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Y-hang

Chocolate fireguard

Active member
Everything you say is correct. But the deviations shown in that diagram would only be guaranteed to settle in those positions, with those forces if there was no friction. With a krab the settling position would probably differ, as would the forces.
But you misunderstand the point of my post, which was to estimate the CHANGE in the tension of the top rope when the krab is pushed upwards and is retained in that position by friction.
The force does increase precisely because with the krab in the higher position the angles have changed.
 

mikem

Well-known member
That is going to be complicated, because it depends on the relative lengths of main rope & deviation cord. However,when deviation is above the horizontal the force will increase - however, this will only be possible at small displacements as krab won't slide up much when there is more bend in the rope (& not at all after 90 degrees)
 

Chocolate fireguard

Active member
That is going to be complicated, because it depends on the relative lengths of main rope & deviation cord. However,when deviation is above the horizontal the force will increase - however, this will only be possible at small displacements as krab won't slide up much when there is more bend in the rope (& not at all after 90 degrees)
That's precisely why I decided to use the idea of maximum friction - it eliminates the need to know the lengths.
But I had to assume a deviation angle to estimate the contact area of rope on krab.
Because I wanted the maximum extra tension in the top of the main rope as the krab is pushed up as far as it will go (without slipping back) I decided to go for what is at the top end of deviation angles - 60 degrees.
Obviously(?) the maximum frictional force that the rope/krab surface will support does not depend on the rope/devi cord lengths, only on the nature of the surfaces, the weight of the caver and the angle (which admittedly must not change much as the krab is pushed up).
 

ChrisB

Well-known member
That's precisely why I decided to use the idea of maximum friction - it eliminates the need to know the lengths.
Ha! And I was working the opposite way, and suggesting using the lengths/angles, to eliminate the need to know the friction!

I did realise you were estimating the change due to pushing the krab up, but didn't have time to do the calculation for that using angles this morning, so I just posted my logic in the hope that it might be useful to you.

I've now done a calculation, just to get a different perspective. As you say, I had to make some assumptions. These were that the bolts are 1m apart, and at the same level, with the pitch rope dropping midway between them after the deviation, and 60º angles as you did. So the upper part of the rope and the dev cord are both 30º from horizontal, and the tension in all 3 sections is the weight of the caver, which I took as 1000N, might be less but easy to follow the numbers.

I then assumed that the krab is pushed 10cm higher. This puts the dev cord at 19º to horizontal. As it moves up on a radius, the krab moves slightly nearer the main belay, so the rope above the krab is at 22.5º to horizontal. Solving the equations for those angles gives 1390N in the main rope and 1420 in the dev cord. So the force in the main rope would change by 390N at the krab. Is that reasonable? It's 0.27 times the force in the dev cord, so a coefficient of friction of more than 0.27 would hold the krab there. The force required to push the krab up the rope would be more than 390N; if the krab were a frictionless pulley, it would start at zero and build up to 390N, so the actually friction force would have to be added to that, during the push.

Initially I assumed the krab was pushed 15cm. That gave me 13.9º, 17.5º, 1830N and 1870N, and a change of 830N at the krab would be too much for friction to hold or to push up that far.
 

mikem

Well-known member
Link from Chocolate Fireguard also includes testing of American Death Triangle (which we all hopefully know that we should never use, except we regularly do on pull through abseils). This shows the difficulty of testing "frictionless" theory v actuality, partly because the load cells don't act as fixed points, partly because of friction (in none of the 4 cases were the loads equal). Link at top (& bottom) of page gives mathematical & practical examples of why we generally shouldn't use it:
 

JoshW

Well-known member
Link from Chocolate Fireguard also includes testing of American Death Triangle (which we all hopefully know that we should never use, except we regularly do on pull through abseils). This shows the difficulty of testing "frictionless" theory v actuality, partly because the load cells don't act as fixed points, partly because of friction (in none of the 4 cases were the loads equal). Link at top (& bottom) of page gives mathematical & practical examples of why we generally shouldn't use it:
There was a thought a while back of placing climbing style anchors at the top of pitch heads of popular pull through routes (the kind with a single loop connected by chains to each anchor to remove the death triangle situation, does anyone know if this has been employed anywhere?
 

mikem

Well-known member
I am unaware of any modern bolts ever having pulled out on a pull through, so seems a bit of overkill - it's only when the bolts are a long way apart that it would be necessary, but that would be an expensive & heavy chain! However, it would make it obvious which point to use...

Also they don't work as well on a single wall, as the individual bolts keep the rope away from the rock, whilst a chain may allow the rope to be trapped between the ring & the rock.
 
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wellyjen

Well-known member
There was a thought a while back of placing climbing style anchors at the top of pitch heads of popular pull through routes (the kind with a single loop connected by chains to each anchor to remove the death triangle situation, does anyone know if this has been employed anywhere?
Various examples in the Peak District. For example, the pull up pitch in to Pilgrim's Way in Oxlow. https://ukcaving.com/board/index.php?threads/oxlow-safety-warning-pilgrims-way.27211/post-333549 though it is an up pitch, rather than a pull through down pitch. There is one of those with double chain anchors as you describe at the top of the aven above Basecamp Chamber in Giant's Hole that can be used for pull through descents after climbing the narrow rift to reach it.
 

langcliffe

Well-known member
There was a thought a while back of placing climbing style anchors at the top of pitch heads of popular pull through routes (the kind with a single loop connected by chains to each anchor to remove the death triangle situation, does anyone know if this has been employed anywhere?
The top of Oliver Lloyd Aven, if I remember rightly. They are commonly used in France. I think all the pull-through pitch heads in the Réseau de la Dent de Crolles are equipped with them.
 

