Note: Descriptions are shown in the official language in which they were submitted.
~ ;~r~ ~L6~
_IND FASTENER
This invantion ralatas to a blind ~astansr and particularly
to a blind fastaner of tha kind eomprising a haadad body having a
taparing nosa, and a tubular slaeve, and which is set by forcing
the sleeve over the noss of tha body so as to radially axpand tha
sleeve, tha expandad sleeva being sscured to the body and providing
a blind head whieh can cooparata with ths hsaded body to fastan
togather apartured mambers of a workpiace.
Tha invantion is conearned partieularly, although not ~xelu-
sivaly, with a fastanar of the kind statad and which has a scraw-
threadad, haadad bolt thraadadly angagad in a screw-threadad bora
in the body, and which bolt can be caused to move along its axis
by rotation ralativa to the body, wharaby tha head of tha bolt can
ba rausad to mova towards tha body and to force tha tubular slsevs
ov0r the taparing nosa so as to provida a blind haad.
Fastanars of tha lattar kind ara wall known and commareially
available under the Ragisterad Trade Mark "~0-50ET", and are wiriely
used in the aerospaee industry, partieularly for fastaning metallie
shaats, sueh as shaats of aluminium alloy, to aach othar or to
othar mambars, 9uch as struetural supporting mambars. Ganerally,
such fasten3rs ara aminently satisfactory when the matarials to be
fastenad ara at least moderately hard and not particularly frangibla
or duetila, but thay are less satisfaetory whan tha work membar whieh
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has to be angagsd by tha blind head is frangible.
In particular, same composita materials, such as carbon- and
glass- fibre reinforced plastics matarials which ars now often
employed in shaat form whers formarly metals wera used, havs desira-
bls characteristics of low density and high tsnsile strength, butara frangible in that they have low compressive and shaar strengths,
and are sometimes unabla to withstand the stresses to which thsy
are subjsctsd whsn fastsned by conventional fastsnsrs of the kind
referrsd to
The main reason for ths unsatisfactory performancs of the con
vsntional fastsnsrs of the kinds rsfsrred to in frangible matsrials
is that the blind hsad providsd by the expandad slasve engagas only
a small arsa of ths workpisce pariphsrally of the aperturs through
which ths fastsnar 0xtands and immediately adjacsnt to ths sdga of
ths matsrial bordering the apsrturs. This rssults in ths frangibla
material bsing subjsct to localissd strassss which it is unabls to
withstand.
This disadvantage of tha conventional fastsnars can ba attribu-
tsd,at lsast in part,to tha fact that,while the external diamsters o~
2û both the sleevs and the part of the body which sxtends through ths
aparture in ths workpiscs are necsssarily limited by ths diameter of
the aperture, the nose of tha body is of simple frusto-conical
shape, and the body is so shaped that, during setting of the fastener,
the expanding sleeva tends to follow closely the contour of the
parts of the body over which it is forced. Consequently, the
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maximum internal diameter of the expanded slaeve is not appreciably
graater than tha diamatar of the part of the body which has been
passed through the aperture in ths workpisce, and ths maximum exter-
nal diameter of the expandsd slaeve is only greater than this by
twice the wall thicknass oF the expanded sleeve. It is therefors
virtually inevitable that the blind head thus provided by convsn-
tional fastsners of the kind raferred to will engage the workpiecs
in the sdgs rsgion closely bordering the aperturs and will lead to
stressing of the matsrial in this region.
We havs now devised a fastensr of the general kind referred to
in which tha body is so shaped as to expand part of the sleeve to
form a blind head having an internal diametsr substantially grsater
than the external diameter of tha part of the body which extends
through the workpisce.
According to the present invention, thara is provided, a blind
fastsnar comprising a tubular body having a praformad hsad for
abutting one face of an aperturad workpiaca, an alongata shank joinzd
to tha praforrnad head and abla to axtend through tha workpiaca so
as to projact beyond the opposite face (herainaftar rafarrad to as
thr~ blind faca) of tha workpiaca, and a nosa at tha end of the shank
remote from the praformad haad, which nosa tapars externally from
a diamatar tha same as that of tha shank to a smallar diamatar at
the and ramota from tha shank, a ductila tubular slaava9 and means
for forcing the slaeve ovar the tapering nosa and towards tha pre-
formad haad so as to causs the slaev0 to be progressively expandedradially to form a blind haacl, wharain that external surface of
tha nosa which oparates to cause axpansion of tha sleave is tapered
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at a greater angle to the longitudinal axis of the body in a region
adjacent to the shank than in a region nearer the smaller diameter
end of the nose, said greater angle being sufficiently great that,
when the sleeve is forced over the nose so that its leading end
passes beyond the nose, the leading end of the sleeve continues to
expand radially to a diameter substantially greater than the external
diameter of the shank, and is thereby permanently deformed to an extent
such that its internal diameter remains substantially greater than the
external diameter of the shank.
