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AeroXan
Feb 1, 2008, 6:48 PM
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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.
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ja1484
Feb 1, 2008, 6:51 PM
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AeroXan wrote: 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.
When new, yes, bartacks are stronger than the sling themselves typically.
I've never heard of a sling failing at the tacks - in fact, it's a pretty rare thing for slings to fail at all under any circumstances. They're usually cut or chemically damaged, because god knows they're not the weakest part of a safety system - that'd likely be either the rock or knots in rope/cordage.
(This post was edited by ja1484 on Feb 5, 2008, 10:08 PM)
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Adk
Feb 5, 2008, 4:15 AM
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Bar tacks are tough! With properly engineered bar tacks your body is going to break before the bar tack.
I would bet that the weakest part of any of your gear would be pro placement followed by your bones breaking!
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?
I bet there is.
I bar tacked by hand, some nylon way back in 1981ish and took some pretty good falls. And yes they hurt! Then, I wasn't even in my teenage years. I'm sure my body would have broke before the tacks did.
I had that piece up until about 8 years ago just to
show folks what I did.
Worry about placing pro and feel confident about those bar tacks. Even when heavily abraded they are still TOUGH!!!
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.
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stymingersfink
Feb 6, 2008, 1:08 AM
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Adk wrote: 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.
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gunksgoer
Feb 6, 2008, 1:24 AM
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I wouldnt want to take a factor 2 fall on a spectra daisey chain! I doubt it would break the bar tacks but it may break bones.
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ja1484
Feb 6, 2008, 1:25 AM
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stymingersfink wrote: Adk wrote: 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.
I'm inclined to think the climber would break before the daisy does, although I can see a truly nasty FF2 full length onto a daisy (say a ~8-10' static fall) potentially doing both.
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stymingersfink
Feb 6, 2008, 3:45 AM
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I've taken one short (18-20") pendulum fall (the good piece was 3' to the left) onto a daisy when a questionable cam-hook proved itself a time-bomb with a lit fuse. Short fall, but VERY jarring to the hips and back. It was more than enough to give me an even healthier respect for avoiding such situations.
No, nothing ripped or broke, but I was glad for the stretchy properties of nylon in at least that instance. Never again (i hope and plan).
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Adk
Feb 6, 2008, 9:35 AM
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slacklinejoe wrote: Adk wrote: 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? 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.
and that info right there folks was acquired through research. Thanks for the info. Interesting.
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knudenoggin
Feb 6, 2008, 6:44 PM
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AeroXan wrote: 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.
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*
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Adk
Feb 7, 2008, 12:39 AM
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In reply to: 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.....
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stymingersfink
Feb 7, 2008, 12:52 AM
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Adk wrote: In reply to: 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.....
The slings I've seen used often wear out at the eye before anywhere else, usually attributable to questionable rigging more than any other factor.
no doubt about it, industrial slings are strong, but they're far too bulky and heavy to even consider taking up a route.
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AeroXan
Feb 7, 2008, 4:59 PM
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no doubt about it, industrial slings are strong, but they're far too bulky and heavy to even consider taking up a route.
good point, there's no way you could fit an x pattern on a spectra sling.
thanks everyone for all the info.
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ja1484
Feb 7, 2008, 5:09 PM
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Not sure about spectra, but take a look at any nylon-slung Tricam you have around - they've always used the X-Box stitching instead of tacking.
Interesting...
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slacklinejoe
Feb 7, 2008, 5:12 PM
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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.
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moof
Feb 11, 2008, 6:20 PM
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Below is from http://www.xmission.com/...ull_tests_11_98.html:
This is the same guy who wrote the high strength cord paper. The gyst si that bar tacks are damn strong, but they ARE stress risers. See also the book "On Rope" which also has some summarized results of different sewing patterns for slings. Strongest is a large section of lengthwise zig-zag lines of stitches.
For our (climber) purposes the drop in strength of the bar tack is on par or stronger than the resulting stress rizer going over the small diamter of a carabiner.
"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
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."
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renneberg
Feb 11, 2008, 8:06 PM
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Bar tack climbing webbing with 138 thread... Not 69.
double fishermans is strongest. Box with X next...
