Safety parameters for descent hitches

[Bradley Ford]

Originally posted by Bradley Ford
I believe that when I descend with a dynamic rope system my hitch is the weakest link in my system.

Is this a statement of the obvious or just hooey? Assume a safe TIP, new condition arborist rope and work saddle, and perfectly TSD-ed knots.

 

[Bradley Ford]

Originally posted by RockyJSquirrel
I do not think of my hitch as the weakest point in my system at all. From what I've been taught, my lifeline would break before my hitch.

These “ballpark” numbers are my guesses:

• My hitch would fail to control descent at a load of 1000 lbs.
  My rope would break at a load of half its rated strength (due to knots), or 3500 lbs.

What should these numbers be?

 

[RescueMan]

Hitch slippage

I do not think of my hitch as the weakest point in my system at all. From what I've been taught, my lifeline would break before my hitch.

I don't think the issue was breaking but rather braking, the ability of the hitch to handle the load (friction capacity).

My hitch would fail to control descent at a load of 1000 lbs. My rope would break at a load of half its rated strength (due to knots), or 3500 lbs.

Most knots weaken a rope by 20-35% - using a 50% stregth reduction factor is very conservative - takes into account normal wear-and-tear of rope as well.

Don't forget that a fall on a slack rope creates a shock load which can multiply your body weight. This is why a 10:1 or even 15:1 safety factor is typically used for lifeline.

I haven't seen any test results on any friction hitch besides the 3-wrap prusik. But that hitch, properly sized (about 3/4 of the mainline diameter) generally slips at about 1200 lbs force.

The 3-wrap prussik uses smaller diameter cord for better "bite" into the host rope, and a total of 6 turns around the host rope. I would think that a Blake's or tautline, with four turns and made of cord similar in diameter to the mainline, would slip at considerably less than 1200 lbs.

Anyone have any data on the Blake's or tautline?

- Robert

 

[Burnham]

RescueMan, in your field do you accumulate theoretical strength losses to a rope with multiple knots? Or is the knot that has the greatest strength reduction the measure used for the system in question? Or is there some other way of evaluating this question?

 

[RescueMan]

Strength Loss due to Knots

RescueMan, in your field do you accumulate theoretical strength losses to a rope with multiple knots? Or is the knot that has the greatest strength reduction the measure used for the system in question? Or is there some other way of evaluating this question?

Fortunately for those of us who hang our lives on rope, strength loss due to knots (and other tight-radius turns) is NOT additive. Think of the rope as a chain made up of rope segments, each with some kind of knot. The weakest of the links is the only one we need to worry about.

Natural-fiber ropes (which had NO inherent stretch) were weakened in a knot because the fibers on the outside of each turn were taking all the tension, so the weakening was proportional to the tightness of each radius.

With synthetic fibers (at least the plastics like nylon, polyesther, or polypropylene), which have inherent stretch, the weakening is due to the internal friction within the yarns which builds up heat when under load, also correlated with the tightness of the radius and with how many turns are sharing the load (figure-8 better than overhand, eg).

A general rule of thumb to avoid rope weakening is for any loaded turn (around a TIP, carabiner or pulley, eg) to have a radius at least 4 times the diameter of the rope (2" for 1/2" rope, eg).

These are some strength reduction factors due to knots:

BENDS
double fisherman's bend - 21%
figure 8 bend - 19%

LOOPS
figure 8 on a bight - 20%
butterfly - 25%
bowline - 33%

ROPE WITH A LOOP (pulled end-to-end)
figure 8 on a bight - 35%
inline figure 8 - 41%
butterfly - 31%

KNOTS IN WEBBING
water knot (ring bend) - 36%
overhand on a bight - 35%

 

[Bradley Ford]

Originally posted by RescueMan
Don't forget that a fall on a slack rope creates a shock load which can multiply your body weight. This is why a 10:1 or even 15:1 safety factor is typically used for lifeline.

I haven't seen any test results on any friction hitch besides the 3-wrap prusik. But that hitch, properly sized (about 3/4 of the mainline diameter) generally slips at about 1200 lbs force.