The nose may be tapered through a plurality of frusto-conical
portions.
The nose may have two frusto-conical portions of which the portion
nearer the shank has a greater apical angle than that remote from the
shank.
The nose may have a frusto-conical portion immediately adjacent
the shank, and two or more further portions of which one may have an
apical angle as great as that of the portion adjacent the shank
provided that the nose has a further portion of smaller apical angle
between the said one further portion and the portion adjacent the shank.
The nose may be tapered to a smaller diameter through a plurality
of portions of which each one nearer the shank is more steeply tapered
to a greater apical angle than il;s predecessor nearer the end of the
nose remote from the shank.
The nose may have an external sùrface which forms, in the direc-
tion longitudinally of the body, a concave curve providing increasingly
steeply tapered portions nearer t:he shank.
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Tha curve may ba continuous.
The curve may be an arc of a circle.
The curve may be an arc of an ellipse.
Spacific ambodiments of tha invention will now ba described
by way af example and with rsferance to the accampanying drawings,
in which:-
Figura 1 i9 an axial section through a first fastener, inssrtedthrough a workpiace, bafore installation therein;
Figure 2 is similar to Figure 1 but shows the fastaner installed;
Figure 3 illustrates the shape of the nosa of the fastansr shank;
and
Figure 4 is similar to Figure 3 but illustratas the shape of
the nosa of a second fastener.
Referring first to Figura 1 of tha drawings, a blind fastensr
10 comprises thrae piecas, namely a tubular nut 12 which constitutes
tha body of tha fastener, a bolt 14, and an sxpansible sleeve 16.
Tha nut 12 comprisas an alongate cylindrical shank 1~, an enlar-
gad hexagonal hsad 20 at ona end of the shank, and a nose 22 at the
and of the shank remote from the head 20. Tha nose 22 tapars to a
smaller diameter away from tha head 20. The nut has a scraw-thraadad
bora 24 which extends axially through the head, shank and nose.
~ D~
The bolt 14 comprisas an elorlgat~ stem 30 and an enlarged head
32 at ona end of the stem. The stam 30 is thraaded for sngagem2nt
with the thread of the bore of the nut, and is longer than the combinad
lengths of the nut 12 and the sleeve 16. Ths head 32 has substantially
the same diameter as the shank of the nut. The stem of the bolt has
a tail portion 40, remote from the haad 32, which is providad with
a pair of diamatrically opposed flats 42 to facilitata gripping and
rotating of the bolt. Th2 tail portion 40 is connected to the remain-
der of the stem by a breakneck 34 at which the stem will break when
subjected to a predet2rmined torque, thus enabling the tail portion
40 to be detached from the remainder of the bolt which remains as an
assential part of tha installad fastaner.
The slseve 16 of this embodiment is a tubular cylinder having
an extarnal diamater substantially the same as that of thz shank 18
of the nut, and has a smooth axial bora which is internally chamferd
at the leading end to enabl2 the smaller diamet2r end of the nose 22
to enter easily ther2into. The diametar of th3 bors of
the sle2v2 16 is sufficiently great to enabl2 the stem 30 of the
bolt to ext2nd through the bora with slight claarance.
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Prior to use, the nut 12, bolt 14 and slsevs 16 ara assemblsd
togsthsr, the stem of ths bolt bsing inserted through the sleeve
and then scrswsd into ths snd of tha nut rsmots from ths hsxagonal
head 20 until the tail portion 40 projscts from ths hsad snd of ths
5 nut. Ths asssmblsd fastsnsr can thsn bs inssrtsd into alignsd
apsrturss 35, 36 in msmbsrs of a workpisce 37 which are to bs fastsnsd
togethsr, so that ths shank 1~ sxtsnds through tha membsrs with tha
hsxagonal head 20 abutting ths near face 3~ of the workpiscs, and
the bolt head, slesvs, nose, and part of ths shank projsct bsyond ths
10 opposits or blind facs 39 of ths workpiscs.
