How much mechanical advantage from this pulley setup

NickfromWI

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I have a chance to buy this on craigslist. There's a thread on here right now for "what are you bad at." My thing is determining MA for pulley setups. I can tell if adding a pulley is going to giving you an advantage, but determining if it's 2:1, 5:1, etc...I'm just dumb at it. :|:

What do you think the answer is?

love
nick
 
In this setup, you just have to count the number of ropes which hold the load.

As in your pic, if the load is on the lower point to lift it, it's held by 4 pieces of ropes. The 4 ropes hold the load each with the same amount of force you put on the free rope. So it gives you a 4:1.

If you want to take out slack in a rigging line (same setup as in the pic), the load is now on the upper point. Like that, you are actually a second tie-in point and there are 5 pieces of ropes pulling on the rigging line. You get a 5:1 MA.
 
Is that the one from Bailey's? We used several of those last summer at Heartwood in the raising and rigging workshop. We made and raised A frames, derricks and gin poles. They looked like good units to me.
 
Even though I understood everything he said in the video, I'm confident that given a stack of drawings, I would fail the test when calculating the mechanical advantage of each system.

2+1=3 and then the 3+3 gets added ending up with 9. Sounds simple but ask me tomorrow and who knows what answer I would come up with. And I always thought I was good at math.
 
The reason the 3 + 3 = 9 is because the first 3:1 system is piggy-backed onto the second 3:1 system. If two 3-sheave pulleys were used, it would yield 6:1.

How many folks on here use fiddle block/block-and-tackle systems very often? I have a couple of double-sheave pulleys that I can set up for 5:1 when needed, but it's faster to break out the come-a-long or the Maasdam rope puller, at least for pulling trees. I suppose the fiddles would be quicker for pretensioning lines or for lifting above a porty.
 
Granted: Inbreds should probably not attempt to contribute to this thread. Nevertheless, I'm with Nick and Soonerfan--What the deuce? I get what you're saying treesmith, but the main thing that I don't understand, is not "how to calculate mechanical advantage," it's "why or what the heck is a mechanical advantage in the first place?"

I understand perfectly well (from experience) that when you are trying to pull a tree over w a bull rope, and you put a bowline on a bight in the middle and put a running bowline around a stump, and pull all the slack through the bowline on the bight--the tree gets WAY easier to pull. What I don't understand is why.

I also perfectly understand that when you use pulleys for mech advantage, you have to pull three times as much rope (as in a 3:1) than you wld if the boys were just "pullin er by hand". What I don't understand is exactly why or how the pulleys help. Did Merlin just set things up that way? Did some ancient wizard--through the dark arts--just dictate that things wld be so?

Does anyone have a metaphor or an analogy or even an allegory that cld possibly help an inbred out?
 
If you see force as a volume in a pitcher of distance x power; you can either spread that volume tall (high back pressure tension) but not far, or spread it very thin(lo back pressure) but far, but must always equate to the same volume amount.

A 3:1/zRig pulley rig allows you to pull 1 end to shorten 3 lines at the same time. So need to pull 3 feet of line to move load 1 foot, so get 3x as much tension return. Same volume of force, but only 1' rather than 3' to 'stack' that volume into, gives higher 'backpressure' / tension.


Nick's rig looks trick, would imagine has high efficient pulleys, last 10% or so efficiency can be expensive. Also, notice, the banjo shape, that allows the lines to run as inline as possible and not 'scrub'. Rope only resists/ conducts force on the inline axis, and only in tension direction. Chasing 100% efficiency, totally inline force etc. is the holy grail. Also, might have lock cam on side, to hold tension, allow impacting etc.

Nick's top pulley has 5 lines to it, if no friction and pulled free end at 100#, top would have 5 100# pulls on it, so is 500#, so is 5:1 pulling down, then likewise on lower; has 4 legs of pull, if all tensed the same, get 4xReturn/ pull on anchor etc.
 
The TreeSpyder is speeder than me !:cry:
I let fall my first answer.

What I don't understand is exactly why or how the pulleys help.
The pulleys have only one purpose: they change the direction of the rope. With the less loss of energy as possible.
Like that, one rope makes several times the same work, each one is added into the system, like there were several ropes (and men) in play, instead of only one.

