Originally published on Fully Threaded Radio, Episode 88, 11/13/2014
Recently, a client of mine had advised a customer who needed some fine thread bolts that they couldn't get, to use coarse thread instead, because they were "stronger", or "better" was the actual word he used.
Well, it turns out, because of the geometry of threads in every case, when the material is the same, a fine thread bolt is stronger than a coarse thread bolt. The reason is because the strength of the bolt is determined by two things. The first is the strength of the material. Whether it is a Grade 5 or a Grade 8, or stainless steel, it doesn't matter. The thing that matters, of course, is the size of the bolt. A half inch Grade 8 is stronger than a quarter inch Grade 8, clearly.
But why is a 1/2-20 stronger than a 1/2-13? Well, the reason is because of the cross sectional area through the threads. The coarse threads dig deeper into the bolt than the fine threads do, therefore generating a smaller cross sectional area through the threads, which is traditionally the weakest part of the bolt and where we expect it to fail.
So, if you ever have any doubt, a fine thread is always stronger than a coarse thread bolt, but not always our best choice because with fine threads come some other implications: Threads nick easier, cross threading is easier, and assembleability is not quite as good. Fine threads are popular in airplanes and race cars, and coarse threads everywhere else.
Learn more about bolt strength and thread specifications through classes at the FTI, www.fastenertraining.org.