3:1 with carabiners only, any advantage?

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3:1 with carabiners only, any advantage?

Postby jeffkruse » Feb 14, 2013 2:55 pm

Hypothetical question, If you made a 3:1 system using carabiners instead of pulleys would there be any mechanical advantage or does the friction negate any advantage?
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Re: 3:1 with carabiners only, any advantage?

Postby Scott McCrea » Feb 14, 2013 3:36 pm

Definitely no mechanical advantage. It would increase the friction quite a bit. The only advantage is it's smaller and lighter to not carry pulleys. And, more people have biners than pulleys, so it might be the only option.
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Re: 3:1 with carabiners only, any advantage?

Postby Jeff Bartlett » Feb 14, 2013 3:50 pm

Scott McCrea wrote:The only advantage is it's smaller and lighter to not carry pulleys.

In the grandest of CaveChat traditions, I started to nitpick this. After all, the Petzl Partner (compact pulley) and Petzl Mini (compact PMP pulley) I carry in my 3:1 kit weigh 56g and 80g, respectively. Hey, a Petzl Attache or Am'D weighs 80g -- that means my two pulleys weigh less than two common locking carabiners...

...and that's when I remembered that, if you're using pulleys, you still need those carabiners to attach them. D'oh! You're not saying carabiners are lighter than pulleys -- you're saying the absence of pulleys is lighter than pulleys. Which is true.

So I'll have to settle for making the following statement:

A 3:1 using no pulleys will not work effectively. A 3:1 using no pulleys in muddy conditions on stiff caving rope (PMI pit rope or similar) will hardly work at all. As such, there is no advantage to building a 3:1 that doesn't work when compared to building one that works.
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Re: 3:1 with carabiners only, any advantage?

Postby jeffkruse » Feb 14, 2013 3:57 pm

I am sorry for being confusing. I meant to ask would there be any mechanical advantage over a direct 1:1? It sounds like your answer is yes but it won't be equal to a 3:1. Would it be equal to a 2:1?
If you’re in a situation far from help and someone is stuck on rope (say unconscious) and you need to get them off rope. You have only a 20’ piece of rope, a few carabiners, and a croll and ascender. Could you rig a 3:1 with only carabiners and use the croll for progress capture and the ascender to grab the rope and pull the person and rope up and over the drop?
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Re: 3:1 with carabiners only, any advantage?

Postby Jeff Bartlett » Feb 14, 2013 4:02 pm

jeffkruse wrote:I am sorry for being confusing. I meant to ask would there be any mechanical advantage over a direct 1:1? It sounds like your answer is yes but it won't be equal to a 3:1. Would it be equal to a 2:1? If you’re in a situation far from help and someone is stuck on rope (say unconscious) and you need to get them off rope. You have only a 20’ piece of rope, a few carabiners, and a croll and ascender. Could you rig a 3:1 with only carabiners and use the croll for progress capture and the ascender to grab the rope and pull the person and rope up and over the drop?

AH, I see what you mean. Looks like Scott and I both misunderstood the original question. That's a very good question... and I don't know, actually. In a self-rescue situation, you'd definitely need another warm body (at least one, preferably lots more) to help with the haul, and I'm not sure what the actual mechanical advantage would be.
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Re: 3:1 with carabiners only, any advantage?

Postby Stridergdm » Feb 14, 2013 7:52 pm

Ok, who has a dynamometer handy?

Honestly, I suspect that this is one of those where "it depends".

Even w/o a mechanical advantage, there may be an advantage if you need to move a load just a little bit.

Think a crack/crevice rescue where you may want to move only an inch at a time. For a 1:1, if you get too many people on the rope, it can be hard to make sure they don't pull too far/too quickly. A 3:1, even w/o any real mechanical advantage will "slow" things down here. Granted, that's a somewhat contrived edge case.

Come to Weeklong in NY this year and perhaps we can try this out with various ropes and biners.
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Re: 3:1 with carabiners only, any advantage?

