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.
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. ============================================================
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.
very interesting and useful bit of info- something I'll try to keep it in mind for the rest of my days, and even pass on to my kids too ( seriously, not sarcastic)
I swear on this day I will not try to tap as deep as I driiled ever again. I will remember your post everytime I tap a hole. Damn...I must of tapped a million useless turns of the tap in my lifetime.
Thanks. Im printing out the first paragraph and pasting it on the board at work. Good job man.
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.