UKC

Hello,


Today while reading "Karabiner Breakings when Using a Figure-of-Eight --
Neville McMillan" I wondered if there had been any incidents regarding melting
through the rope. "About the Ageing of Climbing Ropes -- Pit Schubert"
mentions rope failure accidents, it would seem the only failures to occur in
the real world are from sharp edges and acid.

From this wondering I have four questions:

1. Has there ever been a serious accident due to rope melting? (slings
certainly melt, as mentioned in "Lowering Off and Abseiling - a Huge Difference
-- Pit Schubert"

2. How does elevated temperatures, below melting point, affect the long term
strength (or more importantly number of UIAA falls held) of polyamide? (aka:
does getting my FoE really hot then stopping, weaken that section of rope
permanently?)

3. How do the various common devices compare for heat generated? I've heard an
ATC is much cooler than a FoE, my experiences seem to reflect that.

4. Has anyone ever noticed any signficant rope damage from elevated
temperatures and abseiling?
(I've certainly glazed ropes with prussiks.)


These thoughts are occurring to me because I might setup a 100m abseil one day,
just for something to do.

(For what it's worth I don't own a FoE, though I do own a Petzl Pirana.)

In reply to needvert:

i work as a canyoning guide, so a lot of abseiling, wet and dry.

this comes up all the time.

so, a few years back we did a LIMITED experiment specific to OUR SCENARIOS. capitals reinforce that we in no way covered all possible variations.

we got a petzl fig8, heated it on a bbq till it was hot enough to scorch wood then (holding it with welding gloves and tongs) threaded a length of 10mm static rope thru it.

i dont know how much heat dissipated between the bbq and the rope, but aside from sizzling a few fuzzy fibres it did nothing.

now, we didnt analyse under uv, with a microscope etc, but there was no difference in the feel of the rope (ie no hardening, glazing etc).

we put the rope under as much tension as possible but probably didnt get to as much as geared up climber + momentum forces.

the same thing done to 8mm prussik cord melted thru it like butter.

conclusion: i feel safe about it, but still see the gaps in the experiment.
our 8 was way hotter than any 8 would actually get to, but a dynamic rope under more tension is too big an unknown to ignore.

that said, petzl make a thing called a huit thats an 8 with little rubberized holder to protect your fingers from a hot device.

i cant imagine a piranha is much different.

make of it what you will.

In reply to needvert:
I personally would not worry about it.

If you keep moving then the heat generated is never in the same place for long enough to do any damage.

and from what ice.solo said then it wouldn't seem to make a difference anyway.

I have ran charity abseils that are 100m long with upwards of 70 clients going down it. The heat generated was quite a lot but the ropes showed no sign of any wear more so than usual. I realise that not all of the clients were going down very fast but I and the other instructors went down it at a reasonable pace and even then yes the fig 8's were hot but i cant imagine that they were hot enough to do damage.

The only thing we did differently to any normal abseil was swap the FoE at the top on the safety line to prevent any build up of heat in the device as the others used by the clients had time to cool down.


Chris

In reply to needvert:

The first thought that comes to mind is:

E = mgh

i.e. the potential energy lost by a climber of mass m kg when abseiling through height h metres. This has to go somewhere, and my guess would be that it's dissipated as friction. Some to the rope, and some to the abseil device.

If you took the worst case assumption, that it all went into the abseil device, you could calculate the temperature rise of the abseil device by knowing its mass and specific heat capacity.

Then you have to guess at how much heat is transferred to the rope, and how much lost from the abseil device to the surroundings. But at least you've got the worst case as a starting point.

e.g. a 75kg climber abseils 50m on a DMM Bug (75g, assume all aluminium, c = 0.9J/gK)

E = 75*9.81*50 = 36800J
dT = E/m.c
dT = 36800/(75*0.9)
dT = 545K

Hmmm.... that's rather hot... let's hope it dissipates heat somewhere, rather quickly...

Thoughts, anyone? Spot the error (other than the gross assumptions)?

In reply to captain paranoia:


Well, the obvious one is that I've forgotten the damned great HMS used with the belay device... (d'oh!), but that only brings us down to dT = 240K

In reply to needvert:

Never heard of ropes been damaged by an overheating Fig 8 but I've 'glazed' one when abseiling on a Gri-gri. Apparently this is quite common. Lesson: Don't use a Gri-gri (or similar device) for long abseils.

In reply to captain paranoia:

Heat lost to the surrounding air, i think that will be where a significant amount of the heat goes.

In reply to captain paranoia:

Wait, you have changed from KG to G in the masses of climber and belay device, would that make a difference?

