Why Fasteners Fail: How to Prevent Vibrational Loosening

Our next look into common fastener failures concerns joints which loosen over time, due to vibration.

Many joint applications only see static loading or external loading. These types of loading are erratic and not exceeding the strength of the fastener which holds things together. When tightened, these joints rarely loosen. But there are also joints which see cyclical loading and, in some cases, very high-frequency cyclical loading which causes vibration. Even if the load is not very high, if you use incorrect fasteners, loosening can occur.

Threads of a fastener are ramps that wrap around the shaft in a helix. Tightened fasteners rely on friction to keep from ‘sliding down the ramp’ or loosening. Friction is important not only in the thread flanks of a fastener but also at the bearing surface. Too much friction in a joint and we do not create enough clamp load, but too little friction can lead to loosening.

Some joints are more prone to loosening under vibrational forces:

• Soft joints which cannot create enough clamp load to stretch the fastener without damaging the mating surfaces
• Very short bolted joints which may support higher clamp loads, but do not give the bolt enough length to stretch

Solving Vibrational Loosening

What are some steps to take to solve vibrational loosening?

• Remove the source of vibration: in the case of a rotating member, consider balancing the parts to reduce vibration
• Add friction to the threaded components: thread forming screws or adhesive locking patches may be a good option
• Add friction to the bearing surfaces: serrated nuts, screws or washers may be worth looking into

For more details on solving vibrational loosening problems, contact us at ProvenProductivity@bossard.com.