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Given that loose bolts can have various negative effects during their use, a loose bolt may cause the entire production facility to come to a halt, resulting in losses for the company of thousands or even tens of thousands of yuan. Additionally, loose bolts pose safety risks within the factory. So, what are the main reasons for bolt loosening? Broadly speaking, there are reasons such as spontaneous loosening, automatic relaxation, and fatigue-induced loosening. 5 reasons for loose bolts: 01 Insufficient tightening. Bolts that are not tightened properly or are only partially tightened already have insufficient preload; if they become loose further, the joint no longer has enough clamping force to hold all the components together. This may lead to lateral sliding between the two parts, subjecting the bolt to unnecessary shear stress, which can ultimately cause the bolt to break. 02 Vibration Tests on bolted joints under vibration show that many small “transverse” movements cause the two parts of the joint to move relative to each other; at the same time, the bolt head or nut as well as the components being joined also move. These repeated movements counteract the friction between the bolt and the connected parts. Ultimately, the vibration will cause the threads of the bolt to \"rotate loose\", resulting in the joint losing its clamping force. 03 Embedding: Engineers responsible for the design and development of bolt tension allow for a period of running-in, during which a certain amount of preload is lost; as a result, the tightness of the bolts decreases over this time. This relaxation is caused by embedding between the bolt head and/or nut, the threads, and the mating surface of the components being connected, and it can occur in both soft materials (such as composites) and hard, polished metals. If the joint is not properly designed, or if the bolts do not reach the specified tension at the beginning, the embedding of the joint may result in a loss of clamping force, failing to achieve the required minimum clamping force. There are microscopic irregularities between the mating surfaces; under the preload exerted by the bolts after tightening, these protrusions get crushed, resulting in permanent plastic deformation. As a consequence, the clamping length of the bolts decreases, which ultimately leads to a reduction in the preload of the bolts. 04 Gasket Creep and Thermal Expansion: Many bolted joints include a thin, soft gasket between the bolt head and the joint surface in order to seal the joint and prevent leaks of gases or liquids. The washer itself also acts as a spring, rebounding under the pressure from the bolt and the joint surface. Over time, especially when exposed to high temperatures or corrosive chemicals, the washer may experience \"creep\", which means it loses its elasticity and as a result the clamping force decreases. If the materials of the bolt and the joint are different, large temperature differences caused by rapid environmental changes or industrial cycling processes can lead to rapid expansion or contraction of the bolt material, which may cause the bolt to loosen. 05 Impact – A large impact load exceeds the friction force resulting from bolt pre-tensioning, causing sliding. Dynamic or alternating loads from machinery, generators, wind turbines, etc., can cause mechanical shock – the force exerted on bolts or joints – leading to relative sliding of the bolts. Just like vibration, this type of sliding will eventually lead to the loosening of bolts; moreover, impacts often do not take into account such high loads when designing joint connections. What is preloading? It is a term that has multiple meanings in engineering. One is the tension (load) generated when the fastener is initially tightened; as the bolt stretches, the components between the bolt and nut are compressed, thereby increasing the so-called clamping load until the fastening process is complete. Dangers of loose bolts: 01 Flange leakage; 02 Separation and detachment of the fan rotor from the nacelle; 03 Detachment of the connection bolts due to vibrations in the ship’s engine. The detachment of these connection bolts, along with the rolling of the ship, can cause further damage to the equipment. In bolted connections, tightening the nut actually causes the bolt to elongate, just like pulling on a spring. This pulling force, or tension, generates an opposing clamping force that tightly fixes the two parts of the joint together. If the bolt becomes loose, the clamping force will decrease. Loose bolts are more than just a headache-inducing nuisance. If the joint is not tightened again promptly, it may start leaking liquid or gas, the bolts may break, the equipment may be damaged, or a catastrophic accident may occur. In summary, “the best way to prevent loosening is to ensure that the preload is sufficient, so as to avoid issues such as sliding or opening of the joint.” From the above analysis, it can be seen that there are 3 reasons for loosening due to insufficient or reduced pre-tension; therefore, it is necessary to carefully control the pre-tension of the bolts in order to mitigate the risk of loosening. As long as the pre-tightening force is sufficient to meet the requirements, and provided that the clamping length is not too short (e.g., lk≥3d), the bolts generally will not loosen on their own even in the presence of certain vibration loads. A good bolted joint design, proper clamping force development, and appropriate bolt retention devices combined can reliably secure the bolted joint to address many of the loosening challenges presented here. A good bolted connection will be designed with bolts and nuts of appropriate size and type, and the optimal tension will be specified to achieve the clamping force required to maintain the integrity of the connection. In applications, the appropriate clamping force requires that the tension (preload) in each bolt reach the correct level and remain at that level throughout its service life. Therefore, it is crucial to maintain an appropriate tension on the bolts. Ultrasonic testing can be used during the design phase to measure the axial force of the bolts, ensuring that their preload meets the design requirements.