Ths fastsnsr can then bs sst by gripping and rotating ths pro-
jecting tail of tha bolt rslative to ths nut so as to cause the bolt
hsad to mous axially towards ths nut and force ths slssvs on to and
over ths noss of ths nut until th3 12ading snd of the sleevs clamps
15 the membsrs of ths workpisce tightly against ths hsad 20. At this
stsgs the torque required to turn the bolt furthsr risss abruptly,
and continusd rotating thsrsfors results in ths bolt brsaking at ths
brsaknack, allowing ths tail portion to ba discardsd.
In accordancs with ths invsntion, ths noss 22 of ths nut of this
2û embodimant is so shapsd that, whan ths slasvs is forcsd towards tha
head so that ths lsading snd of ths slesve passes ovsr and bsyond ths
noss, ths leading snd of ths slssvs bscomss axpandsd to a diamstsr
substantially grsatsr than that of ths shank of the nut.
Thus, rsfsrring mors particularly to Figurs 1 of tha drawings,
25 the noss 22 has a terminal frusto-conical portion 44 adjacent tha
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end of the nut remote from khe head 20, and a second frusto-conical
por-tion 46 between the terminal portion 44 and the cylindrical shank
18. The terminal portion 44 extends over about two thirds of the axial
length of the nose, and the second portion 46 extends over the remaining
one third.
As illustrated in Figure 3, in this embodiment the external surface
of the terminal frusto-conical portion 44 tapers towards an apical
angle Of 40, while the external surface of the second frusto-conical
portion 46 tapers more s-teeply, towards an apical angle of 7C. Thus,
the apical angle of the portion 44 is substantially less than that of
the portion 46, and the apical angle of the portion 46 is substantially
greater than the apical angles of single-tapered noses of conventional
fasteners of this general type, which are generally in the range of
30-42. The angle between the surfaces of the two portions should
be greater than 90, and this embodiment is 165.
It can therefore be appreciated that, when the sleeve is forced
over the nose during setting of the fastener, the leading end 48 of
the sleeve first encounters -the gently tapering terminal portion 44
and, with comparative ease, is progressively expanded radially, with
the expansion progressing along the length of the sleeve from i-ts
leading end, so that the sleeve becomes flared. Subsequently, the
already flared leading end of the sleeve encounters the more steeply
tapering second portion 46 by which it is then expanded at a greater
rate per unit of longitudinal travel through which i-t is moved by the
bolt head. Eventually, the leading end of the sleeve reaches and
passes the point of intersection between the second portion 46 of the
nose and the cylindrical shank. As the shank 18 is cylindrical and
has a constant diameter the same as the maximum diameter of the nose,
the shank provides no internal support for the
P.2~5
sleeve. Due to the taper of the second portion 46 of -the nose and
notwithstanding the increasing hoop stress generated in the expanding
sleeve, the leading end of the sleeve continues to move substantially
in line with the surface of the portion 46 for a short distance and
5 thus continues further to expand radially until the hoop stress in the
sleeve overcomes the tendency for the leading end to continue expanding.
The leading end then ceases to expand further.
We prefer that the leading portion of the sleeve which passes
beyond the nose should assume a substantially cylindrical form in the
10 region adjacent to the leading end, while also having a diameter, inter-
nallY,substantially greater than that of the shank, rather than that
it should flare or taper towards the leading end. We therefore make
the angle at which the second portion 46 meets the cylindri-
cal shank 18 such that eventually, and before the leading end 43 will
15 mee-t the workpiece, the tendency of the sleeve to continue to expand
a-t the same rate as when passing over -the surface of the second portion
46 will be substantially balanced by the hoop stress in the sleeve which
simultaneously tends to cause the leading end of the sleeve to turn ra-
dially inward towards the external surface o~ the shank. The precise
20 angle at which the second portion of the nose should meet the shank in
order to balance the forces acting on the leading portion so that it
will become cylindrical must be determined with regard to the length of
the second portion and the physical properties, particularly the ducti-
lity, elasticity and hardness, of the leading end of the sleeve. In
25 practice, we determine the angle empirically.
Continued rotation of -the bolt with respect to the nut causes the
substantially cylindrical leading part of the sleeve 16 to advance,
under the urging of the bolt head at the other end of the sleeve, until
it abuts the blind face 39 of the workpiece and applies a force to it.
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This it does over an annular zona which is spacsd apart radially
from ths 3dgs o~ the aparturs 36, so that strass is not applisd to
tha workpiacs immsdiatsly adjacant this sdgs. The 3xpandsd slsavs
provides a blind haad which~ togsthsr with ths prsformsd haad 20
of ths nut, clamps ths two membsrs of the workpiscs tightly togethsr.
