13 years ago
Here is some validation for the old rule of thumb that 6 threads is
all you need to be engaged in the work.
Best to visit the site because they have a simple graphic showing the
stress on the threads.
As nuts and bolts are not perfectly rigid, but stretch slightly under
load, the distribution of stress on the threads is not uniform. In
fact, on a theoretically infinitely long bolt, the first thread takes a
third of the load, the first three threads take three-quarters of the
load, and the first six threads take essentially the whole load. Beyond
the first six threads, the remaining threads are under essentially no
load at all. Therefore, a nut or bolt with six threads acts very much
like an infinitely long nut or bolt (and it's a lot cheaper).
Stress on bolt threads. Note how the majority of
the stress is on the first thread to the left.
Image from Spiralock.
Thread % %Sum
1 34% 34%
2 23% 55%
3 16% 71%
4 11% 82%
5 9% 91%
6 7% 98%
There is little point in having more than six threads in anything. Nuts
with National Coarse threads typically have 5 threads in them, whereas
nuts with National Fine threads have about 8 threads. Nuts are usually
stronger than the bolts they are on, which is to say that the bolt will
usually break before the nut strips.