Rigging a Pull Through

[Simon Wilson]

I know cavers are generally better rope riggers than mine explorers and have this question about a pull through scenario that we have been debating on aditnow, basically its a pull through off 2 anchors that are in line to each other and horizontal (as for a Y-hang) most of us say it is fine, but one is saying its forming an American Death Triangle)  see:

http://www.aditnow.co.uk/Community/viewtopic.aspx?p=176601#msg176601

My question, is it as bad as made out or perfectly acceptable to use like this? Thanks.

Both views are partly correct. The American death triangle is a theoretical way of explaining how a rope in a triangle will half the angle between the lines of force. The accepted minimum angle for a Y hang is 120 degrees because this is the angle which produces forces on the anchors equal to the force on the live rope. If the rope forms a triangle, the angle at which the force is applied is half of the angle between the live rope and the top of the triangle this is what is described as amplifying the force. An angle between the lines of force of 120 would be achieved if the triangle was an equilateral triangle.

If a rope is passed through two anchors, when the abseiler is near the bottom of the rope the effective angle approaches 90 degrees. If the load is applied close to the anchors and the distance is less than the distance that the anchors are apart then the force applied to the anchors could be greater than the force on the live ropes. In practice it would be impossible to apply the load that close to the anchors. So the angle will always be less than 120 degrees.

Whoever it was that mentioned the American death triangle, they were correct in bringing the phenomenon to people's attention because it is something that people should be aware of although in this case it isn't a problem.

 

[traff]

If a rope is passed through two anchors, when the abseiler is near the bottom of the rope the effective angle approaches 90 degrees. If the load is applied close to the anchors and the distance is less than the distance that the anchors are apart then the force applied to the anchors could be greater than the force on the live ropes.

If I have interpreted that correctly I think you need to go back and have another think about that.

 

[Simon Wilson]

If a rope is passed through two anchors, when the abseiler is near the bottom of the rope the effective angle approaches 90 degrees. If the load is applied close to the anchors and the distance is less than the distance that the anchors are apart then the force applied to the anchors could be greater than the force on the live ropes.

If I have interpreted that correctly I think you need to go back and have another think about that.

Do you think it's wrong?

I was trying to keep it simple so maybe I need to give a bit more explanation. I was assuming the abseiler was abseiling on double ropes, I was assuming the anchors are at the same height (which is the worst case scenario), I was ignoring friction and I was also only being approximate. To be more exact, the horizontal distance between the anchors has to be multiplied by the sine of 60 degrees to give the vertical distance to the point at which the force on each anchor is equal to the force on the double live ropes.

Maybe I have got it wrong but I'm happy to be corrected.

 

[ALEXW]

Its the DOUBLED rope business that had me baffled, using both sides of the ropes to abseil on using either a rack or a figure 8?? Not many cavers do I suspect although climbers seem to. 

If you find the ropes are too short how do you get back up?

Maths is simpler on single rope abseil, knot jammed against one bolt then you just have a 90 degree bend if they are both at the same height so no force multiplier. If you use a crab it will just ride up the rope until it reaches equilibrium or jams against the knot.  Friction makes it difficult to calculate.

 

[andrewmcleod]

In the hideously unlikely event you manage to put enough force on a bolt to pull it out, you will conveniently find the system reverts to a non-multiplying state

 

[mikem]

It does, but may create shock loading on the 2nd bolt...

If you're on the second pull through, you can't get back up the first pitch anyway! You should only abseil on a doubled rope if you can see it's on the floor, although if uncertain, you could tie a knot in one side, allowing you to prussik back up the other.

A knot with maillon which is slightly longer than the gap between bolts means you can't create a "death triangle", but still ensures the line is secure if the knot slots through the bolt - but if that happens it may be impossible to retrieve.

Mike

 

[Chocolate fireguard]

A knot with maillon which is slightly longer than the gap between bolts means you can't create a "death triangle", but still ensures the line is secure if the knot slots through the bolt - but if that happens it may be impossible to retrieve.

It also keeps the 2 ropes apart.

On a long pullthrough pitch (approx 75m) rigged with the ADT method we couldn't pull down. After a few minutes of both of us swinging on one rope after the other (in case we had mixed them up) we really did mix them up. Yes I know it was stupid.
My mate was heavier than me so he clipped onto one rope while I went up the other.
A short way down from the top the ropes were twisted together for a length of about 3 metres.
It took a while to sort out.

