Fastener Distributor Advice | Thread Design and Fastener Failure

Fastener failure results in downtime and increased maintenance costs, two things businesses would rather avoid if possible. Your fastener distributor offers plenty of advice to help you choose products and materials that will reduce the occurrence of fastener failure.
One rule of thumb not widely known outside of the fastener industry is that fine threads have a higher rate of failure. This occurs for two distinct reasons:

• Dilation of the nut shifts stress or pressure up to the weaker tip of the thread.
• On common nut sizes a higher amount of the average stress is focused on the initial fine thread, as opposed to the amount of stress focused on that first thread with coarse thread design.

Combine these conditions and the chance of progressive shear is markedly higher on finer thread fasteners. Your fastener distributor will recommend fine threads for certain applications, but should inform you that the risk of failure is significantly increased with finer thread nuts.

Basic Explanation
Finer threads tend to leave more of the fastener shaft in tact, providing a thicker and stronger bolt. Bolt breakage may be less of an issue, but when coupled with a nut those fine threads are at risk of progressive shear, even without the presence of vibration.
The stress area on fine threads is smaller; at the same time there is less thread material remaining to take the load. That load is spread over a smaller area as well, resulting in a much earlier stripping point.
As a nut dilates and moves the engagement toward the tip of the thread coarse threads provide a longer path and therefore better load distribution. Fine threads, although they admittedly produce a stronger bolt, leave less room for that engagement, meaning the fastener will strip earlier and under less proof load.
The SAE specifications recognize this condition, as your fastener distributor will be able to point out. For hex nuts from 1/4" diameter to 1” diameter, fine thread nuts (grade 5) come with a 109,000psi proof load, while coarse thread nuts (grade 5) offer 120,000psi proof load.
Conclusion
The strength and durability of fasteners is more complicated than it may look at first glance. A thicker bolt (designed with fine threads) offers a greater risk of fastener failure due to a smaller amount of thread material left to engage the load. Stripping strength is one factor of the equation that must be discussed with your fastener distributor.