Prusik cord

Raven

Raven

Branched out member
Location
Northwoods
I hear talk in other posts about prusik cord strength needs to be at least 2500#.
If you double that you get 5000#. Check ANSI Z133.1, 3.5 and 3.23 and you'll see the inaccuracy of that info. Also consider that you've got the cord doubled right, so MBS of 2700# should be OK right? but is it doubled at the point where it passes over the biner? the point where it makes that sharp bend where it's most likely to break? NO!! That spot under the double fish where the biner goes through the eye is only a single line and therefor MUST be 5400# tensile strength!
 
I agree. 2700# is fine to meet ANSI. I would also feel reasonably safe on cord rated to 2500. Also the hitch only sees half your weight, dont sweat the small stuff.
 
There are really two different topics here.
First, what does ANSI say about prussic cords?
Second, what is safe?
It's a little frustrating to see ANSI standards that don't even exsist tossed back and forth. For example, this thing about prussic cords. Some folks have taken the breaking strength requirements of climbing lines and applied it to prussic cord and come up with a new unwritten ANSI standard of 2700#.
Because one use of ASNI standards is legal, arbitrarilly saying the ANSI standard for prussic cord is 2700#s is wrong.
A good lawyer could argue that prussic cord is climbing rope, and needs to be 5400#s. A good defense lawyer might argue that prussic cord is not climbing rope, therefore there is no ANSI standard for strength. A third lawyer might say the prussic cord/fair lead/carabineer is a single piece of climbing gear and needs to be 5000#s. Another may argue for 2700#s.
I could see it go almost anyway, in court.
Consider some arborist climb SRT and some DdRT, how does that play into the 5400#/2700# arguement? How about when footlocking, you have a doubled 5400# rope (10,800#) and your only secured with a 2700# prussic cord? That puts your prussic cord at 1/4 ASNI using the original logic!
Hopfully when the new revisions come out, they will address prussic cord thickness and strength, along with climbing rope thickness. The little line about using lines less than 1/2" is so useless, it shouldn't even be in there!
 
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but is it doubled at the point where it passes over the biner? the point where it makes that sharp bend where it's most likely to break?

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NFPA (national fire protection association) sets rope standards in rescue at 4500 lbs personal use and 9000 lbs general use, but 8mm prusik cords are routinely used in rope rescue by NFPA-compliant agencies.

And, though it might seem at first glance that the "single" cord bent around the 'biner would see the full load on the prusik cord, this is not true. Total strength is always calculated according to the number of linear strands that support the load. Much of the stress on the cord is lost in friction around the carabiner and does not effect the single-strand curve of rope. The full load is only on the linear strands.

However, decrease in cord strengh by the knot must be considered and that strand must be devalued accordingly - because the cord is most likely to rupture at the neck of the knot, not at the bend around the 'biner.

- Robert
 
My choice has been 3/8" Tenex; at 5000# strength; grabbing carabiner with double noose eyes. Tenex also flattens out around the carabiner, so maintains more strength than round cord/rope i think. The point in support chain has some slip of friction hitch, giving some dynamic absorbtion. That is on dynamic leg of DdRT. On static leg of DdRT/ tied to saddle i could have 5400# single leg of line in DBY, as a 1/2" rather than 3/8" line gets more leveraged than the cord, also as round line, gets more leveraged than Tenex type cord; used around the same carabiner. The 2 compounding leverage differances on the 1/2" round vs. even just 1 leg of the 3/8"flat should close the 400# gap i think, let alone, consideration of the matching leg of support at that position. Also, no direct slip for dynamic abosrbtion on static leg(until pulls thru from other side).

@135# dripping wet and dressed; i feel safe on the doubled Tenex choice and application position!
 
