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

Greetings All,
I'm looking for a ball park, back of the envelope, best guesstimate for this situation.. :)
3/4 cube of white Delrin with a 1/4 -20 threaded hole down the middle,
thru which is screwed a long 1/4-20 stainless piece of all thread. (total thread contact is 3/4")
The question is how much weight will it take to shear the threads out of the Delrin from a static load?
For instance, assume Delrin block is secured so it cant move. How much force along the threaded rod (which isn't turning) would it take to "Press" or "Pull" the threads out of the Delrin?
For reference this is NOT a life threatening situation in the event of thread failure, I'd just be pissed off ;)
The Delrin block is being used as part of a fine altitude adjustment for an aluminum structure.
The only time there could be any 'real' stress on this part is during the time of picking it up out of my truck and putting it on the ground, where the Delrin threads may have to support 100 pounds or so, probably less...
Take Care, James Lerch http://lerch.no-ip.com/atm (My telescope construction, Testing, and Coating site)
Press on: nothing in the world can take the place of perseverance. Talent will not; nothing is more common than unsuccessful men with talent. Genius will not; unrewarded genius is almost a proverb. Education will not; the world is full of educated derelicts. Persistence and determination alone are omnipotent. Calvin Coolidge
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My SWAG is 500 lbs. No calculations performed, just an off-the cuff guess.
Jim
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James Lerch wrote:

The shear strength(S)of Delrin is 9000psi. The area (A) in shear is roughly 2 pi r h * .7 or 2*3.14*.125*.75*.7=.412" 2. The .7 factor accounts for the incomplete thread form. The force to strip the bolt out of the delrin is F=SA or 9000 * .412 = 3711 lbs.
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tomcas wrote:

REALLY!!?? My Volvo 240 station wagon weighs 3000 lbs. Would you stand under it while it was being held up by a 1/4 rod threaded into Delrin (assuming the rod would hold it)? I wouldn't stand under it being held by 4 of them. Methinks that a decimal point has been moved, but I'm too lazy to check it 8-)
Bob
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Hmm. That *may* be less than the tensile strength of the steel all-thread.
The steel might fail, not the delrin.
Jim
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jim rozen wrote:

That assumes the bolt is in tension. He specifically stated "press or pull".

The minor diameter or a 1/4-20 bolt is .189". The tensile area is pi r squared. A= 3.14 * .0945 squared= .028" sq. The ultimate tensile strength of a typical stainless steel (304) in the annealed (not cold worked condition) is 85,000 psi. The tensile strength is F=SA or .028 *85,000= 2,385 lb.
If the two parts were loaded with the bolt in tension, and the bolt thread was cut instead of rolled, you are correct, the bolt would fail first. However, most fasteners have the threads rolled, not cut. In austenitic stainless steels this cold working raises the ultimate tensile strength. The smaller the bolt, the greater the cold working effect. Precipitating hardening stainless steels like 17-4PH can easily be hardened to achieve an ultimate tensile strength of 190,000 psi. If the bolt were cut from this material it would have an ultimate tensile strength of 5,320 lbs. The delrin would strip out before the bolt breaks. So, it depends on the material as to which part fails first.

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He said it was all-thread. Probalbly cut threads, so your calculation using the cross-section of the minor diameter is flawed. You need to include a factor that accounts for the stress concentration at the root of the thread, my SWAG is about a factor of two. So the bolt snaps around 1000 lbs tension or thereabout.
Obviously a different outcome if it's a high strenght rolled thread fastener.
Jim
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wrote:

Jim,
Here's the description from smallparts.com where I got the 1/4-20 all thread "Cold Rolled Stainless Steel 18-8"

In the mean time, thanks to everyone for running the calculations!
Bottom line, the item shouldn't see more than 100lbs, unless I drop it! (which I plan to never do, and if that day should ever come, I plan to catch it with my big toe :)
Take Care, James Lerch http://lerch.no-ip.com/atm (My telescope construction, Testing, and Coating site)
Press on: nothing in the world can take the place of perseverance. Talent will not; nothing is more common than unsuccessful men with talent. Genius will not; unrewarded genius is almost a proverb. Education will not; the world is full of educated derelicts. Persistence and determination alone are omnipotent. Calvin Coolidge
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jim rozen wrote:

He means the that the bolt is threaded completely through the delrin block, hence "(total count is 3/4)" in a "3/4" cube". Regardless if a bolt is partially threaded, or threaded up to the head, the tensile strength is the same, being limited by the weakest section- the root diameter of the threads.
Probalbly cut threads, Bolts which are threaded up to the head, just like ones which are partially threaded, are usually rolled. In fact the threads on almost all bolts and screws are rolled.
so your

Rolled or cut, the minor diameter is the same.
You need to include a factor that accounts for

Stress concentration? On a static load? Can you tell me where you are getting this from.
my

http://www.derose.net/steve/resources/engtables/bolts.html bottom of the page- 1/4-20 Grade 2= 2750 pounds Grade 5= 3800 pounds Grade 8= 4750 pounds

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wrote:

Tom,
Here's a picture of the setup
http://lerch.no-ip.com/atm/Projects/DF_Optics-16-Bino_Low_res/Bino_R-L_Detail.jpg
(253KB)
The All Thread is cold rolled stainless 18-8, If it matters
Thanks again
Take Care, James Lerch http://lerch.no-ip.com/atm (My telescope construction, Testing, and Coating site)
Press on: nothing in the world can take the place of perseverance. Talent will not; nothing is more common than unsuccessful men with talent. Genius will not; unrewarded genius is almost a proverb. Education will not; the world is full of educated derelicts. Persistence and determination alone are omnipotent. Calvin Coolidge
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No, he's not useing a bolt. It's "all thread" which is long, continously threaded stock. Not typically made with rolled threads. Seems like some kind of lead screw.

