Note: Descriptions are shown in the official language in which they were submitted.
1 3085~t
1 "Improvement to Flush Head Fastener"
3 Technical Field
The invention relates to the field of fa~teners and,
6 in particular, to flush head fasteners which can be
7 blended into the external contour of 2 vehicle 6uch as
8 the aerodynamic surfaces of an aircra~t~
10~ackground Information
11
12Flush head asteners are used where surface6 must be
13smooth and free of protrusions, for example, the Aero-
14dynamic surfaces of aircraft: where, in order to reduce
15drag, flush head fasteners are almost exclusively used.
16~he ~astener hole is countersunk at a total included
17angle of around 100 degrees for metals and 130 degrees
18for composites with the total underhead included angle of
19ths fastener made approximately equal thereto. In order
20to achieve a smooth aerodynamic contour, tha countersink
21overall diameter is usually made slightly largQr than the
22diameter of the fastener h~ad such that upon installa-
23tion, the top ~urface of the fa6tener head will be below
24the aerodynamic 6ur~ace. This is also neces6ary because
25the fastener at the ~unction of the upper and underhead
26~ur~ace o~ the head normally includes a small cylindrical
27land to eliminate any sharp edges that would otherwise
28exist at the ~unction thereof. What is usually left upon
29in6tallation i8 a 6mall countersunk ring about the
30fastener head. Of cour8e~ if the fastener is being
31installed on a curved surfacQ, the countersunk hole will
32be elliptical in shape and thus the gap around the
33fastener head becomes uneven. The6e 60rt of gaps are
34commonly called "eyebrows". Another surface discon-
35tinuity i~ the typical internal wrenching recess in the
36fastener head, i.e., phillips, slot, tri-wing, allen,
1 ~8581
1 etc. Thus, if it is necessary to have an absolutely
2 6mooth aerodynamic surface or if no electrical discon-
3 tinuities can be tolerated, the recess and the exposed
4 portion of the countersunk hole "eyebrows~ ~ust be
s covered.
7 Typically, puttie~ are u~ed; conductive ones if
8 electrlcal continuity i6 required. Another method i6 to
9 use covers bonded to the fastener head or retained by
means of a protrusion on the cap which i jammed into the
11 internal wrenc~ing reoess in the fastener head. Neither
12 of these methods pr~vide any guarantee of a ~mooth
13 external contour and, of course gaps and discontinuities,
14 while reduced in 6ize, will always be present.
16 The internal wrenching recess can be eliminated if
17 the fastener can be torqued from the shank end. For
18 example, the internal wrenching recess can be provided at
19 the threaded end of the fastener. However, in many
applications, access to the opposite end of the fastener
21 i8 restricted so thi6 solution is not often viable.
22 Another approach i5 to use detachable (frangible)
23 external torquing surfaces attached to the head of the
24 fastener designed to break off after the proper torque
level is reached. Such a fastener ha6 several disad-
26 vantages: there i6 no way of removing it except by
27 mean6 of an easy out (requiring drilling into the
28 fastener head itself) and a rough external head surface
29 remains. Thus, such fasteners are not used on external
aerodynamic 6urfaces.
31
32 Another problem occurs when the aerodynamic surface
33 is made of composite materials, such as graphite Piber
34 reinforced epoxy resins. When used in the wings of
3'5 aircraft which contain fuel, a significant safety problem
1 308581
1 can exist: for while the structure of the wing may be
2 made of composite materials, the fasteners used are
3 typically metal in order to obtain sufficient structural
4 6trength. The difference in electrical conductivity
between the compo~ite material and the fastener causes
6 lightning to become attached to the metallic ~astener
7 head and the fastener can conduct current into the
8 interior of the wing and cause internal arcing in ide the
9 fuel tanX. If fuel vapors are present in the tank, such
arcing can cause an explosion. Therefore, it is nQces-
ll sary to provide some ~ort of protection for external
12 metal fasteners used in composite structures to prevent
13 such lightning ~trikes attaching thereto.
