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per.andersson
Feb 11, 2012, 1:49 AM
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I have a 12mm sling made from a mix of Dyneema and nylon, that I havn´t yet had the opportunity to have bar-tacked onto a slingless cam, since I´m traveling in a less developed country at the moment. I was considering tying it with a knot in the meantime. I´ve read in several books that a double fishermans knot could slip with dyneema, and that instead a triple fishermans should be used for this material. However, a friend and guide just told me that any knot in dyneema will slip open when a certain amount of force is applied, and that dyneema for that reason should never be tied, but always sewn. He was referring to a tour of the BD-lab, where he had watched knots slowly come undone when put under force. I also read somewhere that Beal refuses to sell unstitched dyneema webbing for the same reason. What are peoples opinions on this? What is the difference in results between the short loading from fall and the continuous loading in the testing machine?
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maldaly
Feb 11, 2012, 5:19 AM
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Per,
It depends on the weave. We had some 12mm Dyneema at Trango that would hold a knot. I discovered this by accident when I had tied a knot in a long dyneema sling to shorten it for a test. Our rig didn't have enough draw length for the full-length sling so I needed to shorten it. Damned if that know didn't hold and the sling broke at the test pin. Even more astonishing was that it was a water knot. We then began to perform some more thorough testing and foound that this particular weave would hold under any condition we could think of. Wet, dry, bad knots, water knots, single, double and triple fisherman's all held. We were also able to test it on a regular enstron with a pull rate of 50 mm/min as well as a fast puller which, if my memory is correct, 5M/sec.
I don't know if that stuff is available or where you might be able to get it. As a general rule of thumb, I think the general advice is good: don't rely on knots tied in dyneema.
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JimTitt
Feb 11, 2012, 8:39 AM
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We´ve also broken single strand Dyneema hybrid at the test fixture or in clove hitches by tying a loop in the end, the loop knot never fails. We use a bowline on the bight to form the end loop. Both drop and slow pull testing.
That is with the typical 12mm wide tape with the speckled appearance, not the sort where the nylon only appears as a band of colour down each side.
Jim
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acorneau
Feb 11, 2012, 12:39 PM
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Mal and Jim have much more hands-on experience with testing of this stuff so I'll let their expert answers take care of that part..
However, I believe there might be some confusion here...
per.andersson wrote: I´ve read in several books that a double fishermans knot could slip with dyneema, and that instead a triple fishermans should be used for this material.
The only time I've seen this recommendation is when talking about the high-strength "tech cord" used to re-string older hexes and the like. Blue Water's Titan cord is an example.
"Manufactured from Dyneema high performance fiber and nylon. ... A triple fisherman's knot is recommended for tying 5.5 Titan into loops."
http://bluewaterropes.com/...yKey=&ProdKey=50
It sounds like you have mixed up the information concerning Dyneema webbing and Dyneema cord.
Just a thought.
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per.andersson
Feb 11, 2012, 2:20 PM
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acorneau wrote: It sounds like you have mixed up the information concerning Dyneema webbing and Dyneema cord.
You may be right! Perhaps the texts I´ve been reading have been specifically about cord, without mentioning webbing. Thanks for pointing it out.
Jim and Malcolm: Thanks for taking the time to write those very informative replies! I got the feeling from your replies that you both meant that slings with lots of dyneema in them are not suitable for any kinds of knots. The webbing I have is the "speckled" type with mostly nylon and some threads of dyneema evenly distributed across it (as opposed to the thinner slings with mostly dyneema, and only some nylon on the edges). It´s better to be safe than sorry though, so I guess I´ll just refrain from using the sling until I can have it sewn.
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