I've got a degree in Materials Engineering (formerly known as metalurgical engineering) but I never coverd this topic and now I'm wondering.....

I can calculate the load a cylindrical steel rod can take in tension before stretching BUT if it's a threaded rod is one nut with the threads passing fully through it stronger than the rod's cross section?

speedfreak227

I've done some tensile testing in my mechanical engineering lab with a UTM. If I'm understanding the question right, then yes the area where the nut is threaded on would be stronger. The stretching and eventual failure would happen elsewhere along the threads not covered up by the nut.

But I keep re-reading the question and I'm not certain if I'm understanding it right. Are you asking if the nut is stronger than the rod's cross section or if the nut makes the rod stronger at that one certain cross section compared to the rest of the rod?

I think you understood me the first time. I was asking if the rod would break before the threads would strip away or the nut would shear.

Do you have any numbers comparing the relative strengths of the rod vs. the section with the nut? I'm curious how much stronger the nut is.

speedfreak227

So you are asking whether the rod will break or the threads strip away?

If we look at a typical M8 bolt with a typical nut under tension, we need to calculate the tension diameter (direct translation from finnish). For a M8 bolt this is A1 = 36,61mm^2. If the rod's strength class is 8.8 (typical), an 8mm thick (threaded) rod has Rm = 800 Mpa

The A2 for the diameter of the rod = pi*(d^2)/4 = 36,61mm^2

The thread can then take force of:
F = A1 * Rm = 29,29kN

And the rod can take a force of:
F = A2 * Rm = 29,29kN

So when using rods and nuts that are both cold worked (kylmämuokattu in finnish ), there should be no difference in how much force the thread or the rod can take before breaking. Only if you are designing some sort of stud bolt system, you need to actually calculate that the thread can take it.

Or did I miss the point totally...?

The point where the thread enters the nut acts as a stress raiser, and nearly all failures will occur at this point. You bolts will be graded to 'guarantee' a particular load they can take, so you don't need to work out the effective cross sectional area (and if you do, assume the root diameter minus a bit).

But a nut and bolt should NEVER (ideally) be used in pure tension. They should be used in double shear if at all possible, and in this direction they will be much stronger.

I'm thinking of going and buying two small threaded rods and a nut and taking this to my lab for an actual test. I'm really curious now. Or maybe I'll get a few extra rods and nuts to try different nut positions(near top, middle, near bottom). This would probably take place Monday and I'll grab some pics with the digital camera afterwards.

It will also depend on whether you are cutting your own threads versus buying a threaded rod that has rolled threads. As has been mentioned, the threads themselves cause stress risers at every corner that is formed. A rolled thread reduces this as they will be radiused and work hardened.

For your situation, the rod will stretch to some extend. If the stretch is too great, the pitch of the thread will be too great for the nut. This would cause the nut to bind on the rod. If you plan on putting a tensile load on a thread, check out the ACME thread form.

It sounds like you don't have a copy of "Machinery's Handbook". In America, it is considered the engineers Bible. It is currently in its 27th or 28th printing.

I can see one (Machinery's Handbook) from here (my desk). I last used it on Friday, and I expect I'll need it before the day is out! Highly recommended.

Remember getting my first "Machinery's Handbook" many moons ago when I started my toolmaking appreticeship, brings back some memories

I think this thread is full of nuts
sorry

The Handbook is actually being used as a textbook for a class I am taking. I use that thing on a daily basis at work, too.