I think the hitch is weakest when it is used to control descent (versus being used to belay, or hold in place). 1200 lbs. is the 3-wrap Prusik's belay limit, and its controlled descent limit would be significantly less, right?

Originally posted by Bradley Ford
Do you hold your descent hitch (or device) to the same 10:1 safety factor as the other components of your climbing system?

If so, then the minimum capability for your descent hitch is (200 lbs. climber) X (10 safety factor) / (2 support legs of your dynamic climbing system), or 1000 lbs, right? (Note this is about twice as much as the 551 lbs. that Petzl reports its I'D descender can handle under exceptional circumstances.)

 

[Burnham]

So, Bradley, I've been following this thread with interest, and we've covered some good ground, but I find myself wondering again...what is your point?

 

[RescueMan]

I think the hitch is weakest when it is used to control descent (versus being used to belay, or hold in place). 1200 lbs. is the 3-wrap Prusik's belay limit, and its controlled descent limit would be significantly less, right?

In climbing and in rescue, the prusik is never used as a descent or lowering device. It is a catch-or-release hitch used for ascending or belaying. I know of no friction hitch other than the dog-n-tials (similar to the VT) which is usable as a descent hitch on SRT, though the autoblock as an adjunct on the brake side of a figure-8 descender can be useful to help control speed as well as to lock off.

If so, then the minimum capability for your descent hitch is (200 lbs. climber) X (10 safety factor) / (2 support legs of your dynamic climbing system), or 1000 lbs, right? (Note this is about twice as much as the 551 lbs. that Petzl reports its I'D descender can handle under exceptional circumstances.)

You're confusing MBS (minimum breaking strength) with SWL (safe working load). The 10:1 safety factor is applied to the MBS of "software" (rope and webbing) to get the SWL. The 551 lbs for an I'D is the SWL. The SWL of a prusik is something less than the 1200 lbs at which is will begin to slip. If tied from 8mm cord on a doubled sling, it would have a MBS of about 6,000 lbs after reducing for the knot and a SWL of 600 lbs.

- Robert

 

[Bradley Ford]

Originally posted by Burnham
...what is your point?

Will any commonly used friction hitch provide safe, controlled descending for 2000 lbs. on a static rope? If so, which? If not, which comes closest?

 

[Bradley Ford]

How much force can Joe Climber be expected to be able to apply to release, for the purpose of descending, the holding friction in his hitch?

 

[NickfromWI]

Perhaps you are planning on using a friction hitch in another application? I am sure you could find a combination of line and hitch cord that would meet your needs, but it would probably take a lot of trial and error.

So why do you need a 2,000lb hitch?

love
nick

 

[Bradley Ford]

Originally posted by RockyJSquirrel
What does it matter? In almost 15 years of climbing, I have yet to put anywhere near 2000 lbs of force on my lifeline.

I choose to climb on a rope with a breaking strength rated well over 7000 lbs.--much more than I'll probably ever need--rather than on less capable ropes. Arguably the most important parameter for choosing a hitch (it has to at least handle the fraction of your weight placed on it by your climbing system) is how much weight it can control in descent. Ideally, my hitch would control the descent of much more than I'll ever need, too. 2000 lbs. was my arbitrary (actually 200 lb. Joe Climber mutilplied by 10), much more than I'll ever need, descending load.

Originally posted by NickfromWI
I am sure you could find a combination of line and hitch cord that would meet your needs, but it would probably take a lot of trial and error.

With enough wraps and braids, I think some form of one of the "advanced friction hitches" would work. Since these types of hitches are so responsive, could Joe Climber break the holding friction in the hitch even under a 2000 lb. load?

 

[NickfromWI]

I think Joe Climber could break the hitch, if the hitch-cord/climbing line combination was right.

If you notice with a smaller hitch with less wraps (say a 5/16ths 3/1 shwabisch) you have to pull kinda hard to break that...to get it to slide. All the friction (while it's holding) is on few wraps. If you use a larger diameter hitch cord and more wraps, the friction is spread over a larger surface area, which ends of needing less of a yank to get you descending.