14
Typically, a ceat of paint is applied over the
16 serodynamic surfaces. However, the gap between the
17 ~astenQr head and its counter ink hole can cause the
18 paint to thin and crack around the outer edge of the
l9 fa~tener. This thinning and cracking of the paint in
turn causes a difference in electrical conductivity,
21 which causes electrical streamers to form around the
22 ~dges of the fastener heads. These streamers create an
23 electrical field and increase the chance oS a lightning
24 strike hitting a fastener head. To eliminate this
po6sibility various devices to cover the fastener head
26 have been proposed. For example, U.S. Patent No.
27 4,630,16~ "Lightning Protection Fastener" by J. ~unt,
28 disclose6 a fastener having a metallic head and shaft and
29 a dielectric cap covering a top portion of the head.
When the head and cap are installed into the countersunk
31 hole in the skin, the cap being resilient tends to fill
32 the gap between the top portion of the head and the inner
33 side walls of the co~ntersink. The cap forms with the
34 kin an aerodynamic surface which i5 sufficiently
1 308581
1 continuous to receive and maintain a crack free coat of
2 paint having a uni~orm thic~ne~s.
4 The problem here i6 that the cap encompa6ses a
port~on of the underhea~ conical ~ur~ace and when
6 installed is wedged between th~ fastener head and the
7 counter6unk hole. ~his presents a problem in heavily
8 loaded structures in that the resilient material between
9 the fastener head and coun~ersink will tend to extrude
under cyclic loading and the fastener ~ay eventually
11 become 1008e fittlng therein. Additionally, since the
12 cap $8 $nstalled w~th a given thickness and tolerance it
13 may not always provide an absolutely smooth external
14 contour. Another approach to providing prote~tion for
metal fasteners in composite structures i6 found in U.S.
16 Patent No. 4,681,497 "Encapsulated Fastener" by I.
17 Berecz. Here the head and shank are encapsulated in
18 composite material and thus, it is subject to the same
19 deficiencies as the preceding example. Another approach
can be foun~ in U.S. Patent No. 4,502,092 "Integral
21 Lightning Protection System for Compo~ite Aircraft Skins"
22 by E.T. Bannink, Jr. et al. Here a plastic strip is
23 placed over the outer sur~ace and a potting compound is
24 used to fill the space over the fastener. In U.S. Patent
4,628,402 I'Lightning Protection of Fasteners in Composite
26 Material" by J.H. Covey, one embodiment uses a rubber
27 plug over the fastener to fill the space above the
28 fa6tener caused ~y the layers of dielectric material.
29 None of the above fastener systems designed for use with
composite materials are suitable for use with metal
31 6urfaces. Thus, there is no available fastener design
32 that can provide a smooth continuous aerodynam$c sur~ace
33 in both composite and metal structures and which does not
34 require a paint be ~pplied over the surface, etc.
1 308581
68567-71
Thus, it is a primary object of the subject invention to
provide a flush head fas~ener 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 providiny 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
~ ccordlng to one aspect, the present inventlon is a
flush head fastener for joining two or more structural elements
together, the structural elements havlng a fastener hole therein
wlth a recess for receiving the head of the fastener ln
communication with one surface of the structural elements, the
fastener comprising: a shank; a head ~oined to said shank; and a
layer of non-resilient material permanently joined to and
generally coverlng the entire top surface of said head of the
fastener, said layer of materlal readily reconfigurable relative
to said head and the structural elements; such that upon
installation of the fastener in the hole said layer of material
can be reshaped by deformation filling any gap between said head,
said layer of material of the fastener and the recessed end of the
hole, and the excess material above that needed to blend said
layer of material into substantial conformity with the one surface
of the structural elements can be readily removed.
Preferably, the layer of material has a melting point
substantially lower than the mel~ing 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
1 30858 1
68567-71
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 i~ 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 wlll 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 havlng a metal head and shank, it must be understood that
they could be made of a plastic materlal 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 deslgned for use in a spherical countersink, dimpled, or
counterbored hole and the like.
