The main reasons for bolt loosening are as follows:
1. Undertightening
An undertightened or falsely tightened bolt will not have enough preload to begin with, and if it loosens, the joint will not have enough clamping force to hold the parts together. This can cause lateral sliding between the two parts, which can subject the bolt to unnecessary shear stresses, which can eventually lead to bolt breakage.
2. Vibration
Tests of bolted joints under vibration have shown that many small "lateral" movements cause the two parts of the joint to move relative to each other, as well as the bolt head or nut and the connected part. These repeated movements counteract the friction between the bolt and the connected part. Eventually, the vibration will cause "spin loosening" on the bolt threads, and the joint will lose its clamping force.
3. Embedding
The engineers who design and develop bolt tension allow for a period of break-in, which results in a certain loss of preload, during which the tightness of the bolt will relax. This relaxation is caused by embedding between the bolt head and/or nut, the threads and the mating surfaces of the connected parts, and can occur in soft materials (such as composites) as well as hard polished metals. If the joint is not designed properly, or the bolts are not tensioned at the beginning, the embedding of the joint may cause a loss of clamping force and fail to achieve the required minimum clamping force. There are microscopic unevenness between the mating surfaces, which will cause the convex points to collapse under the action of the bolt preload after tightening, and permanent plastic deformation, so that the clamping length of the bolt will be reduced, and finally the preload of the bolt will be reduced.
4. Gasket creep and thermal expansion
Many bolted joints include a thin, soft gasket between the bolt head and the joint surface to seal the joint and prevent gas or liquid leakage. The gasket itself also acts as a spring, rebounding under the pressure of the bolt and the joint surface. Over time, especially when close to high temperatures or corrosive chemicals, the gasket may "creep", which means that it loses elasticity and causes a loss of clamping force. If the bolt and the joint are made of different materials, the temperature difference caused by rapid environmental changes or industrial cycle processes is too large, which will cause the bolt material to expand or contract rapidly, which may loosen the bolt.
5. Impact
Large impact loads exceed the friction force when the bolt is preloaded, resulting in sliding. Dynamic or alternating loads from machinery, generators, wind turbines, etc. can cause mechanical shocks – impact forces applied to the bolts or joints – that cause the bolts to slide relative to each other.
Just like vibrations, this sliding can eventually cause the bolts to loosen. Even shocks are often not considered when designing joints for such large loads.