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