In reply to captain paranoia:
etc

brilliant stuff - just wish i hadnt slept thru physics.

the heat IS going SOMEWHERE, and its not melting the rope as far as ive seen.
to my (uninformed) mind im assuming a mix of the air and the rest of the 8 that isnt contacting the rope (plus whatever biners are being used).

that said, whenever i feel a hot 8 at the bottom of a rappel, the end on the biner is always cool enough to hold.

mmmm, interesting.
may be time to get an old 8 and the bbq out again.

In reply to needvert:

As the rope is moving through the plate, there is little danger of any heat damage. What about the belay loop of your harness though? rope heats up belay crab, either through conduction (fig 8) or by direct friction (belay plate). The crab remains static on your harness, this is where the danger, if any lies.

I could manufacture some insulating belay crab sleeves from solder mat, attach an arc-hagl-gonia label and prey on peoples fears

In reply to captain paranoia: 240 kelvin is -33 celsius ....

In reply to captain paranoia:

Kev, you do get geeky scientist to great lengths.;-P

Previous threads over the years, along with tests done by rope and belay device makers seem to find that it is unlikely you will melt through a rope, even when all the metalwork is too hot to touch.
However, when we have done long abseils in the past we would try to reduce the pressure of the now stationary rope, just in case it did harm it.

In reply to needvert:

What you have to remember is that 'too hot to touch' is only about 50C or so - its not actually very hot at all. The many danger is scalding your nads when abseiling in shorts IMO.

In reply to minus273degrees:

240K is the temperature CHANGE, so if we started at about 300K (between 20 and 30 celsius) that would increase to 540K or 260-270 celsius, that's quite hot to us, far too hot to touch, but not hot in the grand scheme of things and would not melt a rope easily. That's what i think anyway!

In reply to captain paranoia:

I think the assumption that only the belay device/krab heat up is the main problematic assumption. The friction is between the rope and the belay device so I would expect both to heat up. No individual section of rope gets noticably warmer because the rope is moving through, but overall a lot of the energy will end up heating the rope.

In reply to needvert:

p.s. I'm not sure how frictional heating works - is it a gross assumption to say that 50% of the heat energy ends up in each object that the friction is between? So that would lead to a 120K increase in temperature for the belay plate assuming no heat loss.

Heat lost depends on how fast your abbing. If you go slowly the heat has time to disapate, hence normally you don't get your belay plate up to 100 degrees.

In reply to needvert:

According to wikipedia nylon melts at 190–350 C depending on grade.

I know someone who claimed that they had started to melt the sheath with an atc, and personally have managed to get one hot enough that it produced steam on contact with with water, but no rope damage. Cant imagine it would ever get hot enough to transfer enough heat through a krab back to the harness loop though.

I don't think its a huge danger but maybe something to bear in mind on long abs (slower=cooler). If your in to doing long abseils for fun on a single rope then why not invest in a petzl stop. Excellent behaviour, safety and breaking and a ton of metal to dissipate heat.

On shouldn't glaze a prussick if its used properly- its an emergency stop device that should lock up if you let go, its not meant to be used as a break the whole way down!

In reply to Nic DW: What I would be much more interested in is whether there is damage you can't see.

With many materials, you don't need to get close to their melting point to start affecting their strenght, particularly how brittle they can become. I'm not saying it's the case, I simply don't know. I'd be interested to know though.

In reply to Dan Lane:


No, they're weights of two different things; a 75kg climber, and a 75g Bug. I did think it might confuse...

I used a specific heat capacity using a gramme-basis (J/gK), so there's no scaling error there.

In reply to Dan Lane:


I agree. Transferred to rope on the way down, and to the air.

As I said in my first post, my numerical analysis was deliberately for the worst case scenario, as a starting point for further analysis of where the heat is dissipated.

Modern belay devices incorporate elements that act as heat sinks, to maximize the radiation/conduction losses. And a krab, being basically a long, bent rod, is a pretty good radiator, I suspect.

On the other hand, think about how quickly we get rope burns if the rope passes through our hands when loaded... (that's a two-fold thing, though; thermal burn and mechanical abrasion).

In reply to bigbobbyking:


That was going to be my starting point (aka a pure guess)...

But I suspect that much of the heat ends up in the rope, due to the mechanical flexing of the kern and mantle of the rope that occurs as it passes through the belay device. Again, purely supposition with no experimental evidence to support it.

