Note: Descriptions are shown in the official language in which they were submitted.
1135~8~7
"A BLIND FASTENER"
This invention relates to a self-locking blind fastener and more
particularly to a blind fastener of the kind comprising three main parts,
namely a screw-threaded headed bolt, a nut having a female screw thread
for mating engagement with the thread of the bolt and having a shank and
an enlarged head, the shank provid;ng a nose which tapers towards the end
remote from the head and a sleeve which can be expanded to form a blind
head by screwing the bolt through the nut so that the bolt head abuts the
sleeve and forces it onto the tapered nose of the nut.
Priorto use, the parts are~ assembled, the bolt being inserted into
the sleeve and then screwed into the end of the nut remote from the nut
head until it projects from the head end of the nut. In use, the shank
of the nut is inserted through aligned holes in workpiece sheets to be
fastened together so that the head of the nut abuts one side of the work-
piece and the tapered nose projects beyond the opposite side. The
fastener is set by rotating the bolt relative to the nut so that, by
virtue of the threaded engagement between the nut and bolt, the head
of the bolt is advanced towards the shank of the nut, forcing the sleeve
over the tapered nose and thereby causing the sleeve to be expanded to
form a blind head which is held in abutment with the said opposite side
of the workpiece by the bolt head.
In one ~orm of blind fastener of the type referred to, the stem of
the bolt is provided with a breaker groove at which the diameter of the
stem is reduced to a breakneck at which the stem will break when sub-
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jected eo a predetermined torque. The breaker groove demarcates the
bolt into a main part which includes the bolt head and adjacent threaded
portion of ~he stem, and a break-off portion of the stem, remote from
the bolt head. The break-off portion is arranged to project from the
headend of the nut prior to setting the fastener so that the bolt can be
gripped and rotated reiative to the nut to set the fastener. It will be
appreciated that the torque required to turn the bolt relative to the nut
increases as the expansible sleeve is forced further on to the shank of
the nut and as any workpiece clamped between the head of the nut and the
blind head formed by the expanded sleeve is compressed, and eventually
the required torque exceeds that substainable by the breakneck so that
the bolt stem breaks at the breakneck, leaving the main part of the bolt
as an integral part of the set fastener, and the break-off portion is
discarded.
1~ Blind fasteners of such a kind are used in the aerospace industry
for the assembly of parts of aircraft and space vehicles, and, in
recent years, there has arisen a demand for such fasteners to be pro-
vided with self-locking threads in order to reduce the risk of the nut
and bolt becoming unscrewed in use as can happen as a result of stress
and vibration over prolonged periods of time.
Many kinds of self-locking thread arrangements have been proposed
and used for nut and bolt type fasteners but few are suTtable for blind
fasteners of the kind referred to above. There are two main reasons for
this. First, the strength of the lock obtainable has to meet certain
minimum requirements and it is also generally found that, the stronger
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the lock required, the greater the force needed to engage the lock.
However, the amount of torque available for engaging the lock in blind
fasteners of the kind referred to is limited to that which can be trans-
mitted by the breakneck of the bolt which in turn has necessarily to be
limited and controlled within a fairly precise range in order to ensure
that the fastener can be set and clamp the members of a work piece to-
gether sufficiently tightly?and for the bolt then to break at the break-
neck so as to avoid over-tightening of the fastener which could result
in failure of a different kind.
It will therefore be appreciated that while the torque required to
engage the lock cannot be allowed to approach too closely to that re-
quired to set the fastener and break the bolt, the necessarily lower
torque required to overcome the lock, which represents the effectiveness
of the lock, must be as high as possible. Moreover, Tn manufacturing
the blind fasteners careful control must be exerted in order to ensure
minimum variation between the various torque values concerned.
We have now devised a blind fastener of the kind referred to which
effectively meets the requirements of the market for a self-locking
capability and which can be manufactured to tolerances which are easily
controlled ~o as to provide consistent performance as between one
fastener and another.
