UKC

Hey climbers!

How do you call this knot in english?

http://nudos.bravehost.com/pasabloc.gif

Thanks in advance.

In reply to promotore:

I don't think you can class it as a knot, the closest knot to this would be a 'Sheepshank'; good for shortening over long guy ropes ect.

In reply to promotore:
looks like it's for making adjustable length runners,

In reply to CurlyStevo:

something of a Czech nature perhaps

In reply to promotore:
Pearl one, knit one.

In reply to promotore:
generally we call that class of knot a
'chris tan death knot'

Stevo

In reply to CurlyStevo: good joke...will he understand it?

In reply to promotore:

What do you use the knot for ?

In reply to NIGBEE:

I'd imagine its a shock absorber of some kind.

In reply to GrahamD: I imagine you must be seriously mistaken. Running rope over rope is not a good way to absorb shocks; the most likely result is that the ropes will melt through.

In reply to NIGBEE:

I've not seen one of these before but I just sat and tied one.

When you pull on the ends it says completely static.

You can slide the whole thing around the end karabiners to make it longer or shorter. However the rope will not run over itself when loaded no matter what position in is in.

I would say that it is a technique for tying variable lengths slings so that you can use a couple to equalise belays.

Sam

In reply to nz Cragrat:

It's from a Spanish site.

ps Don't think it qualifies as a CTDK - look too safe

pps Interesting idea though! < clogs in head churning again>

In reply to sam coward:


What's wrong with just tying a knot in a regular sling to get it the length you need? I think there must be another reason for using a setup like this.

In reply to Martin W:
perhaps it's quicker and easier to adjust.

In reply to Martin W:


I didn't say it was a good way ! What do you think its for, then ?

In reply to sam coward:


I would suspect that may not be the case in a dynamic loading situation, although it would be interesting to know what sort of forces it might slip at.

In reply to Richard: If you think about the forces, I don't think it would slip, because the loading on each strand is equalised. Very clever.

In reply to Stefan Lloyd:

Not true - look at the photo again. There're two strands coming from each krab - if we assume zero friction at the krab the forces in these will be equal. Thus the force in the middle middle strand will be zero. Of course the first assumption about friction isn't true, but still, the force in the central strand of the set-up will always have a much lower force in it than those around it.

In reply to Richard: Yes, you are right.

In reply to Richard: I'm not sure that's correct, if you consider it as a series of four pulleys, with a force F acting on the top and bottom pulleys, I get a force of f/4 (or half the force on the strands coming from the krab), acting on the middle strand.

In reply to Papillon:

What force do you get in each thread coming from each Krab?
F/2

On the "internal pulleys", then, you've got two F/2 forces opposed to each other, and an extra strand that cannot have a force in it - or if it does have a force in it, the forces at the krabs will be unbalanced.

If all the junctions were frictionless pulleys, applying a force would pull the rope through to its longest configuration.

In reply to promotore:
It's called Poldo Tackle.

http://www.geocities.com/roo_two/poldotackle.html

It is a self locking adjustable tackle usually used as a quick release tensioner on lashings to hold stuff down on boats and trailers. I am not really sure why anyone would need something so complex for climbing.

In reply to &y_Gee:

Thousands of thanks!!! Finally you are the best! The only one with the solution.

For the other people. This knot or mechanism or what ever you want to say, is used to tensioning a tirolean.

In reply to Richard:


By "force" I assume you mean "tension". For that to be true, friction where the rope doubles back through the knotted rope loops would have to be infinite.

In reply to Stefan Lloyd:


No, it isn't. Imagine that the knotted rope loops were, instead, frictionless pulleys. You can't make the tensions balance in that case. If you pull on the whole thing with tension T then each strand either side of a krab (assumed frictionless) would carry tension T/2. One of those strands from each krab is knotted in to a loop through which a rope is doubled back. If that knotted loop were a frictionless pulley then the tension in each strand of the rope doubled back through it would be T/4 - but we know (from the analysis of the krab end) that one of those strands must carry a tension of T/2, therefore the system cannot be in equilibrium and must collapse into a stable configuration.

