has anyone seen/heard of bar tacks failing on slings? I heard that they are a stress riser point so that's why i figured they put so many on (I've seen six on most slings). I would think they would overengineer the bar tacks so it would be stronger than the rest of the sling.

Is there research as to how close they should be, what thread they should be sewn with and how hong they should be as well as the proximity of one tack to another?

Bar tacks are tough! <snip>
So to answer your specific question..have I ever heard of bar tacks failing? Not in a climbing situation I haven't ...though others here may have.

the bartacks on daisy-chain loops have been known to fail at times when pilot-air (or error, as the case may be) resulted in a daisy-fall while aid-climbing. Those pockets are only rated to 2kn though, so it shouldn't be any big surprise. The daisy-chain itself usually doesn't completely fail catastrophically (AFAIK), but I suppose it could happen were one to FF2 onto the thing.

There are too many variables to have a clear cut answer. It depends on the width, thickness of material, stitch pattern and the width of the thread. Basically you want to keep the tack from having so many stitches that it distorts the weaves natural lines to retain strength but still have enough to get your desired strength level.

Generally speaking the most commonly used threads are bonded nylon or spectra thread in climbing applications. Spectra thread is a absolute PITA to work with as well as very expensive (both in different cutters, needles, increased labor and the thread itself). Bonded nylon (woven strands with a coating to keep it slick, cool the needles and to resist fraying) is readily available online and is often in the TH69 or TH92 sizes for outdoor gear.

TH69 thread is approximately 10 lbs tensile test depending on the supplier and the coating. There are many different types of bartack machine cams but the most common in my experience are the 42 stitch ones for tacks up to 1" wide. That's ~420lbs tensile test per bartack made with bonded nylon TH69.

Spectra is stronger / stitch but I dont' remember the numbers. I think it was closer to 19 lbs / stitch.

has anyone seen/heard of bar tacks failing on slings? I heard that they are a stress riser point so that's why i figured they put so many on (I've seen six on most slings). I would think they would overengineer the bar tacks so it would be stronger than the rest of the sling.

You might want to think about how a joint can be stronger than the
material--I don't buy it (but one can see it said). That said, it
might be that such a joint isn't as weak as the bend of material around
'biners, et cetera, which is a practical measure. IIRC, I've seen some
test reports (Tom Moyer's misc. pull testing?) where the stitched part
failed (and failed at a lower force than a Ring Bend!?).

From what I've read, bar tacking is not the strongest way to stitch
webbing; slings used in heavy lifting have a sort of "box" & "X"
stitching. Given how narrow esp. the "dental floss" tape is, the
bar tacking is simpler to do, I guess--and is good enough.

*kN*

We all have seen those magic sewn "boxed "x's"
No doubt that they are strong.
I'm wondering what performs better if abraded. I'm guessing the bar tack. If the bar tack is the winner than it's the way to go for our climbing purposes.
Those hoist slings I don't think see nearly as much rock as our bar tacks.
This stitching info is out there somewhere.....

You might want to think about how a joint can be stronger than the
material--I don't buy it

It's actually pretty simple. Box tacks are technically stronger because they distort the natural weave of the webbing less and spread their load bearing nature over a larger area.

It's really quite simple - you put so many stitches in there that you effectively fuse two thicknesses of webbing together. With enough stitches, those two layers will be much stronger than (approaching twice as strong as) a single layer of the same webbing, so it breaks at the latter. If the stitches let go, you just re-do it with more stitches.

You can have a joint with a joint efficiency greater than 1, but not for the reason you claim; no matter how many stitches you use to join two pieces of material, one stacked on the other, the two pieces of material do not "effectively fuse" and act as one super-strong piece of material.

(speculation re. parachute construction snipped)

Jay

OK, so maybe my choice of the word "fuse" made you think of "one piece" of material, but that's silly. Webbing is a mass of thousands of interwoven filaments. Two webbings sewn so tightly together that they won't separate under the greatest stress is now 2n thousand filaments acting together. Do you really have a problem with that being much stronger?

Perhaps if you used enough bar tacks, with strong enough thread, then you could achieve a joint with efficiency greater than 1. You might be able to spread the force out over enough of the webbing so that the webbing wouldn't fail at the stitching, and then if the thread itself were strong enough, the joint might be stronger than the native webbing.

That's all I'm saying. I don't think we disagree. Stitches are added until the above is true. If someone has a better explanation, I'm all ears.

According to a 1974 article by Carl Magnussen in the Nylon Highway, the "X-in-a-box" stitch is one of the weakest patterns.

You're not going to introduce enough friction between the two pieces of webbing, in practice, to add much to the strength of the joint.
Jay

Jay,

From my understanding, actually it is possible in certain real world situations. Take a look at belay loops. The tacks are fewer than on an end loop but the resulting strength is higher. Yes the material is stronger because it is a loop but the bond on the webbing between those must support a higher bond than the non-looped counterpart would withstand.

