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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Re: Why knots drastically weaken ropes

Post by dyl_jo » 26 Feb 2019, 08:52

I'm really worried that this discussion hasn't differentiated between MBS and SWL.
http://www.ropelab.com.au/carabiner-specifications/
Additionally, I'm also worried that the discussion has been about weight and not force.

While the MBS for 6mm cords might be in the 0.75kN range, the SWL is much less (by at least a factor of 4, but up 20 for some applications/brands/recommendations). I understand that people like to push the boundaries of their gear and that's okay, but in a public forum like this I think it's really important to make sure that these sorts of things are communicated well.

In short, even at a very "loose" safety factor of 4, your SWL for a 0.75kN rate cord is a little under 0.19kN (without knots, etc).
When you start thinking about the forces that a person can generate on a rope (here's some for ascending - paywall restriction though: http://www.ropelab.com.au/members-ascending-forces/ and here are some for small falls : https://roperescuetraining.com/physics_fall_factors.php ) then 0.19kN is completely inadequate.

Lastly, it's really important to know what makes a cord a cord and a rope a rope. They meet very different compliance standards: https://www.theuiaa.org/safety-standards/ What should be of particular interest to canyoners (especially in Australian sandstone canyons) is the abrasion resistance of cord (which is very low compared to the rope standards).

(Also, this article is a great discussion: https://www.ropelab.com.au/members-8mm- ... ema-ropes/ also behind a paywall)

But as far as the OP knot discussion goes, if I haven't already convinced you of the value of "going behind the paywall" here is one more: http://www.ropelab.com.au/members-derating-for-knots/ Gold stuff by a Blue Mountains local and ex canyon guide with more experience on rope than I'll ever hope to have.

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Re: Why knots drastically weaken ropes

Post by johnmurray49 » 26 Feb 2019, 11:56

wow, all this great information is giving me a headache trying to digest it. By the way, what is the breaking strain on the daggy slings etc. that we feed our ropes through to abseil on.

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Re: Why knots drastically weaken ropes

Post by Kosta » 26 Feb 2019, 22:14

Tape comes in different strengths and the strength is always marked. You might have noticed, that tape always has a few coloured threads on it. Usually one to five of them. Each thread stands for 5000 kN. Thus, all you need to do is to count the number of marker threads the tape has.

Note, that this is only true for the tape you buy off long spools. The sewn slings that you buy as a loop always have a label that tells you their strength. The marker threads are considered ornamental here.

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Re: Why knots drastically weaken ropes

Post by T2 » 27 Feb 2019, 17:53

Dylan, great points about needing to differentiate the point where a piece of gear can be expected to fail in testing, and the safe working load. For metal items like carabiners, this is less of an issue, but I think for anything made of fibres it's a big one. When a manufacturer gives a minimum breaking strength, they're talking about the point where a brand new, unused item could expect to start catastrophically failing. We don't want to be getting anywhere close to that point. That's why the safe working load -- or the maximum force you should actually expose the gear to -- is usually for or five times lower. Once you're talking about ropes which can be weakened by knots and have suffered deterioration due to age, UV exposure, abrasion, and countless other factors, the need to keep a healthy safety margin is even more important.

You also correctly point out that what might be acceptable to abseil on will not handle the larger forces due to ascending, or if bad abseiling technique shock loads the system. Those scenarios also greatly increase the abrasion on the rope, so combine to drastically weaken on point in the rope.

Your point about the difference between "cord" and "rope" is also really important. It isn't a branding thing, or just a way to differentiate products. It has very real impacts on how the product was made, the standards it adheres to, and and its ability to handle abseiling etc. There are some 6mm ropes available, but they are few and far between and are for very specialised purposes. If people do want to experiment in this space, they need to ensure what they have is rope, and that the manufacturer actually supports its use for abseiling / rappelling.

One point of correction though. I think you made a mistake with your kNs. A high quality 6mm cord can come with a 7.5kN rating (rather than .75kN). And if you hang 100kgs on a rope that creates 1kN of force.

Kosta, I'll also call you out. Those markers on tape are not universal. There are plenty of unrated tapes with coloured marker threads. There are also properly rated tapes without them. It's something done by some brands, but not all. Also, where they are used to denote strength each strand marks 5kN (not 5000). But as I say, this isn't universal, so don't blindly trust a tape just because it has a few marker strands.

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