Q: I’m designing a process that must be able to handle a high load at about 1000F. I’m planning to use A286, but I now need to know what bolt length size to use to make this machinery stable. I’d like to use as short small of a bolt as possible so how many threads do I need to make a strong connection? And should I use fine or coarse threads?
A: This is two separate questions. First let’s look at fine compared to coarse thread. Fine threads have more threads per inch so this will give you more gripping area in a smaller length. For example a 1/4″ bolt comes in 1/4″-20 which is coarse thread – and has 20 threads per inch. Whereas a 1/4″-28 finely threaded bolt has 28 threads per inch. A finely threaded bolt will allow you more threads on a shorter bolt. The other major advantage of a fine thread bolt is that it has a larger minor diameter. This will give the bolt a larger cross-sectional area and allow the bolt to take a larger load (both tensile and shear). Because of this – fine thread screws are always stronger compared to the coarse thread option of the same diameter.
Now since you want to use as short small of a bolt as possible it’s important that you have the necessary amount of threads in order to handle your load. The answer here can vary but typically we recommend about 6 threads to provide a secure connection. When you apply load to a bolt, it actually will stretch minutely, which causes the first thread to bear bare the most load, then it decreases from there. After about 6 threads (give or take), it will not improve the bolt’s load capacity