According to the present invention there is provided a blind
fastener compris;ng a screw-threaded, headed, bolt, a nut having a
female screw-thread for matingly engaging the screw thread of the bolt
and having a shank and an enlarged head, the shank of the nut providing a
nose which tapers towards the end remote from the head of the nut; and
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a sleeve which can be forced onto the tapered nose of the shank of the
nut by screwing the nut along the bolt towards the head thereof and
thereby expanded to form a blind head, wherein the bolt has a sub-
stantially cylindrical locking zone, the diameter of the bolt in the
locking zone being intermediate between the crest diameter of the screw
thread of the bolt and the crest diameter of the thread of the nut, the
locking zone extending axially through a distance greater than the pitch
of the thread;
at least that part of the nut thread which will, in use of the
fastener, be traversed by the locking zone of the bolt having a crest
the surface of which forms part of a truly cylindrical surface having a
lesser diameter than the diameter of that helical strip upon the surface
of the locking zone of the bolt with which the aforesaid crest surface
will engage, whereby when the nut is screwed along the bolt to set the
fastener the crest surface of the nut thread interferes to a predetermined
extent with the locking zone along the helical strip and provides a con-
trolled interference giv;ng a frictional lock between the nut and bolt.
The locking zone may be spaced from the head of the bolt by a
distance equal to at least twice the pitch of the bolt thread.
The locking zone of the bolt may have a diameter equal to or
slightly greater than the effective diameter of the bolt thread.
Preferably the diameter of the locking zone is not less than the effective
diameter of the bolt thread.
Preferably, the interference bet~een the cres~ s~rface of the nut
thread and the locking zone is such as to be insufficient to cause
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plastic deformation of the nut thread.
The said interference may be within the range 0.025 mm to 0.1016 mm
and preferably is within the range 0.0381 mm to 0.0762 mm.
.
The truly cylindricalform of the crest surface of the said part of
the nut thread may be formed by an accurate dimensioning operation
performed subsequent to forming the thread of the nut.
The nut thread may be formed by a cutting or roll tapping operation
and the said part thereof may be a subsequently reamed portion thereof.
The mating surfaces of the nut and or the bolt may have a lubricant
coating thereon.
The lubricant coating may be a plating of a lubricant metal such
as cadmium.
Preferably the locking zone is intermediate between the ends of the
screw thread.
The locking zone may be formed as a cylindrical portion of sufficient
diameter to interfere wlth the crest of the thread of the nut without
causing appreciable plastic deformation of the thread of the nut.
Preferably, the locking zone of the bolt extends axially through a
distance equal to at least twice and not greater than four times the
pitch of the screw thread of the bolt.
A preferred form of the invention will now be described with
reference to the accompanying drawings in which:-
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Figure 1 is a sectional elevation of a blind fastener embodyingthe invention;
Figure 2 is an elevation of the bolt part of the fastener of
Figure l;
Figure 3 is a sectional elevation of an assembly, illustrating the
fastener of Figure 1 in use, and
Figure 4 is an enlarged fragmentary elevation, partly in section,
of the assembly of Figure 3.
Referring first to Figure 1 of the drawings, a blind fastener 10
comprises a nut 12, a bolt 14 and a sleeve 16.
The nut 12 comprises a hexagonal head 18 and an elongate shank 20
having an end portton 27 remote from the head and tapered away from the
head to form a conical nose. The nut is, of course, tubular and Is
formed with an internal screw thread 24 throughout the length of its
bore.
The sleeve 16 is a tubular cylinder having an external diameter
substantially equal to that of the shank of the nut and has a bore
which is flared at both ends, the flared ends each reaching slightly
greater internal diameter than the smallest diameter of the tapered nose
22 of the nut so that the nose 22 can easily enter either of the flared
ends.
The bolt 14 has a head 30 at one end of diameter substantially equal
to that of
/the shank of the nut, and an elongate stem 32 of greater ax.l:al length
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than the combined lengths of the nut 12 and sleeve 16. A break-off
wrenching portion 34 of the bolt stem most remote from the head 30 is
provided with a pair of diametrically opposed wrenching flats 36 whereby
the bolt may be rotated, and the portion 34 is joined to the remainder
of the stem 32 by a breakneck 38 at which the stem may be broken at a
predetermined torque, enabling the wrenching portion 34 to be detached
from the installed fastener as illustrated in Figure 3.
