Machine Design

Bolted joints take on vibrations and shock

Almost any fastened joint will fail if subjected to strong enough vibrations for a long enough time.

A new approach, the Symmetry Bolt from Botex Inc., Torrance, Calif. (, uses an external thread that includes both a coarse right-hand thread and a fine left-hand thread to remain in place despite harsh vibrations. The main bolt uses the right-hand thread while the jam nut uses the left-hand thread. So the two nuts lock against each other with opposing forces, with vibrations actually increasing the locking force while the overall fastener maintains the clamping load. In fact, it can take more torque to remove the jam nut than to install it. In tests, the bolt has survived NAS 3550 tests of 1,780 cycles/min for 17 min without loosening. It also has been lab tested for severe vibration and impact based on National Aerospace Standard 3550.

Current bolts are metric, but Unified threads can be ordered. Bolts come in sizes M8 to M24.

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