Maybe if you tied something like a 8 over 8 VT out of 12mm vectran, you might get close to that....but this is all speculation.

I see where you're getting that 2000lb number. Makes a bit of sense, but probably not accurate for this purpose. I wonder how hard it'd be to descend If I had another person's body-weight hanging from my saddle....

love
nick

 

[Bradley Ford]

Originally posted by RescueMan
In the cave rescue community, we've been experimenting with the Dog-'n-tails as a means (actually the ONLY means) of descending a loaded rope (a rope with a casualty hanging on it), as any mechanical rappel device requires slack below the device.

What criteria does the cave rescue community use to judge the Dog-'n-tails (or any other) hitch? Specifically, what load was used on the Dog-'n-tails hitch in these experiments?

 

[RescueMan]

Bradley Ford:

Arguably the most important parameter for choosing a hitch (it has to at least handle the fraction of your weight placed on it by your climbing system) is how much weight it can control in descent.

Actually, more force is exerted on a hitch in ascent because of the dymanic element of bouncing. It is not hard to place a 500 lb load on a hitch during ascent, depending on how smooth your technique is.

Ideally, my hitch would control the descent of much more than I'll ever need, too. 2000 lbs. was my arbitrary (actually 200 lb. Joe Climber mutilplied by 10), much more than I'll ever need, descending load

You're still missing the distinction between MBS (minimum breaking strength) for rope and SWL (safe working load) for hitches and ascent/descent devices. MBS is often 10 or 15 times the working load because of all the weakening potential of knots, tight radii, wear & tear, aging, and dynamic loading. SWL only has to be equal to or slightly greater than the load (your weight and the weight of equipment) that you expect to hang on it.

...could Joe Climber break the holding friction in the hitch even under a 2000 lb. load?

I haven't seen any test data on arborist hitches, but of all the rock climbing hitches, the prusik can control the most weight (as much as 2300 lbs in some applications), with a 4-wrap Bachman close behind, and the autoblock and kleimheist down at 400-600 lbs. But, because the prusik "bites" so well, it is also the most difficult to release under load (in fact all but impossible - which is why we have to use load-release hitches in series with them in rescue). So I don't believe that any hitch that could control 2000 lbs (lowering a major stem section?) could possibly be released under load. And then, the better the hitch "bites", the less suitable it is for gradual lowering or descent.

NickfromWI:

Maybe if you tied something like a 8 over 8 VT out of 12mm vectran

You keep touting technora and vectran, two of the new breed of "super synthetic" cordage. I'd be interested in knowing why?

In rescue, these fibers are generally verboten. Independent test results for all the high-strength cords, presented at the 2000 International Technical Rescue Symposium, showed that there is nothing better than good old nylon rope.

While Technora, an aramid like Kevlar but without the self-abrasion problem, is significantly stronger then nylon in tensile strength, Vectran, a liquid-crystal polymer, is actually a bit weaker. And, while nylon weakens only 8% with a figure-8 knot in it, Vecrtran and Technora weaken by 48% and 40% respectively - making them both significantly weaker than nylon once knotted. In a loop tied with a double fisherman's knot, none of the super-cords were as strong as nylon.

Anecdotally, there have been reports of sudden catastrophic failure of high-strength cords under shock loading, particularly the Kevlar cords which are used today only for fire-fighter's emergency escape lines because of their high melting point and are for single use only.

 

[NickfromWI]

Originally posted by RescueMan
You keep touting technora and vectran, two of the new breed of "super synthetic" cordage. I'd be interested in knowing why?

...while nylon weakens only 8% with a figure-8 knot in it, Vecrtran and Technora weaken by 48% and 40% respectively - making them both significantly weaker than nylon once knotted. In a loop tied with a double fisherman's knot, none of the super-cords were as strong as nylon.

RescueMan, I keep bringing up these lines because I want people to think about them and to think about the applications where they can be useful.

Using your numbers...

Yalex (which is polyester, not nylon...but they are comparable) has an ABS of 6,000lbs, at 3/8th" diameter. If you cut down 8% of that (which sounds very generous, if you ask me), you end up with a strength of about 5,520lbs.

