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Dr_Crash

5 mm perlon okay for all my prusiks?

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Hi, I have a question about prusiks. It looks like the standard prusik diameter with people like the Mountaineers is 6 mm. Yet, many do glacier travel with 8.1 mm or similar ropes (typically a strand from a twin), and they agree that 5 mm would be better on those.

 

I actually have two questions I guess:

 

- Is 5 mm what you'd pick for an 8.1 mm line, or would you stick with 6 mm and make an extra wrap (like 4 wraps maybe)? I won't be trailing a sled or other heavy stuff, just myself and a week-end pack.

 

- If yes, would it make sense to have 5 mm for everything, including the school's 11 mm ropes, or do I want another set for those, in 6 mm. I'm thinking a single set should work with all of the ropes.

 

Thanks,

drC

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A single strand of 5 mm cord has a breaking strength of 5.6kN or so. In a prusik, there are two strands holding a load, one with a knot in it, so a prusik with 5 mm cord should in theory hold about 9.4kN. In reality, there will be some movement as the prusik sets, etc. That's a pretty healthy margin for something that is basically supposed to support body weight and change. For an 8.1mm rope, 5mm seems like a reasonable choice to me. It would be illogical to trust 5mm prusiks on an 8.1mm rope, but not on an 11mm rope.

 

That said, I use 6mm cantfocus.gif 5mm is pretty thin stuff and seems to wear out faster. If you are ever using prusiks to get over a rock edge, you will be happy to be using 6mm. You could just try seeing if 6mm will work on your 8.1mm ropes. Just buy some and try hanging on it, it's cheap. See what happens. 3-wrap it if need be. As the prusik material gets worn a bit, it will "stick" better too.

 

-Iain

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w00tIw00t w00ttriedw00t w00ttow00t w00tusew00t w00t6w00t w00tmilw00t w00tonw00t w00tmyw00t w00tBealw00t w00tIcew00t w00tLinew00t (w00t8w00t.w00t1mmw00t) w00tandw00t w00titw00t w00tworkedw00t w00tpoorlyw00t. w00tIw00t'w00tllw00t w00tbew00t w00tusingw00t w00t5w00t w00tmilw00t w00tonw00t w00tmyw00t w00t8w00t.w00t1mmw00t w00tlinew00t.

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w00tIw00t w00ttriedw00t w00ttow00t w00tusew00t w00t6w00t w00tmilw00t w00tonw00t w00tmyw00t w00tBealw00t w00tIcew00t w00tLinew00t (w00t8w00t.w00t1mmw00t) w00tandw00t w00titw00t w00tworkedw00t w00tpoorlyw00t. w00tIw00t'w00tllw00t w00tbew00t w00tusingw00t w00t5w00t w00tmilw00t w00tonw00t w00tmyw00t w00t8w00t.w00t1mmw00t w00tlinew00t.

 

Ryan, you need to get some help for that w00t tic , Man. wave.gif

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w00tIw00t w00ttriedw00t w00ttow00t w00tusew00t w00t6w00t w00tmilw00t w00tonw00t w00tmyw00t w00tBealw00t w00tIcew00t w00tLinew00t (w00t8w00t.w00t1mmw00t) w00tandw00t w00titw00t w00tworkedw00t w00tpoorlyw00t. w00tIw00t'w00tllw00t w00tbew00t w00tusingw00t w00t5w00t w00tmilw00t w00tonw00t w00tmyw00t w00t8w00t.w00t1mmw00t w00tlinew00t.

 

damn!

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No idea how that happened...point is, 6 mil doesnt work on my 8 mil line. 5 mil does.

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w00tRyan,w00t w00thoww00t w00taboutw00t w00ttryingw00t w00t5.5w00t w00tmmw00t w00tprussiksw00t w00tonw00t w00tyourw00t w00t8.1mmw00t w00tBealw00t w00tIcew00t w00tLinew00t?

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w00tAnd,w00t w00tohw00t w00tyeah,w00t w00tI’mw00t w00ttalkingw00t w00tregularw00t w00tplainw00t w00toldw00t w00tperlonw00t w00taccessoryw00t w00tcord,w00t w00tnotw00t w00tKevlar,w00t w00tSpectra,w00t w00tGemini,w00t w00torw00t w00tTitanw00t w00tcordw00t.

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"And, oh yeah, I’m talking regular plain old perlon accessory cord, not Kevlar, Spectra, Gemini, or Titan cord. " Excel is good at removing "w00t".

 

I wouldn't have any problem with the strength of 5mm for my personal prussiks, but for crevasse rescue, I'd want 6 mm because you can exert more force than body weight using a 3:1 Z-Pulley. To get the necessary grip on 8 mm, you might have to go with a kleimheist.

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I wouldn't have any problem with the strength of 5mm for my personal prussiks, but for crevasse rescue, I'd want 6 mm because you can exert more force than body weight using a 3:1 Z-Pulley. To get the necessary grip on 8 mm, you might have to go with a kleimheist.

 

Is that force exerted on the prusik? My weight doesn't change so the force *I* exert doesn't change. The force on the rope does change (going in the up direction) but it's pretty static. Do you think the rope could slip through the prusik then?

 

A kleimheist is out of the question given the environment is school where we have to use prusiks.

