UKC

/ Shock loading prusik

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phil456 on 02 Dec 2018

Hi

Does anyone have experience of shock loading a prusik. ?

Normal Prusik or French

Thanks

 

AlanLittle - on 02 Dec 2018
In reply to phil456:

I've done the prusik to safeguard a sketchy lower-off thing as recommended by Petzl, on the basis that it's got to be a better than nothing & Petzl generally know what they're talking about. Still felt well sketchy though.

The gear I was lowering off didn't fail so no, I haven't ever actually shock loaded one.

Mark Haward - on 03 Dec 2018
In reply to phil456:

When would you envisage this happening?

    The closest I have come is holding crevasse falls. The prussik has been used to lower the 'pulling' point on me from the chest coils to the harness. However, I'm not sure this would count as shock loading because of the 12-15 metres of dynamic rope involved, snow sliding, rope cutting into crevasse edge etc.

oldie - on 03 Dec 2018
In reply to phil456:

No personal experience. I vaguely remember an accident when a prusik knot failed (may have melted through) when used by someone to protect themselves using a fixed rope, 1970s?. Of course the connection between climber and rope was probably using thin perlon to enable the knot to grip. The almost forgotten Tarbuck knot, once recommended as a way of tying on, was a friction knot but obviously with full weight rope and would only have slipped a short distance before the its loop closed.

Post edited at 11:13
AlanLittle - on 03 Dec 2018
In reply to oldie:

> The almost forgotten Tarbuck knot

I used to use those on my tent guylines.

pass and peak - on 03 Dec 2018
In reply to phil456:

To many variables in your question I'm afraid. We need a scenario, under what circumstances you would envisage this happening? in order to give an answer!

M

 

petegunn on 03 Dec 2018
In reply to phil456:

As stated by pass and peak, can you give more detail on the situation?

Prusiks tend to be used on dynamic rope, so your prusik loop would work along the same lines as if using a sling.

I have used a prusik loop as a spare quickdraw many times, but again as the rope is running through the quickdraw this is the dynamic part.

Most prusiks are made up out of either 5/6/7 mm cord .

6mm has about 8kn breaking strain on average.

Rick Graham on 03 Dec 2018
In reply to phil456:

> Hi

> Does anyone have experience of shock loading a prusik. ?

> Normal Prusik or French

> Thanks

Like earlier replies , have only seen the petzl test and vague memories of an accident back roping with a prussik knot.

There is also the Purcell prussik knot. There is published info on testing these.

As both the petzl and Purcell use variations of the standard knot, this is perhaps preferable to French prussik.

Though thinking about it, the noble tarbuck knot is a variation of a french prussik so who knows?

You may need to persuade someone to do some testing, not easy, for numerous reasons.

jimtitt - on 03 Dec 2018
In reply to phil456:

There´s plenty of testing on Prusiks but we do need to know the intended scenario before we can point you in the right direction.

phil456 on 05 Dec 2018

 

To those that are asking for a scenario.

I have commonly used a prusik on a safety line when setting up a awkward top rope at the top of a quarry, principally as it was easy to adjust when moving around.

I have also used one one to protect a sport lower off as per Petzl Instructions.

Recently I was learning some self rescue systems using a minding prusik by hanging a rope from my loft and the prusik failed to catch, which got me thinking, are there examples of failures in the real world.?

Either fail to catch or more likely melting.

I looked for the Petzl lowering technique which seems to have disappeared from their website which made me suspicious, hence my curiosity.

 

 

 

SteveD - on 05 Dec 2018
In reply to phil456:

I was recently on an Alpenverien Hochtouren Course, the Austrian guides only used the classic prussic and more or less implied any other type would involve dying to death.  They had a rule of thumb calculation for accessory cord (Dia of cord squared x 20)

5mm = (5x5)x20 = 500kg so 5kn
6mm = 720kg 7.2kn

Seems about right

They also used long prussic 1.5m and 3m not tied, initially I resisted but came to realise how much more versatile they were and basically less faff than using slings to extend for foot loops, ETC.  In Cliff/technical rescue the Americans use Prussics to reduce overloading on haul systems as the Prussic will slip before the system fails.

Not actually answering your question though.....

Steve

summo on 05 Dec 2018
In reply to phil456:

A prussik might fail to catch if it's not set properly, too loose, badly tied etc..

Once set if used correctly a prussik will slip and dissipate energy long before the cord breaks. An example of a badly tied would be say putting 10 turns in a French prussik of 5mm cord, clearly the friction and grab will exceed it's strength. 

If you have a standard well tied 7mm French prussik on 10/11mm rope, the amount of force required to first get the prussik sliding and keep it moving long enough to generate enough heat to melt through it would likely kill the person in the harness. 

Whilst petzl have a device for everything, it's hard to out class the simplicity and functionality of a few prussiks. 

wivanov - on 05 Dec 2018
In reply to phil456:

I have no personal experience but don't fire and rescue groups use a tandem Prusik to backup belay pretty large loads? I'm pretty sure that if you Google Tandem Prusik, you'll find a fair amount of test results.

phil456 on 07 Dec 2018
In reply to wivanov:

>  if you Google Tandem Prusik, you'll find a fair amount of test results.

Thanks there is loads of info with that searching Tandem Prusik

http://itrsonline.org/wordpress/wp-content/uploads/2015/03/Laidlaw.2006.pdf

The above was informative 

 

phil456 on 07 Dec 2018
In reply to SteveD:

Not actually answering your question though.....

 

Food for thought though ????

 

L randy987 on 14:54 Mon
In reply to phil456:

If I remember correctly, there had been an accident with friction (heat) induced failure mentioned in Pit Schubert's books (founder and former head of DAV safety commission), alas not translated into English as far as I know. Let me check my library - one climber was rope soloing some 12m above deck with prusik instead of a (then used) ascender on his rope. Not a very safe practice... At the crux he neglected to push the prusik up the rope (since he "had his hands full"), creating some slack in the system, and finally falling, shock loading the prusik severely, breaking it. Schubert mentions the fall as ~4kN load, while the prusik cord was assumed to hold ~6kN. Theoretically it should hold (under the best circumstances), but with subsequent testing, DAV found out that even in smaller falls, the prusik knot on the same cord could slip along the rope up to half a metre before locking fully, creating enough friction and heat to stress the cord up to the point of failure. On a drop tower with 80kg dead weight, the cord tested (5mm) broke in the knot (friction heating) with certainty on 2.5m falls (not that much slack at all). The amount of slip before locking, how much friction and heat is generated during the slippage of course depends on how the prusik is tied, friction with the rope, and is hard to quantify. Obviously, nobody in their right mind would rope solo with prusiks only, especially with slack involved, but it happened. So keep your prusiks tight and tidy, and watch the slack 

summo on 15:06 Mon
In reply to randy987:

Also 5mm is ridiculously thin. Two thirds the diameter of the rope you intend to use it on is the rough guideline. 

L randy987 on 15:25 Mon
In reply to summo:

It is. It was the only sort-of-documented example of cord melting and failing due to slippage before locking fully that I could find in the literature, though. Obviously, a stronger prusik cord would be safer (given rope compatibility).


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