/ Slings and spikes

This topic has been archived, and won't accept reply postings.
Stuart William - on 18 Sep 2012
Just wondering, I've always assumed that the best practice when putting a sling over a thin spike to use as a runner (particularly in places like Avon where there are old iron spikes to be slung) is to use a clove hitch to stop it from lifting off when you climb past it. However, does anyone know what the risk is of a dyneema sling melting through itself if you fell on it when using a clove hitch?

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.

Cheers, Stu.
Gordon Stainforth - on 18 Sep 2012
In reply to 2PointO:

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.
Stuart William - on 18 Sep 2012
In reply to Gordon Stainforth: The melting popped into my head only because I think that it would be unlikely for the fall would generate too much force and snap the sling but the clove hitch suddenly tightening and running over itself could possibly generate enough heat/friction to cut through dyneema.

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!
jkarran - on 18 Sep 2012
In reply to 2PointO:

Why would it melt?
jk
Duncan Bourne - on 18 Sep 2012
In reply to 2PointO:
> (In reply to Gordon Stainforth) The melting popped into my head only because I think that it would be unlikely for the fall would generate too much force and snap the sling but the clove hitch suddenly tightening and running over itself could possibly generate enough heat/friction to cut through dyneema.
>

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
Stuart William - on 18 Sep 2012
In reply to jkarran:
> (In reply to 2PointO)
>
> Why would it melt?
> jk

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.
HardenClimber - on 18 Sep 2012
In reply to 2PointO: When ropes / slings snap the potential energy has to go somewhere. Part of this is noise and part is heat. 'Failure' can generate enough heat to cause local melting, which can then be misinterpreted as the primary cause of the failure (rather than a product of the breaking).
Stuart William - on 18 Sep 2012
In reply to HardenClimber: Really? I would have thought that friction is far more likely to cause melting than snapping or tearing. I find it really hard to believe that melting can happen in the way you suggest, but I could well be wrong.
caravanshaker on 18 Sep 2012
In reply to 2PointO: I wouldn't be worried about a sling melting when the knot tightens... There would need to be yards of slack in the knot for this to happen, which you could easily mitigate by pulling the knot tight as you tie/dress it.

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/
Jon Stewart - on 18 Sep 2012
In reply to 2PointO:

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...
bpmclimb - on 18 Sep 2012
In reply to 2PointO:

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.
jimtitt - on 18 Sep 2012
In reply to 2PointO:
The only tests Ive 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.
Stuart William - on 19 Sep 2012
In reply to Jon Stewart: Well congratulations on unquestionably accepting everything in your life, that must be fascinating. Your whole "without thinking" thing sounds great but although I happily trust slings and have no real doubts about they way I use them I feel it can't hurt, and can even be interesting, to know a little bit about things in our world. But I'm sure condescension is far more fun.

It was a slow day, I thought I would ask UKC about an idle curiosity. I will remember not to bother in future.
Stuart William - on 19 Sep 2012
In reply to jimtitt: Thanks, that's pretty much what I was wondering.
Kemics - on 19 Sep 2012
In reply to 2PointO:

I usually use a larks foot in this situation, mostly because it's a lot easier to do one handed....while shaking :)
Michael Gordon - on 19 Sep 2012
In reply to 2PointO:
> (In reply to Jon Stewart) Well congratulations on unquestionably accepting everything in your life, that must be fascinating. Your whole "without thinking" thing sounds great but although I happily trust slings and have no real doubts about they way I use them I feel it can't hurt, and can even be interesting, to know a little bit about things in our world. But I'm sure condescension is far more fun.
>
> 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
Simon Caldwell - on 19 Sep 2012
In reply to Michael Gordon:

nor me
purplemonkeyelephant - on 19 Sep 2012
In reply to 2PointO:

Would the clove hitch cause more friction than an overhand knot?
Gordon Stainforth - on 19 Sep 2012
In reply to purplemonkeyelephant:

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.
David Coley - on 19 Sep 2012
In reply to 2PointO: how would the single movement of tightening cause enough heat? Heat melts slings because the same area is cycled. This is why if you lower someone over a sling the sling melts but the rope doesn't. The knot won't even be under tension until it stops moving and without tension, nothing will melt. Hence you can pull an ab rope many times through the same sling.
Michael Gordon - on 20 Sep 2012
In reply to David Coley:
> (In reply to 2PointO) without tension, nothing will melt. Hence you can pull an ab rope many times through the same sling.

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!
jimtitt - on 20 Sep 2012
In reply to David Coley:
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!
purplemonkeyelephant - on 20 Sep 2012
In reply to Gordon Stainforth:

I was trying to point out that I didn't think a clove hitch would cause more friction than the commonly used overhand knot?
David Coley - on 20 Sep 2012
In reply to Michael Gordon: yes some friction, but not much. I'm with Jim on this, pressure has to be the key.
martinph78 on 21 Sep 2012
In reply to Kemics: I wtend to use a larks foot as well. Not that I have many opportunities for using spikes where I climb.

ads.ukclimbing.com
martinph78 on 21 Sep 2012
In reply to Jon Stewart:
> (In reply to 2PointO)
>
> 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.


This topic has been archived, and won't accept reply postings.