Safety parameters for descent hitches

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Originally posted by RescueMan


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

And here is the problem, there are almost infinite numbers of possible combinations of ropes, sizes, hitches and weather to be able to say such and such hitch reacts this way when loaded.
Perhaps the best way to figure it out is to find the combination you like to use, then test it.
 
Not to mention contaminants such as oils from your hands, dirt, etc.

Sounds like a cool project for MythBusters!
 
> I haven't seen any test data on arborist hitches,
> but of all the rock-climbing hitches,

Let's be careful here: what's "all"? --name names! E.g.,
was the 2-wrap Hedden H. tried? --how about a 3-wrap one?
(You can see that this is an infinite progression, going
well past practicality, yes, but not initially.) And how
about the series of reversed Half-Hitches, as shown and
briefly discussed on Gary Storrick's site?! Cf.
http://storrick.cnchost.com/VerticalDevicesPage/Ascender/AscenderKnots.html

The point is: there is much "information" about knots about that has the form
"is the strongest ..." which fails to describe the universe of knots that have
been considered--which might turn out to be few!

> the [P]rusik can control the most weight (as much as 2300 lbs
> in some applications), with a 4-wrap Bachman[n] close behind,
> and the autoblock and kleimheist down at 400-600 lbs.

How about using some friction hitches in sequence?
(But given the heat generation for a really big load such as 2000#, one might
want them to be of Vectran or even a more heat-resistant aramid!)

> You keep touting technora and vectran, ...
> 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.

Was polyester among the fibers tested? (no, not if you're thinking of Moyer)
Actually, in the marine world, there is much use of poly-combo/"Poly-DAC" ropes.
Among the various sorts are those in which the PS surrounds core PP fibers
(and arborists have some like this). I should think that some of these, which
have both flexibility but also some bit of *spring* might serve well in friction
hitches, where the spring would help on release. (But I've not played around
much w/them.)

> While Technora, an aramid like Kevlar but without the self-abrasion problem,
> is significantly stronger than nylon in tensile strength, Vectran, a
> liquid-crystal polymer, is actually a bit weaker.

Whoa! --no way, maybe not even after flexing (though that's an open issue).
It is simplistic to call the cords tested by Moyer "Technora", "Vectran",
"Kevlar/Spectra", "Spectra", because they are kernmantle constructions with
these hi-mod fibers limited to the core, which is a fraction of the rope size.
AND, the cord size was smaller than the nylon cord's size. So, you're comparing
the apple-y Sterling 7mm nylon kern (where core and sheath both provide strength!)
to the orange-y Sterling Nylon-sheathed Vectran (where core alone provides the
strength, unless it's slipping internally to load the sheath--something that
does happen and isn't always recognized by test reporters!). So that's about
a 6/32" Vectran (or less) vs. 11/32" nylon. Moreover, it's just ONE test.
(That said, a striking result was that the Sterling Vectran-cored cord broke
at less than its advertised loop strength (strength of a sling made by tying
ends w/Grapevine or Triple Fisherman's is of common relevance to climbers
--nevermind the nominal tensile of material)! I think that Sterling and other
climbing cord vendors are avoiding Vectran, now.)

(For a good overview of the history of synthetic cordage fibers, see the
impending book _The Outdoor Knots Book_ by Clyde Soles (Mountaineers Press),
due out ca. April.)

> And, while nylon weakens only 8% with a figure-8 knot in it,

Robert, you elsewhere sling about CMC's test data, also for nylon, so note
that their Fig.8s weaken at about double this figure!? (And the 70-80%
range for the knot seems a pretty common reading--though one never knows
1) how the knot is tied & loaded, and 2) how pure tensile was determined.
(Moyer, e.g., did his own testing, of 5 samples for everything; his images
of the actual Fig.8 knots--he tied top one way, bottom knot another--aren't
entirely clear, but seem to be of a non-symmetric form; his data, however,
are remarkably similar, with a std.dev. approx. 100#.))

> Vectran and Technora weaken by 48% and 40% respectively

These precise results are for one testing of a few samples of particular ropes.

