MORE SCREAMER INFO
Below you will find a letter by John Yates published to rec.climbing about his product. I am trying to collate as much information as I can about screamers and links to relevant sites. If anyone has any additional knowledge please pass it own.
Physics of Screamers by YATES
Author: John Yates/ Pro Design USA
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
. My name is John Yates, and I have done extensive work with various load
limiting "Stitch Ripping" devices since the invention of the Screamer's
almost ten years ago. I thought I could shed some light on this subject for
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 "BRAKE" 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 "WEEK LINK" 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 "absorbed" in the system when a screamer is used. If the "True" 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 "Dynamic" 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 "Dynamic" 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 žScreamerÓ 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.
Yates Climbing Equipment
ice climbing by state
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