface. (2) can provide a surface with no
electrical discontinuities. (3) can be used with both metal and composite
structures (4) can provide lightning strike protection Other advantages
are a reduced tendency to rotate and loosen, in some applications the
tolerances on the countersink end of the hole can be relaxed and it may be
possible to eliminate corrosion resistant coatings or sealants on the
fastener or hole where such coatings or sealants are presently required.
Furthermore, while primarily designed for use on aircraft the fastener has
application on external surfaces of land vehicles and ships, internal
fluid ducts in general or on any surface for cosmetic purposes In this
latter application it is apparent that the concept is applicable to
fasteners having plastic heads and shanks as long as the plastic has a
higher strength and/or higher melting point than the layer of material.
While the invention has been described with reference to particular
embodiments, it should be understood that the embodiments are merely
illustrative as there are numerous variations and modifications which may
be made by those skilled in the art. Thus, the invention is to be
construed as being limited only by the spirit and scope of the appended
claims.
INDUSTRIAL APPLICABILITY
The invention has applicability to vehicles and structures and, in
particular, to aircraft.
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