Machinest Handbook lookup request (Pleeeeze)

It's stronger than that, Jim. Three is a guess, but that's the range over which there is a roughly balanced wall section on both sides of the joint. Where the wall section is thinnest is at the ends; neither end is the determinant of strength.

Oh, I wouldn't dispute that. It's just that it isn't necessary to show that the joint is weaker than one with parallel threads. And it's always problematic because it depends on the threading tool that cuts the thread. The standard is one thing; actual strength is another, in other words.

In fact, I just took a look, and there are ANSI, ISO, and JIS standards for mechanical strength of tapered threads -- apparently. I say "apparently" because they're mentioned in the abstracts, but it costs money to get the full standards, and I'm not buying.

It should be noted that parallel pipe thread standards say "mechanical joints," and tapered pipe threads say "sealing joints," but both mention mechanical strength.

Ed Huntress

Uh, in that respect the taper is actually an *advantage*, not a disadvantage. The thin walls at the end of the taper, where there is less strength, are also more stretchy, so they don't attempt to bear as much load. The normal rule of thumb that only about three threads bear the stress (and that it is actually concentrated on the first or last thread) can be improved by using a taper.

(Consider, as a thought experiment, two tubes of the same OD and ID screwed together using an extremely fine tapered thread, with the taper extending all the way from the OD to the ID, so that looking at the joint as a whole, there are no stress risers on the inside or outside. The whole thing stretches like a solid mass; there is no stress concentration on any first three threads or last three threads. Of course this situation -- both OD and ID matching, and ultrafine threads -- is not the situation we're considering here, and any other situation will be worse, but it does serve as the extreme example of the advantages a taper can bring.)

OK Stu , my profound apologies for being one of the reprobates ragging you about multiple posts. Your project looks like a very creative job.

My curious mind forces me ask:

A: I don't recall seeing you mention why you built this rig Stu. Is there a handicapped person living there who can't do stairs?

B: What are the alternate means for this person (if they are disabled) to safely get down and out if there's a fire and the electric power is lost?

C: The offset position of that lifting pipe on the lift platform makes for quite a lot of bending load on the joint in question. And maybe a bit of "bounce" when it stops at the top or when someone steps on at that location. How is that pipe flange under the platform fastened to it so that it won't tear loose.

D: I didn't "get" the safety brake concept from the photo, but it sounds interesting. Does it work something like the jam washer on a screen door closer? Can you amplify the description for me? I realize there's very little chance of it ever being needed, but did you actually test its performance with a dummy load by snipping a temporary link in the winch cable while the elevator was in motion?

Finally, I myself wouldn't trust the 3/4" black iron pipe threads for this job; not with a person's safety depending on them. Assuming you can accomode assembling things in place with the platform fastened to the pipe, I'd go with something better than hardware store pipe there and spend the bucks to have it welded into a 1/4" thick steel plate (about one half the area of your platform) by a certified welder, with a collar or sleeve welded to it and the plate as well.

Just my .02.

Jeff

Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"If you can keep smiling when things go wrong, you've thought of someone to place the blame on."

Stu wrote:

I think he tried to copy a post response to you and got a message back from your "Automatic e-mail verifier" Stu, the one for "Stu or Jan". It looks a somewhat like it might be of e-mail harvesting scam. This is what it just sent to me:

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Jeff

-- Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"If you can keep smiling when things go wrong, you've thought of someone to place the blame on."

If you look at an actual pipe fitting, unless it's one of the thin "merchant" couplings, the wall is much thicker than that of the pipe. I still maintain that the weakest point is the pipe wall where it exits the fitting, per my exchange with Jim.

Here the NPT thread has an advantage over a UN thread. The shear area is a function of the minor diameter of the external thread, and for a tapered thread the minor dia of the ext and internal thread are equal. Measuring in the middle of the thread engagement on a 3/4 fitting, I get a minor dia of about .92

(.92 in X pi) x (3 threads / 14 threads/in) = .62 in^2

Assuming a shear strength about 1/3 of UTS (very conservative, I think)...

.62 in^2 x 20000 lb/in^2 = 12400 lb.

Which is pretty close to the yield of the full (unthreaded) wall of the pipe.

Ned Simmons

The major, minor, and pitch diameters all follow the same taper, so the thread form remains constant throughout the length of engagement.

You must have a different MH than I do - mine (22nd ed) says,

"While external and internal taper pipe threads are recommended for pipe joints in PRACTICALLY EVERY SERVICE, there are mechanical joints where straight pipe threads are used to advantage."

And from Kent's Mechanical Engineer's Handbook (11th ed),

"Taper male and female threads are recommended for threaded joints for ANY SERVICE. ... Straight male threads are applicable only to special purposes, as long screws and tank nipples."

My emphasis added in both cases.