A screw thread 40 which is generally complementary to the screw
thread 24 of the nut so as to matingly engage the nut thread, extends is
throughout the length of the bolt stem 32, except in a locking zone
42 and over the surfaces of the breakneck and wrenching flats. The
locking zone 42 is spaced from the head of the bolt by a distance
approximately equal to the length of the sleeve 16. The locking zone
42 is in the form of an unthreaded cylindrical land accurately concentric
with the axis of the bolt thread and having a diameter which iç greater
than the crest diameter of the complementary thread of the nut and which
is less than the diameter of the stem taken across the crest of the thread
on the bolt. The land 42 extends axially through a distance which is
equal to twice the pitch of the thread 40. It is not objectionable if
2n the surface of the land has a raised vestige of thread provided that the
land has a helical path over which the mating thread of the nut can pass
which path has a surface which is part of a cylinder concentric with the
axis of the bolt thread~
Moreover, at least ~hat part of the thread 24 of the nut which, in
use of the fastener, will be traversed by the locking zone of the bolt
.
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has a fiattened crest. As shown in the drawings, the entire length of
the nut thread has a flattened crest since this is more convenient to
achieve in the manufacturing process than over a limited length of the
thread. In particular, it is here stressed that the flattened crest of
the nut thread is formed with precision so that the surface of the crest
is part of a truly cylindrical surface.
The mating threads of the nut and bolt in this embodiment are
basically of the form known as Unified National Fine and as is well
known, the basic form of this thread has a flattened crest, although in
normal manufacture it is usual to tolerate some rounding especially of
the longitudinal edges of the crest.
However in forming the nut of the invention we take especial steps
thread
to ensure that the crest of ehe nut/is truly cylindrical and free from
burrs and irregularitTes such as are normally formed in manufacture by
tapping or thread rolling and to this end we first form the thread in a
normal manner by means of a cutting tap, and then ream at least that
portion which will be traversed by the locking zone of the bolt so as to
remove a portion of the crest of the thread and leave a truly cylindrical
crest surface.
Furthermore, the nut thread is reamed to a minor diameter such that
the crest will interfere with the cylindrical surface of the land of the
bolt to a predeter~ined extent within close tolerances. More specifically
the amount of interference is arranged to be such that the deformation
of the thread of the nut when engaged with the locking zone of the bol~ is
within the elastic limit of the materTal of the nut so that the nut
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thread is not caused to undergo any plastic deformation.
Thus, in this embodiment, both the nut and the bolt are formed of
steel, and the shank of the nut has a nominal external diameter of A
4.83 mm. The mating threads of the nut and bolt have the form of
6 - 40 UNF with an effective diameter for the bolt thread of 3.02 mm.
The cylindrical land of the locking zone of the bolt has a diameter
which is made the same as (or can be very slightly greater than) the
effective diameter, and the thread of the nut is reamed to a minor
diameter so as to interfere with the land of the bolt and with a
precision such that the interference is within the range of 0.038 mm to
0.076 mm.
Prior to use, the blind fastener 10 is assembled so that the
cylindrical sleeve 16 is disposed on the stem of the bolt 14 adjacent
the head 30, and the stem of the bolt is entered into threaded engage-
ment with the nut 12 so that the conical nose 22 at the end of the nut 12
just reaches the nearer end of the locking zone 42 on the bolt. The
wrenching portion 34 of the bolt then projects beyond the head 18 of the
nut.
Referring to Figures 3 and 4, in use of the blind fastener 10, the
bolt head 30, sleeve 16, and nut shank 20 are inserted through aligned
holes in workpiece members 50, 52 to be joined together so that the head
18 of the nut abuts a surface of the member 50, and the head of the bolt,
the sleeve and the conicat nose of the nut shank project beyond the face
of the other member 52 remote from the head of the nut. The nut 12 and
bolt 14 are then relatively rotated, conveniently using the wrenching
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flats 36 of the bolt and hexagonal form of the head of the nut ~8 to
obtain a suita~le grip~ so as to bri~ng the head 30 of the bolt nearer
to the head ~8 of the nut. In doing so, tfie sleeve ]6 ts forced over
the conical nose 22 of the nut shanR and tfiere~y expanded to form a
blind head 54 whicn ab~ts the ad)acent face of the workp~ece member 52
and causes the two ~orRpiece ~embers to ~e ti~g~tly clamped ~etween the
b`lind fiea~ 54 and tfie head ~8 of tfie nut.