Dunno, there is likely something at play there that I'm forgetting but my understanding was that the friction from the wrapped webbing in the belay loop was such that even with minimal sewing the friction provided adequate bonding.

I shouldn't post without caffeine.

Basically in the belay loop scenario the webbing on webbing friction of the wrap adds a significant amount of strength. Similar to that of any friction hitch or the "no knot" tie off method.

Thus, less stitching can be used than a standard rabbit ear style loop and still yield a much higher break strength.

Finally, who says that you really can attain the same strength from a seam in a belay loop by using fewer bar tacks than would be required in a runner?
Jay

Real world field testing. With all due respect please keep in mind, I do design and test high tensile sewn webbing goods for my full time job.

Jay,

Perhaps you'd be better off asking how we did the tests rather than implying that the tests were unrelated, misleading or incomplete.

As to my "speculating" about parachute seams, as it happens, I'm an FAA licensed Senior Parachute Rigger, and though I don't understand the physics underlying greater-than-one seam efficiency parachute seams, I am not speculating when I say that they exist.

... a single wrap of webbing on itself adds a more than significant measurable of additional pull strength.

I didn't say you didn't have real test data. You were the one making claims based on "real world field experience." It was that I was calling bullshit.

Edit: Although the fact that you chose to try and dazzle us with a lot of technical lingo instead of just posting the data, suggests that you probably don't have relevant data.

Jay

Egregious mis-quote there Jay. Scroll up and see for yourself. BIG DIFFERENCE between "experience" (anecdotal, usually) and "testing" (controlled evaluation, usually). I have to wonder if you're blinded by the desire to argue.

I don't misquote. That wasn't the paragraph I was quoting. Try again.

Jay

Nope, sorry...can't find where SLJ ever used in this thread the phrase you put in quotes. What'd I miss?

(Just trying to referee the dialog here...not fan the conflagration)

The guy's profile says he's a college student, so I assume he does not have much experience in technical writing. I'm used to reading scientific literature, where a phrase like "real world field testing" would be never be used.

Comments like these belong in a different forum. It's semi-personal stuff, not related to the question.

Not that it matters - Graduate actually. Masters in Communication/Administrative Studies 2005, undergrad degrees in business / computer information systems 2003. My brother in law is wrapping up his PHD program as an mechanical enginneer and one of my employees is a engineering student at CU. I like to leave self stimulating academia where I left it.

Jay, I've withstood a lot of personal attacks on this forum for my mere existance at what I do. I don't have enough invested in this particular debate to make it worth it. Thus, I am drawing the line and saying no - you've dished out too much disrespect, been too discourtious and your desire to argue has blinded your better judgement. I know very well what will probably be said at my expense following this but I am going to unsubscribe myself from this thread and tell you to shove it.

I also want to let you and the RC admins know I have received emails from other industry professionals who have decided not to post up, basically saying you, or people's posts like yours, are the reason it isn't worth it to them to post potentially valuable information. Goodnight.

Jay, I disagree with you.

This is supposed to be a highly moderated technical discussion

I don't see your comments as adding anything to a discussion of how bar tacked slings failed above the webbing's mean breaking strength.

If you want to start a methodology thread, go for it.

Wow, more creative quoting here. Jay, YOU were the first to use the term "friction" in this thread.

If it were about friction, I imagine we'd be hearing the manufacturers citing stitch tension and webbing surface finish rather than stitch count and thread tensile strength (see moof's post).

You're right. I was the first to use the word friction as a succinct way to express your awkward phrase "sewn so tightly together that they won't separate." If you don't want credit for realizing that you're describing friction, so be it.


Well, now you're agreeing with me, and disagreeing with your previous claim, since I have been saying all along that friction is probably unimportant, whereas you had implied (whether you realize or not) that friction is important.

Jay

Fercryinoutloud, I can nail a couple greased boards together with a few hundred nails and you'll never pull them apart lengthwise, but friction ain't got nothing to do with that either.

Jeesis F'n Cripes...there's no end to your making up quotes, is there? I never said "tightly", let alone the rest of what you put there in quotation marks, so if that's where you INFERRED anything to do with friction, it's all in your head.
.

OK, so maybe my choice of the word "fuse" made you think of "one piece" of material, but that's silly. Webbing is a mass of thousands of interwoven filaments. Two webbings sewn so tightly together that they won't separate under the greatest stress is now 2n thousand filaments acting together. Do you really have a problem with that being much stronger?

Fercryinoutloud, I can nail a couple greased boards together with a few hundred nails and you'll never pull them apart lengthwise, but friction ain't got nothing to do with that either.

Point, Set, Thread.

