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Vibrational Loosening

What is vibration, and how does it contribute to the loosening of fasteners? The answer may seem obvious, but understanding the mechanics of vibrational loosening can help us take steps to prevent it.

Imagine a block of wood on a ramp. The angle of the ramp is low enough that the block of wood does not slide down the ramp. Now, we repeatedly wrap on the ramp with a hammer, not too hard, but enough to make the block of wood jump a bit and slide down the ramp. This is like how vibration causes threads to rotate loose, or “down the ramp”. When vibration occurs, it briefly, but repeatedly, lessens the pressure between the block and the ramp (or thread flanks) and the block naturally slides down the ramp.

Now, if we use a heavier block of wood it takes more vibration to cause the block to slide down the ramp. This is similar to adding more clamp load into the joint. But, given enough amplitude and frequency of the vibration, we still get the same result – the block sliding down the ramp, or rotational loosening.

Lastly, if we clamp the block of wood to the ramp, so that there is pressure on the top of the block, it is not allowed to bounce and lose friction on the bottom side, and it does not slide down the ramp. So, how can we simulate this condition in a threaded joint? Your normal nut and bolt joint will always have thread tolerances to ease assembly, creating gaps on the back side of the threads. However, if we can use thread forming screws, which make their own threads into the mating part during assembly, we have no thread gaps.

Thread forming screws, while not the solution for every joint, work very well in situations where vibrational loosening is a high risk.

Contact us at ProvenProductivity@bossard.com for more information on solutions for vibrational loosening.

Doug Jones
Applications Engineer
djones@bossard.com

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August 11, 2017

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