You guys are killin' me here. I don't make the rules, I just follow them. ANSI Z-133.1, 3.23 clearly states that "The breaking strength of the (Prusik) loop shall meet minimum strength criteria for an arborist climbing line". As far as where it would break I think it's anyones guess until we actually destroy a few of them with hydraulic pressure.
Now, ANSI is supposed to be an industry standard that we in the industry adhere to. I know 2500# is fine, I've got my kid's swing at home hanging on some stuff I bought at Wal-mart, maybe 200#tensile,
I know it's perfectly safe.
When I go to work, I follow the industry standards, and for the sake of professionalism I'm just trying to get everyone here to do the same.
 
Re: Professionalism

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When I go to work, I follow the industry standards, and for the sake of professionalism I'm just trying to get everyone here to do the same.

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Being a professional means being able to make intelligent decisions in the field. That's what ppl are doing when they use 2700# MBS cord in a continuous loop.

Being professional is not pontificating about blindly following a literal interpretation of the standards. Predicting where lines will fail is not "anybody's guess." It's physics and the data are in, lines will fail at the neck of the knot.

The ANSI standards are there to set minimum benchmarks. Professionals make decisions.

Be careful.

BAB
 
[ QUOTE ]
ANSI Z-133.1, 3.23 clearly states that "The breaking strength of the (Prusik) loop shall meet minimum strength criteria for an arborist climbing line"

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If that's how the standard reads, then it clearly states the LOOP, not the cord, shall meet climbing line strength. Which means the CORD needs to be approximately half as strong.

- Robert
 
[ QUOTE ]
If that's how the standard reads, then it clearly states the LOOP, not the cord, shall meet climbing line strength. Which means the CORD needs to be approximately half as strong.


[/ QUOTE ]

Unless it's not used as a loop, such as with a climbing hitch!
Consider the difference between an MT and a VT, as defined by The Tree Climbers Companion. (For those that don't have the book, the MT is said to be tied with a prusik loop and the VT with a prusik cord.)
So first, we need an industry distinction between a cord and a loop. Then, how would the strength of the loop be tested? Would it be a simple pull to breaking of a loop of tied cord, or would it be a pull simulating how it is attached to a host line? Big difference!
And a more subtle question; is diameter a strength issue? If not, why is it included in the specification for climbing lines? If so, does prusik cord need to be 1/2"?
Lastly, why is the ANSI standard worded like this? Why don't they just say the minimum strength and diameter of prusik cord they want the industry to use, instead of some vague reference to a percentage of climbing line strength and possibly size?
 
Mike, the climbing hitch is effectively a loop, right? Either way, unless you spliced the cord into a loop, you're going to have two knots in it. I don't see this as being a problem, so long as the cord is being used with two "legs".

Glen
 
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Mike, the climbing hitch is effectively a loop, right?

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Are you asking me if it's safe, or if a loop of rope is "legally" exactly the same as a cord of rope attached at two points to a carabineer?
Because if you 're asking legal advice, you may want to contact a lawyer.
Seriously, I'm just trying to make a distinction between ANSI standards, and what is safe. A question originally brought up by the thread starter.
 
I'm not asking for legal or safety advice, Mike.

Robert stated in an authoritative manner that when used as a loop, the cordage could be downgraded. You then made a distinction between a "loop" and a length of cord used to form a friction hitch.

I was merely expressing (in the form of a question) my opinion that so long as both ends of the friction hitch cord were affixed to the same device, you'd effectively have the "loop", thus removing the distinction.

Glen
 
[ QUOTE ]
I was merely expressing (in the form of a question) my opinion that so long as both ends of the friction hitch cord were affixed to the same device, you'd effectively have the "loop", thus removing the distinction.

Glen

[/ QUOTE ]

Right on Glen. It does form a loop. Just like your system forms a loop with the climbing line in DdRT. You can attach with a split tail if you want. Using a piece of the same climbing line and an open hitch, all lines are identical and everything is by the book. With a prussik cord (minimum 2500-2700#s) and a closed hitch (both legs affixed to the same device) you get another loop at the second point of attachment. And the load is split again (quartered if that helps) at this second point of attachment, and again, everything meets the intent of the standard.

I like it that way.

BAB
 
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