Rssistancs to rotation of ths bolt incrsasss, and ths tail portion
40 breaks off, lsaving ths fastsnsr installed in ths workpiscs, as
illustratsd in Figurs 2. Ths nut, bolt and sxpandsd slssvs ars
effsctively sscured togsthsr by frictional sngagsmant. Ths distinct
intsrnal chamfar on tha leading end of tha sle~va is no longer
apparsnt, having bsen smoothsd out, by ths passage of tha slssvs
along ths nut nosa.
Ths anglss chossn for this smbodimsnt and statsd abovs havs
bsen found appropriate for a slssvs formsd of a stainlsss stasl
spacifisd as A.I.S.I.304 (i.s. Amarican Iron and Stsal Instituts
dasignation No.304). Gsnsrally, we havs found that ths apical angls
of ths portion 46 should bs grr3ater than is usual for ths singls
tapar ussd in convsntional fastsnsrs of this kind, and prsfsrably
should not bs less than about 60 .
It is to be noted howsvsr) that if ths apical angla of tha
portion 46 is too grsat, not only will sxcsssivs Forcs bs raquired
to forcs the sl3svs ovsr this portion of ths noss, with ths attsndant
risk of buckling ths slssvs or causing pramature braaking of ths
breaknr3ck, but also ths hoop strassss in ths lsading snd of ths
slssvs will not bs sufficisnt to prsvsnt continusd expansion of ths
lsading 3nd of ths slssvs within a rsasonabls distancs of longitu-
dinal travsl, with ths r3sult that ths lsading snd portion of ths
slssvs will hava a flarsd shaps rathsr than a cylindrical shape.
This is not dssirabls.
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Instaad of only two differently tapered portions, the nosa
may have a plurality of portions of which each ons nearer ths shank
is more steeply tapered than its predacessor nearer the eno remote
from ths hsad 20.
Thus, in a further embodirn3nt by way of an sxtr3me but practical
example of such a construction~ a fastener generally similar to that
of Figure 1 and illustrated fragmsntarily in Figure 4 has a nose 22 of
which the external surface forms, in the direction longitudinally
of the nut, a continuous concave curve which can be ragardsd as
1û being constituted by an infinite number of such increasingly staeply
tapered portions.
In this embodiment, ths curve is formed as an arc of a circle
of radius r of which the centre C is spaced forwardly of the end
of the nose remote from the head of the nut by a short distance d1,
and radially from the axis A of the nut by a distance which is greater
than ths sum of the radii of the bore of the nut and the circle
itself by a small distance d2.
The distance d1 is chosen preferably to be such that, at the
point whsre the curved external surfacs of the nose meets the end
face of the nose, a tangent to the curve meets ths end face at an
angle greater than 90, so that there is an immediate increase in
diameter of ths nose away from tha snd face.
The distance d2 is chosen prefsrably to be such that thers is
a finite distance radially across the snd facs batwesn ths intsrnal
surfacs of ths bors of ths nut and ths external surface of the nose,
so that ths snd facs is not a kni~s-edge.
: P.295
12
The length of the radius of curvature r of the curved external
surface of the nose is chosen so that the rate of change of diameter
of the external surface of the nose is acceptable in relation to the
rate of change in diameter which the sleeve can be forced to undergo,
having regard to the size of the fastener, the ductility of -the sleeve,
and the force available for deforming the sleeve.
In a specific example of this emdodiment, the shank of the nut has
an external diameter of 7.9mm, the major diameter of the threaded bore
is 5.6mm, and the minimum diameter of the nose is 5.9mm. The radial
width of the end face of the nose is therefore, (5.9 - 5.6) 2mm =
0.15mm. The curvature of the external surface of the nose, in the
longitudinal direction, is such that lt forms part of a circle having
a radius of 5.7mm, and the distance dl, which is determined by the
angle at which the external surface of the nose meets the end face,
is l.Omm, and the distance d2, which is determined by the radial width
of the end face and the value of dl, is slightly less than 0.15mm.
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13
The effect of the curvingly tapered nose upon the sleeve, when
the sleeve is forced on to and bayond the nose, is substantially the
same as in the previous embodiment in that the leading end of the
sleeve is expanded progressively and at a rate which increases as
the leading end progresses from the tip of ths nose towards the shank.
