Mechanics of a threaded fastener & why threaded assemblies fail

Standard

Being involved in any kind of engineering or mechanical assemblies in general, we often think of threaded fasteners, or as we would call them more colloquially – bolts & screws & nuts – as a rather basic thing and rarely think about how they work and what can make them fail.

Tightening the nut on a bolt stretches the bolt, much like a spring. In technical terms, we’d say it elongates it. Elongation is necessary for creating the clamp load, which results in friction between the two joined parts. More torque results in more elongation and, in turn produces more clamp load.

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There are three further points of friction that are responsible for the bolt to stay elongated:

  • Between the bolt head and the clamped part
  • Between the nut and the part
  • Between the threads of the nut and bolt

There are certain cases where threaded fasteners keep together static assemblies. And in those cases, they’ll hold just fine. But typically, they are also used on many assemblies subjected to load and vibration during the work cycles of the machine or device the assembly is a part of.

Vibration will cause side to side movement of the nut and bolt, which is possible because the threads don’t adhere tightly enough to entirely fill the space between them (no matter how tightly pressed together they seem to the naked eye). Once vibrational stress becomes higher than friction holding the pieces together, the bolt will loosen. It usually takes 50-100 cycles for the threaded assembly to completely fail.

You can see in a very simplified, visual way, what exactly happens in the following animated video.

Apart from vibration, there are several factors that can affect the reliability of a threaded assembly. For example, relaxation in bolt tension can occur due to settling of the nut head and creeping of the gasket. In case of different substrates being joined, variable service temperatures can cause different thermal expansion of the materials, which will affect the fastener performance negatively, too. Finally, the good old corrosion, a well-known enemy of metal assemblies will inevitably happen due to humidity and elevated temperatures. And we all know corrosion is no good whether it’s from the perspective of affecting the functionality of the assembly or making a disassembly harder when repair or maintenance is needed.

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Luckily enough, a lot of these issues can be eliminated by simply making sure the gaps between the threads are fully filled. And liquid Threadlockers are a solution that has been around for almost 70 years now – Loctite being the original among them. Have a look:

As you can see, it extends beyond Threadlocking, but if you want to learn more about Loctite as technology, feel free to visit Loctite online (you can access content in different languages from there).

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