Here, again, note that we're talking about core components of a cord; one
might conjecture that pure hi-mod will be weaker, yet. Indeed, in an article
for SAIL mag., Brion Toss reported testing of hi-mods in which the highest
figure of any knots--and this was for a hitch--was about 45%; the Bowline
got no higher than 27% or so. However, his report leaves many questions
(such as Why didn't you test a Dble.Bwl and a Fig.8 loopknot?); e.g., I
tried to get the slippage in a Bwl he asserted obtained at about 15% in
Vectrus (Yale), and with a (???? skinny!) "1/4-inch" rope and a 5-to-1
pulley, bouncing my 180#, I could see no threat of slippage!? YMMV ?
(5 x 180 = 900 = 25% of 4500, approx. nominal tensile) Brion's results
seem dubious in light of those by Moyer and others (and even some of the
laughable Practical Sailor copycat testing (Prac.Sailor Sept.2001) suggests
quite different results).

> Paolo Bavaresco in "Landscaper" magazine 2000, writes: ...
> Loading a braided polyester rope over 40% of breaking strength
> will result in permanent stretch (damage)."

This is contradicted by an Oil Companies Int. Marine Forum (OCIMF) break-test
method which stipulates that a rope be cycled ten times to 50% of its
estimated breaking strength before stretching it to rupture--the cycling is
done to ensure that fibers are settled, as a sort of acceleration of the
normal in-use conditioning. This process tends to reduce data scatter
(as opposed to testing uncycled new rope). [Cordage Institute tech.man.]

> And here is the problem, there are almost infinite numbers of possible
> combinations of ropes, sizes, hitches [and how they are drawn & set!]
> and weather to be able to say such and such hitch reacts this way when loaded.

This is a key point. But, it might be that some careful analysis of hitches and
close variations, and the slight or greater changes in behavior, can reveal some
helpful principles. E.g., Heinz Prohaska, (1st) originater of the ProhKlem Hitch,
aka (Jason) Blake's Hitch (HP pub'd in '81 & '90; Blake in '94), advises to add
an extra lower wrap to improve gripping in stiff Prusik rope if it otherwise slips,
and to add a turn at the top end (which turns around only the object rope) if the
slippage is attributable to load alone. Given some guidance by principle, then
perhaps the vast array of possible test cases could be traversed by intelligent
sampling (doing fuller testing here or there only when sample results didn't
fall within expected ranges). (This is a problem for more than friction hitches.)

Knotting seems fertile ground for myths.
I hope to cut through a lot of them.

Cheers,
knudeNoggin*
 
Myths?

Knotting seems fertile ground for myths. I hope to cut through a lot of them.

knudeNoggin,

Cutting down other's research is easy. Building up useful information and data on rope and knot characteristics is a much larger challenge, and your lengthy obfuscation doesn't contribute to that.

All test data has a limited scope and applicability. Good researchers (such as Moyer) indicate those limitations and offer their results as pieces of a larger puzzle.

Taking the puzzle apart and scattering the pieces doesn't help anyone's understanding.

How about contributing something positive to the discussion, rather than just tearing down what you consider to be "myths".

- Robert
 
> How about contributing something positive to the discussion,
> rather than just tearing down what you consider to be "myths".

RescueMan, save us--toss us a line! What in the world are you saying?
My msg. was pretty clear, and, to my thinking and I think others' too,
a contribution that shed light on this topic; so, you'll have to help us
understand how you see it as "obfuscation"!?

> Cutting down other's research is easy. ...
> Taking the puzzle apart and scattering the pieces ...

Huh? First of all, research is intended to be examined and, if found
wanting, cut down (like a bad tree that might otherwise lead to injury).
And if it were so "easy", there'd be less bad research & other nonsense
published (we should hope). Rather, it seems that most people are lazy
and willing to accept conclusions w/o scrutiny. In the realm of knots
and cordage, this seems especially true. Otherwise, how could something
like the Practical Sailor issue (Sept'01) stand the light of day? How
could knot books repeatedly put forth the things they do--errors not
only not corrected, but copied!?

You made some quite particular assertions about materials & knots,
some of which were clearly wrong (Vectran weaker than nylon), and
others that needed some perspective (Fig.8 loopknot weakens nylon
by (only) 8%). So, I contributed information to improve understanding
(such as CMC's test results showing the Fig.8 reducing by 18%, and I
might add the Lyon report (of three static & one dynamic kernmantles)
range of 23..34%. How is this obfuscation? --or scattering of pieces?
Perhaps in that neither CMC nor Lyon tested hi-mod ropes, to have
their take on that (maybe they'd have shown reductions more severe
than did Moyer?!) Well, I did cite the Sail issue's test report on the
quite severe results, though that article was less than overwhelming
with data. But a key point was to see that one can find quite different
figures out there, and just pulling out one that seems to suit you is
less than playing fair (in your challenge to Nick's championing hi-mod
stuff (though he has in other situations taken the low-tech side :eek:)).

