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How Vibration Can Lead to Bolt Failures

The ongoing trend toward lighter, smaller products leads to an increase in the amount of vibration in a joint. These two trends can sometimes work in tandem with each other, leading to loosening of bolts and other fasteners.

For example, lighter materials vibrate more, while smaller fasteners, in general, can handle less vibration. As a result, fasteners in small, lightweight products are susceptible to loosening, unless design options are considered to address this risk.

"The safety and reliability of bolted joints often determine the overall reliability and safety of mechanical and structural systems,” notes Dr. Cheng Siong Phua, vice president of technology for Stanley Engineered Fastenings Asia Pacific & Global Electronic Division. Stanley recently published an overview on the subject, “Prevent Bolt Self-Loosening by Keeping Torque in its Place.”

Bolt self-loosening is caused by any type of dynamic load, such as vibration or changes in temperature, insufficient clamp load and poorly fitting parts, allowing relative movements to increase the risk of self-loosening, according to a 2017 article in Engineer Live magazine. The sum of these very small movements ultimately results in the loosening of the threaded assembly.

By far the most frequent cause of loosening is side sliding of the nut or bolt head relative to the joint, resulting in relative motion occurring in the threads, according to Bolt Science, a U.K.-based consultancy and training company. A consultant there, Bill Eccles, notes “a loosening problem is indicative of insufficient preload that results in joint movement. 

Relative motion occurring in the threads can be attributed to three common problems: bending of the parts; differential thermal effects; and external forces on a joint. Stanley Engineered Fastening, which recently rebranded, provides technical insights into self-loosing bolts: