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nafod
May 26, 2011, 11:56 PM
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For the techie wonks, some reading material.
http://etda.libraries.psu.edu/theses/approved/WorldWideIndex/ETD-4491/index.html
In reply to: This thesis describes an analytical and experimental investigation into the energy absorption performance of textile-based load limiting devices. The primary technology considered is the stitch ripping device (SRD). Development of analytical models for predicting SRD behavior is presented. The models account for energy absorption due to webbing stretch, thread rupture and stitch slippage. The models predict the force-displacement behavior of the device, the total amount of energy absorption, and the amount of absorption due to frictional losses from stitch slip. Experimental testing conducted to validate model results exhibits close agreement for the applied load and displacement required to induce thread rupture. Over multiple stitch breaks, the models slightly over estimate the amount of absorbed energy. It is predicted that a lower amount of stitch slip decreases activation force as well as fluctuation of force in the plateau region of the force-displacement curve. Decreasing slip increases the amount of energy that can be absorbed for a given peak force and stroke distance. In addition, the effectiveness of adhesive treatments in preventing stitch slippage and pull-out, and the role of stitch pattern and stitch density on energy absorption are examined. Results indicate that certain adhesive treatments increase total energy absorption and reduce stitch pull-out. However, decreases in specific energy absorption result due to increases in total mass from the addition of adhesive. Hand-stitched thread tests show that altering stitch pattern and reducing pull-out increase rip force and energy absorption by 23% and 16%, respectively. Finally, to assess the performance of SRDs under simulated crash conditions, an instrumented dynamic drop rig was constructed. Tests were conducted with a drop sled moving at free-fall velocity, and a higher, elastic-cord-assisted velocity. For velocities from quasi-static to 6 m/s, no significant trends or changes in SRD average activation force and energy absorption were observed in the experiments.
Also related...
http://etda.libraries.psu.edu/theses/approved/WorldWideIndex/ETD-5690/index.html
(This post was edited by nafod on May 26, 2011, 11:58 PM)
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Rudmin
May 27, 2011, 4:32 AM
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Huh, I thought the idea wasn't to absorb energy through stitching, but to reduce peak forces by delaying the rope stretch at the right moment.
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JimTitt
May 27, 2011, 5:48 AM
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No.
To reduce the peak force you want to start stretching the rope as soon as possible. You can start slowing the faller immediately such as when top-roping or wait until the faller has achieved terminal velocity and then try to stop them, which do you think is better?
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nafod
May 27, 2011, 1:16 PM
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The ideal system for catching a fall while putting a cap on the peak force the anchor experiences is, oddly enough, a static line rope used with a screamer. The elastic aspect of a dynamic rope stores energy and allows you to fall farther than you would otherwise. The rope also absorbs lots of energy, but not as efficiently as a screamer.
Of course you're not going to use a screamer at every single anchor point, but it is an interesting piece of info to store away for a rainy day situation.
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patto
May 27, 2011, 7:03 PM
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nafod wrote: The ideal system for catching a fall while putting a cap on the peak force the anchor experiences is, oddly enough, a static line rope used with a screamer. The elastic aspect of a dynamic rope stores energy and allows you to fall farther than you would otherwise. The rope also absorbs lots of energy, but not as efficiently as a screamer. Of course you're not going to use a screamer at every single anchor point, but it is an interesting piece of info to store away for a rainy day situation.
No really. That IS NOT the ideal system. What do you think happens when the screamer gets fully deployed? THIS IS DANGEROUS.
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patto
May 27, 2011, 7:07 PM
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To my knowledge there not a whole lot of evidence, research or otherwise that screamers are an effective tool to protect marginal placements.
Their energy absorbing capacity is very small compared to the climbing rope. Thus their benefit is debatable.
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nafod
May 27, 2011, 8:35 PM
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patto wrote: nafod wrote: The ideal system for catching a fall while putting a cap on the peak force the anchor experiences is, oddly enough, a static line rope used with a screamer. The elastic aspect of a dynamic rope stores energy and allows you to fall farther than you would otherwise. The rope also absorbs lots of energy, but not as efficiently as a screamer. Of course you're not going to use a screamer at every single anchor point, but it is an interesting piece of info to store away for a rainy day situation. No really. That IS NOT the ideal system. What do you think happens when the screamer gets fully deployed? THIS IS DANGEROUS.
I' m presuming you have enough screamers to absorb the total energy of the fall.
It' s an academic thing, I know. But if you want or need to minimize the distance fallen, that is the configuration that will allow you to do it. Stack the SRDs in series to increase total energy absorbed. Stack them in parallel to increase activation force.
For example, if you took a factor 2 fall on a 60 meter rope (yikes) you'd expect to stretch it about 20 meters at max bottom, and see 12 kN at the anchor. If you used a static line and 12kN activating SRDs, you only go 10 meters extra rock bottom. Of course you'd need 10 meters of total screamer length. Again, an academic exercise.
Not completely, though. Climbing pard' took a FF1 fall back in 97, and although he was decelerating once catching the rope he was still moving fast enough when he landed that he broke both his legs, one an open tib-fib fracture. This, two pitches up on Seneca's south pillar. Epic self-rescue ensued.
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nafod
May 27, 2011, 9:05 PM
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The rope works both elastically (spring) and inelastically (damper). If it were a pure spring and ignoring all the other ways energy gets dissipated like friction, you'd bounce up and down until the sun set. There is also internal friction in the rope so you don't bounce as much, and the energy is dissipated some internally, but it is not ideal. The elastic aspect of a rope stores up energy, not dissipates it.
The thing about a spring is you don't see the peak tension in the rope (and therefore peak force on an anchor) until the rope has stretched as much as possible. With a screamer set to the anchor's max loading coupled with a static line, you see that peak force immediately and it stays at that level until you stop moving. Because of that, you can stop far sooner for the same maximum peak allowed force.
We demonstrated this experimentally for tiedown systems for cargo in aircraft, designed to deal with a crash. We showed that SRDs in line with steel chain was better performing than SRDs in line with partially elastic nylon webbing. Since it was designed for a crash, we weren't worried about the one-shot nature of the rig like you are with a climbing rope, where you keep falling on it over and over. Can't do that with screamers obviously.
Maybe velcro screamers? 
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rgold
May 29, 2011, 5:29 PM
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A very interesting paper, Nafod, thanks for the link.
As Nafod says, this is all academic, but the idea that the screamer is the most efficient way to absorb the energy associated with a particular peak load assumes that the peak load is known in advance.
Put a 12 kN activated screamer on a static line, take a short fall that would only generate a few kN with a rope, and instead you will get a load and an impact to the body far in excess of what the rope would have provided, since the screamer doesn't activate at all and your fall is held by the static line. Ouch.
In fact, it isn't realy the peak load you have to know in advance, rather, it is the length of the fall. A rope provides "scalable" energy absorbtion---this is codified in the fall-factor---but a screamer is can only absorb a fixed amount of energy (roughly the activation load times the tear length). So it's not as if any one 12 kN screamer would work for for any fall-factor two fall. If the potential energy involved in the fall is greater than what the screamer can absorb, then you are back at the mercy of your static line. Ouch again.
The most interesting experimental results about screamers in the the climbing context are the drop tests done by the CAI that suggest that screamers are not effective in reducing peak loads when their full deployment is not enough by itself to absorb all fall energy. In other words, if the screamer fully deploys and then the rope has to stretch further to stop the fall, there appears to be little benefit to having had the screamer present at all.
This is obvious without experiment in the case of long falls, because the fraction of the fall's potential energy the screamer can absorb is small and so the majority of energy absorbtion still falls to the rope. But the CAI tests suggest that even in more moderate falls the screamer is not much help.
I might add that I have never seen or heard of any test, anywhere, that even remotely replicates the peak load reductions claimed on the Yates site.
Of course, the literature is full of claims of screamers deploying and keeping all kinds of marginal protection from pulling. But, for obvious reasons, none of these experiences includes a test of exactly the same gear without the screamer, so no valid conclusions are possible from any of these anecdotes.
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JimTitt
May 29, 2011, 7:00 PM
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Summed it up nicely.
The problem with introducing a screamer into the system and then overloading it (ripping it completely) when working with conventional belaying devices is that the screamer `stealsīsome of the benefits of slip in the device and eventually increases the impact force as the CAI describe.
However screamers have their place, in aid climbing possibly but primarily they are a cool, trendy and utterly harmless item for climbers to buy and replace! 'There are reasons why the industry, the UIAA and CEnorm have never introduced a standard for this obviously "important" safety item whereas the performance for screamers in fall arrest/via ferrata applications are strictly controlled.
Jim
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patto
May 29, 2011, 9:58 PM
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Nafod's reply and explanation of operating a static rope with individual screamers did make me stop and think for a second. It certainly is an interesting theoretical tidbit but hardly relevant or practical for climbing. It could also be noted that you don't even need to place the screamer devices on the pro, just place them in line with the rope.
One thing this academical tidbit doesn't account for is the importance of a smooth increase in force. A sharp edge acceleration profile is less tolerable to the human body that a smooth increase.
Anyway the study does add to the body of knowledge surrounding energy absorbers but the relevance to climbing is mostly academic.
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healyje
Jul 7, 2011, 9:14 AM
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patto wrote: To my knowledge there not a whole lot of evidence, research or otherwise that screamers are an effective tool to protect marginal placements. Their energy absorbing capacity is very small compared to the climbing rope. Thus their benefit is debatable.
I can personally testify to their benefit in marginal free climbing protection. I've gone through dozens of them in free climbing falls in the kind of Edisonian research that counts. I've ripped them partially and completely. I razor blade them down at a varying angles to adjust their initial loading to suit the placements and sport tape partially blown ones back together for multiple goes. They are absolutely key to marginal pro. I've taken many multiple dives onto nests of RPs / Crack N Ups / #1-3 Loweballs that would never hold a fall otherwise.
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patto
Jul 7, 2011, 9:55 AM
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That is hardly a representative test. For christ sake there was only 2.74m in the system and a FIXED non movable belay. This is a HIGHLY unrealistic situations with very little energy in the system and a static belay. This sort of scenario completely exaggerates the benefits of screamers.
healyje wrote: I can personally testify to their benefit in marginal free climbing protection. I've gone through dozens of them in free climbing falls in the kind of Edisonian research that counts. I've ripped them partially and completely. I razor blade them down at a varying angles to adjust their initial loading to suit the placements and sport tape partially blown ones back together for multiple goes. They are absolutely key to marginal pro. I've taken many multiple dives onto nests of RPs / Crack N Ups / #1-3 Loweballs that would never hold a fall otherwise.
None of that is evidence of the screamers being effective. Climbers at Arapiles, the birthplace of RPs, regularly fall on RPs without screamers. I've seen plenty of small RPs hold falls with no screamers.
Ball nuts are rated quite highly and don't need screamers. #2 & #3 are rated at 8kN so are pretty damn strong.
(This post was edited by patto on Jul 7, 2011, 9:55 AM)
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healyje
Jul 7, 2011, 4:02 PM
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patto wrote: healyje wrote: I can personally testify to their benefit in marginal free climbing protection. I've gone through dozens of them in free climbing falls in the kind of Edisonian research that counts. I've ripped them partially and completely. I razor blade them down at a varying angles to adjust their initial loading to suit the placements and sport tape partially blown ones back together for multiple goes. They are absolutely key to marginal pro. I've taken many multiple dives onto nests of RPs / Crack N Ups / #1-3 Loweballs that would never hold a fall otherwise. None of that is evidence of the screamers being effective. Climbers at Arapiles, the birthplace of RPs, regularly fall on RPs without screamers. I've seen plenty of small RPs hold falls with no screamers. Ball nuts are rated quite highly and don't need screamers. #2 & #3 are rated at 8kN so are pretty damn strong.
The pieces in all cases were capable, the placements however were not. You don't have to believe me, but I can assure you none of those placements would have held without the screamers. I know the difference because I've fallen on and pulled all those pieces without screamers which is why I started using them.
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patto
Jul 7, 2011, 6:26 PM
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We'll have to agree to disagree then....
Its not that I distrust you or your experience it is just that I trust science more.
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Rudmin
Jul 7, 2011, 6:29 PM
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patto wrote: We'll have to agree to disagree then.... Its not that I distrust you or your experience it is just that I trust science more.
I'll bet that you don't work in academia.
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healyje
Jul 7, 2011, 6:42 PM
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patto wrote: We'll have to agree to disagree then.... Its not that I distrust you or your experience it is just that I trust science more.
Cool with me, but climbing isn't about 'science' and science is both worthless and pointless once you leave the ground.
Climbing has never relied on science and I personally hope it never will - it has always taken an Edisonian approach to sorting out what works and what doesn't.
I'll stack my direct experience blowing lots of marginal pro and several dozens of screamers against anyone's 'science'.
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JimTitt
Jul 7, 2011, 6:58 PM
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patto wrote: That is hardly a representative test. For christ sake there was only 2.74m in the system and a FIXED non movable belay. This is a HIGHLY unrealistic situations with very little energy in the system and a static belay. This sort of scenario completely exaggerates the benefits of screamers. In reply to: What, you mean you arenīt impressed that having removed the one item usually used which would have drastically reduced the impact force an industry professionals estimate of the screamers efficiency was completely haywire and so he had to re-jig the test to give it at least a chance of respectability. Shame on you! (Interestingly the second test to get a moderately respectable result which used a fall from a foot above the gear, that is nipple height, matches quite well the results one of Europes leading manufacturers told me, "if you could grab the gear a screamer will help, otherwise your f*cked"). The real question is not why doesnīt one of the world leading gear manufacturers make screamers themselves, nor why donīt they at least have a database of data. The real question is why is a representitive of the worlds leading and best piton manufacturer looking down at a bent and nasty pin! Jim
(This post was edited by JimTitt on Jul 7, 2011, 7:15 PM)
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healyje
Jul 7, 2011, 7:12 PM
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In reply to: xxxx are rated quite highly and don't need screamers
I should again point out almost no gear requires screamers in perfect placements, in solid rock. All my uses of screamers have been on placements which were marginal in some way or another.
The quality of a piece of protection isn't governed by the rating of the pro, but by that rating in combination with the quality of the rock, the suitability of the geometry to the pro and, in 'nests' of small pro, the relationship of the pieces to one another and any method of equalizing them if employed.
I'm always interested to hear the opinions of people who do testing, but 'testing' is no substitute for direct experience on rock in real climbing situations. This is very similar to the 'testing' done around belaying (both hip and device) that comes to conclusions squarely at odds with direct climbing experience. I'm definitely a science, engineering, and technology aficionado, but have no problems calling bullshit on climbing-related testing when the results are so removed from experience as to bring the methodology or conclusions into question.
And in this case I consider any folks conclusions on the lack of benefits of screamers as clueless. I will concede that off-the-shelf screamers without pre-slicing will have load curves that are unhelpful in some applications, but then I rarely if ever use them as sold.
(This post was edited by healyje on Jul 7, 2011, 7:56 PM)
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JimTitt
Jul 7, 2011, 7:54 PM
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Well, as you know Iīm probably as much a climbing realist as the next man and a lot of theory doesnīt match up with our daily experience which is partly my job, sorting the wheat from the chaff.
With screamers there are three camps amongst the manufacturers. There are those who promote them as the answer to everything and make claims which even the most deluded find a bit dubious. Then there are those who hum and haw around the benefits with plenty of "cans" and "mays" (the Petzl website is a masterpiece for this). Then there are the ones who never got them to work, got pissed off with the whole business and donīt make them, who are incidentally in the majority.
From my chats with the technicians in industry the most positive thing you will hear is "they might help" which is not exactly a positive endorsement. Some, who have spent a long time trying different sewing patterns and cold days on the test rigs are less charitable.
Since there is no UIAA/CE test for these things all we can fall back on is our intuition, a bit of private testing, some back of the envelope calculations, information leakage from the industry and the Italian testing which was about as complete and credible as we can hope for at this stage. From all these the manufacturers claims donīt come out well and some braver souls such as myself would be prepared to brand them as hype or worse to sell an effectively worthless product.
Since the issue has been open for many years and no manufacturer has ever published any verified test results from an independent test laboratory to clearly establish where the undeniable benefits of screamers stop and the undeniable negative aspects start then I feel the public can say or do what they think and so they should.
Time perhaps for one of the manufacturers to step up to the plate?
Jim
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healyje
Jul 7, 2011, 8:05 PM
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JimTitt wrote: Time perhaps for one of the manufacturers to step up to the plate?
Maybe to better inform themselves on better designs. I specifically think they need much more gradual loading curves. Right now the stock initial loading on all of them is still way too high for falls onto really small or marginal gear.
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JimTitt
Jul 7, 2011, 9:38 PM
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This is in fact the point, for each impact scenario there will be a perfect screamer. It is obvious that a screamer set to say 1,5kN is going to be good for a short fall on your marginal pieces but it will have to be hundreds of metres long to catch that giant whipper.
As I have said before, for aid falls a screamer with the right characteristic will be a good thing and industrial fall arrest system work within the limits they require but with much higher trigger force, as do via ferrata systems. But mix them about and you are deep in the s*hit which is not what we are being told and we certainly arenīt seeing the information on when the negatives outweigh the positives.
Jim
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healyje
Jul 7, 2011, 9:47 PM
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JimTitt wrote: It is obvious that a screamer set to say 1,5kN is going to be good for a short fall on your marginal pieces but it will have to be hundreds of metres long to catch that giant whipper.
My comment was confined to loading curves, not the body of the screamer. The issue is how 'smoothly' (fast) you go from 0k to whatever the screamer's rating is.
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