Postby Carl Amundson » Feb 14, 2013 9:16 pm

Here are the calculations on the difference between 3:1 with pulleys and one made with biners. This is with absolutely no friction (a world that does not exist inside a cave). In the real world biners suck as pulleys. With a small pulley and a micro-traxion, you can build a great 3:1 or 2:1.


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Re: 3:1 with carabiners only, any advantage?

Postby Scott McCrea » Feb 14, 2013 9:36 pm

This one time, at a vertical practice session, we made a 3:1 with biners. It worked. Used clean 11mm PMI. There was MA, but it was obvious that a pulley or two would have been a big help. Also, if you only have one pulley, put it on the travelling biner.
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Re: 3:1 with carabiners only, any advantage?

Postby Carl Amundson » Feb 14, 2013 9:40 pm

Scott McCrea wrote:This one time, at a vertical practice session, we made a 3:1 with biners. It worked. Used clean 11mm PMI. There was MA, but it was obvious that a pulley or two would have been a big help. Also, if you only have one pulley, put it on the travelling biner.

Correct or in other words; put it closest to the hauler.
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Re: 3:1 with carabiners only, any advantage?

Postby Jeff Bartlett » Feb 14, 2013 10:10 pm

junkman wrote:Here are the calculations on the difference between 3:1 with pulleys and one made with biners. This is with absolutely no friction (a world that does not exist inside a cave). In the real world biners suck as pulleys.

So, if I'm reading this correctly, the quick-and-dirty answer to the OP's question is:

In a perfect world, building a 3:1 with carabiners in place of the pulleys gives you a ~2:1 mechanical advantage (F = 0.48 L).
In the real world -- mud, friction, etc -- doing this gives you something more like ~1.5:1 MA ...and perhaps less.

Yes?
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Re: 3:1 with carabiners only, any advantage?

Postby Carl Amundson » Feb 14, 2013 10:19 pm

Jeff Bartlett wrote:
junkman wrote:Here are the calculations on the difference between 3:1 with pulleys and one made with biners. This is with absolutely no friction (a world that does not exist inside a cave). In the real world biners suck as pulleys.

So, if I'm reading this correctly, the quick-and-dirty answer to the OP's question is:

In a perfect world, building a 3:1 with carabiners in place of the pulleys gives you a ~2:1 mechanical advantage (F = 0.48 L).
In the real world -- mud, friction, etc -- doing this gives you something more like ~1.5:1 MA ...and perhaps less.

Yes?

Yup I have built 3:1 with biners during small party rescue practice and find that's not much better than a 1:1 with a redirect.
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Re: 3:1 with carabiners only, any advantage?

Postby jeffkruse » Feb 15, 2013 8:33 am

So going by the last chart from junkman it looks like a 3:1 with pulleys will give almost a 3:1 advantage (.38). Using only biners would give a 2:1 advantage (.48) BUT I don’t understand your statement of “absolutely no friction”. If there is no friction then why is there a difference? Isn’t the main reason to use a pulley to reduce friction?
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Re: 3:1 with carabiners only, any advantage?

Postby Carl Amundson » Feb 15, 2013 8:41 am

jeffkruse wrote:So going by the last chart from junkman it looks like a 3:1 with pulleys will give almost a 3:1 advantage (.38). Using only biners would give a 2:1 advantage (.48) BUT I don’t understand your statement of “absolutely no friction”. If there is no friction then why is there a difference? Isn’t the main reason to use a pulley to reduce friction?

Sorry, I meant ideal conditions; clean rope, no edge friction, shifting loads, etc.
The calculations just take into effect the differences between pulleys, biners and the placement of pulleys in the system with no external influences.
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Re: 3:1 with carabiners only, any advantage?

Postby jeffkruse » Feb 15, 2013 8:43 am

OK, that makes sense. Thanks!
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Re: 3:1 with carabiners only, any advantage?

Postby LukeM » Feb 15, 2013 11:27 am

I string up slacklines with a 3:1 using only carabiners. It would certainly not be possible to achieve the tension needed if I used a 2:1 or 1:1.

I wonder, is any MA achieved when tying a trucker's hitch? It is a 2:1 running rope through other rope after all.
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