Screamers = Shock Load!??!?

[layton]

Polish Bob went off on me about this theory and said BD won't make em cuz of this and testing results. Anyone have any remarks or better yet, Evidence?

 

Polish Bob's Screamer Theory:

 

1.You fall onto a screw w/a screamer leading on twins.

2.Rope streches then..

3...then the screamer rips after the strech is out of the rope

4.screamer shock loads screw cuz the rope stretch is gone, and extension occurs cuz the screamer is now huge.

 

Makes sense IF:

1.Screamer rips after rope stretch, not concurrently

2.force absorbed by rope stretch far outweights force absorbed by screamer ripping.

[ketch]

I'd have to think on that one a little more. My immediate thought is that if you fully extended your rope and than fully extended a screamer as well, that is a lot of force resisted and you wouldn't want that to hit your screw directly anyway. There may be a slight shock load in this scenerio as the rope would not recover any significant amount while the screamer is popping.

[Marko]

Naw.

 

You're falling and hit the end of the ropes with a certain amount of energy. The ropes dissipate some of that as they stretch. If there's a screamer, it dissipates more energy until it fully zippers and then the screw and surrounding ice get to deal with the rest. (Or not.)

 

If there's no screamer, the screw and ice get hit with whatever energy the ropes haven't dissipated.

[cracked]

Naw.

 

You're falling and hit the end of the ropes with a certain amount of energy. The ropes dissipate some of that as they stretch. If there's a screamer, it dissipates more energy until it fully zippers and then the screw and surrounding ice get to deal with the rest.

 

If there's no screamer, the screw and ice get hit with whatever energy the ropes haven't dissipated. (Or not.)

Not quite true either, since the screw doesn't 'absorb energy'. The work done on the screw is zero. Energy is aborbed only in the climber's body, the knot cinching up, the rope as it stretches, etc. Now if the screw pulls partially, then some work is done on it.

 

Screamer activates at 500lbs? Which is roughly 2kN. Which is far less than the max impact force of a rope. So while the rope stretches, the screamer activates, so there will not be a shock load, nor does the screamer absorb whatever energy the ropes didn't.

 

Now let's look at this more closely. Climber, mass of 75kg, falls 10m. KE=PE=mgh=7357.5 J

Screamer activates at 2kN, Work W=f(delta h). Say the screamer, when activated, extends, 1/3m. Then W=666.666J. Which is only 9% of the energy of the falling climber. Might make a difference.

Ahh, semantics.

[Marko]

Well, I didn't actually say "absorb" but if the screw flexes at all, then it does actually dissipate energy. In fact anything in the system that flexes, stretches, or heats up is dissipating energy. I did simplify the system a little I guess. More energy dissipaters: Knots tightening, harnesses stretching, climber's beer gut sloshing around, crampon catching on ice and shattering ankle, rope whipping through carabiners, rope slipping through belay device and belayer's gloves, belayer sinking onto anchors, cordalette stretching, pins shifting in manky rock, etc.

 

The point is if there's a screamer in the system and it activates at all, it will reduce the energy, and therefore the shock loading, imparted to the pro. Yeah, 9% maybe isn't a whole lot, but like you say it could make the difference...

[snoboy]

From the Yates webpage. (bold mine)

 

Screamers not only absorb energy directly because of the stitch ripping effect, they also allow your rope to absorb more energy from the fall by increasing the time interval of the fall. Screamers can reduce shock loading in any rescue system. The standard ?Screamer? can effectively reduce peak loads by 3-4kN in any climbing or rescue system.

 

 

And from the

Fish Products Tech Weenie page (click webbing strengths and scroll down about 2/3):

 

Physics of Screamers by YATES

Author: John Yates/ Pro Design USA

Email: prodesig@snowcrest.net

Date: 1998/12/08

Forums: rec.climbing

 

Hello all,

 

I friend of mine rang me up this morning and told me about a discussion

going on in this news group about forces involved with screamers and air

voyagers <<snip>>

My name is John Yates, and I have done extensive work with various load

limiting ìStitch Rippingî devices since the invention of the Screamerís

almost ten years ago. I thought I could shed some light on this subject

for those interested.

<<snip>>

 

Dan (Osman) and I had many lengthy discussions on how to limit the loads on his

rope jump systems. We talked about the use of a Tyrolians, Screamers to

limit loads and use of High Strength Tie-off Pulleys to terminate the

rope ends. It is hard to believe that forces could be generated that

would ìBRAKEî a climbing rope if Dan had the system set up as usual. Dan

usually had Screamers set in the system in a tandem configuration (ie.

two side by side). This configuration would limit the load and the

Screamers would elongate if forces over 950-1000 lbf. were reached

during a jump. If the system was configured right there was a ìWEEK

LINKî ie. Screamers that would activate if forces reached any thing

close to a critical level. I hope the investigation will reveal how Dan

had his system set up, only then can we really speculate on what really

happened.

 

Anyway about Screamers.

Screamers are a stitch ripping device that allows forces to be

decelerated over a longer time interval than they would be if the

Screamers were not in the system. Standard Screamers or ICE-Screams are

configures from stitch patterns consisting of 6 rows of zigzag stitching

sewn into each wing of the unit. This stitching is done by means of a

computerized sewing machine. The machine can be configured to allow the

Screamer to activate basically anywhere between 1 and 650 lbf. We chose

to use an average activation of 550 lbf. because it seamed to be about

the right force to use as a upper limit for marginal protection and Ice

screws. Not every thing is a real science.

