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Why Threaded Assemblies Fail

Functions of a Threaded Assembly

  • Create clamp force
  • Maintain clamp force
  • Allow disassembly

Why They Fail

Threaded assemblies fail because clamp force is not maintained. Threaded assemblies loosen because of:

  • Gaps: In order to make the assembly possible, nuts and bolts must have some tolerance, which creates gaps between the threads.
  • Vibration & Side-to-Side Movement: These gaps allow the parts to move from side-to-side when exposed to vibration.
  • Expansion / Contraction: Expansion and contraction can also cause side-to-side movement. This, in addition to vibration, leads to loosening and ultimately disassembly of parts.

Disassembly is Not Always Possible

This failure happens because, in certain conditions, a nut and a bolt can seize together. This seizing effect is caused by:

  • Corrosion, rust, when dealing with humidity, high temperatures, assembly of different metals (galvanic corrosion)
  • Galling (friction welding)

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