As in the previous embodiment, the angle at which the leading end of
the sleeve eventually leaves the nose, at the intersection of the
nose and shank, is sufficient to cause the leading and to continue to
expand radially before the increasing hoop stress in the lsading end
of the sleeve appreciably restricts the radial expansion. As in the
previous embodiment, ths hoop stress eventually restricts the expansion
sufficiently to cause the part of the sleeve following tha leading end
to assume a substantially cylindrical shape, surrounding but radially
spaced from the external surface of the shank. The cylindrical
leading part of the sleeve continues to advance, under tha urging
of the bolt head at the other end of the sleeve, until it abuts the
workpiece and thus constitutes a blind head which, in cooperation
with the head of the nut, serves to fasten the members of the work-
piece.
In contrast to the nose of the first embodiment, in which the
taper is provided by two distinct frusto-conical portions of which
the second has an apprsciably greater apex angle than tho first, or
terminal, portion, the rats of change from small diarneter to maximum
diameter of the nose of this embodiment is srnooth throughout the nose,
and consequently tha amount of force required to force the sleeve over
the nose does not increase suddenly. There is therefore less risk
with this embodiment of a shock reaction causing thH breakneck to
break prematurely.
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$~ ~'~
14
In this embodimsnt, the concave curvatura corresponds in function
and effact to the intarnally-angled relationship of the two frusto-
conical portions of the first embodiment.
Instead of the concave curvature of the nosa being in the form
of an arc of a circla, it could be in the form of an arc of an ellipsa
without substantially altering its parformance, provid0d the criterion
as to ths angle at which a line or tangsnt to the curve of tapar
meets the external surface of the shank, and, desirably,the end face
or tip of the nose~ is met
Although, in the foregoing embodiments, the construction of the
fastener is such that it can be sat by rotation of the bolt to urge
the sleeve on to and over the nose by reason of the lead-screw effect
achieved by the threaded interengagement between the nut and bolt,
other constructions whereby the sleeve can be forced over the noss
and, once it has been expanded and urged to fasten the members of a
workpieca together, be secured to the body of the fastener, are possi-
ble.
Thus, it is not necessary for the stem of the bolt and the bore
of the nut to have interengaging scrsw-threads, but instead, the nut
could be a haaded body having a shank, a tapered nose, and a smooth
bore instead of a threaded bore, and the stem of the bolt could then
ba pulled axially along the bore to cause tha bolt haad to urge tha
sleeve over the nosa and to engage the workpiece, khe stem of the bolt
being secured subsequently to the body so as to retain the sleeve in
position in any suitable manner such as, for example, by being deform-
ed to form an abutment able to cooperate with the body to prevent
retraction of tha bolt and loss of the expanded sleeve.
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In embodiments of the invsntion in which the tapering nosa is
formed of a plurality of frusto-conical portions, as for example in
th0 embodiment of Figure 1, it is to be noted that the length of
the frusto-conical portion naarest the shank has an influence on the
extent to which the lsading end of the sleeve continues to expand
radially after leaving the supporting surface of the nose. Thus,
it has besn found that, within limits, tha greater the length of the
frusto-conical portion nearest the shank, the greater the extent
by which the diameter of the expanded sleeve will exceed the diameter
of the shank.
Similarly, in ambodimants in which the nose prasents a roncavaly
curved surface which meats the external surface of the shank, the
radius of curvature of the curved surfaca will have an influence
on the extent to which the slaeve continues to expand.
In a still furthsr ambodiment, the body of the fastener has a
nose having a frusto conical portion adjacent to the shank, and
naarar the tip of tha nose a further frusto-conical portion of which
tha apical angle may ba as great as, or possibly even greatar than,
that of the portion adjacant the shank, the two frusto-conical
portions baing separated from each othar in tha axial diraction by
an intermediata portion of smaller or zero apical angle.
In considering the magnitude of the angle formed in any embodi-
ment between the longitudinal axis of the bolt and the surface of
the tarrninal portion or the tangant to tha curvs as it approachas
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16
the end face or tip of the nose, any angle form~d by any bavelling
or radiussing of the tip of the nosa for avoiding sharpness of the
nose-tip or assisting its location in the bore of tha sleeve, can
ba ignored.
Although we have disclosed some specific embodiments and
dimensions, it will be appraciatsd that ths invention is not restricted
thereto, as it is possible to make variations in the details of shape
and dimensions. For instance when tha tapering nose is form3d of a
plurality of frusto-conical portions, one frusto-conical portion may
merge into tha next by means of a portion of suitably concavely
curved profile.
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