Safety parameters for descent hitches

[Bradley Ford]

the hitch for descending SRT

I use "static" and "dynamic" below to indicate whether or not the rope moves through the tie in point (TIP) of the climbing system, not to describe the rope.

If a hitch could control descent on a static rope system of twice the maximum weight capability of either the Tautline (TLH) or Blake's (BH) hitch on the same rope (2 X (min(max TLH, max BH))), then would descending a static rope with this hitch be at least as safe as descending a dynamic rope system with a "frictionless" TIP and the corresponding TLH or BH? (Jepson implied that the answer is absolutely not: OK.....Let's Get Into The Meat Of SRT !!!!)

Ever measured the force required to pull a TLH or BH, complete with stopper knots, to failure on a static line? This force would provide an upper limit to how much the hitch can control in descent. So how much force did it take?

2000 lbs. was an arbitrary weight that I thought might be a safe parameter. I'm guessing the (2 X (min(max TLH, max BH))) weight is less than 2000 lbs, i.e. that the TLH can't control descent of 1000 lbs. on a static rope system.

 

[Tom Dunlap]

All of you arbos need to broaden your views. The rest of the rope world has done a fantastic amount of research into climbing and rigging systems. Even uisng a basic search engine like Google will bring loads of info to your finger tips.

I read a test report about the bounce from ascending. If I remmeber right, Robert will help me here, the climbers were using full on SRT systems. If they would have added a footlock ascent the bounce would have been much higher. Every time you sit on the rope you'll do a mini slam-dunk on the anchor point. It would be pretty simple to setup a test like this.

Tom

 

[Bradley Ford]

On page 98 of 166 (page numbered 93) from Industrial rope access - Investigation into items of personal protective equipment, a Blake's hitch is reported to have held a 4 kilonewton = 899 pound-force load. This makes me think a Blake's hitch could control the descent of a 2000 lb. load on a dynamic rope system where the load on the hitch is ~1/2 the actual load. This also provides some rational for my earlier, arbitrary use of the 2000 lb. value.

Tom,

I tried googling "ascend bounce test climb SRT", but I didn't see the link to which you referred. Please help.

 

[Tom Dunlap]

be creative with Google. If I were on a payroll I would find theinformation. To me, the search is sometimes more important that the find.

Go to SARBC and root around in the links, it might be there, don't know for sure.

Tom

 

[RescueMan]

Bradley Ford:

a Blake's hitch is reported to have held a 4 kilonewton = 899 pound-force load. This makes me think a Blake's hitch could control the descent of a 2000 lb. load on a dynamic rope system

That test was for a static pull to determine ONLY the holding power of a variety of friction hitches. That says NOTHING at all about using a hitch as a decent device.

Some figures on forces from ascending a rope can be found at:Anchor Loads

There is also a reference to these forces in the test data I posted earlier at the top of page 6 of:
High Strength Cord
- Robert

 

[Eagle1]

Word up Rock!
I have been reading this 5 page topic for some time now, still have not understood the true question and or why?

 

[NeTree]

Maybe he's expecting a mssaive weight gain?

 

[rbtree]

Originally posted by netree
Using a low-stretch line as a tress cord or split-tail would be inconsequential.

Using a low-stretch line as your lifeline could easily break you in half in a fall.

Isn't that why we don't climb on static line?

Eric, that's where you're wrong. Arbo lines, both climb and bull, are low stretch static lines. Most have 50-70 less stretch than kernmantle static line.

So, do not fall fall on arbo line, it might hurt. But the limited stretch should save your hide, as opposed to the near zero stretch of synthetic fiber high tech lines like Spectra, kevlar, and Vectran

 

[MasterBlaster]

Originally posted by rbtree


So, do not fall fall on arbo line, it might hurt.

The three hard falls I've had suprised me with their lack of violence. Each time, my rope/saddle snapped me upright and I didn't even drop my saw.
This was on safety blue.

 

[NeTree]

Originally posted by rbtree
Eric, that's where you're wrong. Arbo lines, both climb and bull, are low stretch static lines. Most have 50-70 less stretch than kernmantle static line.

So, do not fall fall on arbo line, it might hurt. But the limited stretch should save your hide, as opposed to the near zero stretch of synthetic fiber high tech lines like Spectra, kevlar, and Vectran

Say what??

 

[Lumberjack]

RB, I still don't understand where you get that static kernmantle ropes are more strechy than arbo lines.

You got a site or something with the info on it?



Carl

 

[RescueMan]

Rope Stretch

I still don't understand where you get that static kernmantle ropes are more strechy than arbo lines

The National Cordage Institute defines three categories of rope according to their stretch characteristics:

Dynamic - >10% @ 10% MBS (= 1 SWL)
Low Stretch - 6-10% @ 10% MBS (= 1 SWL)
Static - <6% @ 10% MBS (= 1 SWL)

The stretch characteristic of arborist rope varies considerably from the relatively stretchy NE Safety Blue to the very static 32-strand double-braid (about 1%).

