Rigging a Pull Through

Mr Mike

Mr Mike

Active member
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.

 
M

MJenkinson

Member
I am planning a pull through trip this weekend and was thinking about this on the train this morning (caving nerd).

I think I would use it (and probably have in the past as most pitch heads are rigged for y hangs, and I would use the same bolts for a pull through) as it still shouldn't generate failure loads. but....it does seem to go against generally accepted advice when you think about it.

Also I am guessing that as you only descend the rope (assuming some basic skill) it should be a nice smooth application of loads and have no bounce loading as if you were prussiking.
 
Simon Wilson

Simon Wilson

New member
Mr Mike said:
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.
Click to expand...

It is not an American death triangle. On the contrary it is a safe way of loading a pair of anchors because the effective angle of forces is 90 degrees. However, pulling ropes through which are in direct contact with an anchor should not be done because it will wear the anchor.
 
MarkS

MarkS

Moderator
I've not heard of an American death triangle before this morning (https://en.wikipedia.org/wiki/American_death_triangle), but I find it hard to see how a pull-through like the one on the link above doesn't form the same configuration.

I think the link on aditnow is not a correct comparison because that's essentially giving the forces in a Y-hang, not in a full triangle, and they are much greater in the triangle configuration (as shown in the table in the wikipedia article above).

I'm surprised I've not seen mention of this before...
 
Rob

Rob

Well-known member
A key difference of this pull through setup is the friction at each of the anchors and on the krab. This friction makes it uniquely different, although i'm not sure of a way of putting values to it!

If you take it to its extreme and assume the friction on each anchor to be 100%, then all you have is a deviation, which would be nicely loaded.

If no friction on the anchors, and full friction at the krab, then i think you have an ADT.

Therefore the actual setup will presumably be somewhere inbetween...  :confused:
 
M

mikem

Well-known member
If the anchors were receiving a dangerously amplified load then they would usually turn towards each other before failure (assuming they are not loose hangers, which will turn anyway) - climbers use retrievable set ups far more often than cavers & I'm not aware of any incidents being blamed on outlandish loadings. Having said that, it sounds as though forces will be kept to a minimum by using a figure 8 with both ends of the rope threaded through it.

Have any caving bolts had to be replaced due to wear from pull throughs - the additional cost of adding rings may not be worthwhile, compared to the cost of replacing the bolt?

Mike
 
Mark Wright

Mark Wright

Active member
Mr Mike said:
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.
Click to expand...

I don't see the loads on the ADT method of rigging being all that bad at all really, certainly well within the capabilities of the installed anchors and associated rigging gear. I would be a little concerned if we were talking about taking big falls onto such a system but we aren't, we are only applying predominantly static loads.

Having said that, it is always important to ensure both descents and ascents are carried out as smoothly as possible. Industry testing has shown that bouncing forces up to 2.5 x the cavers weight can easily be applied when descending and ascending.

mikem said:
Have any caving bolts had to be replaced due to wear from pull throughs - the additional cost of adding rings may not be worthwhile, compared to the cost of replacing the bolt?

Mike
Click to expand...

I think Simon has explained that anchor wear is probably the main issue.

Mark
 
M

MJenkinson

Member
For trips that are regularly done as a PT should we (cavers / CNCC) be putting some "disposable" maillons in place for the rope then if rope wear is an issue?
 
M

mikem

Well-known member
Mark Wright said:
I think Simon has explained that anchor wear is probably the main issue.

Mark
Click to expand...
I know that wear is an issue, but is it enough of an issue to require the fitting of rings on any pull through routes, as worn bolts can now be reported & replaced before that wear becomes dangerous?

Climbers regularly thread the rope through the bolts & get lowered on that, creating far more wear than abseiling & pulling the rope down after (although their ropes don't tend to be as gritty).

The problem with leaving maillons is they may cause faster corrosion of the anchor by having 2 different metals in contact & are harder to pull the rope through as they sit flatter against the rock.

Mike
 
Mark Wright

Mark Wright

Active member
Finding people to replace anchors on crags is a lot easier than finding people to replace anchors in a cave so it probably is worth considering installing easily replaceable rings, just as they do in popular pull through trips on the continent.

Mark
 
M

MJenkinson

Member
Is there a preferred type of maillon for this task? Suppose the bigger the better really.  Might leave some in Rowten.
 
M

MJenkinson

Member
mikem said:
The problem with leaving maillons is they may cause faster corrosion of the anchor by having 2 different metals in contact & are harder to pull the rope through as they sit flatter against the rock.

Mike
Click to expand...

Good points....
 
M

mikem

Well-known member
& the maillons (that were fitted before the rings) either disappeared or were damaged!

Thanks for your reply paul - so have any Yorkshire bolts had to be replaced due to wear?

Mike
 
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