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Using springs

When considering a bolted joint, and when explaining how it works under load to a novice engineer, it is often helpful to consider the components as springs. The joint and the bolt ‘share’ any imposed loads depending on coefficients that are functions of the components’ stiffness, as is the case with nested helical springs.

It does not take a huge leap of imagination to look at the actual applications of helical springs as fasteners. There are applications where the imposed displacements of various components are large, and the amount of compliance required in the joints is very large. In such situations, using a threaded fastener becomes difficult owing to its stiffness. We need something that still maintains a controlled load, but over a much larger range of joint displacements.

The main application of helical springs as fasteners in motorsport is in exhausts. It can be difficult to manufacture the various separate pipes with flanges or lugs that are accurate enough to use traditional bolted joints. Radial lugs on adjacent pipes are sometimes joined using short ‘kinked’ plates that have their own compliance, along with threaded fasteners.

However, it can be simpler and more convenient in practice to use extension springs to pull adjacent parts of an exhaust system together. The requirements in terms of the accurate positioning of the lugs or loops between which the springs are stretched are less exacting than for joints using plates and threaded fasteners. If you are in the position where you need to quickly disassemble and then reassemble part of a hot exhaust, you may find it difficult to wield spanners and ratchets. A simple spring puller may be easier, faster and more pleasant to use in such situations.

The problem with using springs to pull a joint together on an engine where a range of frequencies are generated during service is that a spring will tend to resonate when excited by a number of frequencies of vibration. Once this happens then the stress in the spring is likely to lead to premature failure. It is a similar situation to spring surge in valve springs, although in this case the spring is an extension spring rather than a compression spring.

I have seen a couple of methods to counteract the effects of resonance, on motorcycles used for 24-hour endurance racing. The first is simply to run a ‘bead’ of high-temperature silicone along the spring. It works but looks messy, and the silicone bead can fall off if adhesion is poor. The second method is to use polymer heat-shrink over the entire length of the extending portion of the spring. This is more reliable, as the heat-shrink is essentially in tension around the spring and is very unlikely to become detached; it also looks much better than using silicone sealer. However, service temperatures need to be borne in mind because if the heat-shrink is too close to a hot exhaust then it will melt or burn and will no longer offer any damping.

Written by Wayne Ward

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