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10/01/2005
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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.
In 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.
Guy Croft Racing Engines
Email Guy Croft Racing Engines
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
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Author
Tom Shelley
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