Once che workpiece mem6ers 5~,52 are tigfitly clamped ~tween the
head 18 of the nut and the blind head 54, the force required to rotate
the bolt further relative to the nut exceeds that which can be sustained
6y the breakneck 38 and the bott stem breaks at the breakneck allowing
the wrenching portion to be discarded, leaving the set fastener in the
condition illustrated in Figure 3
In entering the bore of the nut, the locking zone of the bolt
]5 interferes with the thread of the nut, producing some elastic deformation
of the nut. However, due to the close tolerances within which the locking
zone and the reamed thread of the nut are formed, neither the locking
zone nor the nut thread undergo any plastic deformation and no galling
occurs The friction between the Interfering surfaces of the nut thread
2û and tfie locktng zone o6tained with the prescribed amount of interference
for tfiis embod~ment ~namely o.a38 - û.076 mm) i5 low enough to enable
the fastener to apply an adequate clamping pressure to the workpiece
mem6ers which, as preYiously expla;ned, is limited by the torque trans-
misstble by the breakneck, but sufficient to ensure that the fastener
will not ~ecome unscrewed even under rigorous condittons of use.
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Other similar fasteners have been made with dimenstons differing
from those of the foregoing example. In all those tried, tt has been
found that the dimensions of the fastener can be varied slightly but
that the interference between the crest surface of the nut tfiread and
the locking zone should always 6e ~ithin a broad range of 0.025 mm to
0.~016 mm. There is however, an overriding need to avoid the interference
6eing so great tha~ the bolt will break before the fastener has clamped
the workpiece mem6ers together, and also the interference ought not to be
so great that plastic deformation occurs at the interfering surfaces.
The friction between the interfertng surfaces reaches a maximum once the
locking zone has entered fully into engagement with the nut thread and
does not increase as the locking zone passes further along the nut thread.
Slmilarly, once the fastener has been set, the torque required to
turn the bolt in a directlon to unscrew the bolt from the nut remains
~5 constant as long as the locking zone of the bolt is entirely within
and engaged by the nut thread. Consequently, and especially in order to
meet user requirements for a thread lock which maintains a predetermined
strength of locking after the bolt has been unscrewed through up to two
full turns, we arrange the locking zone of the bolt at a position so
spaced from the head of the bolt that when the fastener is fully set, the
locking zone has entered the nut and passed beyond at least two full turns
of the nut thread.
In order that the transverse width across the flattened crest of
the nut thread is great enowgh to have substantial resistance to plastic
deformatton when tnterfered ~ith by the locking zone of the 601t, we
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arrange that the dianeters of the locking zone of the bolt and the crest
of the nut thread are such as to interfere with each other at a diameter
which is substantially the same as or slightly greater than the effective
diameter of the bolt thread, 6ut not so great that the height of the crest
of the nut thread above its root is so small as to render the nut thread
of inadequate strength to strongly engage the thread of the bolt.
After manufacturing the parts of the blind fastener to the dimensions
and tolerances referred to above, the parts are then provided with a
coating in the form of an electrolytically deposited layer of cadmium to
serve as both a lubricant coating on the surfaces of the frictionally
engaging parts and the interfering parts and to act as a rust proofing
coating.
- Although the bolt 14 of the self-locking blind fastener hereinbeforespecifically described and illustrated by way of example incorporates
break-off portion and a torque-limiting means in the form of a breakneck
which is adapted to transmit sufficient torque to engage the self-locking
threads and set the fastener and to break when the torque applied exceeds
a pre determ;ned value, It will be appreciated that Tt is not essential
for the bolt of a self-locking blind fastener according to the invention
to have these features since the fastener can be set and the threads
brought into locking engagement without any need to limit the torque
applied, or for the bolt to break. As previously explained, however,
it is very desirable to have some means for limiting the torque which
can be applied,in order to avoid over tightening the fastener: such
torque limiting means can be incorporated in the tool used for se~ting
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the fastener or in the fastener itself, as in the illustrated embodiment.
We prefer that the torque-limiting means be incorporated in the fastener,
since this avoids relying upon proper adjustment of the tool used.
Furthermore the breaking-off at an appropriate position of the part of the
bolt which is engaged by the tool when rotating the bolt, enables the set
fastener to have a neat appearance without any portion of the bolt pro-
jecting from the head of the nut.
It will of course be appreciated that when, as in the illustrated
em60diment, the 601t of the self-locking blind fastener has a torque-
limiting breakneck, then it is of particular importance to be able reliably
to control the torque required to engage the locking threads and set the
fastener in order that this torque shall not exceed that transmissible
by t~e ~reakneck. The thread-locking arrangement incorporated in a
fastener according to the invention is susceptible of close control in
manufacture and is therefore particularly suitable for the particular
kind of fastener having a bolt with a torque limiting breakneck.
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