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strength of limiting knots in thin webbing?
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linvillelover


Apr 5, 2008, 8:37 AM
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strength of limiting knots in thin webbing?
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I am curious as to how much the single overhand knot will weaken the bight of the thin Mammut (8mm?) webbing.

Specifically, as a limiting knot in a sliding X or the equalizer/ equallete..

The tape is rated to 22kn.. and i would expect the knot to weaken it 20-30%, but is it worse since the webbing is so thin (sharp)?

I have seen it used with knots in many books, but i just worry as many times it is the full size webbing (9/16) used.

I also worry about the strength of the knot itself holding the load, i wonder how strong the knot holds when pulled on directly as would happen in the case of an anchor leg failure.


Aric.. care to break some?

Am i worrying too much?

Please feel free to discuss...
thanks.


ja1484


Apr 5, 2008, 9:16 AM
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Re: [linvillelover] strength of limiting knots in thin webbing? [In reply to]
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Best case scenario knots steal ~30% strength, usually more like 40%.

There is some question about whether these ratios are higher in thin webbing due to the recent John Sherman rappel anchor failure incident:

http://www.supertopo.com/...msg=273885#msg273885

FWIW, if it's a sling you're going to be knotting, I'd stay with at least 12mm dyneema or Nylon. I carry maybe two or three slings on the rack of the butt-floss-thin stuff. the rest is nylon, tied cordage, or metolius ultratape.


majid_sabet


Apr 5, 2008, 9:19 AM
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Re: [linvillelover] strength of limiting knots in thin webbing? [In reply to]
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Generally knots are the weakest part of the system so just take 30% off.


knudenoggin


Apr 5, 2008, 9:49 AM
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Re: [ja1484] strength of limiting knots in thin webbing? [In reply to]
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ja1484 wrote:
There is some question about whether these ratios are higher in thin webbing due to the recent John Sherman rappel anchor failure incident

This nonsense should not keep popping up: it was dealt with by Mammut,
who confirmed what appeared obvious--the tape was c u t . Can we kill
this myth?

*kN*


maldaly


Apr 5, 2008, 9:51 AM
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Sherman's sling was cut. Follow that thread and you'll see Mammut's report. I took a hard look at the sling before it was sent to Mammut and I also pull tested all of John's other slings from that same batch and we couldn't duplicate that failure.
Mal


ja1484


Apr 5, 2008, 9:55 AM
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Fair enough fellas...put down the pitchforks and torches.

I didn't follow the situation that in depth after the BD tests of sling on sling as Mammut was taking for frickin' ever to get their report out the door. I'll go give it a looksie.

Mal, good to know you guys couldn't duplicate the failure, at least not at loads low enough to be worrisome. That's pretty much the same conclusion Kolin Powick over at BD came to, and a little more reassurance is always nice.


adatesman


Apr 6, 2008, 3:17 PM
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knudenoggin


Apr 29, 2008, 9:51 AM
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adatesman wrote:
linvillelover wrote:
Aric.. care to break some?
Sounds like formal tests have already been done on this by people more qualified. I'll see if I can find published results, but if I can't, sure, I can pull some (would need samples though). -aric.
Actually, no it doesn't: the apples at issue in the question would be the
Offset Ring Bend in tape & the Becket Hitch you photographed for me,
among possible others; the oranges of testing referred to above are
whatever (?!) versions of the infamous "Girth Hitch" were made.

Someone posted to RC.com about testing the ORB in 1" tubular, IIRC,
and the results were significant in two respects: the failure was at lower
forces than for the non-offset RB (aka "Water Knot"/"Tape Knot"), and
the failure came by a(n audible) tearing, incrementally (well, incremental
to slow-/steady-pull testing, not necessarily to a dynamic loading) instead
of a sudden bang & burst. From this, I'd surmise that the skinny tapes
would have LESS of this difference, having less proportionately that
tape-wise, flat aspect. (But given the relative thickness of skinny HMPE
tape, one will likely find ORB's taking a variety of orientations, as the
material doesn't so comfortably lie flat upon itself!?)

Ah, here's my saved snippet:

http://www.rockclimbing.com/...ost=1503079;#1503079
In reply to:
[ Dec 16, 2006]
Not too long ago I used my low-tech dynamometer to test EDK'd 1" slings to destruction:

Today I tested 1" tubular webbing with edk [OOB ORB] vs water knot, two specimens each.

The edk[ORB] tore at the knot at 16.4 kn and 16.9 kn.

The water knotted sking broke both times right at 23 kn.
Both times it broke away from the knot, at the place where it was looped
around quicklink that secured it to the test chassis.
[Which is to say that the knot did NOT break! geeesh, ... *kN*]

The webbing tore at the knot at around 17 kn as opposed to 23 kn in a water-knotted sling.

The thing that got my attention was not so much the reduction in strength as it was the failure mode. While the water knotted sling let go all at once with a bang, the EDK'd sling tore through progressively, starting at one edge and going through to the other.

What that test told me is that the EDK tends to load one edge of the the webbing more than the other. And once the fibers at that edge reach their breaking strength they let go, transferring the stress to the next fibers over. And so on until there are no fibers left.

My suspicion, based on what I saw in that test, is that the reduction in strength between the EDK and the water knot, while not so bad in brand new webbing like I tested, might be much worse in aged material. Such material would probably be less elastic, and thus even less able to distribute stresses when loads are unevenly applied as they are with an EDK.
[And one might suggest that the lower elongation of HMPE would *model*
this stiffening of nylon, but then the difference of actual shape (less wide
vs. thick) works to reduce this weakening. *kN*
]

[& later]
That's one funny thing about the tests that I mentioned earlier. In four tests of EDK'd webbing to failure, not one of the knots ever rolled. They all just cinched down and then tore through at the knot. That doesn't mean that EDKs won't roll in webbing, but it suggests to me that they don't roll as much as with round cordage.

Thanks to all, Bob "BoKu" K.

To the OP, the cited testing gives a hasty read of 1" tubular nylon slings
with limiter Overhands. NB: It's a test of really exactly what you're
asking, NOT a test of the knot per se, which alone should be about half
the figure (as what was tested was a closed loop in which the knot was
on one side taking about half of the stated figure!).

*kN*


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