I reckon that would be a configuration with the pulleys next to each other and the "middle middle" strand (to borrow Richard's term) effectively removed. I suspect that if you constructed something like this using DMM Revolvers both for the krabs and for the knotted rope loops, it would do this. The only reason it doesn't collapse when it's built the way it's drawn is because the friction at the "pulley points" (where the rope doubles back through the krabs and the knotted loops) is (a) non-zero and (b) higher in the knotted loops than at the krabs. In other words, it only works the way sam coward observed because there's enough friction in the various components to allow the tensions either side of the "pulley points" to be different.

I can imagine that, subjected to a shock load, the "pulley points" would slip - and as we all know, rope running against rope under high tension is A Bad Thing. On that basis I think it qualifies rather well as a CTDK, at least if used as part of a belay.

By the way, I reserve the right to change my mind about this afer I've thought about it a bit more and maybe even modelled it myself at home (although I only have two revolvers).

As to what this setup is supposed to be for, I have no idea. Perhaps it's an elaborate troll aimed at people who think they understand physics?

In reply to Martin W:

1. you can use the climbing rope to make it

2. only need to guess a max length then you get sliding adjustment latter

3. dynamic hence less shock load.

In reply to CurlyStevo:

Sorry, what is the meaning of chris tan death?

It's a play of words?

Thanks

In reply to Martin W:


Yes, I know; I was just trying to demonstrate that the forces were unbalanced and that the assumption of a)stability and b) zero friction at the krabs don't really add up.

If you notice I summed up that post with the milder "much lower force than the other strands"...



I'd agree. If nothing else it's the longest configuration and hence the one with the lowest energy while a force is being applied.


Another one! Now what happened to that aeroplane?

In reply to promotore: Chris Tan is a regular poster here, he has an intrest in the more experimental side of climbing equipment, as a result he has give us a series of very useful applications for various knots which may or may not improve your safety in any given situation. As most of them fall outside normal accepted practise or usage of climbing gear we call them "Chris Tan Death Knots".

In reply to Richard: Yeah I agree the whole thing relies on friction, but once stable, the middle rope would have a force on, but now I realise you weren't saying it wouldn't.

In reply to davidwright:

You can do this with a clove hitch on a single krab. KISS.


1. The main dynamic part of the system is the rope between the belay device and the climber. This aborbs the shock load from a fall - if it didn't, it would never be safe to build belays from tape slings.

2. Since the whole thing relies on friction of one rope running over another, using it as part of a fall arrest system is a bad idea.

The enigmatically-named Mr &y_Gee, who revealed the true name and purpose of the knot, said: "I am not really sure why anyone would need something so complex for climbing." The link he provided mentions: "...it works best in static situations". Finally, the OP clarified that he uses it for tensioning tyroleans.

I can't think of any reason to use it in a normal climbing fall arrest system.

In reply to Martin W:


And when you look at the entire statement on that link:

"Excessive motion can overcome the friction of the Poldo Tackle and cause it to lengthen out to its maximum span, so it works best in static situtations."

it makes it quite clear that dynamic loading will overcome the friction, thus the rope will slip, resulting in frictional heating, leading to potential melting.

I have to say that, at first, I couldn't see how it could serve as an adjustable element; it seemed to me that it would simply slip to its maximum length. Only when I figured that it had to be statically loaded to retain its set length, relying on friction, did I realise how it worked.

Definitely counts as a CTDK...

In reply to promotore:


Anyone notice a distinctly musty smell on this thread? How do tapirs smell? I know how this one talks anyway. :¬P

People uses the poldo tackle for linking two protections. For example when one of them is very precarious because you can link perfectly with this system of tensioning.

Climb now, fall later!

And sorry for my english, very bad, i'm sure.

In reply to Richard Smith: Its not a sheepshank, no way!
http://www.42brghtn.mistral.co.uk/knots/42ktshep.html

In reply to &y_Gee:

Wiki has a better picture of the knot en.wikipedia.org/wiki/Poldo_tackle

In reply to Chris Tan.Clone II:
Yes, I saw that too, nice photo but not much explanation.

In reply to Martin W:

There's nothing enigmatic about my user name, it's just text talk. Spotty teens and small children get it straight away.

Andy Gee.