> 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*
)).
