# Percentage Strength of Number of Threads Engaged

• posted
To All:
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.
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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.
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.
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• posted
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Hi BottleBob,
A few other things to take into account:
1) What you post holds true for when both thread elements are made from the same material, but when you have a property mismatch (think steel screw into a teflon block), you can increase the overall holding strength with a longer thread engagement.
2) When properties match, lowering thread/head interface friction is the best way to get consistent joint strength, but this usually means using a moly sulfide or equivalent lube on the mating surfaces (uncluding under the bolt head), and doing some "torque to yield" tests with your fastener to find the ideal torque to use. A rough estimate is that ~90% of force applied to a dry fastener can be absorbed by joint friction.
3) Within limits, a longer screw will have a better functional life (remain tight) than a shorter screw, as the longer one will exhibit more "ideal" elastic behavior. It will have more torsional windup during tightening (increasing clamping force scatter), which makes it more critical to reduce friction in the joint.
4) Fine threads when the bolt is weaker, course threads when the nut/block is weaker.
5) Rolled threads good, rolled threads after heat treat better. Cut thread fasteners should be used with caution in any critical application. Be careful of "Grade 9" type fasteners, sometimes the metal is over-hardened to achieve tensile strength, and may fail due to poor elastic properties.
Hope this was of some use,
GPN
• posted
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