Two webbings sewn so tightly together that they won't separate under the greatest stress is now 2n thousand filaments acting together. Do you really have a problem with that being much stronger?

[from Tom Moyer at www.xmission.com/~tmoyer/testing/pull_tests_11_98.html ]
Test Methods

We used a vehicle winch on a Hummvee to apply forces. A second Hummvee was initially used as an anchor. However, with an end-to-end pull, and with all four wheels locked, we were able to drag both vehicles across the concrete floor with 5000 lbs force. (good number to know if you use vehicles as anchors.) We ended up anchoring one vehicle to a tank (yes - really) and the other to eyebolts mounted in the wall. A Sensotec load cell was used to measure forces. It is calibrated internally with a shunt resistor.

Results

11/18 Test #1:
Pull an old 9/16" tubular web sling to failure. This sling was found girth hitched around a tree on Mt. Olympus and had obvious burn marks where ropes had been pulled through it. The sling was loaded with an end-to-end pull on the loop.

Result: Material failure at one of the burn marks at 4410 lbs.

11/18 Test #2:
Pull an old 1" tubular web sling to failure. This sling was found tied around a tree on Mt. Olympus. It had no obvious burn marks on it, but it has to be assumed that at least one rope had been pulled through it. The sling was cut off and retied with a water knot. It was loaded with an end-to-end pull on the loop.

Result: Material failure in the middle of the sling (presumably at an unseen burn) at 4300 lbs.

11/18 Test #3:
Pull a new 1" tubular sewn web sling to failure. One of our members sewed the sling on his home machine with a random stitching pattern (for testing purposes only!) The sling was loaded with an end-to-end pull on the loop.

Result: Failure of the stitching at 5260 lbs.

11/23 Test #3

Repeat of 11/18 Test #3
Result: Failure of the stitching at 5920 lbs.

11/18 Test #4
Pull a new 1" tubular sewn web sling to failure. The sling was sewn professionally with 5 bar tacks. The sling was loaded with an end-to-end pull on the loop.

Result: Failure of the stitching at 4720 lbs.

11/23 Test #2
Repeat of 11/18 Test #4 prof.sewing
Result: Failure of the stitching at 4730 lbs.

11/18 Test #5
Pull a new 1" tubular web sling to failure. The sling was tied with a water knot and loaded with an end-to-end pull on the loop.

Result: Material failure in the knot at 4980 lbs.

11/18 Test #6
Pull a new 1" tubular web sling to failure. The sling was tied with a [grapevine bend]and loaded with an end-to-end pull on the loop.

Result: Material failure at the bend over the shackle at 6210 lbs.

---------------------------
Discussion:
The relatively high breaking strength of the two old and burned slings surprised me. Nevertheless, don't expect to see me rapping off a single old sling any time soon. I was also surprised that the home sewing job outperformed the professional bar tacking - and that the bar tacked slings were so weak. I have always been told that sewn slings are stronger than tied ones.

The sling tied with a water knot failed at 4980 lbs. If the two sides are loaded equally, each side carries 2490 lbs of tension. The CMC Rope Rescue Manual claims a 36% strength reduction for a water knot in webbing. Mountain Search and Rescue Techniques also lists a 36% strength reduction, although in nylon rope. If these are correct, the webbing has a material strength of 3890 lbs - very close to the 4000 lb rating usually assumed for 1" webbing.

The sling tied with a [Grapevine Bend] was the strongest of all slings. It is notable that the sling did not fail at the knot.

Bar tack climbing webbing with 138 thread... Not 69.
double fishermans is strongest. Box with X next...

It's actually pretty simple. Box tacks are technically stronger because they distort the natural weave of the webbing less and spread their load bearing nature over a larger area. The reason they aren't used in our applications is that to get enough stitches to maintain strength will require a large area of the sling for sewing as well as increase labor (and thus prices). It would also make the sewn joint 5 x larger, thus making it more bulky, stiff and unmanagable.

Industrial slings are sewn with cordage, not standard thread. This cordage when used in 1" or smaller webbing would distort the weave. It has a much higher tensile strength 30+ lbs so you need far fewer stitches. Together, that makes it practical on large items to use a box tack.

At the first order, say just putting in a dozen stitches the literature says you get about 2-2.5X the threads tensile strength per stitch (due to loading in shear, and there being two strands of thread in cross section).

My office is next to that of the world's most advanced parachute manufacturer, I'm going to wander over there and ask a few questions. But it's not a priority for me, and my wife is having surgery on Thursday, so I'll get around to it when I do. I do know a bit about sewing standards and I do manufacture critical application materials in webbing and in fabric.

If you really want to find out everything there is to know about sling construction, try researching the US Military's voluminous experimentation in ways to put webbing together for climbing slings and a plethora of other applications.

moof- where is this strength estimate from (the 'literature').?