Bolted joints are popular in the majority of engineering sectors as they are simple to assemble and do a great job. Bolted joints can be critical making it pivotal they remain tightened and do not loosen during their life. If not, detrimental failure can occur. Here, we will cover the 4 most common reasons bolts loosen.
4 Most Common Reasons Bolts Loosen
1) Vibration
In practice, bolted joints will undergo vibrations creating transverse movement. Although only a small amount of movement occurs, over a long period of time, the accumulation overcomes the friction between the threads. As a result, the bolt starts to loosen by unwinding itself.
2) Embedding
This mechanism is different to creep. In some joints, some peaks are present that will witness a local stress concentration. The stress concentration exceeds the materials yield and results in plastic deformation on a micro-scale. Over some time, the surface beneath the bolt will flatten, which then removes the peaks. Although, by this time, the bolt will have lost pre-load.
3) Gasket creep
It is usual for a gasket to be used in a joint for several reasons. For example, to prevent a leak or provide sufficient clamping area on uneven surfaces. After the initial tightening, the gasket will start to relax due to the compressive force induced from the bolts. Positively, the majority of gasket creep seen experienced is soon after use. Luckily this means, if we don’t see any issues with the joint shortly after tightening the bolts, there’s a good chance gasket creep won’t lead to the bolt loosening. However, there are suggestions that gasket creep occurs over the materials life, although it is minuscule.
The material used for the gasket will influence the amount of creep witnessed. It is considered that denser materials produce less creep than softer materials.
4) Elastic Interactions
If there are multiple bolts in the joint, elastic interactions will occur. When a bolt is tightened, it compresses the material beneath it as well as a small (micro) scale of material beneath other bolts. This further compression of material under bolts reduces the pre-load present in the bolt. It is critically important to use bolt tightening sequences on joints with multiple bolts. The bot tightening sequence reduces the elastic interactions seen when tightening bolts.
Summary
We have covered the 4 main common reasons why bolts loosen once tightened. However, there are many other reasons why bolts may loosen. Bolted joints have been the subject of vigorous research, and there is still a lot to understand. The 4 main reasons for bolt loosening, covered here, are vibration, embedding, gasket creep and elastic interactions. If investigating a failed bolted joint, it may be best to consider these four factors initially, if they are applicable.
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