Tom Shelley reports on a fastener which offers
benefits in an unusually wide range of potential applications,
include joining to composites and wood
A simple fastening expands a collar inside a hole so as to
improve stress distribution while maintaining an exceptionally
firm grip.
Devised by a race engine specialist, it consists of only two
parts in its most basic form, a stud and a collar.
It nonetheless offers striking advantages when used to join metal
parts to each other, parts to tubes, and metal parts to wood and
other non metallic materials, doubling possible effective holding
forces.
The idea comes from Guy Croft, the proprietor of Guy Croft Racing
Engines in Lincoln, one time chief engineer with Napier
Turbochargers.
The stud has a tapered head, which fits inside an expandable
collar, with a corresponding internal taper and a slot in one
side. A pin on the stud engages in the slot. When a nut on the
stud is done up, it pulls the stud backwards through the collar,
increasing its effective diameter and causing it to grip on the
inside of a hole or tube into which the fastener has been
inserted. The pin in the slot prevents the stud from rotating
inside the collar.
The collar has a chamfer on its outermost extremity to facilitate
insertion into the hole.
n the samples
supplied to Eureka for evaluation, it was evident that the
internal friction between the underside of the collar and the
part to be attached had been made to be greater than that between
the nut and the other side of the part. To attach, all that had
to be done was to do up the nut finger tight, and insert the
arrangement into the hole. Provided the part was initially
prevented from rotating, tightening with a spanner produced a
firm bond to the substrate or tube to which the part was being
attached.
The main advantage of the approach over a conventional bolt
inserted into a threaded hole is that allows a much more even
stress distribution, and thus a higher maximum permissible
holding force. All threads include natural stress concentrators
at the bases of thread grooves, and are prone to damage. Typical
damage results from over tightening and entrained dirt. Repair
requires use of thread inserts. A round role is cheaper and
quicker to manufacture than a threaded hole. Increasing the depth
of thread engagement beyond certain finite limits yields no
additional benefits. The expanding collar stud requires a shorter
hole than a conventional threaded bolt to achieve the same
holding force, allowing attachment to a thinner substrate.
A further advantage of the expanding collar concept is that it
also works with substrates such as wood, in which it is not
feasible to cut a threaded hole. Experiments with pine show that
it is possible to double maximum holding force without splitting
the wood. Similar results can be expected when used with plastics
and composites.
Expanding fasteners, are of course, very widely used, and are the
favoured means of attaching to brick and concrete. Those
commercially available fall into a number of categories. Many are
one use only devices, which expand sleeves plastically. Some have
assemblages of parts that are forced away from each other inside
the hole. These are re-usable but do not apply their holding
stress completely evenly over the inside surface of the hole, and
can be expected to form indentations in the sides of the hole.
What sets Guy Croft's expanding collar fastener apart from the
others is that it applies force evenly to the entire inside
periphery of the hole throughout its length, it needs only a very
short hole, and it works equally well no matter how many times it
is dismantled and re-inserted.
Three sample fixings have been supplied to Eureka. One attaches
one plate to another. One attaches a plat to the end of a tube,
while the third attaches to a tube, using a flat nut beneath the
main nut, making the remainder of the stud shaft available for
the attachment of other items. All were interference fits with
holes in the substrate, but ideally, Guy Croft favours having the
collar of very slightly greater diameter than the hole. This
gives the collar natural outspring and sufficient friction within
the hole to prevent it turning during initial tightening. It is
an important feature of the design that the frictional force of
the stud head in the collar is substantially less than the
frictional force of the collar in the hole.
The circumference of the collar can be smooth, as in the supplied
samples, or ribbed. The expanding collar and stud works without
any lubricant. The stud may be through drilled so as to permit
cabling to pass through the fixing. In applications where the
collar is fitted into relatively thin walled tube, a reinforcing
collar outside the tube can be used to augment the holding
strength and prevent failure of the tube.
In order to facilitate dismantling, the inner face of the collar,
the face opposite that in the base of the hole, incorporates two
small holes so that the collar can be squeezed by a means of
circlip pliars or similar to aid withdrawal. Additionally the
collar is partially notched along its length, opposite the slot,
in order to weaken it.
The collar and stud are not necessarily made of the same
material. The use of a collar material with high thermal
expansion relative to the stud enhances the holding strength of
the fastener at elevated temperatures, since the collar will
expand axially, tending to maintain preload as the stud itself
expands.
The fastener looks ideal for use in motorsport, where very rapid
and repeated assemblies and disassemblies are constantly
required. However, Guy Croft sees it as being even more suited to
duties in hot, wet and corrosive environments, where failure to
dismantle a joint is not an acceptable option. It may be,
however, that its big future lies in fastening to wood and
composites, where fast, repeatable assembly and disassembly
systems which make optimal use of wood's particular mechanical
properties are somewhat lacking at the present time.
A patent has been applied for.
http://guy-croft.com/page5.html
mailto:Gcengines@aol.com
Eureka says: Despite the vast range of fasteners
available on the commercial market, it is still possible to
invent better ones for particular applications.
Pointers
* Fastener achieves a greater holding force using a shorter hole
than is possible using conventional bolt and threaded hole.
* It applies force evenly to the entire inside periphery of the
hole throughout its length, and it works equally well no matter
how many times it is dismantled and re-inserted.
* It also works extremely well in wood and other non metallic
materials
For more technical
developments see www.eurekamagazine.co.uk