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What Causes Loosening of Bolts?

Bolts are the preferred fastener in a variety of industries and applications due to their ease of disassembly. This renders them susceptible to self-loosening and preload loss. Depending on the application, the loosening of a bolt might have significant repercussions. In certain situations, a single loose bolt may shut down an entire manufacturing facility and cost a corporation a large sum of money, while in others it might constitute a severe safety risk.

So, what causes the loosening of the bolt?


Spontaneous loosening, also known as rotational self-loosening, occurs when a bolt rotates loose as a result of stress, vibration, or dynamic loads. Even a minor rotation may cause a bolted joint to lose its whole preload. This is the most common cause of bolt loosening.


There are three mechanisms that induce slackening: creep, settlement and relaxation.

  • Creep refers to a permanent deformation caused by long-term exposure to high amounts of stress below the yield strength of the joint's components. It is particularly severe in applications involving high temperatures.

  • Settlement refers to the permanent deformation of clamped material that occurs when the joint is exposed to an increase in stress from dynamic operating loads.

  • Relaxation occurs when the microstructure of a joint's materials restructures over time, changing current elastic deformation to plastic deformation. In contrast to settlement or creep, the clamp length does not alter, making it more difficult to detect.

It has become more important in recent years to reduce fatigue because of the growing demands on numerous bolted joints and the greater usage of lightweight materials. Correct joint design is the key to obtaining a high strength friction grip connection with a high degree of preload, and consequently a high level of slip resistance throughout the whole lifespan.

Avlock International is a leading manufacturer and distributor of specialised fastening systems to both the domestic market and other global destinations. Contact us today!

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