Read a copy of Carroll Smith's "Nuts, Bolts, Fasteners and Plumbing" because they give a good discussion. An introductory text on mechanical engineering will have a short section on this as well.
Stress concentration is NOT the same thing as dynamic loading. Two different animals.
Stress concentration happens when an object has a sharp corner or ridge where lines of stress are nearby.
The classic example is a die- or lathe-cut thread which will fail at about 1/2 or 1/3 the strength predicted by calculating ultimate strength based purely on the minor diameter of the thread. Basically the sharp V form concentrates stress at the root. Even if the thread form is radiussed at the bottom (whitworth threads) there will still be inherent weakness because of the microscopically torn and fragmented material at the root from the lathe tool.
True high strength threads are *rolled* threads so that the grain structure of the material is oriented and cold-worked at the point of highest strain (root of thread) and this allows one to approach the calculated tensile limit based on the minor diameter and the material properties.
Other examples are shot-peening of cranks, etc, to mimize surface defects and to leave the material under tension at the surface.
One can also see engineering to miminize stress concentrations in the following places:
High strenght bolts often are 'waisted' where the fastener is relieved down to the root diameter of the thread, for some portion of its length behind the thread. A bolt with the unused portion of the thread turned off is actually *stronger* than one with an unthreaded shank sized at the major thread diameter. The lines of stress will flow smoothly into the minor diameter portion and not concentrate at the last engaged thread if the unused threads are removed.
Shafts with sharp-edged corners will concentrate stress at the edges, and be two or three times weaker than one engineered properly, with radiussed or undercut corners. Often a lucite model of the part or fastener in question can be made to good advantage and placed between crossed polarizers, to actually visualize the lines of stress.
Smith's book is an excellent reference for this issue.
My own particular personal experience was fabricating 3/8-16 bolts out of SP-1 Vespel, and trying to get the strongest ultimate strength. By modifying the design I was able to increase the ultimate strength by three times, from a sharp V form to a greatly radiussed thread form, with a relieved shank.
Jim
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jim rozen wrote:

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.

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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
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jim rozen wrote:

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

stainless steel screw is fully annealed after rolling.

I don't know what you typically fly on, but the chances are pretty good if it's military or commercial, and has a GE engine, its too late. You have already needlessly subjected yourself to my engineering. Why don't you ask a mechanical engineer at your place of employment to explain to you why stress concentration factors are not applicable to a statically loaded threaded member in pure tension. Perhaps you will believe it if you hear it from someone else.

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I strongly recommend this book.

Stress concentration is *not* only about fatigue. Try snapping a few bolts with an instron. You will see the difference, depending on the thread type and fastener design.
Jim
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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
tomcas says...

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THink you missed something there. I don't see the tTPI or the thread width and .7 would definitely not be it.
How about Pi*D*TPI*L*(D1-D2)/2 where D1 is the major diameter. of the screw and D2 is the minor diameter of the hole. For 1/4" UNC that would be .25*3.14*20*.75*(.25-.196)/2 =.318 So the threads would strip at about 2800 lb. and a safe working load would be around 280 to 560 lb.
I have a lot of acetal parts in my steering system and while I have not tested to distruction I do torque the bolts and know that a 1/4" stainless hex bolt will take a straight pull well over 1,000 lb without a shrug because I used the K factor for dry steel when Delrin is a lot slicker.)
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James Lerch writes:

_Machinery's Handbook_ section, "Formulas for Stress Areas and Lengths of Engagement of Screw Threads", which includes complicated formulae for cases like this where the two materials are of different strengths.
As a guess, I would estimate 1/5 to 1/10 of the steel version. A C-1030 1/4-20 forged eyebolt has a breaking strength of 2500 lbs and safe working load of 500 lbs, while the threads themselves are supposed to be a bit stronger than the bolt itself. So I would estimate a safe load is 50 to 100 lbs, and a pullout strength of 250 to 500 lbs.
I have some suitable scraps of polyacetal if anyone would care to join in a wagering pool, to be awarded by an actual test.
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I took a quick shot at it: Bolt breaks at 1600 pounds =pi*(root radius)^2 * 60,000 psi. Shear out is (root diameter)*pi* height* shear strength or about 3700 pounds. Net: bolt will break before you pull out of the block. That calc is not particularly accurate since the Delrin will compress way more than the stainless will stretch under load, puts all the stress on the far end, shears out the threads sequentially.
My guess is that it will break the bolt before it pulls out. For an unbalanced load with potential twisting, I'd be leery of any load over a few hundred pounds.
Now that you have my interest up, I might stick it in the tensile tester next week! :)
James Lerch wrote:

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Now THAT would be cool! Espeically if you could get video of it :)
Take Care, James Lerch http://lerch.no-ip.com/atm (My telescope construction, Testing, and Coating site)
Press on: nothing in the world can take the place of perseverance. Talent will not; nothing is more common than unsuccessful men with talent. Genius will not; unrewarded genius is almost a proverb. Education will not; the world is full of educated derelicts. Persistence and determination alone are omnipotent. Calvin Coolidge