Having thus described the fastener it is now necessary
to describe the method of installing. The steps are basically as
follows:
forming a countersink fastener hole in said structural
elements, said countersink located on an external surface of one
of said structural elements; providing a fastener with a shank and
1 308581
68567-71
head portions and a layer of material permanently joined to the
top surface of said head, said layer of material readil~
reconfigurable relative to said head and shank and structural
elements; positioning said fastener in said hole with said head of
said fastener in contact with said countersink; deforming said
layer of material such tha~ any gaps between said countersink and
said head are filled; and removing excess materi.al of said layer
of material such that the upper surface of said deformed layer of
material is continuous with the external surface contiguous to
said fastener.
-` 1 30858 1
1 In one preferred embodiment, the head of the fastener
2 incorporates an internal wrenching recess and the layer of
3 material includes a wrenching tool access hole therethrough
4 in communication with the internal wrenching recess in
the fastener head. A plug is provided, made of the same
6 material as the layer of material or other compatible
7 materials, which can be inserted ~nto the hole and/or
8 recess a~ter torquing the fa6tener in place and which will
9 become part of the fa8tener after the 6tep of deforming.
11 In another preferred embodiment the layer of material
12 incorporates frangible external wrenching surfaces ~oined
13 to the layer of material which can be broken off when the
14 proper torque level is reached.
16 ln still another preferred embodiment of the invention the
17 underhead surface of the fastener is conical in shape having
18 a total included angle substantially equal to the total
19 included angle of the countersink hole. The layer of material
extends out from the head of the fasténer having a conical
21 shaped underslde contiguous with the conical underslde of
22 the head and having a total included angle substantially equal23 thereto. Thus, when this fastener i8 ln~talled lnto the
24 countersunk fastener hole, no gap will exist between the
materiel layer and counter~ink, and, little or no deformation
26 of the layer of material need t8ke place, although some
27 deformation may be required to insure line-to-line contact.
28 Thereafter, the excess material can be removed in the manner
29 previously described.
31 The novel features which are believed to be charac-
32 teristic of the invention, both as to its organization
33 and method of operation, together with further ob~ects
34
--" 1 30858 1
1 nnd advantages thereof, will be better understood from
2 th~ ~ollowing de~cription in connect~on with the accom-
3 pany~ng drawing~ in which pre~ently preferred embodimQnt~
4 of the invention are ~llu~trated by way of exampleg. It
is to be expre~sly understood, however, that the drawings
6 are for purposes of illustration and description only and
7 are not intended as a definition of the limits of the
8 invention.
10Brief Description of the Drawinqs
11
12Illustrated in Figure 1 is a partial cross-6ectional
13view of the fastener installed in structural elements;
14the right ~ide of the figure showing ths fa~tener a8
15provided while on the left side the ~astener is Rhown
16after lnstallation is complete.
17
18Illustrated in Figure 2 is an enlarged partial
19cross-6ectional view of the fastener shown in Figure 1.
21Illustrated in Figure 3 is a partial cro~s-sectional
22view of a second embodiment of the fastener installed in
23~tructural elements; with the right side of the figure
24showing the fastener as provided while on the left side
25the fastener iB shown after installation i~ completed.
26
27Illustrated in Figure 4, is a partial cross-section-
28al view of a third embodiment o~ the fastener installed
29in structural elements; with the right side of the figure
30showing the fastener in the as provided and the left side
31of the fiqure the rastener i~ shown a~ter installatlon is
32complQted.
33
34Illustrated in Figure 5, 6, 7, and 8 are partial
35cros~-sectional views of the fastener illustrating the
1 30858 1
1 process by whlch the fa~tener i6 ~ nstalled ~n the
2 structural elements.
