UKC

Hey,

I know taking any fall directly on Dyneema is extremely bad unless you do not want to continue living. However, my question is about falling onto an anchor which uses Dyneema when you are attached to a rope (E.g. A DMM Cam/Hex or Sling anchor). I assume that this is fine but am unsure as to why this is any better? Is it because the rope absorbs a lot of the force from the fall?

Thanks

In reply to d2038501:

Yes

In reply to d2038501:
Yes it is.
Generally if using either in a belay system I try to get some of the rope into it too to dynamically absorb any forces.
And I mean substantially more than a few inches of rope. Clearly not understood by a previous responder to an earlier post of mine. Try not to allow any slack between your harness and any points of belay attachment either to stop snatching forces.

DMM have a good video relating to nylon and dyneema strengths.

In reply to brianjcooper:

After I watched those videos and was prompted by that to do some further reading, I decided that whenever I replaced my slings it would be with nylon. Any advantages of dyneema (such as their lighter weight) seem trivial.
Nylon slings are cheaper as well

In reply to d2038501:

Dell pretty much explained it, but since i think this comes from the thread i posted before i'll expand a bit to try and clarify.

A rope does indeed absorb a lot of the force, in fact, it is designed to do so and that is a requirement for climbing ropes. You can fall all day long to your content onto a rope.

A nylon sling is not designed to absorb the force, but it does. However it's bad form to be taking falls however small directly onto a sling.

A dyneema sling is not designed to absorb the force, and neither it really does (only a little bit).

So if you are going to be loading something dynamically, you really want that to be the rope. I.e. A rope clipped to a piece of gear is fine, you clipped directly to that piece of gear and falling not so much. As far as the rope is there, you can ignore everything past it as the rope will do its job.

In a belay you can be attached directly with a rope, with a nylon sling (clip it to an anchor higher than the waist), or dyneema keeping in mind that you really dont want to be loading it dynamically, so sit back to be hanging from it and keep it under some tension, and never clip it to an achor below waist height. Ideally though, don't clip yourself directly to an anchor with dyneema.

Hope that clears it up

In reply to d2038501:

Yes - essentially if you have a dynamic element in the system, there's no problem. Not that I'd advise climbing above your anchor and leaping off, but the force generated even by a high fall factor with rope in the system will be much less than if you fell directly onto the dyneema.

In reply to brianjcooper:

The main advantage of modern dynema slings is that they can be threaded through smaller features than wider slings.

In reply to GrahamD:
True.
Although I have been doing that for years with thinner nylon slings before dyneema arrived.
Wires are also useful in pocket holes too if you move the 'chock' down the cable out of the way.

In reply to d2038501:

There is in general some confusion between force and energy but the simple message is if you're going to fall then you want to slow down gradually, if you hit the floor which is quite rigid you slow down very fast resulting in very high forces* which overload bits of your body breaking them. If you fall is arrested by a high-modulus material like dyneema or steel then again, the forces generated are very high like hitting the floor but in this case it's likely to be the equipment the sling is connected to that is overloaded... potentially leading to hitting the floor. Think about landing straight legged vs bending your knees, the forces are very high when your skeleton is rigid and the mass of your body just stops, when the knees bend and you crumple the forces are limited. Having a length of dynamic rope in your belay especially one with a couple of knots in it provides the stretch required to keep forces generated by realistically foreseeable falls within relatively safe limits. The rope stretching acts much like knees bending, it deforms absorbing energy while remaining in one piece.

*energy = force x distance

The energy released by a given fall is basically fixed (an oversimplification but a harmless one in context) so to keep force low we have to increase the distance over which the energy is absorbed, in the case of a climbing fall we do this by stretching the rope. Slings don't stretch much by design so aren't good at keeping force low when asked to absorb energy from a fall. High forces break things.

Climbing rope absorbs energy when stretched, it's what it's designed to do and as a result, limits peak impact force safely. It's a very good idea to have some rope in any connection between you and rock that might have to arrest your fall.

jk

In reply to jkarran:

No, it's still going to be the human body that takes the brunt of the damage - the forces experienced by the gear are also experienced by the body attached to the gear.

In the examples in videos online of slings being broken by the dropping of a test weight, the test weight is generally a lump of steel or concrete, which deforms very little under the kind of forces that will break a sling. A human body deforms much more - initially without taking any damage, then further as soft tissue is crushed and bones bend and break. Overall in your 'force x distance' calculation, the force is reduced as the centre of mass of the body travels on over a greater distance, moving away from the belay loop of the harness as the body changes shape.

You can break a dyneema sling by dropping a concrete test weight on it but not by dropping a human body on it for the same reason that you can drive a nail into a piece of wood with a brick but not with a grapefruit.

In reply to deepsoup:


I disagree. I understand and agree with the basic point you're making: people are fragile and the force applied to the gear is also applied to the body. However, I'd argue it's quite conceivable the force required to snap a small wire or strip wires out of soft rock, perhaps 10kN would not cause serious injury when applied through a sit harness, that's 'only' 12g for an 80 kilo climber. People survive enormous 'straight up' accelerations in motoring and aviation accidents, often relatively unscathed.


I'm not certain this is true under all conditions but it seems likely to me and I've argued similarly in the past, that drop tests with rigid proof weights are unrepresentative of the dynamics of a real fall involving a big floppy climber. Either way, it's a bad idea to fall on a sling, somethings gotta give and I didn't wish to encourage it.
jk

In reply to jkarran:


They do. But 10kN/12g is a *lot*, especially applied to the body through a sit harness as opposed to an ejector seat. If you've not seen it before this (already slightly dated when it was published) paper might interest you:
http://www.hse.gov.uk/research/hsl_pdf/2003/hsl03-09.pdf

The data in there hints at some pretty gnarly post-war experiments. It puts 12g towards the upper end of what's endurable by a military pilot in a parachute harness, and sets out the reasoning behind the decision to set the maximum "safe" load on a human body in a (full body) harness at 6kN in the European standards for industrial fall-arrest lanyards. (In practice the shock absorbers in those lanyards generally start to blow out at about 4kN)

In the post I was replying to you didn't mention any wires though, you were talking about overloading a sling. Unless that's a skinny dyneema sling with a knot in it, that would mean a fair bit more than 10kN.


No doubt you're right. I've certainly discussed this on here before and I'm sure you have too.


Can't argue with that!
The above is really just a pedantic digression; the point is entirely moot.

In reply to Mr Lopez:




You can absorb force?

In reply to jimtitt:

Oh come on, i was using the terminology he used in the OP to try and keep it clear. No need to delve into laws of conservation of energy and how energy is dissipated, transferred, or converted into heat.