Why Do Bolts Usually Break? Most Common Reasons

When it comes to mechanical fasteners, bolts are one of the most commonly used types. They are used to hold two or more objects together and are designed to withstand high levels of stress and tension.

However, bolts can break for a variety of reasons, which can lead to costly repairs, equipment downtime, and even safety hazards. One of the main reasons why bolts break is due to overstressing. This occurs when too much load is placed on the bolt, causing it to fail.

Why Do Bolts Usually Break?

Another reason is fatigue failure, which happens when the bolt has not been tightened properly or has loosened up during its service life. This can cause bending stresses that weaken the fastener, eventually causing it to fail.

Hydrogen embrittlement is another common cause of bolt failure. This is a serious type of failure that can occur when a bolt breaks off and projects like a rocket due to the high load on it.

This typically occurs within the first hour of the initial torquing of the bolt and is caused by the electroplating process. Understanding the causes of bolt failure is crucial for ensuring the safety and reliability of mechanical systems that rely on bolts for fastening.

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Types of Bolt Failure

Various types of bolt failure can occur in mechanical systems. Understanding these types of failures can help prevent them from happening in the first place. The following are some of the most common types of bolt failure:

Overstressing: This is the most prevalent cause of bolt failures. Bolts are constantly under pressure from the internal stress (aka preload), external pressure (the tensile load), and static stress (tensile load) pulling on both ends of the bolt. Even high-strength bolts can fail due to overstressing.
Hydrogen Embrittlement: This is a very serious type of failure that can take place when a bolt breaks off and projects like a rocket due to the high load on it. This typically occurs within the first hour of the initial torque of the bolt. The reason this issue occurs can be linked to the electroplating process.

Fatigue: Fatigue failure occurs when a bolt is subjected to repeated loading and unloading cycles. This can cause microscopic cracks to form in the bolt, which can eventually lead to its failure.
Corrosion: Corrosion can weaken the bolt, making it more susceptible to failure. This can occur due to exposure to moisture, chemicals, or other corrosive substances.
Installation Error: Bolts can fail if they are not installed correctly. This can include not tightening the bolt to the correct torque or not using the correct lubricant during installation.

It is important to note that bolts can fail due to a combination of these factors. For example, a bolt may fail due to overstressing and corrosion. Therefore, it is important to consider all potential causes of bolt failure when designing and maintaining mechanical systems.

Causes of Bolt Failure

Bolts can fail due to a variety of reasons. One of the most common reasons is overstressing. Bolts are constantly under pressure, and if they are subjected to excessive stress, they can fail.

This can happen if the bolt is tightened too much or if it is subjected to loads that are beyond its strength. Even high-strength bolts can fail if they are overstressed. Vibration is another common cause of bolt failure.

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, which can result in a total loss of control.

Another cause of bolt failure is hydrogen embrittlement. This is a very serious type of failure that can take place when a bolt breaks off and projects like a rocket due to the high load on it.

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This typically occurs within the first hour of the initial torque of the bolt. The reason this issue occurs can be linked to the electroplating process. Quality issues can also cause bolts to fail. If the bolt had not been sufficient during installation, had lost its preload during service, or had been understrength (quality issue/wrong grade), it can fail.

Finally, corrosion can also cause bolts to fail. If the bolt is exposed to moisture, it can rust and weaken over time. This can cause the bolt to break when it is subjected to stress or load.

Preventing Bolt Failure

Preventing bolt failure is crucial to ensure the safety and reliability of any equipment or structure. Here are some tips to prevent bolt failure:

Proper installation: Bolts should be installed with the correct torque and tension to ensure they can handle the load they are designed for. Under-tightening can cause the bolt to loosen and fail, while over-tightening can cause the bolt to break due to excessive stress.
Use the right grade of bolt: Using a bolt with a lower grade than required can lead to failure due to insufficient strength. It is important to use the correct grade of bolt for the load and environment it will be exposed to.
Prevent corrosion: Corrosion can weaken bolts and cause them to fail. To prevent corrosion, bolts should be made of materials that resist corrosion and should be coated with anti-corrosion coatings. Additionally, they should not be exposed to chemicals or environments that can cause corrosion.

Regular maintenance: Regular inspection and maintenance of bolts can help prevent failure due to fatigue or wear. Bolts should be inspected for signs of corrosion, cracking, or other damage. They should also be retightened if they have loosened over time.

By following these tips, the risk of bolt failure can be significantly reduced, ensuring the safety and reliability of equipment and structures.

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Conclusion

There are several reasons why bolts break, including overstressing, hydrogen embrittlement, fatigue failure, misassembly, and damage to mating surfaces. Overstressing involves placing an unjustifiable load on bolts, which is one of the most common causes of bolt breakage.

Hydrogen embrittlement is a serious type of failure that can occur when a bolt breaks off and projects like a rocket due to the high load on it.

Fatigue failure is the most common type of bolt breakage, which occurs when the bolts have not been tightened properly or have loosened up during their service life. Misassembly and damage to mating surfaces can also cause bolts to break.

Preventing bolt breakage requires proper installation and maintenance. It is important to ensure that bolts are adequately stressed to improve their longevity. Additionally, choosing the right type of bolts for the application and ensuring proper torque during installation can help prevent bolt breakage.

Regular inspection and maintenance of bolts can also help detect any issues early on and prevent catastrophic failures.

Overall, understanding the causes of bolt breakage and taking preventative measures can help ensure the safety and reliability of various structures and equipment that rely on bolts. By following best practices for bolt installation and maintenance, individuals can minimize the risk of bolt breakage and prevent costly damage and downtime.

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