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
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(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
Reply to
James Lerch
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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.
Reply to
tomcas
_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.
Reply to
Richard J Kinch
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
Reply to
Bob Engelhardt
Hmm. That *may* be less than the tensile strength of the steel all-thread.
The steel might fail, not the delrin.
Jim
Reply to
jim rozen
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.)
Reply to
Glenn Ashmore
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.
Reply to
tomcas
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:
Reply to
RoyJ
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
Reply to
jim rozen
Here's a Huge Thank you to the group!
I'm replying to myself as a simpler method than reply in kind to each of the excellent responses.
First I'm amazed that a 3/4" long female section of threaded Delrin could possible support 1 ton of weight dangling off a 1/4 - 20 threaded rod (not that it ever will!)
Secondly, here some more details.
A) The threaded rod is "Cold Rolled Stainless Steel 18-8" (If that helps)
B) I threw some pictures of what's going on into a web-page, which I'll link to after I fill in the important details and Caveats.
Caveats:
A) This is the first Al welding I've ever done, so yea My welding could use some work! . A friend of a friend has a Miller Syncrowave Tig machine which he was kind enough to let me use. (wow what a nice machine, and no doubt why I was able to make 'anything' looking like a weldment :)
B) I have two pages setup, Both pages are just 320x240 thumbnails, but each thumbnail links to a larger image. The first page thumbnail links are about 500KB, while the lower res page thumbnail links are about 250KB
Details:
The images show the beginnings of a 16" Binocular Telescope. Basically I have two square boxes, each soon to support a 16" mirror. The two boxes are attached to a common "T" shaped frame by compliant metal hinges.
A) The Top left image shows the detail for one of the two Delrin / Threaded rod hinge adjustment schemes I dreamt up.
B) The Top Center Image show the setup for the main framing of the bino-scope. The left box will Adjust for Up/Down miss alignment of the two scope (only needs to move a few degrees). The right box adjusts for Left / Right miss alignment between the two scopes
My Concern, and original motivation for the Delrin strength question, arose from the concern that Two guys are going to want to grab the "ends" of this thing to haul it around the field. The only thing keeping the unit from Flopping around are the two pieces of threaded Delrin :)
Here's the links to the images
Hi-Res thumb nail links:
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Lo-Res Thumb nail Links:
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Thanks again for the help gentleman!
"Now all that's left is EVERYTHING, then hopefully it will work, then hopefully I can sell the thing and I buy my Friend of a Friend's Tig Machine :)
Take Care, James Lerch
formatting link
(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
Reply to
James Lerch
Jim,
Here's the description from smallparts.com where I got the 1/4-20 all thread "Cold Rolled Stainless Steel 18-8"
I've posted more details and links to pictures in another reply.
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
formatting link
(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
Reply to
James Lerch
Now THAT would be cool! Espeically if you could get video of it :)
Take Care, James Lerch
formatting link
(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
Reply to
James Lerch
On Sun, 24 Oct 2004 03:47:24 GMT, snipped-for-privacy@no.spam.tampabay.rr.com (James Lerch) calmly ranted:
James, what's the X stitching in this picture, holding the two sides of the mounts together, just left of the delrin parts?
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That'd be cool.
I like it!
Reply to
Larry Jaques
It's my version of this:
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a "Split X Metallic Compliant Hinge" Works great if you only need +/- 15 degrees of motoin, and not very often...
What's amazing about them is they absolutly positivly prevent motion in any other direction other than the hinge line. (well up untill you collapse one leg of an X somehow)
Take Care, James Lerch
formatting link
(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
Reply to
James Lerch
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
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of the page- 1/4-20 Grade 2= 2750 pounds Grade 5= 3800 pounds Grade 8= 4750 pounds
Reply to
tomcas
Tom,
Here's a picture of the setup
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(253KB)
The All Thread is cold rolled stainless 18-8, If it matters
Thanks again
Take Care, James Lerch
formatting link
(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
Reply to
James Lerch
Consider that the Delrin is not constrained from expanding radially and is a low modulus (soft, flexible) material compared to the steel. Under load, a radial component of force is exerted by the contact pressure on the sloped face of the thread cross-section. This force could be calculated, remembering that friction and creep (in the plastic) will complicate the calculation somewhat. The radial force, distributed over the bore of the block averages out to an effective radial pressure and the block may be treated computationally as a thick-wall pressure vessel. If, under this pressure, the threaded ID of the Delrin block expands radially, even slightly, then the Delrin and steel threads will start to disengage, and bending, not shear, may be the limiting mode of ultimate failure. Initial clearance between the male and female thread (class of thread fit), part wear, and presence of lubrication or dirt would also enter into this. So would the likely deviation of material strength from published values. Were this a critical component, (or just for the fun of it) I like Roy's suggestion of a well run test to verify any calculations. Hint; how is the Delrin component attached to, and supported by the frame? Structural engineers are expected to consider all these kinds of details.
A (well run) test could also be a very practical answer to the original question. Always use safety factors to account for difference between idealized calculations (or idealized test conditions) and actual field conditions. The worse the consequences of any failure, the higher should be the safety factor. Safety factors as high as ten or more are not uncommon. Some things just shouldn't be done at all; for example, one wouldn't suspend a heavy object by a cast lifting eye and let people walk underneath it.
David Merrill
Reply to
David Merrill
Part of the 'safety factor' issue has to do with dynamic loading.
If hardware is loaded dynamically then that can multiply the force applied by many many times, often five times.
Part of the temptation to strengthen items like this has to do with the difficulty of figuring out exactly what the dynamic loading really is. So folks just toss in a bunch more metal and hope for the best.
Jim
Reply to
jim rozen
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
Reply to
jim rozen

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