Vectrus, a rope of almost identical construction has an ABS of 16,000lbs at the same size. Subtract the 48%, and you have a strength of about 8,320lbs.

So using your numbers, if breaking strength is your only concern, then high tech fibers still win.

Unfortunately, strength is not the only issue. One of the most important factors that plays in to the high tech lines losing so much strength in a knot is stretch. Nylon, as many know, is quite stretchy. Imagine 2 parallel strands. You lay the two strands parallel and tie them to something. Start pulling on the other end. Chances are, when you tied them, one of the strands ended up a hair shorter than the other.

When you start pulling, the shortest one will take the load first, then stretch until it is the same length of the longer strand. Now keep pulling and soon both strands will be sharing the load.

This is what happens in rope, but with more strands.

Now same scenario, but with Vectran. You tie, and pull. The shortest one takes the load...you keep pulling. It has such little stretch, it cannot elongate itself until the longer one can help out! Soon, it just breaks, leaving the longer strand to take the load, but it's the same load the just broke the first strand, so it, too, breaks!

This is what leads to the catastrophic failures you mention.

It needs to be remembered that these lines are not just STRONG ROPE. They are different and need to be treated as such. Splices are practically ALWAYS reccomended as the form of attachment, not knots. Proper splicing allows all strands to share the load, maximizing the useful strength of the rope.

Now, if we are talking about a vectran friction hitch...I use it! I have some that are knotted with a double fisherpersons knots and some that have spliced eyes. Either is fine. Even if it lost 75% of the strength, it'd still be stronger than what some people are using for hitch cord.

You raise a good point. That being that in the wrong hands, these high tech lines could be quite dangerous. Be careful what you buy and how you use it.

love
nick

 

[RescueMan]

Nick,

I just looked up the breaking strength of Vectrus, and it IS insanely strong. But that's braided rope and none of the testing I refered to was on arborist-type rope. It was on specific brands of kernmantle rope used in climbing and rescue, so the results are not transposable to the rope you're using.

If you're curious about the testing, go to:

Comparative Testing of High Strength Cord

Another caution about high-strength cords is that if you fall on them with any slack in the line, it's like falling tied to a steel cable. Because there's no stretch to absorb the force of the fall, a tumble on as little as 4' of slack can create enough force to fracture your spine.

- Robert

 

[Lumberjack]

Originally posted by RescueMan
Another caution about high-strength cords is that if you fall on them with any slack in the line, it's like falling tied to a steel cable. Because there's no stretch to absorb the force of the fall, a tumble on as little as 4' of slack can create enough force to fracture your spine.

- Robert

I never have 4' of slack in my rope.

But even if I did, how much does your tress cord really stretch in comparison to the many feet plus feet of rope that it is tied to?

IMO you could tie it with steel rope, and there wouldn't be any appreciatable difference.

The only diff. in material would be, IMO, their tendencies to slip at a givin weight.

IMHO

Carl

 

[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?

 

[Bradley Ford]

Originally posted by RescueMan
Actually, more force is exerted on a hitch in ascent because of the dymanic element of bouncing. It is not hard to place a 500 lb load on a hitch during ascent, depending on how smooth your technique is.

I do not think I bounce when I ascend, but maybe I don't understand what bouncing means. Please explain.
I still think it will be more difficult to find a hitch that controls descent of 2000 lbs. than one that will allow ascent for the same load (e.g., as you mentioned, 2000 lbs. could ascend with a prusik, but it could not descend with one).

Originally posted by RescueMan
SWL only has to be equal to or slightly greater than the load (your weight and the weight of equipment) that you expect to hang on it.

I do not think that a hitch that can only support my weight or a little more is safe. If I did "bounce" on such a hitch, wouldn't it fail? And if that was all that was required to be safe, then:

Originally posted by Eagle1
I asked that question because i do believe, and practice decending on a single line, with a hitch (Blake).

climbers of Eagle1's weight or less could safely use a Blake's hitch for descending a static rope.