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    Default Kilonewtons overkill

    Gear bags get over-weighted with all the steel in them. NFPA 40Kn carabiners seems like a lot to me. My question is this; we have a system with 30-40Kn biners and pulleys, yet our 8mm prusiks are rated around 12Kn, so would not the lower rated prusik be the weak link? Why use high strength rated biners when our prusik will fail long before? Aluminum biners with 26Kn (6000lbs.) seems sufficient to me, but I never took physics so ...
    Hope that was not dumb question.

    Thanks

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    Quote Originally Posted by MichaelXYZ View Post
    Gear bags get over-weighted with all the steel in them. NFPA 40Kn carabiners seems like a lot to me. My question is this; we have a system with 30-40Kn biners and pulleys, yet our 8mm prusiks are rated around 12Kn, so would not the lower rated prusik be the weak link? Why use high strength rated biners when our prusik will fail long before? Aluminum biners with 26Kn (6000lbs.) seems sufficient to me, but I never took physics so ...
    Hope that was not dumb question.

    Thanks
    Good morning Michael,

    After being in the fire service for 45+ years I can definitely say that YES there are some stupid questions out there. I can also definitely say from reading your posts that you haven't asked any stupid questions yet.

    The strength of the hardware used relates to the desired system safety factor you are trying to achieve. The "ideal" safety factor varies widely from team to team and location in the world. Some mountain rescue teams I know of are comfortable with a 4:1 safety factor while some fire departments insist on a 15:1 safety factor and others are happy with a 10:1. OSHA is happy with a safety factor of 2:1 in some situations and I know of a cliff/beach rescue team that is comfortable with a 1:1 safety factor.

    A lot of consideration comes into it. What is the expected/estimated load on the end of the line? One person load or two or three person load? What is the slope angle? 15 degree mud or loose rock or 90 degree free-hanging? Are you going to be using single line techniques or will a belay line or a 2-tensioned rope system be used? How many personnel are available and what is their skill level? What anchors are available? Twigs, pebbles and blades of grass or structural steel? How far do you have to carry the equipment to reach the rescue location. How high above the ground is the rescue location? Is it an industrial location that has chemicals that react vigorously with aluminum hardware? What is the potential of a shock load to the system? Have swivels been used to eliminate torsional force on carabiners? Is the rope padded to protect it from sharp edges and abrasion? Is the rope new or used? Clean or dirty? Wet or dry? Static, low-stretch or high-stretch? What diameter rope is being used for the main line? What kind of knot is tied in the rope? Is it tied right? Is it dressed and set?

    Typically, when a prusik fails it fails by slipping and acting as a clutch to protect the main line. When mechanical rope grabs fail it is often catastrophic to the main line. How much abuse will the system receive on a regular basis? Kind of like the theory that if you put a firefighter in a padded rubber room alone with two steel ball bearings he will lose one and tear the other one up. The ultimate strength of the system depends on all of these variables plus a few more I probably forgot. There is no 1 perfect answer as to what is the "best" rescue system.

    Mike Dunn

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    Thanks Mike. In my personal bag, I have biners ranging from 26Kn to 50Kn So we have a range of ~6000lbs. to 12000lbs. Even the lowest rating biner can hold a 3 ton truck (theoretically). So I tried to figure in the fall factor, honestly it seems the force applied that would be enough to break a 26Kn biner would snap a man in two. I understand the need for headroom and safety factor, but I just don't get a 15:1 rating. Name:  carabiners.gif
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    BTW, have you checked out the Rock Exotica Pirate Orca Carabiner? The autolock is real nice, the feel is good too, but it is a tad on the small size. It is rated at 26Kn so no good for rescue work, I keep it on my belt clip as my backup.

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    In search for more info on this subject, I came across this system analysis paper on the PACI website. Just a FYI if anyone is interested.
    http://www.paci.com.au/downloads_pub...Rescue_Std.pdf
    Last edited by MichaelXYZ; 07-19-2012 at 09:24 PM.

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    Quote Originally Posted by MichaelXYZ View Post
    Thanks Mike. In my personal bag, I have biners ranging from 26Kn to 50Kn So we have a range of ~6000lbs. to 12000lbs. Even the lowest rating biner can hold a 3 ton truck (theoretically).
    Also remember that a kN is a measure of force, not pure weight. Say for example you are lifting that truck, if the load is bouncing, swaying or moving in any direction but straight up, the force is greater than the weight... I know; F--- Science!
    ~Drew
    Firefighter/EMT/Technical Rescue
    USAR TF Rescue Specialist

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    Yeah the science. According to the article a Newton is expressed as: Force (N) = Mass (kg) x Acceleration (m/s2). Where acceleration is gravity. So I guess a 15:1 would account for gravity standing and higher accelerations. Still the article also pointed out that the prusik would be the weak link in the system so the biner strength is negated. So would a tandem prusik be cumulative, ie. 12Kn + 12Kn = 24Kn? Sorta like current through resistors in parallel.

    Thanks

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    Quote Originally Posted by MichaelXYZ View Post
    Yeah the science. According to the article a Newton is expressed as: Force (N) = Mass (kg) x Acceleration (m/s2). Where acceleration is gravity. So I guess a 15:1 would account for gravity standing and higher accelerations. Still the article also pointed out that the prusik would be the weak link in the system so the biner strength is negated. So would a tandem prusik be cumulative, ie. 12Kn + 12Kn = 24Kn? Sorta like current through resistors in parallel.
    Thanks
    Your article references a static system safety factor of 9:1. Their math is fine but it is unusual to include the ratchet prusik in the SSSF. The prusik has a breaking strength but it also operates as a load limiter/clutch. When rigged correctly, the ratchet should never see dynamic loading. Your belay side may have a dynamic system safety factor of as little as a 2:1 depending on the load you are moving. I'm not sure where you are seeing a 15:1 safety factor being referenced. A tandem prusik belay is not designed to be cumulative, it's designed to limit the maximum arrest force to 15kN or less. With our rescue equipment there is a relationship between applied force and elongation. At about 17-18kN you begin to encounter what's known as the yield point. That's when permanent change to our equipment begins to occur. Tandem prusiks typically arrest a 200kg test mass at around 10-12kN. Also, since you are on the topic of SSSF. You should also be able to put a force number on tri-links for system analysis. The math you listed above is the same but you will need to plug in a few other items that are listed below. This conversion is for components that are stamped with SWL. A typical Tri-link is SWL of 900kg. When converting a SWL mass unit to force unit you will need to multiply the SWL number by 5. This is because commercially designed products like tri-links have a SF of 5:1.

    900kg SWL Tri-link
    As you stated above: F(force)=ma(massxacceleration)
    =900kg(5:1)
    =4500kg(9.81m/s2) or just round up to 10
    =44145N move your decimal over 3 to convert to kN
    =44.1kN
    So you needed to find F(i.e. a kN). You already had the number for m(a mass of 900kg) then you multiplied that by a(the acceleration of gravity - in this case 10) and out comes a force. As you can see, this process can also be reversed if you needed to find out what a products mass strength was - including it's SWL. But thank God, most products are already marked in kN.
    Last edited by bottrigg; 07-20-2012 at 05:41 PM.

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