# [SOLVED] Delrin and Stainless 1/4 - 20 all-thread question

• posted

Well, you can skip the theory and measure this, w/ a cupla calc's.

Fasten a tight-fitting washer, 1" OD, to resist the delrin as the bolt/delrin are tightened. Hold/turn whatever/however is most convenient, and turn w/ a wrench/pipe of known length, w/ some estimable applied force, to the stripping point. Use r1F1 = r2F2. Multiply F2 by (20*12) (the pitch and foot conversion), to get the force due to the mechanical advantage of the thread pitch. r1, F1 are the applied radius/force of the wrench, and r2 is that of the bolt, somewhere between the major/minor radius of the bolt.

If the whole thickness of the delrin can't be used, use whatever thickness is practical, and extrapolate linearly to the full thickness, as this should be linear.

I think.

• posted

Right. That would be my first guess. A ordinary nut has a thickness of about 80% of the thread diameter. In this case the thread engagement is three times the thread diameter. Dan

• posted

You are wrong. It is exactly the strength predicted.

Basically

All of this means nothing to a statically loaded thread. Stress concentration due to the thread form will have absolutely no effect in this case.

In the aerospace industry they are called "J" threads, as in UNJ thread form. They were designed to increase fatigue life by reducing the stress concentration. The effect on static load capacity is nil.

Grain orientation has got nothing to do with it. It is simply the rolling process which induces cold working into the stainless and in turn raises the ultimate tensile strenght.

Wrong.

I suggest your test data was flawed. No way are you going to get 3 times higher static load strength by changing the thread form from a standard truncation to a radiused root, not for a static load. The fatigue life can be greatly enhanced but as I said before, the static load is a function of the root diameter.

• posted

Then you had better take that up with the authors of the mechanical engineering texts, and specifically with Carroll Smith.

Go right ahead and use lathe-cut threads rather than rolled threads. You are correct they are just as strong. Please let me know if anything I ride, fly, or drive on has any of your design work in it.

Jim

• posted

I don't have the Carroll Smith book you refer to. But I do have several mechanical engineering texts.

only if the rolled thread of a austenitic stainless steel screw is fully annealed after rolling.