weight limit?

TCI uses a Rescue Randy to train with rescue techniques. Randy is plastic with articulated joints (used by fire depts.) and weighs 145 pounds. One rescue technique we have practiced in the past is to climb up next to the victim, clip him to our harness, and then cut the bridge which puts his weight full on the climber doing the rescue. We had a ground belay holding the rope on the ground (DRT system). The shock on the climber was less than I thought. The friction hitch held fine (B-53 knot – Blake’s with 5 wraps and 3 tucks). It made for a jerky ride down though. We do not practice this style of rescue anymore. We put a carabiner above the victim with an adjustable knot (pick off strap, lanyard with ascender, or daisy rope) and let our descending weight pull down the victim’s friction knot automatically. It is easier to cradle the victim while descending using this system.

To test for the limit I need to define a "safe, controlled descent", maybe in terms of distance slipped before hitch holds. Is there a common definition for this that I could use? How would you define this for this test?

Brad

Bradley Ford says:

To test for the limit I need to define a "safe, controlled descent", maybe in terms of distance slipped before hitch holds.

My experience with "rescues" both in training with TCUSA and when assisting climbers with disabilitites, is that if the Blake's slips, it is going to keep on slipping. I haven't experienced it sliding for a while, then catching. (Using a 5-3 Blake's). It would seem that you could use a Munter hitch on the down rope to slow yourself even more.


From the original post, it sounds as if you want to know the holding power of a knot if you clipped the other climber to your belt and now had their full weight and your full weight on your knot system. It sounds as if you're trying to prepare in advance for something happening in the tree, which is good. But I'm not sure that you can get a definite weight limit that will work in all circumstances (as you pointed out in your first post). Having a specific TECHNIQUE(s) that would accomodate a myriad of circumstances may provide you with the best (i.e. safest) result.
Maybe you could do some tests in different circumstances (diff knots, diff amounts of wraps on Blakes, diff types of trees, wet vs. dry rope) and then post the results?

It should be pointed out that the blakes, and other friction hitches work by providing friction within the hitch to slow the line. The more weight put on the hitch the more friction it will apply. I'm sure that this friction increases exponentialy early on and then flattens off at some higher load, yet continues to increase. Slipping under load occurs when the load is greater than the friction availible. I doubt the weight of THREE individuals is enough to cause slippage of a properly tied knot, but the hitch wont help after you break it's hold, at that point you and your two poor rescue victims are hurtling to the ground. To control the descent constant friction must be added by way of a control decvice (rack, munter, 8, I'D) the hitch wont ever slow you down after the friction has been released.

By the way, a "safe controled descent" is one that you walk away from without broken bones, burned hands, or sudden unexpected onset of DEATH.

Originally posted by Icabod

By the way, a "safe controled descent" is one that you walk away from without broken bones, burned hands, or sudden unexpected onset of DEATH.

Although Icabod's definition coincides with my general ideas, more specific criteria are needed to test for the weight limit that can safely descend on a friction hitch with a dynamic climbing system.

What defines a safe, controlled descent? Is it maintaining the descent rate below some level? Is it limiting the distance covered between full stops?

Brad

It seems a factor of safety would be the way I judge any friction hitch. It should lower you (and the other person, in this case) when you want to be lowered, stop when you want stop. No exceptions, no slipping, minimal maintanence (dressing the wraps on the hitch, etc).

love
nick

I'm thinking that Joe or Peter would be best to comment for sure, but it is my impression that the more weight you apply the LESS likely the blake's hitch is to slip. That is to say, the more weight applied, the tighter it holds. I was taught that for very light climbers, an extra wrap in the blake's will make the system safer. I know that in my experience, with a pulley assisted system with a blake's hitch, we have had problems with them slipping, presumably because the climber is only loading it with 1/2 or 1/4 of their weight because of the mechanical advantage.

Joe, Peter, what say you?

Bob

This is a good question with no good answer. There are so many variables that it will be hard to give a very definite answer.

My experience with friction hitches shows that there are some that will slip/lock and others that will slip/fail. Think of this as a spectrum.

My friend Paolo Bavaresco has done a research project on friction hitches. In the next few weeks he's going to send me the results. When I get them, I'll post a note on here.

Arbos need to think more about safety factors. If we are choosing a hitch because it will work with a 200# load but not a 175# load It seems like we're cutting things a little close.

Tom

I think my 250 pounds has tightened the blakes 4-2 hitch so tight on safety blue (all white rope) it is hard to move sometimes. I added a prussik foot loop to help me to be able to release some of the tightness on the knot, this helped alot.

After Tie Dress Setting the knot it holds very well. Even when there is a little swinging goin on.

When we want a little faster RAP out o the tree we use some kind of device 8 or ATC.

Just wondering if the 5-3 might not tighten so much.