4 Best Mode for carryi~q Out the I~vention
6 Illustrated in Figure 1 i~ a partial cross-sectional
7 view of two structural elements, indicated by numerals lo
8 and 12, respectively, joined together by the improved
g flush head fas~ener~ generally indicated by numeral 14.
lo The fastener 14 is primarily designed for use on air~
11 craft, and, in particular, to She fastening of external
12 skins or other external structural elements to interior
13 structures. Thus, ~or purposes of illustration, struc-
14 tural element 10 can be considered the outer skin of an
aircraft having an exterior surface 16 exposed to
16 aerodynamic forces and lightning strikes as well as
17 illumination by thrsat radars. The fastener 14 is shown
18 installed as provided on the right side of the figure and
19 after install~tion i8 compl~ted on the left.
21 The fastener 14 i6 mounted in a countersunk fastener
22 hole 18 with the counter~unk portion 20 in communication
23 with the surface 16 and having a total included angle
24 indicated by numeral 22 ~typically lOo degrees for metal~
~nd 130 degrees for composites). Fastener 14 includes a
26 head portion 30 having a top surface 32 and a conical
27 ~haped underhead sur~ace 34 having a total included angle
28 substantially equal to the an~le 22 of the countersink
29 portion 20. The fastener further includes a small
cylindrical land 36 at the ~unction of the top and
31 underhead 6urfaces 32 and 34, respectively. Thi~ land 36
32 is machined to ~liminate any sharp edges that would
33 normally exist at the junction of the surfaces 32 and 34.
34 However, this land 36 is extremely small and is ~hown out
of proportion for illustration purposes only. Thus, a
~ 1 3nsssl
1 volume indicated by numeral 38 i~ created between the
2 fastener head and countersink. Also the counter~ink
3 portion 20 ~, typically, larger than the overall
4 diameter of the land 36, thus, the volume 38 is ring
~haped. On a contoured ~urface the volume 38 takes on a
6 more crescent shape.
8The fastener 14 further inclu~es a shank portion 40
9having a threaded end 42 with an internal wrenching
10recess 43 and is held in place by means of a nut and
11washer, indicated by numeral6 44 and 46, respectively.
12Thus, a fastener can be locked in place by use of tools
13applied from the shank end of the fastener.
14
15Still referring to Figure 1 and additionally to
16Flgure 2, which is an enlarged view of a portion of the
17left side of Figure 1, a layer of material 50 having a
18thickness 51 (right sidQ of Figure 1) i6 ~oined to the
19top surface 32 of the fastener 14. If the structural
20element 10 is steel, or aluminum, the fastener is
21typically high strength heat treated steel or titanium.
22The layer 50 would be made of a soft aluminum or titanium
23material or a ~older such as those covered by Federal
~4Standard QQ-S-571, Tin Alloy, Tin-lead Alloy and Lead
25Alloy Solders. If ~older is used for the layer of
26material it can be heated to a very soft, readily
27deformable state (having a melting point well below that
2Rof the aluminum or titanium structural element 10 or the
29head and 6hank of the fastener) such that the structural
30element or fastener are not effected at this tempera-
31ture. By a process to be subsequently di~cussed, the
32layer of material 50 is deformed 80 as to ~ill the
33volume 38 about the head portion 30 (between the land 36
34and counter6in~ portion 20) and, thereafter, the excess
35material, indicated by dotted lines and numeral 52 on the
11
-" 1 3Q858 1
1 left side of F$gure 1, is removed. The remaining materlal
2 layer 54 and surface 16 are blended into one smooth and contlnuous
3 surface. The overall thickness 51 of the layer of material 50 is
4 therefore selected eO provide sufficient material to fill the
volume 38 and the remaining unfllled portion of the counterslnk 20
6 above the surface 32 of the head portion 30. This thlckness will
7 depend upon the allowable (selected) tolerances of the various
8 dimensions of the countersink portion 20 and the head portion 30.
9 The end result ls that no discontinuities will exist ehat could
reflect incoming radar slgnals, the surface is aerodynamically
11 smooth and there are no electrical discontinuities.