Paolo Bavaresco in “Landscaper” magazine 2000, writes: "Don’t choose a rope that mimics Steel or Bungee properties! Under no circumstances should high strength fibres like Kevlar be used as a climbing or rigging line. It requires very high bending radii to operate safely, and has extremely poor energy absorption properties, therefore eliminating its safe use in any operation other than pulling! High stretch ropes will not provide sufficient control in close proximity to targets. Stretch in a rope is not recoverable after severe shock loading – it will stay stretched rendering it useless for energy absorption. Loading a braided polyester rope over 40% of breaking strength will result in permanent stretch (damage)."

see:

SELECTION AND USE OF ARBORIST ROPE

- Robert

 

[NeTree]

yea... but who climbs on DOUBLE BRAID?

When I said "Low Stretch", I was referring to static line in relation to climbing line. I should have said "no stretch".

I was comparing "static lines" in relation to arborist CLIMBING lines in specific.

 

[Lumberjack]

I have climbbed on super braid before, and it really wasn't that bad, I liked the lack of strech for work positioning, but it would be a bugger on a fall. I also will repel down it, so I don't have to use another method of pulling my climbing rope down.

I also remember someone on here saying that super braid was the rope of choice for a climning rope.

Carl

 

[NeTree]

I dunno, I've tried all kinds of line, and I still like Safety Blu HI-V best. Maybe I'm just ATTACHED....

get it?



attached?


HA! I crack myself up!

 

[Stumper]

BradleyFord, I have read this thread with interest but I understand Brian's frustration with your question. Slippage in a friction hitch does not equate to failure. One of the theoretical benefits of the V.T. is the fact that it can be expected to slip at 800-1200lbs of load(dependent upon cordage diameter and material). The theory is that slammming a V.T will result in a shock absorbing slippage without breaking the rope or (hopefully not) killing the climber. You'll have to look a while to find anyone who has tested the theory though!. If any friction hitch controls the climber and his gear without slipping during normal use then it is an adequate /safe hitch. A study comparing the point at which various hitchs slip would be useful but looking for the hitch that will support 2000 lbs without slipping is looking at the wrong end of the spectrum for maximum safety. The hitch that slips at just a little more than the climber's weight is the "safest" in the event of a fall since it will absorb the shock by slipping. As Brian has pointed out-good tree climbing techniques preclude setting oneself up for a free fall so in the final analysis whatever holds us in normal use makes the grade.


I think that Mike Maas reported testing an Open Prussik/Improved Tautline To 4000lbs a couple of years ago-The only friction hitch that didn't slip at around 1000lbs of those tested--but I may be remembering incorrectly or Mike might have had that hitch aroud our legs.


RB, I think that you may have confused things a bit. Dynamic kernmantle is very stretchy. Static kernmantle is very stretch resistant-it varies but the numbers are very small for 200-400lb. loads. I have read references to Arbo lines as "Semi-static". They stretch but not a lot. I agree that falling onto slack lines is a very bad idea.

 

[Kneejerk Bombas]

An interesting thing that happens with a prussic is the more weight applied, the tighter it grabs. Then, the more pressure it takes to break it loose.
That's why JPS (350 lb.s) and Kenny (90 lb.s) can both climb on the same set up and it will work great for both.
When we climb on a doubled line, the hitch only takes a small percentage of our weight, and works great. Install a friction saver and things start getting tighter, hitches grab more and are harder to release.
Try the same hitch on a single line and it grabs and holds, but you can't get it to slide, and if you do, it burns and is hard to control.
To hold and control 2000 lb.s would not work with a rope hitch. Now, if you just want it to handle a fall, a prussic will slip if it encounters a 2000 lb. load, then grab once the shock is ended.

I did do some pulling last year on a few different prussic hitches on arborist ropes, the results were interesting, but most importantly they all started to slip at some point, they don't hold until the line breaks.

 

[Bradley Ford]

Originally posted by RockyJSquirrel
You have yet to explain why you find it imperative to have a hitch capable of LOWERING 2000 lbs on a static system (or is it a dynamic system today?).

I do not find it imperative to find a "2000 lb." hitch; in fact I'm open to the possibility that such a hitch would not be safe (referring to my next post). I would like to objectively define a "safe" hitch with a set of criteria that each hitch could be measured against. Such criteria would allow one to judge hitch A safe to use, or hitch A safer than hitch B. The criteria should be applicable to both dynamic and static rope climbing systems. I added "descent" to the thread title because I'm not as interested in hitches used only for ascent.

 

[Bradley Ford]

Originally posted by Mike Maas
A good hitch would run when overloaded, then slowly grab, to reduce shock loading in case of a fall.

Does a "safe" hitch have a maximum weight holding ability so that by slipping it can help absorb some of the shock load from a fall? How would this maximum weight be determined?

 

[RescueMan]

Does a "safe" hitch have a maximum weight holding ability so that by slipping it can help absorb some of the shock load from a fall?

This is exactly the reason that tandem 3-wrap prusiks have become the belay of choice in the rescue community (for handling up to 600 lb loads and the possibility of some shock loading).

Mechanical rope grabs have been shown, in drop testing, to severely damage or sever a static kernmantle rope. Prusiks - which tend to slip and then heat up, partially melt and grab - can safely catch a 200kg load in a facter 1/3 drop (1 meter drop on 3 meters of rope).

But the weight-holding ability of a prusik (or any other hitch) depends on the type of cord and type of rope used (both the material and the "hand" or stiffness), the relative diameters of the cord and rope, whether the host rope is new or used, wet or dry, etc...

How would this maximum weight be determined?

By carefully controlled static pull testing and dynamic drop testing with various combinations of cord/rope.