/ Slings and spikes
Obviously I am talking about falling with a dynamic rope in the system when using the sling as a runner, not falling directly onto the sling.
I don't see why melting comes into this particularly. Surely it's just a question of the impact of a fall on that very static knot. I would suspect that modern slings clove-hitched around typical Avon Gorge metal spikes are approx. 100 per cent safe for any kind of fall whatever.
Either way I doubt I'm going to stop doing to; I'd far rather risk the sling breaking than just lifting off and being completely useless!
Why would it melt?
Not over that short a distance.
Incidently I have seen some demonstrations recently where old slings took a hell of a lot more loading to snap than old metal karabiners
> Why would it melt?
Dyneema and nylon (and many other materials) can theoretically melt themselves if tied in a knot which is then suddenly tightened causing the fabric to rub over itself under pressure and at speed since movement=friction and friction=heat, although I imagine the safety margins used in production obviously take this into account. However it makes sense that in a clove hitch the available range of movement is too small to cause enough heat to be generated anyway.
I was not necessarily saying that this is relevant in a climbing context but it is technically true and I was simply wondering if anyone knew specifically how much a clove hitch can weaken dyneema, through heat generation or other means.
Besides, if this were of concern then it would appear prominently in the user manuals (which it does not): http://dmmclimbing.com/instructions/Slings_0120CE_Oct09.PDF
If you want a valid worry about slings, check this out!! http://dmmclimbing.com/knowledge/how-to-break-nylon-dyneema-slings/
Is it just me that never, ever considers that their knot will collapse, their slings will melt, their face will fall off, etc etc?
I do worry about inverting and smacking my head, gear in crap placements not holding, lifting wires out with the rope and that sort of thing, but stuff that I've paid good money to work as intended I just trust without thinking.
If any bastard sells me a rope that snaps or whatever and I plummet hundreds of feet to a gory and partially tragic death, I'll be straight on the phone to the manufacturer and I'll believe me I'll give them a piece of my mind...
Just in case anyone doesn't already know this - the clove hitch is best flipped over before being dropped over the spike/stake, so that the knot is at the back.
The only tests I´ve read are drop-tests on single strand clove hitched dyneema where the material is considerably weakened compared with slow-pull testing. Whether this will be of concern in a double strand application would have to be tested.
It was a slow day, I thought I would ask UKC about an idle curiosity. I will remember not to bother in future.
I usually use a larks foot in this situation, mostly because it's a lot easier to do one handed....while shaking :)
> It was a slow day, I thought I would ask UKC about an idle curiosity. I will remember not to bother in future.
didn't sound condescending to me
Would the clove hitch cause more friction than an overhand knot?
You need the clove hitch so that the sling does not slip on the spike (and quite a few of the Avon ones are slightly downward-pointing.) Friction not an issue in either case.
When you pull an ab rope through a sling there IS some tension, hence there is friction and as a result there IS often some melting!
The difficulty is that in the act of tightening there is both pressure and friction, the pressure is more the problem as polymers "melt" under pressure in that the glass point is reduced. Combined with the heat of the friction of the parts of the knot tightening and things get weaker.
The most common way to achieve this is to double a nylon sling to shorten it, if one strand lies over the other at the karabiner the end result is weaker than in the single strand configuration as the outer strand neatly cuts the inner one due to the inner being under pressure.
The clove hitch seems particularly prone to weakening as the inner crossing strand is directly on the karabiner (or the spike in this case) and the outer crossing strand forces it onto the karabiner.
Stuttgart Uni did some tests on this, for a single strand of 8mm Dyneema which would normally be rated at 15kN a slow-pulled clove will fail at 8.9 but drop-test it the heat cannot escape fast enough from the knot and heats up locally failing at 5.3kN. Where we lie in the fast or slow testing using a rope in the system in anyones guess!
I was trying to point out that I didn't think a clove hitch would cause more friction than the commonly used overhand knot?
> Is it just me that never, ever considers that their knot will collapse, their slings will melt, their face will fall off, etc etc?
> I do worry about inverting and smacking my head, gear in crap placements not holding, lifting wires out with the rope and that sort of thing, but stuff that I've paid good money to work as intended I just trust without thinking.
No, you're not alone. I trust the manufactures will have done all of the relevant testing for the equipment and it's intended use.
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