The Experts: Why do bolts rotate loose?

Q: Why do bolts and nuts rotate loose when a bolted joint is subjected to vibration and/or dynamic loads?

A: Loosening of threaded fasteners due to vibration and/or dynamic loads is an important phenomenon since it is one of the most common causes for failure of bolted joints.

A tightened bolt is elastically elongated but strives to assume its original length, much like a spring. This can only be accomplished by rotation of the bolt/nut in the loosening direction so that the bolt preload is lost. However the bolt/nut is prevented from rotating by friction between the contact surfaces in the threads and under the bolt head/nut.

If a joint is subjected to vibrations and/or dynamic loads that cause relative movement between the contact surfaces the friction is reduced because dynamic friction is lower than static. Therefore the bolt/nut is then no longer captivated by friction and rotates loose. The most common causes of movement between contact surfaces are vibration and external dynamic forces.

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