Thread: Kilonewtons overkill
07-14-2012, 05:43 AM #1
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.
07-16-2012, 10:50 AM #2
- Join Date
- Oct 2002
- Port Allen, LA
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.
07-17-2012, 01:29 PM #3
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.
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.
07-19-2012, 09:21 PM #4
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.
Last edited by MichaelXYZ; 07-19-2012 at 09:24 PM.
07-19-2012, 10:23 PM #5~Drew
USAR TF Rescue Specialist
07-20-2012, 11:53 AM #6
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.
07-20-2012, 05:26 PM #7
- Join Date
- Feb 2006
900kg SWL Tri-link
As you stated above: F(force)=ma(massxacceleration)
=4500kg(9.81m/s2) or just round up to 10
=44145N move your decimal over 3 to convert to kN
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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