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Composite Materials

fastenersThe use of composite materials in racecars is not new; nor does it represent a particular novelty for race engines. The strength, stiffness and low density make them ideal for many components, both structural and decorative.

It is now pretty rare to find a race engine airbox, certainly on 'formula' cars, made from anything other than carbon-fibre reinforced polymer (CFRP) composites. It has also been used to good effect for plenums on turbocharged engines, on structural covers for race engines and in parts used specifically to increase the stiffness of the engine in order to provide a performance benefit to the vehicle as a whole. It is widely used for electrical boxes and cosmetic covers too.

One difficulty with using fibre-reinforced composites though is that they cannot be directly tapped in order to provide a strong thread. To provide a thread, we really need to provide some metal into which the thread is already cut, or in which is can be formed. There are a number of methods of doing this, some of which we will discuss briefly here.

Clearly it is possible, where access allows and using a conventional nut, to provide a metal thread, but this can be fiddly. A flanged nut can be bonded to the composite panel, and there are companies who make nuts that are specially designed with a large flange and features to improve the reliability of the bonded joint. These bonded fasteners can be used during the lay-up of the part if required.

The most basic way to provide a permanently fixed thread is to bond in a piece of metal into which a thread is formed. These 'hard points' can be made in a material that best suits the application, and have the further advantage that they can take whatever form is required. The chief disadvantage is that they are more costly than the proprietary parts discussed.

'Rivnuts', as the name suggests, are nuts that are riveted to sheet parts. They are basically a thin-walled tube, provided with a thread at one end and a flange at the other.

The tube is pushed through a clearance hole until the flange is against the composite panel, and then a special tool crushes the tube against the other side of the panel, thus restraining the rivnut. They are easy to use, but not all types leave the flange flush with the panel.

Anchor nuts are fasteners with a female thread and which are riveted to sheet metal or composite parts, and can be fixed or 'floating' types. The floating type is pictured here and, as the name suggests, the thread element has a certain amount of lateral movement and can accommodate inaccuracies in the hole positioning on the composite part or in the mating piece. They are also available with multiple nuts on a single plate, meaning that fewer individual parts are needed.

Fig. 1 - This floating anchor nut is typical of the type of fastener available for composite applications

Written by Wayne Ward

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