 

Some interesting things happen when you look at how much energy is

ìabsorbedî in the system when a screamer is used. If the ìTrueî

absorption is measured in a completely static system, lets say doing a

drop test with a steel cable and weight we will see that about 5-600 lbf

was absorbed by the stitch ripper(Screamer). When a Screamer though is

put in a system which uses dynamic climbing rope instead of static steel

cable the amount of energy which is absorbed is increase by 25-40%. We

see that the absorption of energy increases to 800-900 lbf. I can

attribute this extra energy we see being absorbed to the fact that the

ìDynamicî climbing rope in the system is allowed to elongate and remain

dynamic for a longer time interval than it would be, if there was no

screamer in the system.

 

An example: A dynamometer or load cell is placed on a bolt hanger. A

climber takes a fall which generates a fall with a factor of .5. This

generates a force of 2000lbf as seen on the dynamometer. When a

Screamer is hooked in the system below the dyno. the same fall only

shows a peak force of 1200 lbf. We know from extensive testing that the

Screamer can only absorb 500 lbf. So how do we account for the extra 300

lbf seen in this example???

The increased time interval(duration) of the fall allowed the climbing

rope to be more absorptive!!

Thus Screamers limit loads and dissipate energy over a an increased time

interval. This increase in the duration of the fall is most important

in a ìDynamicî systems because it allows the rope to do its job even

better than it was designed to do.

 

Stitch ripping devices have been used in many other areas besides just

climbing. Many industrial application have been developed for the use of

Stitch ripping devices. The most common is the Fall Arrest lanyard

which uses a woven type of screamer device to decelerate a industrial

worker if he falls in the workplace.

 

Over the last ten years I have worked on numerous projects with various

Aerospace companies such as McDonnel Douglas and Bowing to develop

various types of Stitch Ripper ìScreamerî type devices. They use them

to decelerate objects which seperate from one another during controlled

testing. The coolest project I worked on was a rocket fairing which was

to house a communications satellite during launch on a Titan rocket. The

fairing separation test was conducted in a vacuum chamber which took

about 120 rippers in an elaborate configuration to decelerate the

various parts of the fairing after the explosive bolts separated them.

Anyway the point is that climbers are not the only ones using this type

of technology to their benefit.

 

If any of the other physics nerds in this group would like to discuss

specifics on the use of Screamers in various climbing applications, I

would be more than happy to respond.

 

Sincerely,

 

JOHN YATES

Yates Climbing Equipment

prodesig@snowcrest.net

 

There used to be some more detailed info on the web, but I can't find it right now. Graphs showing the force over time of systems with and without screamers.

[cj001f]

Well, I didn't actually say "absorb" but if the screw flexes at all, then it does actually dissipate energy.

I was thinking of the extreme case - the screw pulling(it "absorbed" energy, just not enough).

[JoshK]

Naw.

 

You're falling and hit the end of the ropes with a certain amount of energy. The ropes dissipate some of that as they stretch. If there's a screamer, it dissipates more energy until it fully zippers and then the screw and surrounding ice get to deal with the rest.

 

If there's no screamer, the screw and ice get hit with whatever energy the ropes haven't dissipated. (Or not.)

Not quite true either, since the screw doesn't 'absorb energy'. The work done on the screw is zero. Energy is aborbed only in the climber's body, the knot cinching up, the rope as it stretches, etc. Now if the screw pulls partially, then some work is done on it.

 

Screamer activates at 500lbs? Which is roughly 2kN. Which is far less than the max impact force of a rope. So while the rope stretches, the screamer activates, so there will not be a shock load, nor does the screamer absorb whatever energy the ropes didn't.

 

Now let's look at this more closely. Climber, mass of 75kg, falls 10m. KE=PE=mgh=7357.5 J

Screamer activates at 2kN, Work W=f(delta h). Say the screamer, when activated, extends, 1/3m. Then W=666.666J. Which is only 9% of the energy of the falling climber. Might make a difference.

Ahh, semantics.

 

 

I see you paid attention in physics class,eh? I'll be asking to copy your homework when I get around to that subject.

[catbirdseat]

The data that I've seen suggests that screamers have an effect that is more than additive. Its not just energy absorbed by rope plus energy absorbed by the screamer. The screamer extends the time over which the rope is allowed to stretch. I know that the elasticity of some materials is time dependent, that is, they stretch under a steady load, but resist stretch under a shock load. Think of the Silly Putty you played with as a child. Pull on it slowly and it stretches. Pull sharply and it breaks. Rope isn't nearly this extreme, but I'd bet there is some time dependency with regard to impact force.

[dylan_taylor]

I watched an old screamer save a life at squamish a few years ago. A beginner friend of mine augmented his rack with a bunch of my gear and draws (including an ancient screamer), then headed up on "up up and away". He thought he was getting on a 5.6, not a 5.9. He placed a yellow (#3?) HB offset backwards! in a little flair, and clipped my screamer to it. This was near the top, where, for those familiar, you have to lieback for a few feet along a nice crack. I don't know why he didn't put a bomber cam in there. His last piece of gear was about 20' lower. I was around the corner at the time, on witchdoctors apprentice, and I heard an awful scream, the sound of gear crashing, an impact, and the sound of a giant unzipping his fly. When I went around the corner, my buddy was hanging upside down, the screamer was 3/4 extended, and the air was full of little pieces of white thread falling like snow. I thought the offset would have been welded, but when I went up and cleaned it, it plinked right out with barely any effort. Not much deformation on the brass, considering the amount of energy in the fall. The moral is, screamers work, and a lot of people above have hit the nail on the head - the screamer lengthens the amount of time available for the rope (and the rest of the system) to absorb energy.