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The perils of fastening thin components

There is very often a need to fasten thinwall parts to other components. In the case of engines and transmissions, this might be cosmetic covers, airboxes, airhorns, electrical enclosures or any number of other items.

The fastening of thin components is not technically difficult to achieve: we can use the same type of fasteners that we use in normal circumstances, but we need to be aware of the difference in behaviour of threaded fasteners, which can tend to lead to fastener loosening or accelerated failure. In the case of airboxes being fastened to cylinder heads with threaded fasteners, loose fasteners or bolt heads that have fallen off due to fatigue can be swallowed by the intakes and cause a catastrophic engine failure.

As readers may already appreciate from reading previous RET-Monitor articles on fasteners or technical papers on the subject, most of the load is taken by the first few threads closest to the applied load. In the case of fastening thin components, this means that the clamped length of the joint is much shorter than normal. This has two effects, neither of which help towards the success of the joint.

The first effect is to increase the internal load coefficient, which dictates the fraction of the applied service load to which the fastener is subjected. The internal load coefficient increases as the ratio of fastener stiffness to joint stiffness; in simple geometry the joint stiffness is calculated using the ideal stiffness of truncated cones. (For a recap on this you may find it useful to refer to previous articles or mechanical engineering/fastener textbooks.) For thin joints, the stiffness of the joint is lower than usual compared to the stiffness of the bolt. This leads to an increased load on the bolt due to cyclic service loads, and lower fatigue life. Using a washer can improve this situation a little.

The second effect is that the fastener itself physically stretches very little in developing the pre-load, so any slight relaxation of fastener ‘stretch’ (elongation) due to the joint ‘settling’ can easily lose a significant proportion of the desired pre-load. If the cyclic service loads are sufficient to then allow joint separation, the fastener itself is subjected to 100% of the applied load.

Unless the fastener is sized for this – which is certainly not the case in any optimised joint – you are likely to find either broken fasteners (with fracture surfaces characterised by fatigue marks), loose fasteners or missing fasteners. Again though, use of a washer is a good way to improve the situation. This increases the working length of the bolt, which increases the stretch in the bolt (for a given strain, the stretch is in direct proportion to the working length of the fastener).

Where two thin components are being fastened together, you should consider using washers on both sides of the joint. Where a thin component is being fastened to something more substantial that’s provided with a tapped hole, counter-boring or counter-drilling the tapped hole again increases the working length of the fastener.

Written by Wayne Ward

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