mechanical advantage

LOWSIX

Hey Gang

I have done the quizzes and understand the force on the legs, the pulleys, and the total force.

I see some fiddle blocks offer a 4:1 MA and the next ad has the same device offering 5:1. Which is correct??

Same goes for pulleys??

Using the example If the tree you want to pull is tied off at the top and a pulley is attached to the base of a tree nearby and you pull with 100 lbs of force... when you pull the rope away from the pulley you have only "redirected" the rope/force and you have no MA on the tied off tree. True??
But the trunk with the pulley has 2:1 force being applied to it or 200lbs.

Here's what I am having an issue with. If I attach a second pulley to the line and pull toward the first pulley, I create a 3:1 MA. However, the second pulley only has a force of 200lbs being applied to it from my pull but the line to the tree is still getting 100lbs making it 3:1. Is this correct??? I think it is, but it just seems strange that it changes from 0 to 3:1 by running through one more "redirect"

I look forward to your replies

Thanks

Dean

Eric Frei

I got a headache reading that.

Stuff like that needs diagrams and less question marks.

Here, go to this thread and see if you can work it all out.

Mechanical Advantage Pulley Quiz

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LOWSIX

HA!! I'm glad I rewrote it so it wasn't so wordy!!!

I did the quiz and understand the answers. I'll be back after I create some diagrams. Thanks

Rescueman

As you noted, the redirect merely passes the input force through to the load. Adding the 2:1 travelling pulley doubles the input force on the load and adds that to the 1:1 that passes through the redirect, for a total of 3:1 MA.

Both are correct. It depends on whether the block & tackle is "reeved to advantage" or "reeved to disadvantage". That doesn't refer to the MA - which in both cases gives a mechanical advantage.

But if the haul line (input end) is pulling towards the load, it has to go through one additional redirection pulley, which adds friction but no additional mechanical advantage - so you've lost some force.



You can see in this diagram of mariner's block & tackle systems that each one can be turned around to change it from an even to an odd MA. Even MA systems are always terminated at the anchor, while odd MA systems have to be terminated at the load.

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LOWSIX

:signs101

That's the wording I was looking for. If the haul line (input end) is pulling away from the load then it creates more MA than the same setup pulling toward the load. Awesome Thank you!!!