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What is a Safety Bolt?

By Paul Scott
Updated May 17, 2024
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A safety bolt is an industry standard fastener that makes use of a tensioned locking wire system to prevent unintentional loosening due to vibration and impact forces. It has one hole drilled through the head to accept a locking safety wire. The locking wire system also serves as a visual inspection aid to verify that fasteners are tensioned correctly. The safety bolt is commonly used in applications that place extreme mechanical demands on the machinery such as aviation and motor sports.

Fasteners used in machinery exposed to extreme structural stress during regular operation are prone to work loose with time due to continuous impact, vibration, and torque distortion. The failure of a critical fastener in these high-stress applications could have serious implications. A particularly simple and effective way of preventing this from happening is the use of a safety bolt system. This system is cheap, quick, and easy to use, and provides both an extra level of security and a clear visual indication of fastener integrity.

Safety bolts have a small hole drilled through their heads that allows for a length of purpose made wire to be looped through it and an adjacent fasteners head or other anchor point then twisted up to maintain tension on the two. The wire is inserted in such a fashion as to maintain tension in the tightening orientation of the bolt thus preventing it from vibrating loose. The wire is a malleable material that holds it shape once tensioned and will continue to maintain the locking action until removed.

This wire locking loop also allows anyone inspecting the installation to ascertain at a glance whether the fasteners are still secure. In applications such as aircraft and car racing, the use of safety bolts is often a legislated safety requirement and a standard part of pre-use checklists. The same locking principle is also used to secure clevis pins to shackles and other applications where fastener loss could lead to injury or damage. The simplicity of the safety bolt system also makes it a particularly cost effective way of locking fasteners which is an attractive option considering the expense of the systems they typically protect.

Safety bolt wire is available in a wide range of different gauges and materials and only requires one simple tool to apply. The most commonly gauge and material used is 0.032 inch (0.8 mm) stainless steel wire, and the tool used to tension the wire is a simple, purpose made device that resembles a regular screwdriver. Safety wire tabs are also commonly used to enhance the efficacy of the safety bolt system. These tabs resemble a standard tabbed washer with one tab being longer than the other with a hole in its end. The bolt is tensioned over the washer in the normal fashion, and the washer tabs bent up to contact the bolt head then locked in place through the tab hole with the safety wire.

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