Ned Simmons

Again, the trouble arises with the sharp V form at the root of the thread, and also the die-cut threads. Those are the two *worst* situations to have when one is trying to optimize the strength of the joint.

Each of those will substantially reduce the load carrying ability of a connection like that.

A proper fastener will have a) rolled threads, with b) a bit of a radius or flat at the root.

Jim

================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================

The power was out here for a few hours, so I did a bit of reading and playing after cleaning up in the dark.

First, like you, I recalled the NPT thread as sharp, which is not the case. It's truncated, though obviously not nearly as much as the UN thread. Neither requires, though both permit, a radius in the root.

Socket caps over 2", and nonstandard lengths, have cut threads. Even socket cap screws are allowed to have certain "discontinuities" (laps and seams) in the thread, though not below the pitch diameter.

For a ductile material like low carbon steel, stress raisers have little effect on static strength. They do have a marked effect on fatigue, though the stress multiplication factor used for figuring fatigue (Kf) is often less than for static stress concentration.

The endurance limit for carbon steels varies from 25 to 75 ksi.

Using the 3/4 pipe from before, and assuming a Kf of 3, the joint should not fatigue below about .138 x 25000 / 3 =

1150 lbs. The 25 ksi is pretty conservative. Kf=2~3 seems typical for cut threads in low strength fasteners. The allowable load could be higher if you could predict the number of cycles and/or avoid strain reversals.

Failure under static load would be closer to .138 x 60000 =

8280 lb.

While the lights were still out, I did a quick experiment. I threaded a length of 3/8 pipe from the scrap bin thru a hollow 12 ton porta-power cylinder and put some heavy washers over the pipe, then screwed a couple random CI fittings onto the ends. Upon pumping up the cylinder, initial yield occurred at about 5400 lb tension; tensile failure was at approx 6800 lb. The failure was exactly where we both predicted, where the threads exit the fitting. This corresponds to 57 ksi and 72 ksi respectively. The material spec for black pipe is 35 ksi yield and 60 ksi tensile, minimum.

Ned Simmons

Here of course the *peaks* of the male thread, and likewise any part of the female thread, don't count. What only counts is the most highly stressed point, which is the root of the male thread.

So there is likely to be a fair amount of variation in the thread quality. That makes sense.

There's a lot of substance in that last statement, and I'm not sure I understand all of it. My impression is that in any kind of material, including low carbon steels, microscopic tears and fractures do reduce the static strength. Your statement contradicts that impression and I would be interested in following the reasoning behind it.

Here I was thinking of the two strength-reducing mechanisms (stress risers, and the sharp-V stress concentrating feature) as being two separate but related mechanisms. Maybe not. But I do know that stress concentrations from non-optimal geometry - like sharp inside corners or sharp V thread forms *do* reduce the ultmate strength in all kinds of materials.

I found this out when making some bolts out of SP-1 Vespel. :)

Here I would try to say that the factor of 3 I was proposing was present even in static load. That's the sort of number I've seen in practice. Not a fatigue factor, but due purely to stress concentration.

Did you apply any correction for stress concentration to go from the 6800 lb to get to the 72 ksi number? If not then it may be that either a) it's not as big as I recall, b) the threads you are using are well formed (rolled?) or c) maybe the black iron pipe was steel?

Interesting experiment. It would be easy to duplicate it for some home depot cut thread 3/4 inch pipe, and give this guy a *real* number. The failure you observed where the root of the thread crosses the top line of the female thread is a classic, this is where all the stress shows up. One reason why designers take pains to avoid putting *other* stress concentration features in line with that surface in bolted-up assemblies.

Thanks for taking the time to do this, and to post up the results. Fascinating.

Jim

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Isn't it? I thought all black and ganvanized pipe was welded steel tubing.

Tim

-- "That's for the courts to decide." - Homer Simpson Website @

Agreed. Both the crest and root are truncated, though only very slightly.

And I can't claim to completely understand either. Presumably it only applies to ductile materials because they have the ability to deform locally, thus reducing the stresses without fracturing.

I think ceramics would be an extreme example of this. I've never used Vespel, isn't it similar to Ultem? If so, I'm not surprised it would be sensitive to sharp threads. I'd expect something like nylon to be less so.

Determining stress concentration factors appears to involve a lot of voodoo and empirical formulas. Most of the values I found for the stress concentration factor for fatigue seemed to come from experimental results and were a function of material as well as geometry. Roark has pretty extensive formulas for simple elastic stress concentration for various geometries.

72 ksi is simply force/cross sectional area at the root of the thread (6800 lb/.094 in^2).

I just took a look at the threads with a magnifier and compared them to an NPT gage. They're pretty rough looking, obviously die cut, though they do appear to have a slightly wider flat at the root than the gage.

Black iron is a misnomer, it really is steel.

I learned a lot myself.

Ned