Bar tacks are easily inspected, redundant, do not loosen and are less bulky than knots.
Bar tacks are obviously strong enough...
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AeroXan
Feb 11, 2008, 8:27 PM
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neat stuff! that's interesting that the amateur bar tacks held more than the professional ones. maybe that was because there were 5 bar tacks instead of 6? it would be interesting to compare the number of stitches between the professional tacks and the home grown ones. I imagine the big difference with the professional machines is repeatability and continuous use. i was surprised to see that the water knot held more than the pro bar tacks. I've never been sketched about bar tacks, I think they are plenty strong for their purposes. but this is really neat stuff to see. thanx for the info.
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moof
Feb 11, 2008, 8:45 PM
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Even though some home grown samples outperformed professionally bar tacked ones doesn't mean we should all go sew our own of course. The difference between 5 and 6 tacks is likely a red herring. Without getting a larger sample size, the results are mostly just "interesting" To compare joints you would want all the slings sewn/tied off of the same rolls of webbing.
Professional ones usually go through batch testing, and get a 3 sigma strength rating. If you just cobble to together at home, you have no guarantees, and no control.
I sew a lot of NON-life support, non-full strength gear for myself and others. Lately it's been Russian Aiders, which have to be strong, but are not holding falls, or acting as the sole life support like a harness. From all of this I've become keenly aware that a few bar tacks would take a couple second, while it takes me ~5-10 minutes per zig-zag joint to laboriously go back and forth. Mine are also all unique. Even though I have tons of stitches such that I know they are strong, I can't ever claim an average or 3 sigma strength without both testing them AND having a very repeatable pattern (ie, not done by hand).
At the first order, say just putting in a dozen stitches the literature says you get about 1.8X the threads tensile strength per stitch (due to loading in shear, and there being two strands of thread in cross section). So the first order approximation for on my my 2" joints with about 250 #69 (11lb) stitches is ~5000 lbs. Reality is that once you approach the strength of the webbing there are other phenomenon that kick that determine the overall strength.
(This post was edited by moof on Jun 20, 2009, 3:38 AM)
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gunkiemike
Feb 11, 2008, 10:01 PM
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knudenoggin wrote: You might want to think about how a joint can be stronger than the
material--I don't buy it
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.
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moondog
Feb 11, 2008, 10:25 PM
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slacklinejoe wrote: 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.
According to a 1974 article by Carl Magnussen in the Nylon Highway, the "X-in-a-box" stitch is one of the weakest patterns. See below for link; the article is titled "How Strong Is A Stitched Splice In Nylon Webbing" and begins on page 11. Carl was REI's QA/Testing manager at the time of publication.
http://www.caves.org/...ical/nhback/NH03.pdf
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jt512
Feb 12, 2008, 1:03 AM
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gunkiemike wrote: knudenoggin wrote: You might want to think about how a joint can be stronger than the
material--I don't buy it 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. There are seams in parachutes with joint efficiency greater than 1. I assume that this is achieved by using seams that change the way that the material being joined is stressed when the parachute canopy opens.
Jay
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gunkiemike
Feb 12, 2008, 1:21 AM
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jt512 wrote: 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?
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jt512
Feb 12, 2008, 1:54 AM
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gunkiemike wrote: jt512 wrote: 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?
I have a problem with the idea that the tightness of the joint is very important. 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. Your analysis makes no mention of the material used to join the webbing, or the stitch used. We're talking about sewing two pieces of webbing together, not weaving them together, so there is an interaction between the stitching and the webbing fibers that ought to generally weaken the joint relative to the material. So whether the webbing is stacked tightly or loosely (or not stacked at all) you still have the strength of the seam limited by the strength of the webbing and the strength of the thread (or whatever you're using to join the webbing).
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. Again, I don't think this has a whole lot to do with how tight the webbing is stacked.
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.
Now, in light of the fact that there are qualified materials engineers on this site, who can probably answer this question definitively, I have the sense not to take this much further. Do you?
Jay
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gunkiemike
Feb 12, 2008, 2:39 AM
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jt512 wrote: 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.
(Sorry for the implied cheapshot re. parachutes)
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