Five Causes of Loose Bolts and Ways How to Prevent Them

November 5, 2019

Almost all equipment being used in manufacturing products, mining, transportation, and power generation have a lot of bolted joints involved. For security, a bolted joint is a little bit stretched by tightening the nut. The stretching, or tension, will result in a contrasting clamp force that holds both sides of the joint together.

If the bolt loosens, the clamp force ultimately weakens. If the bolted joints are left unattended, they can end up breaking apart, which can cause not only damage to your equipment but also catastrophic accidents to your workplace.

So, what causes loose bolts?

Over/Under Tightening

Sometimes people tend to either over tighten or under tighten bolts. Over tightened bolts may seem to be at the right place, but they create additional force to the clamps that can entirely strip the bolt. On the other hand, under tightened bolts are already loose by default, which weakens the clamp force holding the entire bolted joint. Under tightened bolts could eventually break since the sections of the loose bolt have already slipped sideways.

Shock and Vibration

Even if the bolt is correctly tightened, external factors can still affect the integrity of the clamp force. Machines that regularly move, such as wind turbines and generators, can generate mechanical shocks to the bolted joint, causing the threads to slip. Machines that vibrate frequently can also create small movements to the bolted joints. This vibration can affect the friction between the bolt and joint thread, causing the bolt to unwind from the threads.

Embedding

Even the design specifications of a bolted joint can also lead to the loss of clamp force. A certain break-in period, where the tightness of a bolt relaxes to a specific degree, is caused by micro-embedding of the bolt head and/or nut into the joint surface. If the specified tension of a bolt was not met from the very beginning of the tightening operation, a partial collapse may happen to the surface of the bolt. This collapse is known as embedding. 

Gasket creep

Many bolted joints have a thin, elastic gasket in between the bolt head and the joint’s surface that acts as a spring. This pushes back against the pressure of the bolt and the joint face. However, when near heat or corrosive chemicals, these gaskets may lose its springiness that leads to the loss of the needed clamp force.

Differential thermal expansion

The difference in material composition used for the bolt and joints can affect the behaviour of the combined pieces. Rapid environmental changes or repetitive industrial processes can cause either one of them to expand or contract rapidly, which loosens the bolt in the process.

Despite the regular occurrence of loose bolts, they can still be prevented by the following added tools and devices:

• Washers – a thin wide plate that can be used to add extra friction and distribute the load of a bolted joint. Some examples of these are Belleville washers and wave washer.
• Nuts – a type of locking fasteners that can be used with a mating bolt in holding multiple parts together. A few options for nuts are locknut and castellated nuts.
• Adhesives – both liquid and solid adhesive patches can strongly help bolted joints retain their ideal placements.

Aside from these devices, a loose bolt can be prevented if you pick the proper size and type for both your bolt and nut. The optimal amount of tension must also be identified so that you can monitor and maintain the same level of tension all throughout its operation.

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TCI Fasteners - Topcope
13 Slater Parade, Keilor East VIC 3033 Australia

Telephone: (03) 9336 0155