Broken Bolts

5 Causes of Broken Bolts – And How To Prevent Them

Source ThomasNet.com

Mustansir Haji Jun 24, 2019

1. Overstressing

As the name suggests, overstressing involves placing an unjustifiable load on bolts, one of the most common causes of bolt breakage. If you ensure that your bolts are adequately stressed, then you can rest assured that your bolts’ longevity drastically improves. Bolts experience three types of stress:

• Preload, or the internal stress keeping the bolted joints intact
• Service load, which is the external force of the joint material to exert separate stresses
• Tensile load, the most important stress, is static stress that pulls both bolt ends separately. If this load is overapplied to the bolt, it can cause a shift in the load-bearing capabilities and ultimately cause bolt breakage. To prevent the additional force, refrain from tightening and overturning your bolts often. The tensile force exerted should never exceed the bolt’s tensile strength.

2. Fatigue

Fatigue failure mainly pertains to shear bolts, which do not have a lifetime value. In simple terms, the bolts experience a significant amount of fatigue during their lifetime, which causes their efficiency to decrease gradually. Like overstressing, fatigue can also be tied to preload and service load. The preload and service load experienced can be estimated in cycles; bolts go through a predetermined preload and service load cycle. Fatigue is then calculated by the number of preload and service load cycles experienced by the bolts. To ensure that your equipment does not succumb to fatigue, replace bolts every two years, especially if your equipment is exposed to stress factors in modeling equipment and stamping machines.

3. Corrosion

Corrosion refers to the exposure of the bolts to incompatible substances like oxygen, naturally occurring chemicals, or even other metal products. Bolts in automobile parts are more susceptible to chemical corrosion where bolts can be exposed to engine oil leaks or other fluid leaks, therefore causing them to deteriorate over time. Incompatible metal or galvanic corrosion is rare but can happen in mechanical, mining, or construction applications. The most straightforward way of reducing the effects of corrosion is to use pre-applications designed for specific types of fasteners and causes of corrosion. This method is simple, efficient, and economical compared to other sophisticated methods of reducing corrosion.

4. Hydrogen Embrittlement

This process occurs when the metal of the bolt becomes brittle due to the presence of atomic hydrogen in the structure of the metal. Hydrogen embrittlement is largely the most difficult to prevent of all the factors outlined here. Hydrogen embrittlement can also be tied to overstressing; once blisters are formed on the structure of the bolts due to overstressing, the unprotected parts of the bolt are exposed to atomic hydrogen present in the atmosphere, causing it to degrade faster and more significantly. The only solution to preventing hydrogen embrittlement from occurring is to replace fasteners as soon as they crack due to overstressing. Another solution could be altering the bolts’ atmosphere to ensure they are free of the anodic solvents and chemicals that cause hydrogen embrittlement.

5. Bolt Locking

In applications where the bolts face large amounts of vibration, it can cause an uneven amount of stress on the bolts, causing them to break. For example, in a shift fork, vibrations cause either the loosening of bolts or even complete breakage, resulting in total loss of control. In cases where the equipment is deployed in an operation with high vibrations, bolts should be used with locking washers, locking nuts, wire wraps, cotter pins, and other fasteners. After locking the fasteners, you can rest assured that vibrations won’t degrade the quality of your bolts. Considering these factors while designing, building, and maintaining your equipment can drastically improve bolt life and decrease the possibility of failures.