As for "cutting down", removing deadwood IS positive. Consider the
Lyon report (www.hse.gov.uk/research/crr_htm/2001/crr01364.htm)
and its assertion/conclusion about the relative strength of the
Overhand loop's two possible loadings; Lyon says "In all cases,
failure occurs ... wher the loaded rope first rounds the loop.
Whether it rounds the loop above or below the loose end can
affect strength by up to 10%. In the overhand knot, it is stronger
if the working rope lies above the rope end." We can wonder what
"above"/"below" mean, but a natural interpretation would be that
if the loaded part lies above the end, it bears against it (rather
than pulls away from it). We can also remark at "by up to 10%":
the natural interpretation in this context is "10 percentage points"
--something easily seen in result figures (as opposed to 66% vs.
60%, it would be 66 vs. 56). In any case, in the appendix with
test data, Lyon's wording is "live rope on {top/bottom} as it
entered the knot"--which I'd take to equate "top" with "above",
etc.. But in that case, their data if anything give a contrary
conclusion, with "on top" cases having a slight edge. Here's
their results, converted to percentages; they tested the loops
with knots on both ends, two specimens having the same of
whichever orientation (on top or on bottom), and the middle
case one of each(!). For the 3 low-elongation & 1 dynamic
ropes they got:
66-69-68 ; 64-60-66 ; 59-59-62 ; 59-59-63
Now, should we let this stand w/o question? I don't see
either the bias for the stated orientation (but there is some
dubious description in terms), and certainly don't see the
"10%" in any sure way! And with so few test cases, there
really isn't good ground to stand on in drawing conclusions.

knudeNoggin
 
Knud,

Can you direct me to more information about this:

E.g., Heinz Prohaska, (1st) originater of the ProhKlem Hitch,
aka (Jason) Blake's Hitch (HP pub'd in '81 & '90; Blake in '94), advises to add
an extra lower wrap to improve gripping in stiff Prusik rope if it otherwise slips,
and to add a turn at the top end (which turns around only the object rope) if the
slippage is attributable to load alone.

If you want to email me off forum that would be great.

Thanks,

Tom
 
Based on feedback from Mike Maas and RescueMan, a safe hitch should slip (and then grab when the load is sufficiently reduced) at some shock load to help prevent injury. Can this shock load be defined relative to Joe Climber's weight? For example, is it reasonable to suggest that if, after all load absorbing effects have occurred, the remaining load is (this percentage is just made up) 150% of Joe Climber's weight, his hitch should slip to prevent Joe from absorbing any more of the shock load? If so, what is a safe percentage?
 
I would say 3x's the weight of the climber, possibly 4. At only 1.5x's his weight, the hitch would slip too much, IMO. 3-4g's is a good number IMO, alowing some stiff stops (repeling, or parachuting to a spot on the tree) but it isnt enought to hurt the climber.
 
The "limit of survivability" for a human body is considered to be 12 kiloNewtons (2650 lbs. force). Dynamic ropes are required to limit maximum shock loads to 12 kN, and long falls on dynamic rope generally do not generate more than 9 kN (about 2000 lbs).

The actual shock load on the climber depends on the fall factor (the distance fallen divided by the amount of rope available to absorb the shock), the modulus of elasticity of the rope, and the amount of intermediate friction or dyamic slippage in the system.

Just bouncing on a fixed rope can create 2g's of force (double the climber's weight). A fall factor of 1 (e.g. falling 5 ft on 5 ft of rope) on static rope can generate 15 g's of force.

I'd agree with Lumberjack that a hitch should be able to hold 3-4 times the climber's weight if it's going to absorb the usual bouncing of ascending. And that would still be well within a safe zone for shock load to the climber.

- Robert
 
A safe hitch should start to "slip and grab" when loaded with a 4g shock load. So for 200 lb. Joe Climber, a safe hitch slips at 800 lbs., but for 50 lb. Joe Jr., a safe hitch slips at 200 lbs.

Originally posted by Mike Maas
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. :)

If a safe hitch is dependent on the climber's weight, will JPS and Kenny require different hitches in order to both be safe? Or will the same set up slip at 4g's for both 350 lbs. (at 1400 lbs.) and 90 lbs. (at 360 lbs.) due to the actual load difference?
 
Originally posted by Bradley Ford
A safe hitch should start to "slip and grab" when loaded with a 4g shock load. So for 200 lb. Joe Climber, a safe hitch slips at 800 lbs., but for 50 lb. Joe Jr., a safe hitch slips at 200 lbs.

