Discussion Topic |
|
This thread has been locked |
rgold
Trad climber
Poughkeepsie, NY
|
|
|
Apr 21, 2008 - 03:35pm PT
|
Del X wrote "There have been a few of what I called "hints" that there may be more to the story. John Yates wrote about extra energy absorption, although his description was muddled. John Middendorf attested to this extra absorption as well in an earlier ST thread. And Craig Luebben described tests in his ice climbing book where measurements did not match those predicted by simple theory. Did these guys all get it wrong?
I'd be prepared to say that Yates got the explanations for extra energy absorbtion wrong, and I have neither seen or heard informally about any test that comes anywhere near confirming the numbers he has posted. Moreover, his observation that more energy was absorbed than predicted was made in a vacuum devoid of any account of the predictive theory, the protocols for obtaining data, and the data itself, so there is no way to form an independent judgement about the existence of this extra absorbtion from anything Yates said. I don't know about the other two; are they in part or wholly repeating Yates? Do they have data other than what Yates has on his website, which does not even appear to be experimental data? When Luebben mentions "the simple theory" that is not matched, does he also actually give an account of that theory? Does the failure to match whatever simple theory Leubben is using occur in the direction of additional unexplained energy absorbtion?
Del X wrote: "Rope slippage is another issue. I've read that a tube style belay device will slip at 2-3 kN (Petzl's fall simulator assumes 2 kN). Is this realistic?
As what? An average over all climbers? If so, I'm betting on a very large standard deviation.
"If it is and if the rope isn't redirected through the power point, then in a factor 2 fall the force on the entire anchor is limited to 2-3 kN. So why bother with the questionable virtue of screamers in the anchor? Why not just wear a belay glove instead?"
Indeed! But remember that the screamers were, at least originally, proposed by Karl Baba as much for their equalization potential as for their load-limiting contribution.
And if rope slippage really does keep the anchor load down to 2 kN, then one does have to wonder how, for example in the Middle Cathedral accident, a 2-3 kN force managed to extract a four-piece anchor constructed by experienced climbers.
---------------------------------------
Footnote added in edit:
Theoretical calculations suggesting that screamers will be of marginal utility for reducing maximum impact to protection are confirmed by tests performed by the Italian Alpine Club. A translated page can be found at http://home.pacbell.net/takasper/ital_screamer_test.htm.
|
|
rgold
Trad climber
Poughkeepsie, NY
|
|
|
Apr 23, 2008 - 02:02pm PT
|
Del, thanks for reminding me about the thread entitled "help with some math please." I have indeed read Middendorf's comments and replied to them there. He too seems to refer to tests he did a long time ago that indicated that screamers did absorb some "significant" amount of fall energy. He grants that this isn't actually possible, and then searches for explanations, like arresting the fall over a longer period of time, that don't agree with basic physics.
Luebben's account is interesting. He reports taking factor-1 falls on a climbing wall, which should have resulted in him hitting the floor every time. Maybe the belayer was up on a balcony? He says the belayer was anchored to "the ground." Even if they did get the belayer up off the ground, was Leubben really willing to lend his body to science by taking that big a fall? I'm impressed.
He also reports on drop tests with weights. His findings appear to say that screamers performed as predicted (i.e. negligible advantage) in the drop tests with weights but, when real climbing falls are taken by real people with real belayers, peak loads were reduced more than any theoretical analysis of screamers load-absorbing capacities would predict. He takes this extra load reduction to be the "real" reality and does not discuss reasons for the discrepancies between theory and his observations.
So we seem to be left with at least some of the original confusion (or, as you said, "interest. The most obvious explanation is that the deploying screamer changed the belayer behavior in some way. Other side-effects are possible, such as the rope stiffening after multiple falls and then causing the belayer to give a much more dynamic belay, resulting in a lowering of the anchor impacts.
The amount of the reported reductions, near or beyond 50%, seems really too good to be true. On the other hand, if memory serves, it is consistent with Yates' claims (which were, however, made, as far as one can tell, with weights not humans).
|
|
deuce4
climber
Hobart, Australia
|
|
|
Apr 26, 2008 - 03:37pm PT
|
rgold-
Haven't really studied this entire thread, just noted that I was being quoted from the old days of slide rules ;) I'll be happy to hear when someone does the full analysis, and finally solves the mystery, of screamers.
My engineering guess on bar-tack screamer type devices is NOT that they themselves absorb a significant amount of energy, but that they allow the rope (and other energy absorbing elements in the system, ie. body, knots, friction, etc.) to do so.
Thus, my hypothesis:
(1) The use of screamers with a dynamic rope system can result in a longer absorption period of the fall energy.
(2) With a longer absorption period, the peak forces can be reduced (energy under the curve type-of-thing).
(3) Therefore, the use of screamers can reduce peak forces.
Now, you state: "arresting the fall over a longer period of time, that don't agree with basic physics." and of course you may be right, but I do recall seeing John Bouchard's tests on bartacked screamer type devices way back when and it did seem that the force loading was extended in time when the screamer was utilized. The force loading looked like this:
(lower peak, multiple peaks, longer time)
---00----------00----------00
-00---00---00---00---00---00
00-------00--------00----------00
rather than this:
(higher peak, single peak, shorter time)
---------0000--
-----000-------000
---00--------------00
00-------------------00
In both cases the area (energy) under the Force/Time curves is equal.
|
|
rgold
Trad climber
Poughkeepsie, NY
|
|
|
Apr 30, 2008 - 03:56pm PT
|
John wrote: "In both cases the area (energy) under the Force/Time curves is equal."
The area under the force-time graph is often called the impulse. It represents the net change in momentum, not energy absorbed.
The mechanism for energy absorbtion in an idealized rope is just the work done in stretching it, and this quantity depends on elongation but not on the time required to realize the elongation. To the extent that a screamer reduces anchor loads, the rope stretches less, so it is difficult to see how giving the rope "extra time" increases the energy it absorbs when the net result is less energy-absorbing behavior.
Of course, the answer to all this is that a rope is not an idealized rope, and Del has mentioned some hypotheses that could conceivably explain the results of those tests that do not find a negligible screamer effect. But there is also the possibility that the tests are either flawed in some way or, because of the small number of trials, have mistaken some aspect of systemic variation for an actual effect.
|
|
deuce4
climber
Hobart, Australia
|
|
My bad-
I was thinking force x time = work, thus...
It's been too long since I did any real engineering.
Ask me about Geodetic Datums, I think I could probably be more reliable.
Hope you guys figure it out.
cheers
http://www.lynxgeos.com
ps: still, my design intuition, which often serves me well, tells me that it has something to do with the extended TIME of absorbing the fall energy with a screamer doohickie in the system. Perhaps it has something to do with a variation of the role of other energy absorbing components of the system...?
|
|
Karl Baba
Trad climber
Yosemite, Ca
|
|
|
May 22, 2008 - 07:58pm PT
|
I wish a got a massage!
Are you saying she's a screamer?
I don't get it
PEace
karl
|
|
|
SuperTopo on the Web
|
Let us know!