12
13 If the structural element 10 is a composite material
14 having an organic matrix, such as a thermoplastic or thermosetting
resin matrix with fiberglass or Kevlar filamentary reinforcements,
16 then the layer 50 could be a thermoplastic material such as a
17 polycarbonate; one ehat melts upon heatlng and does not cure
18 (with a melting point well below the melting point of the matrix-
19 material). Another suitable material is Nylon 6 manufactured by
the Dupont Corporation, Wllmington, Delaware. In either case
21 the metal fa6tener i8 protected from lightning strike sttachment.
22 If the structural element 10 were made of graphite reinforced
23 PEEK (polyetherether~etone) manufactured by ICI, Americas,
24 Inc., Wilmington, Delaware, then the layer of material could be
carbon flber reinforced Nylon 6 to eliminate electrical
26 discontinuities. As can be seen, whether the layer of material is
27 a metal, dielectric material or a dielectric material loaded with
28 conductive fillers will depend upon the particular application.
29
Illustrated ln Flgure 3 is 8 partial cross-sectional
31 view of a second embodlment of the fastener indlcated by
32 numesal 14A. ~he fastener 14A differs only in havlng an
33 internal wrenching recess 60 in the head 30A and the
335 * 1~ 4~k
)~
1 308581
1elimination of the internal wrenching recess 43 (see
2Figure 1) from the shank 40A. All of the other pre-
3viou~ly me~tioned dimensions of the fastener are identi-
4cal, and thus, have the same identifying number. The
5layer of material, indicated by numeral 50A, having a
6thickness 51A, incorporates external wrenching ~urfaces
762 ~onnected by means of a necked down frangible portion
864. Thu8, the nut 44 cDn be prevented from rotation AS
9the fa6tener is torqued. However, in most applications
lothe fastener would be engaqin~ a plate nut ~not shown).
11When the proper torque level is reached the wrenching
12surfaces 62 can be "torqued off" 6eparating at the
13nec~ed down portion 64. Therea~ter, the same procedure
14i8 used, which again will be subsequently discussed, to
15produce the completely installed fastener with excess
16material 52A removed leaving remaining layer 54A, as
17illustrated on the left side of Figure 3.
18
19Illustrated in Figure 4 i8 a th~rd embodiment of the
20fa6tener. The ~astener 14B includes a wrenching recess
21 60B in the head 30B and ~he ...... layer of material 50B
22includes an aperture 70 in commun~cation with the rece6s
2360i3. Thus, a8 in the embodiment illustrated in Figure 3,
24the fastener 14B can be prevented from rotation when the
25nut 44 is torqued ~or vica versa). A plug 71, preferably
26made of the same materi~l ~8 the 12yer of material 50B
27or another suitabl~ ~aterial iB provided which can be
28inserted lnto the recess 60B after the fastener 14B has
29been locked in place ~left side of Figure 4). The layer
3050B, having a thickness indicated by 51B, include~ an
31underhead surface 72 with an included angle 74 egual to
32angle 22 of the countersin~ hole and extends upward and
33outward so as to almost completely fill the countersink
34(leaving only the small volume 38' between the land 36
35~nd countersink 20). $his makes the filling of the
13
~ `
-` 1 30858 ~
1 countersink by the process to be ~ubsequently discussed
2 much eas~ er. Once the fa~tener ~5 installed, the ~nsert
3 . or plug is installed (driven in place by a hammer or the
4 l~ke) prior to rsmoving excess layer material S2B,
s leaving remaining l~yer 54B. If the fastener needs to be
6 removed, the plug can be "dug cut", preferably after
7 heating to soften it.
9Having thus described three embodiments of the
10improved fastener, it is readily apparent that t~e
11features of each can be interchanged. For example, the
12concept of a conically shaped underhead of the layer of
13materials 50B in the embodiment illustrated in Figure 4,
14can be used in the embodiments illustrated in Figures 1
15and 3. Since the discussion of the physical embodiments
16of the fasteners prior to installation and after final
17installation has been completed, it is nacessary to now
18discu6s the procedure for installing fastener.
