What is the weight limit for safely descending on a friction hitch with a dynamic climbing system? I know there are variables such as type of hitch, type of rope, and friction from the tie in point, but I'm hoping to find some reasonably determined guideline.
First off, there are so many types of hitches a person could choose. And if you find one that slips at low loads, many of these you can just add a wrap or two to give a little more friction.
There are reports of prusiks and klemheists not slipping until the 1000lb range. This is not to say, "A prussik can hold a thousand pounds." Under the right (or should I say wrong) circumstances, they can slip at significantly lower loads.
Maybe we should set up a test.
Were the tests you reference done on a static or dynamic climbing system? And were they determing the weight limit for safely descending (controlled slipping with the option of stopping) or for causing the hitch to fail?
I have safely descended (knock on wood ) with a dynamic climbing system on 1/2'' XTC using a 5-3 Blake's; my climbing weight is ~175 lbs., so I can offer that the limit is at least 175 lbs.
My climbing weight is about 195, on the same system, and I've never worried. When I was taking my facilitator course, I also descended with Tim Kovar's, who's a lot bigger than I,weight on my rope, and was fine.
There is a lot of pulling going on there. The prussik info is down towards the bottom.
I've seen prussik hold many hundred pounds, and I've seen others slip at way less than a hundred. It's just a matter of trying it out low and slow. Get used to your materials. And take it easy when you try new things!
... I want to understand more about tree climbing systems. Part of the education I need relates to this guideline.
I want to know the capabilities and limitations of the climbing systems I use. If I descend using SRT and a Petzl I'D, I know from Petzl's specifications for the I'D that the maximum "normal working load" is 150 kg. I'd like to know the corresponding weight limit for safely descending on a friction hitch using a dynamic climbing system.
Since the limit is above normal use, here's a hypothetical question that stresses the need to know this limit: (Referencing Joshua's post earlier in this thread) Could Joshua have safely "rescued" Tim if they each weighed 350 lbs.?
I guess this wouldn't be too tough to test. Climb up, set up in a dynamic double rope system, then clip a buddy to one of your d-rings!
However, you wouldn't be testing it if there was the possibility of failure. It might be a good idea to have other friends there to put one or both of you on a super loose "just in case" belay.
What climbing hitch would you use for this?
I think your question shows an unclear understanding of how a friction hitch works. You do not descend on a friction hitch as you do on a descent control device such as an 8, rack, tube, or other device such as the I'D. Descent on a DdRT system, using a friction hitch, is controled by friction between the control hand of the person descending and the friction between the rope and the TIP. The friction hitch acts to prevent downward motion by providing enough friction inside the hitch when loaded to arrest motion of the running line. During descent you are forced to release the load on the hitch to make any downward motion.
This said, in a rescue situation (which is not what TEAR was describing) the incapicated individual should be lowered to the ground on his own rope and descent control system. The chances of an individual being able to control the weight of two humans on the same DdRT system using only the friction provided by his hand and the rope/cambium friction to control descent are poor. The addition of a descent control device, I would recomend a rack system whose friction can be adjusted per need, in place of the friction hitch would make it a safer, but still dangerous, opperation.
A little misunderstanding: Tim was definitely on his own rope and knot for our practice rescues. The specific instance I was thinking of was one exercise when he was "unconcious," and I had to have some of his weight on me in order to support his neck and back. The majority of his weight was still on his own rope, but some was on mine, since I was holding him.
Following TCI's guides of having a rescued individual's weight primarily on their own rope, I would say yes. Because obviously, we got into the tree somehow, so our knots have been holding each of our individual 350 lb. weights. There shouldn't be any real difference between the stress on the rope while climbing and being rescued/rescuing. Does that make sense to anyone besides me?
What is the weight limit for safely descending on a friction hitch with a dynamic climbing system?
Nick's idea of testing this myself may have to suffice as a single point of information, although I would be surprised if I were the first to perform such a test. Suggestions about performing this test welcome; I don't like the idea of using myself (or anyone) as weight for this test, since to find a limit I must produce at least one rapid, uncontrolled descent.