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Why Do Fasteners Self-Loosen

Why Loosen?

Loosening fasteners have been an issue for decades. Because a fastener coming loose can often lead to an accident, it is important to understand the possible factors that could contribute to the loosening of a fastener.

Typical applications for fasteners are in clearance holes, and the tightening of the fastener helps maintain the structural integrity. If loosening occurs, the preload is reduced or eliminated completely, resulting in structural failure.

Causes of Loosening

Research on the topic of loosening fasteners has led to the conclusion that transverse joint movement is one of the central causes. The transverse force acting on the joint exceeds the frictional resistance from the fastener’s preload, resulting in relative movement. This movement occurs between the mating threads and the fastener bearing surface, and when repeated, causes the fastener to loosen.

Differences in temperature or differences in clamped materials can often also result in this movement occurring. Ultimately, vibrations are what effectively cause fasteners to loosen, in the simplest of terms.

When this happens, fatigue failure can often occur. Loosening reduces the clamp force acting on the joint, causing it to slip and subject the fastener to bending loads.

Embedment can also mean the loss of preload and cause the fastener to loosen. Embedment that occurs during torqueing causes the flattening of the mating surfaces and loss of friction in the interfaces which in turn causes loosening.

Solutions for Self-Loosening

There are no universal solutions for loosening fasteners, but designers and engineers are constantly taking preventative measures during the build process. Ensuring that there is sufficient clamp force on the joint interface and also crafting a design that accommodates the effects of embedding and stress relaxation are two important measures. Proven thread-locking devices are also available to help deter loosening.

Loosening is a significant issue, and not one that you want to learn about from experience. It is important that you be knowledgeable on the subject, as researchers work to find innovative methods for addressing this problem. To learn more about the factors that cause fasteners to self-loosen, contact Bossard at ProvenProductivity@bossard.com.


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October 31, 2014
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