Why knots drastically weaken ropes

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T2
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Why knots drastically weaken ropes

Post by T2 » 13 Jan 2019, 16:08

I've always known that knots drastically reduce the rated strength of ropes (by 50 per cent or more in some cases), but I'd never really considered why this is. I was just reading something that gave one of the best, plain-language explanations I've seen.
A bent rope means some of the fibres on the outside of the curve will have to carry more load while fibres on the inside may take none of the load. In addition, parts of the rope in a knot may be compressed and the fibres unable to move to share load.
It's interesting to consider this given canyoners are constantly getting ropes to operate in ways where the forces are not longer evenly spread between the fibres. A knot is an obvious one, but so too is the rope bending over an edge (particularly sharp edges where the rope bends more quickly) or running through a descender.

According to one manufacturer website I saw, the assumption for knots like a figure of eight or overhand in nylon or polyester ropes should be that they will result in about 50 per cent reduction in rated strength. Other fibres, types or ropes, and knots can reduce strength even further.

I recently saw a discussion where someone was talking up using a 6mm pullcord to abseil on due to its rated strength of 1000kgs (most canyoning ropes are rated to about three times this). It might sound like a good safety margin, but the moment you run it through a descender (particularly one that turns the rope sharply like an ATC / tube style device) you're going to take it down a lot. Likewise, if abseiling over a sharp edge you'll have the combination of reduced rope strength with higher cutting risk. Extend that to situations where you may use the rope in a rescue, whether hauling someone or abseiling tandem with an injured person, and it starts getting very dicey.

Something to think about when assessing the real-word situations our gear finds itself in. Also a good reason to research the benefits of different ropes and fibres. Depending on what a rope is made from, and how it is made, it can have vastly different properties.



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Re: Why knots drastically weaken ropes

Post by Kosta » 13 Jan 2019, 23:26

Abseiling on a rating of 1000kg (10kN) isn't too bad if you run the numbers. You lose 50% in the knot. As an engineering principle, a safety factor of 5 is acceptable for human loads. Thus, in theory, a 100kg person can hang on it. This would rule you out, of course. ;-)

That said, typically, 6 mm accessory cord is only rated to 7.2 kN (720 kg). I've seen some that are a little stronger, but not by much. But then, if you double this, you get 14.4kN which means you can hang 144 kg on it. And that's actually common practice - after all, 6 mm is a common strength for prusiks.

With such a thin rope, my main concern would be the mechanical damage. The prusiks are short (which means you can inspect them for damage quickly) and usually don't touch the rock. The rope has to suffer much more wear and tear and the thinner the rope, the more critical any of this gets.

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Re: Why knots drastically weaken ropes

Post by T2 » 17 Jan 2019, 22:11

Good points Kosta. I probably could have worded my post better. I definitely don't think all 6mm ropes are inappropriate to abseil on. The point I was trying to make was more that people need to realise that items of gear such as ropes perform very differently in real-world settings than in tests. So safety margins need to be considered in the context of how rated strength is modified by different applications.

The 1000kg rating I had in mind was based on the Imlay 6mm pullcord. Even with this rating, Tom Jones specifically tells people not to abseil on it. I believe his reason is in line with your comments about mechanical damage. That much smaller diametre would be much easier to slice on a sharp edge.

There are 6mm ropes now available that are suitable for abseiling on, but most still seem to be recommended more for emergency use than everyday use. The common feature of these ropes is that they use high-end fibres like Technora and Dyneema. Using those fibres they can get the rated strength up into the 22 - 25kN range. Cut-resistant fibres like Technora also address the mechanical damage risk. Austri Alpin have even produced a very small, light-weight descender specifically for use with 6mm abseil ropes (https://www.austrialpin.at/en/products/ ... -figure-8/). I'm actually planning to buy a 6mm rope to use as part of my emergency abseiling setup for more remote, exploratory bushwalking.

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Re: Why knots drastically weaken ropes

Post by Turfa » 21 Jan 2019, 20:13

Also important to remember that these figures relate to static loads. It's not that hard to put some additional dynamic load on your rope, eg. if your descender is not running smoothly, or (heaven forbid) you are prussiking

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Re: Why knots drastically weaken ropes

Post by Flynny » 25 Jan 2019, 16:58

Also remember the rope is significantly weaker when wet. Not sure why, I assume less friction between strands = less laod transfer between stands

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Re: Why knots drastically weaken ropes

Post by T2 » 25 Jan 2019, 17:21

Flynny wrote:
25 Jan 2019, 16:58
Also remember the rope is significantly weaker when wet. Not sure why, I assume less friction between strands = less laod transfer between stands
This is particularly the case with Nylon, which can lose more than 20% of its strength when wet. It's not a mechanical issue (loss of friction) but an issue with how Nylon fibres form weak bonds with water molecules. Yet another reason why the cheaper Nylon canyoning ropes don't last as long. They also need to be thicker to achieve the same strength. I now use Polyester tape for anchors instead of normal Nylon tape for the same reason. They retain more strength in wet canyon settings, plus have better UV resistance so will remain safe for longer.

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