A loose bolt may bring the entire production plant to a standstill and cost the company thousands or even tens of thousands of yuan. In addition, loose bolts may also cause safety hazards in the factory. So, what are the main causes of loose bolts? Broadly speaking, there are causes such as spontaneous loosening, automatic loosening and fatigue loosening.
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5 Reasons Why Bolts Are Loose
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Under tightening
Under-tightened or falsely tightened bolts inherently have insufficient pre-tightening force. If they become loose again, the joint will not have enough clamping force to hold the various parts together. This can cause lateral sliding between the two parts and the bolt will be subject to unnecessary shear stress, which can ultimately cause the bolt to break.
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vibration
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Tests of bolted connections under vibration have shown that many small "lateral" movements cause the two parts of the connection to move relative to each other, as well as to the bolt head or nut and the part being connected.
These repeated movements counteract the friction between the bolt and the parts being connected. Eventually, the vibration will cause the bolt to "rotate loose" on its threads and the joint will lose its clamping force.
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Embed
Engineers who design and develop bolt tensions allow for a break-in period, a certain loss of preload, during which the bolts can loosen in tightness.
This loosening is caused by the embedment between the bolt head and/or nut, threads, and 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 improperly designed, or if the bolts are not initially tensioned to the specified tension, embedding of the joint may result in a loss of clamping force that does not achieve the minimum required clamping force.
There are microscopic unevenness between the joint surfaces. Under the action of the bolt pre-tightening force after tightening, the bumps will be crushed and permanently plastic deformed. As a result, the clamping length of the bolt will be reduced, ultimately leading to a decrease in the pre-tightening force of the bolt.
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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 washer itself also acts as a spring, rebounding under the pressure of the bolt and mating surface.
Over time, especially when near high temperatures or corrosive chemicals, a gasket can "creep," meaning it loses elasticity, resulting in a loss of clamping force.
If the bolts and joints are made of different materials, excessive temperature differences due to rapid changes in the environment or industrial cycling processes will cause the bolt material to expand or contract rapidly, possibly causing the bolts to loosen.
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impact
Impact - The larger impact load exceeds the friction force when the bolt is pre-tightened, causing sliding.
Dynamic or alternating loads from machinery, generators, wind turbines, etc. can cause mechanical shock-an impact force exerted on a bolt or joint-causing the bolts to slide relative to each other.
Just like vibration, this slippage can eventually cause bolts to loosen, and even shocks are often not considered when designing joints for such large loads.
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What is preloading?
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A term with multiple meanings in engineering. One is the tension (load) created by the fastener when it is initially tightened. As the bolt stretches, the components between the bolt and nut compress, increasing what is called the clamping load until the end of the tightening process.
Dangers of loose bolts
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Flange leakage
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The fan rotor separated from the engine room and fell off
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Ship engine vibration connection bolt fell off
The vibration connecting bolts of the ship's engine fell off and caused further damage to the equipment as the ship rolled.
In a bolted connection, tightening the nut actually causes the bolt to lengthen, like pulling a spring. This pulling force, or tension, creates an opposing clamping force that tightly holds the two parts of the connected piece together. If the bolt is loose, the clamping force will be weakened.
Loose bolts are more than just a headache. If joints are not quickly retightened, fluid or gas may begin to leak, bolts may break, equipment may be damaged, or a catastrophic accident may occur.
Summarize
"The best way to prevent loosening is to ensure that the pre-tightening force is sufficient and will not cause problems such as slipping or opening of the joint."
From the above analysis, it can be seen that there are three reasons for loosening due to insufficient or reduced pre-tightening force. Therefore, special control of the pre-tightening force of bolts is required to control the risk of loosening.
As long as the pre-tightening force is sufficient to meet the requirements, and as long as the clamping length is not too short (such as lk ≥ 3d), the bolts will generally not loosen automatically even if there is a certain vibration load.
The combination of good bolting design, proper clamping force development, and appropriate bolt locking devices can reliably secure bolted joints against many of the loosening challenges presented here.
A good bolted connection will be designed with appropriately sized and typed bolts and nuts, and specified with optimal tension to achieve the clamping force required to maintain the integrity of the connection.
Proper clamping force in the application requires that the tension (preload) in each bolt reaches the correct level and remains at that level throughout its service life.
Therefore, it is crucial to maintain appropriate tension on the bolts. Ultrasonic waves can be used to measure the axial force of the bolts during design to ensure that the pretightening force of the bolts meets the design requirements.