The anchors were nearly a metre apart so I don't think it could have happened with Mike's method.

It was 9mm rope but I would imagine a Fig 9 knot used to form the loop wouldn't go into the anchor (?).

 

[mikem]

The American Death Triangle is a sling clipped to both anchors & then loaded on one side, so has minimal friction compared to ropes & importantly, if one anchor fails then slings do not cope with shock loading as well as rope does.

A fig 9 knot will fit through much the same gap as a fig 8...

Mike

 

[Chocolate fireguard]

The American Death Triangle is a sling clipped to both anchors & then loaded on one side, so has minimal friction compared to ropes & importantly, if one anchor fails then slings do not cope with shock loading as well as rope does.

A fig 9 knot will fit through much the same gap as a fig 8...

Mike

Yes, by ADT I meant the pull-through method used by a lot of cavers. Rope through both anchors so both ends reach the bottom, knot with krab on one rope, clipped round other rope, abseil down rope passing through krab. The one you referred to as a "death triangle" a couple of hours ago.

Yes I see your point about the 2 knots.
Perhaps some sort of metal disk with a hole for the rope so the disc is between the knot & the anchor?
A stitch plate?

 

[mikem]

Using the rope rather than a separate sling means the load cannot be evenly distributed like a true ADT, friction thus becomes a much greater factor - somewhere else on this site a maillon is recommended rather than a krab.

Mike

 

[PeteHall]

A short way down from the top the ropes were twisted together for a length of about 3 metres.
It took a while to sort out.

The anchors were nearly a metre apart so I don't think it could have happened with Mike's method.

This is the classic reason for a pull-through to jam and it can still happen if you have a long loop.

As the rope pulls through the anchors, it twists so no matter how long the loop, if the two sides are joined it is always a risk. The bigger the pitch the more chance of if jamming. I think the anchor placement can make a big difference to twisting too...

I avoid this problem by using a double rope through my rack and no knot at the top (I do tie the ends together though). I have one rope in each hand and make sure not to rotate on the pitch. When I get down, I release one side first, then move as far away as possible with the other side before pulling down. Doing this, I have never had a pull through that is even slightly difficult to recover

 

[Simon Wilson]

In forty odd years of caving and climbing I have never had a rope jam when pulling through. Maybe that is because I have done more pulling through on alpine routes when the stakes are high.

I have often been alarmed by the cavalier attitude of other people to pulling through in caves. It really isn't difficult to get organised, plan your trip and make sure you know the cave and the pitch lengths and have the right ropes and the right gear. At least one person should be able to abseil on a double rope, ideally have ropes of different colours like climbers do and keep the two sides separate.

 

[Simon Wilson]

I think we can forget this American death triangle. None of the rigs described in this thread could form an ADT. There are really only two ways; a double rope through both (the best way by far) and the knot and crab way of abseiling on a single rope. With the knot and crab rig it will automatically equalise to an equilateral triangle with the force on each anchor being the same as that on the single live rope. That is in theory ignoring friction. In practice in both of these rigs friction will act to reduce the force on the anchors but by how much we can only guess.

The third rig mentioned was where the rope is jammed against one anchor and the live end is passed through the other anchor. That would be a bad way of rigging because there is a mechanical advantage which increases the force on the live side anchor above the force on the live rope.

 

[mikem]

The third rig mentioned was where the rope is jammed against one anchor and the live end is passed through the other anchor. That would be a bad way of rigging because there is a mechanical advantage which increases the force on the live side anchor above the force on the live rope.

Although it is the only one that can be easily tested with a load meter to find out...

Mike

 

[GT]

I get bored easily...

https://youtu.be/ixHFt2aghds

 

[Mattrees]

Yes I see your point about the 2 knots.
Perhaps some sort of metal disk with a hole for the rope so the disc is between the knot & the anchor?
A stitch plate?

I've always just tied a huge knot, fig of 16 or 22 or something!

Your metal disc idea would work but here's an alternative, clove hitch onto the back bar of a krab:

 

[topcat]

At last; a picture!  My mind is tied in knots trying to understand all the words)

 

[ianball11]

I get bored easily...

https://youtu.be/ixHFt2aghds