 

drC

Edited by Dr_Crash

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The prusik you use to haul with will experience a peak of force when the haulers start yarding on the rope, and your "progress capture" prusik will peak when the haulers quit hauling. These forces can be quite large if you have haulers who don't pull smoothly and continuously and just try to jerk the person up. However, you still have almost a 10:1 safety factor with the 5mm prusiks (assuming your subject, hanging in air, is putting about a 1kN force on the rope to begin with). That would be a 100kg person, or 220lbs, seems reasonable for your "average" climber with ton of gear. If you are heavy, maybe that is something to consider. But 10:1 is pretty conservative margin.

 

There are only a few pulleys out there that will reliably mind prusiks made from 5mm. Something to consider.

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There are only a few pulleys out there that will reliably mind prusiks made from 5mm. Something to consider.

 

I hope the Petzl Mini is one of them. Do you know? If not, I guess I'd have to use my belay device to do the minding.

 

RE: your safety margin of 10:1, Strong's book notes that 5mm perlon has a breaking strength of ~1150 lbs, which would be a 5:1 safety margin then (which he does consider good, btw). Not sure who's right smile.gif

 

drC

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While we're at this, what length do folks use for their prussiks?

 

If on a glacier I rig up a Texas Prusik, the length of which is strictly dictated by FOTH. Geek_em8.gif

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I hope the Petzl Mini is one of them. Do you know? If not, I guess I'd have to use my belay device to do the minding.

 

Sorry, I have no idea what pulley would work with 5mm. Perhaps someone can ring in with some experience with 5mm.

 

RE: your safety margin of 10:1, Strong's book notes that 5mm perlon has a breaking strength of ~1150 lbs, which would be a 5:1 safety margin then (which he does consider good, btw). Not sure who's right smile.gif

 

1150 lbs force equals about 5.1 kN, so it's in the same ballpark as the number I put down (that was from REI). That's the breaking strength of the material. But take a look at the prusik. Is all of the force being held by a single stand of that material? I doubt it's split perfectly 50:50 between the two strands, but neither is bearing the entire load. So if you said it could support 10.2kN (2 strands, each holding 50%), you're doing quite well in terms of safety margin. You have a knot in the line too, which, on average, reduces the cord strength by 1/3rd. So if you take that into account, you would have about 8.5kN to work with. You would have to generate over 8 times the force of your 1kN load to break that material. Pretty good margin of safety from strictly a material strength standpoint. Perhaps this is overanalysis, but maybe it helps (I hope I calculated that right!) yellaf.gif

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you can exert more force than body weight using a 3:1 Z-Pulley. To get the necessary grip on 8 mm, you might have to go with a kleimheist.

confused.gif

Catbird,

 

Huh? Draw a free-body diagram for the Z-pulley. If the tension in the rope in your hand is T during pulling, the maximum tension on the prussik closest to the crevasse is going to to be roughy T, during pulling (assuming you've attached your prussik in a normal loop to the pulley with a carabiner). The maximum tension on the ratchet prussik is going to be half the weight of the victim, when you are not pulling. This neglects transient effects when you start and stop pulling, which might briefly give a slightly higher tension.

 

Of all the things to worry about during a crevasse rescue, the 5mm perlon prussiks breaking is about the most unlikely, unless you have the strength of a gorilla and are using some kind of insane 24:1 pulley system.

Edited by Stephen_Ramsey

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Huh? Draw a free-body diagram for the Z-pulley. If the tension in the rope in your hand is T during pulling, the maximum tension on the prussik closest to the crevasse is going to to be roughy T, during pulling (assuming you've attached your prussik in a normal loop to the pulley with a carabiner).

 

Actually, it will be 2T.

 

 

The maximum tension on the ratchet prussik is going to be half the weight of the victim, when you are not pulling. This neglects transient effects when you start and stop pulling, which might briefly give a slightly higher tension.

 

Ignoring friction, the tension of the ratchet prussik will be the weight of the victim. The rope holds the victim up, the prussik holds the rope to the anchor.

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Stephen, I'm not sure if I understand you exactly, but the force on the hauling prusik in the standard "z-drag" would be 2T if you are pulling with T, as it is connected to a pulley.

 

edit: whoops, slow reply

 

Ideally, the prusiks will begin to slide to some extent before they will fail. Prusiks can even be used to act as a clutch, slipping on purpose to make sure the system is not overloaded. This is generally not necessary.

Edited by iain

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and the weight of the victim (ignoring friction) should be about 3T. Hence the 3:1 ratio of the Z-drag system.

 

So, floating prussik max load = 2T = 2*weight/3.

 

Ratchet prussik max load = 3T = weight.

 

Assuming all standard assumptions of course.

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Reread my post. I said TENSION in the prussik. If the force on the center-point of the pulley is 2T, then since the prussik is in a loop around the carabiner, the actual TENSION in the Prussik is T. So it isn't 2T, it is T. It is the TENSION in the Prussik that matters.

Edited by Stephen_Ramsey

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Are you saying a single strand of the prusik cord is experiencing T at that location because there are 2 strands of material? I could agree it is close to that. Otherwise, if you are pulling on a z-drag with force T, that prusik (as an entire unit) is experiencing 2T worth of force.

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Iain,

 

Yes, it is because with a Prussik loop, there are two strands going from the carabiner to the "winding part" of the Prussik. This is why, if the carabiner is pulling with a force 2T, the tension in the prussik is very close to T.

The net force on the Prussik (as an entire unit) is not what is important, in terms of the rated breaking strength. It is the tension in the Prussik that matters. This is the same reason why when people are rapping off questionable cord, they sometimes wrap it a few times around a tree or whatever the rap anchor is.

 

-Steve

Edited by Stephen_Ramsey

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