Babyhagrid

Well-known member
Various examples in the Peak District. For example, the pull up pitch in to Pilgrim's Way in Oxlow. https://ukcaving.com/board/index.php?threads/oxlow-safety-warning-pilgrims-way.27211/post-333549 though it is an up pitch, rather than a pull through down pitch. There is one of those with double chain anchors as you describe at the top of the aven above Basecamp Chamber in Giant's Hole that can be used for pull through descents after climbing the narrow rift to reach it.
Is this the pitch that drops you down into the crabwalk? The ring there seems a little overengineered from memory.
 

mikem

Well-known member
Discussion here. Can't get the above link to work.
That site also has links to Over the Edge Rescue's ADT Theory page, but not the Real World one. The "Lowering from an adjacent route?" section helps explain why the Real World results for Test 2 are higher in RHS than LHS, & under "What are the real world forces?" explains why Test 1 is mostly lower than expected:
The theoretical force on each bolt would be the same as the load, or 100 pounds. But because the cord is running through the rappel rings, this friction actually reduces the force going to the hangers, which is a good thing. With a 60° equilateral triangle, about 80% of the load goes to the bolt
It's also why the further across a deviation pulls the rope, the less load there is on the main anchor.

Where you have to pull up a rope then the chain has real advantages, although it can be more difficult to determine if the cord is twisted. (French cavers are probably more used to using chains as their climbing cliffs are covered in them.)
 
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mikem

Well-known member
Don't forget that what you are used to using & what you have available are big factors in what gets put in, rather than what is ideal.
 

wellyjen

Well-known member
The top of Oliver Lloyd Aven, if I remember rightly. They are commonly used in France. I think all the pull-through pitch heads in the Réseau de la Dent de Crolles are equipped with them.
The Dent de Crolles is where I first remember seeing them, back in the last century.
 

andrewmcleod

Well-known member
Is this the pitch that drops you down into the crabwalk? The ring there seems a little overengineered from memory.
I think it was 'engineered' because of concerns about the amount of wear happening on the anchors from pullthroughs (or possibly lowering which is obviously very naughty)? A ring that can rotate freely spreads wear around and so lasts much longer than an anchor or a maillon.
 

Chocolate fireguard

Active member
It's also why the further across a deviation pulls the rope, the less load there is on the main anchor.
I don't understand that.

My take on this is that the distance pulled has no bearing on the main anchor load.
In that diagram in post #90 the tension in the top section of rope (the main anchor load) is always 100%.
No matter how far the rope is pulled.
That diagram is for zero friction (a good pulley).
With a krab in the same position nothing would change.

Both ChrisB and I, in our different ways, have shown that with the krab fixed in a higher position by friction the load on the main anchor is increased.

The 2 ways of looking at that are:

that the devi cord is applying a downwards force to the rope via friction, which adds to the weight or

that with the krab in a higher position both devi cord and the main rope make smaller angles with the horizontal so the tension in both must increase (as per normal Y-hang theory).

The opposite is the case with the krab in a lower position.

But again, I can't see how the distance pulled has any bearing on the main anchor load.
 

mikem

Well-known member
The distance doesn't, it's the fact that more krab is in contact with the rope, so friction increases - this means that (slightly) more of the load is transferred to the deviation (& we haven't even mentioned the effect of rope stretch or deformation of it's cross-section, under greater load, which also increases friction - as they are impossible to calculate theoretically!). The main anchor load is only 100% in a frictionless system (but it's a good enough proxy for most calculations as it's a worst case scenario until you get above the main anchor).

The change as you push the deviation up is also difficult to calculate, as it is a variable curve (depending on the relative lengths from the anchors, but again not really significant for what we need to know). Anyway, for lower part of devi radius the main line will be pretty much the same angle, then it will increase slowly until the devi line is horizontal, above which it will increase more rapidly until you can't physically push it up anymore.
 

Chocolate fireguard

Active member
The distance doesn't, it's the fact that more krab is in contact with the rope, so friction increases - this means that (slightly) more of the load is transferred to the deviation (& we haven't even mentioned the effect of rope stretch or deformation of it's cross-section, under greater load, which also increases friction - as they are impossible to calculate theoretically!). The main anchor load is only 100% in a frictionless system (but it's a good enough proxy for most calculations as it's a worst case scenario until you get above the main anchor).

The change as you push the deviation up is also difficult to calculate, as it is a variable curve (depending on the relative lengths from the anchors, but again not really significant for what we need to know). Anyway, for lower part of devi radius the main line will be pretty much the same angle, then it will increase slowly until the devi line is horizontal, above which it will increase more rapidly until you can't physically push it up anymore.
If you say so!
 
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