In reply to needvert: Reading an account of this yesterday in Lynn Hill's book 'Climbing Free' when she did a stunt for an american TV show 'That's Incredible'(see Page 155). The abseil from a helicopter used ropes 1800 ft long! And she had trouble trying to slow down the abseil until after 1000ft she could hardly hold the descender because ir was so hot! There were scorch marks on the rope and the exterior fibres had begun to melt. She lived to tell the tale...

In reply to captain paranoia:

The potential energy is being converted into kinetic energy.

Let's take E=mgh, and combine it with the equation for kinetic energy, E=0.5mv^2
Conveniently, the masses cancel:
gh=o.5v^2
taking a height of 50m, and a value of g as 10 (close enough) gives
v=SQRT(1000)=100m/s

Clearly, the climber is not moving at 100m/s. This is due to some amount of the kinetic energy being lost due to work done against friction, both air resistance, and between the belay plate/climber's hand and the rope.
While movement speed effects the proportion of energy lost to work done against the friction with the rope and that lost due to air resistance, I can't be bothered there.

Let's say that half of the heat energy is given to the plate and half to the section of rope directly in contact while the plate at that moment.

So, yeah. I'll let someone else pick up my slack here, I gotta bounce real quick.

In reply to Sean Kelly:

That little story makes me cringe quite a lot....

In reply to ice.solo: a similar artificialy heat the belay device and apply rope style test here: youtube.com/watch?v=APreETBFDHw&

In reply to needvert: I don't imagine a belay device is ever likely to melt through a rope whilst you're abseiling on it. The issue is at the end of a long abseil the device has heated up, then you stop which leaves some time before you unclip for the hot device to heat the same piece of rope as its no longer moving through the device. This isn't going to melt the rope but could it invisibly weaken that point so that in a big leader fall at a later date it could break at that point?

In reply to needvert: One of the local walls does a charity climbing event where teams have an hour in which to do as many ascents as possible of the 8metre wall. The ropes are held by a maillon at the top and belaying is done with gri-gris, lowering the climbers off very quickly. The gri-gris will get hot enough to allow the plastic handle to soften and deform(!), but the ropes do not get melted. Yes, they do have to be ditched after 10 hours of this abuse 'cos they're so stretched and knackered, but neither the maillon or the grigri 'melt' them.

In reply to pec:

Nope. 'Invisible' damage wouldn't be a problem. The outside of the sheath would melt before the core is affected at all, any damage done that way would be pretty obvious.

In reply to MRJ:


In a fall, yes... but in the context of an abseil, no; you come to a stop at the end of the abseil*, so initial and final velocities are the same, and there's no change in kinetic energy.

* I prefer to come to a controlled stop, rather than an uncontrolled stop (i.e. decking out...)

In reply to sutty:


Geeky engineer, if you please...

In reply to needvert:

Hi
FFS. Sorry to say but 95% silly replies. Typical UKC.

In general, the sheath is to protect the core, the core provides the strength.
However, a rope with an undamaged sheath is far safer than a rope with a knackered one. Someone more knowledgeable than me could write a book on this subject.
Rope failure = death.
Look after your rope as best you can.

In reply to needvert: Interesting to note that the Petzl Stop, which we use at work, is marked up with a "100 m Max" precaution, ie 80kg load for 100m descent maximum (normal use). I've used them on 200m drops in the past, and you need to balance the speed with the heat build up, and boy, it does get hot and will glaze a rope sheath easily enough when stationary.

In reply to deepsoup:


I'm not disagreeing, but can you give any reference for that? I was having this argument in the context of people pulling ropes directly through ab tat, and whether or not such damage would be spotted by visual inspection of the ab tat. My instinct is to agree that there would be visible damage to the sheath before any core damage, but I'd be interested to hear any evidence to back that up...

In reply to needvert:

So I wonder, what are the melting points of the outer sheath and the core?
The outer sheath can dissipate heat to the air. What about the heat trapped in the core? Could the fibres get brittle or partially melt together, causing a weak point?
Like a burn gets worse if you don't cool it straight away, the surface gets scorched but inside cooks.

In reply to jonny taylor:

The heat energy from pulling through is concentrated on the surface because a)nylon itself is a very poor conductor b)nylon rope is about 60% air by volume and so even poorer, you will see glazing and melting on the surface long before any damage occurs inside.
Work by McCartney et al showed there is no loss of strength in the core but some rope characteristics change as the outer fibres will have passed through the glass transition point.

It´s complicated!

Jim

In reply to Rick Graham:


Many thanks for those detailed and helpful guidelines. I'm sure we'll all be safe now.

In reply to jimtitt:
Thanks!