19
20Referring to Figure 5, it can be seen that the
21fastenQr which was ~llustrated in Figure 4 i8 used a~ an
22example. After the fastener has been locked in place, ~n
23curved structural elements 10A and 12A, the plug 71 is
24driven into place as illustrated. Thereafter, as
25illustrated in Figure 6, heat is applied to the material
26layer 50~ and plug 71 until, the materialS are 80ft and
27ea5ily deformable (as illustrated, a heatlamp 75 is
28uced). Illustrated in Figure 7, the layer 50B and plug
2971 are tapped down to firmly drive the layer of material
3050B into contact with the countersink 20 and filling
31volume 38'. This step typically deforms the layer of
32materials 50~ 60 that it has an irregular surface,
33indicated by numeral 76. This can easily be accomplished
34by use of a rivet gun, partially illustrated and indi-
35cated by numeral 77. In many ca es, little or no impact
14
-` 1 308581
1 will be requixed and the ~tep can be eliminated. However,
2 with the Qmbodiments illustrated in ~ and 3, cons~d~rable
3 impact may be neces~ary to achieve filling of the volume
4 38'. It must also be noted that if the layer of the
material and plug is ~oft at room temperature heat may
6 not be neCes~ary. Furthermore, if heat ~6 applied, it
7 may be po~sible to soften the layer of material 50B
8 sufficiently 60 that little tapping ox driving is
9 neces~ary. Regardless, of whether the layer of material
50B raquires heatin~ and drivin~, heatin~ alone or
11 driving alone, the next step as illustrated in Figure 8
12 is to remove the excess material 52BI leaving remaining
13 layer 54B~. This is easily accomplished by use of a
14 chisel 80 having a knife edge 82 conforming to the
contour of the curved surface 16'. It is important to
16 note that if the structural element lo is a compo~ite
17 material with an organic matrix, the layer of the
18 material 50B and plug 71, ideally would be a thermoplas-
19 tic material, which would require heating to make it
deformable.
21
22 In some applications it may be desirable to heat the
23 layer o~ material to A temperature wherein it melts and
24 i~ easily flowable provided the 6tructural elements can
withstand the temperature. This would eliminate the need
26 for impact. The main advantage of melting is the
27 elimination of any gaps between the ~astener head portion
28 and the countersin~ improv~nq electrical continuity, etc.
29 However, ~ome difficulty may be encountered in removinq
excess material and providing a smooth surface. Melting
31 could easily be accomplished when using a solder,
32 thermoplastic or even thermosetting materials such as an
33 epoxy.
34
1 3085~1
1 Thus, the main advantages of the fastener are
2 readily apparent: 1) it can provide a ~mooth aero-
3 dynamic 6urface. 2) can provide a 6urface with no
4 electrical discontinuities. 3) can be used with both
metal and composite ctructures. 4) can provide lightn-
6 ing strike protection. Other advantages are a reduced
7 tendency to rotate and loosen, in some applications the
8 tolerances on the countersink end of the hole can be
9 relaxed and it may be possible to eliminate aorrosion
resistant coatings or sealants on the fastener or hole
11 where such coatings or 6ealants are presently required.
12 Furthermore, while primarily de~igned for u~e on aircraft
13 the fastener has application on external surfaces of land
14 vehicles and ships, interna; fluid ducts in general or on
any surface for cosmetic purposes. In this latter
16 application it ls apparent that the concept is applicable
17 to fnsteners having plastic heads and shanks as long as
18 the plastic has a higher strength and~or hi~her melting
19 point than the layer of material.
21 While the invention ha6 been described with reference
22 to particular embodiments, it should be under6tood that
23 the embodiments are merely illustrative as there are
24 numerous variations and modifications which may be made
by tho6e skilled in the art. Thus, the invention is to
26 be con6trued as bein~ limited only by the spirit and
27 scope o~ the appended cla$ms.
2~
29 Industrial Applicability
31 The invention has applicability to vehicles and
32 ~tructures and, in particular, to aircraft.
16