If a safe hitch is dependent on the climber's weight, will JPS and Kenny require different hitches in order to both be safe? Or will the same set up slip at 4g's for both 350 lbs. (at 1400 lbs.) and 90 lbs. (at 360 lbs.) due to the actual load difference?

I would doubt it. I would require a different number of wraps and crosses (VT and such). The added weight would make some difference in the amount it grabbed, but not enough to take the climbers weight, so a variation would be required.
 
Originally posted by Mike Maas
.....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. :)...

Jest when ya think ya squeaked by, i'm catching up on mah'reading!! After the line about thighs as huge as my waist, m'Lady and i been wondering about an ol'pair of his Levi's for a dual sleeping (mostly) bag......

Fantastic thread, surely a "5" on the ratings!! One view might be to inquire why someone can ride a friction hitch on a DdRT; yet it jams on someone half the original climber's weight on an SRT.

Part of the stabilization of a setup over differnet weight ranges might be in the stabilization of speed in 2 falling logs of different weight, yet hitting the ground at the same time. The heavier one hits harder, because it has more raw force potential; but yet does not acces that to speed up. IMLHO, that same weight that gives it the heavier log/body more force; also takes more force to move/change speed; so the 2 items bound as principals (more force yet more reluctant to change)invoked by the weight cancel each other out (assuming same shape/air resistance, wind etc.); and the 2 items fall at the same speed (?)

i think that the most clear point is that there are a lot of things going on, in directions and views that surprise most all and have lead to lots of thinking:eek: ! Just more reasons for keeping the requirements for strengths etc. high; just bumped up some to cover everything etc. In the same spirit as they have been handed down to us. Knowing that whatever is said and written; variations, mistakes etc.; and someone outrightly just going with less than is said everytime. The guidelines set should cover all these things and still have safe ranges i think.

The fancy fibers and their stiffness etc. have yet to prove themselves to me; many i think are weak on the bend. Like some super carbon fiber threads that can hold tons, yet can be snapped easily by hand once tied in a Knot . But, then i don't burn out chords and tails descending...............

RescueRob (et al)- How much more strength do you quote on a double bowline w/Y tie off? And on hitches whose strength would be dependant on host item; when that is a carabiner; does that tilt things towards 8's and bowlines causing less interuption in the line? What of (i guess ) the "Stevedore" and + of an '8' brought to a '9' or '10' (?) strengthwise; and the strength loss in arching the wrong way with that strategy?

Enquiring Minds Knead to Know!
:alien:
 
Originally posted by Bradley Ford
A safe hitch should start to "slip and grab" when loaded with a 4g shock load.
Has anyone tested their climbing system for this functionality? How did you do it?
 
Do all hitches slip in response to a shock load? Do the Tautline and Blake's hitches slip or do they just grab? Do all of the French Prusik family hitches slip?

Can all hitches be categorized into either "slips and grabs" or "just grabs" in response to a shock load, or are there other responses to consider? (Careful with those "just slips" hitches.)

Is a properly configured "slips and grabs" hitch inherently safer than a properly configured "just grabs" hitch? Or is it vice-versa?
 
Do all hitches slip in response to a shock load?

All hitches slip, not just in response to a shock load but whenever the load exceeds the friction of the hitch on the host line. And the friction of any hitch on a host line depends on the composition, the stiffness, the texture, the size, and the surface condition (new/old, wet/dry/frozen, coated/uncoated, etc.) of both the hitch cord and the host line.

The results of slippage depends on the extent and the speed of the slippage. The sudden dramatic slippage from a shock load can cause melting of the hitch cord (the host rope is moving through the hitch and so does not overheat at any one point) and consequent fusing to the host or possible failure of the hitch.

The friction capacity of any hitch/host combination will vary from the low of a Munter hitch (about 600 lbs. force) to the high of a 3-wrap prusik (up to 2400 lbs. force).

I suspect that most of the arborist climbing hitches would have friction capacities toward the lower end of that spectrum.

- Robert
 
Another Correction

knudeNoggin,

That link shows an interesting drop test, the only one I've ever seen that included the dog-n-tails (similar to the VT hitch but with just braids).

However (and here's the editor in me again), there's a typo on the picture for drop test #10 which indicates "wet" when the chart correctly identifies that test as "dry".

I emailed Corvallis Mountain Rescue a while ago to find out which condition was correct. They said the chart is the one to go by.

- Robert
 

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