I understand perfectly well (from experience) that when you are trying to pull a tree over w a bull rope, and you put a bowline on a bight in the middle and put a running bowline around a stump, and pull all the slack through the bowline on the bight--the tree gets WAY easier to pull. What I don't understand is why.
Sorry, I don't see exactly your setup. Can you post a quick drawing?
 
I understand perfectly well (from experience) that when you are trying to pull a tree over w a bull rope, and you put a bowline on a bight in the middle and put a running bowline around a stump, and pull all the slack through the bowline on the bight--the tree gets WAY easier to pull. What I don't understand is why.

I also perfectly understand that when you use pulleys for mech advantage, you have to pull three times as much rope (as in a 3:1) than you wld if the boys were just "pullin er by hand". What I don't understand is exactly why or how the pulleys help. Did Merlin just set things up that way? Did some ancient wizard--through the dark arts--just dictate that things wld be so?

When you run the rope back through a loop, you create a z-rig, or 3:1, simply without pulleys. What the pulleys do is reduce friction to near (or almost) nothing. This saves wear on the rope as opposed to running it through a loop, as well as yields more pull for your effort. It's true that you have to pull 3 times as much rope, but you're getting 3 times the amount of pull (not allowing for loss to friction). Just look at it as if you had it in a lower gear. Your truck engine has to turn far more RPM's in first gear than it does in high gear, but the advantage is power. Pulling a tree directly by hand would be like having it in drive, 1:1. You can add as many pulleys (redirects) as you like, converting it to 2:1, 3:1, 4:1 or 5:1, stepping your pulling power up, just like your transmission shifting gears as you go into a hard pull.
 
Thanks Spider: I somehow got the "pressure spread high or thin." Thanks. And thanks to treesmith for explaining to Marc what an inbred pulley system is.

I will, however, have to think a lot more this week abt how a pulley, by simply redirecting a line, "spreads pressure out thinner." Despite everyone's charitable efforts--still a little confounded. :|:
 
I will, however, have to think a lot more this week abt how a pulley, by simply redirecting a line, "spreads pressure out thinner." Despite everyone's charitable efforts--still a little confounded. :|:

Jed, think about natural-crotching a 100# limb down. If the limb hangs and must be lifted by the groundie, it's quite difficult, due to friction at the crotch. If the same limb were lowered with a block or pulley, the reduced friction would allow him to lift the limb with much less effort. Same limb, same rope, same groundie. Only difference would be the amount of friction encountered at the bending point of the rope.
 
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Jed, think about natural-crotching a 100# limb down. If the limb hangs and must be lifted by the groundie, it's quite difficult, due to friction at the crotch. If the same limb were lowered with a block or pulley, the reduced friction would allow him to lift the limb with much less effort. Same limb, same rope, same groundie. Only difference would be the amount of friction encountered at the bending point of the rope.

Your example is easy to understand. But I don't think any of us pulley-dummies don't have problem understanding how pulleys reduce friction. Where the problem comes in is how do you explain that if you set up a Z-rig, it's EASIER to pull that just with 1 pulley alone. Then start adding more pulleys and it gets even EASIER. That's where the merlin the magician crap starts happening.

Dark Arts.
 
The issue of eliminating friction is not what enables pulleys to compound pulling power. Eliminating friction prevents the LOSS of pulling power, but does not CREATE additional pulling power. I would describe things a little differently. Think what would happen if you tied a rope to a tree and pulled on it with 100 lbs of force - the tree would feel 100 lbs. But if you tie two ropes to the tree and pull on them both with 100 lbs of force, then the tree feels 200 lbs even though the force on each rope was only 100 lbs. And if you tied three ropes to the tree and pulled on each one with 100 lbs, then the tree would feel 300 lbs. And so on. Adding pulleys just allows you to pull on multiple strands of line all at once. The more strands connecting the anchor point to the load, the more times you compound your pulling power. This is why you can simply count the number of strands supporting the load to calculate MA.
 
I should clarify and say that in a simple (not a complex) MA system, the above is why you can simply count the number of strands supporting the load to calculate MA.
 
Another way to consider it is using the example of our DRT climbing system. I think we all know, if not understand why, that we have (taking friction out of the discussion) 2:1 MA in that system, allowing us to lift ourseves into the tree by only pulling one half of our weight aloft.

But we're not applying only one half of our weight to our anchor point, are we? Each leg of the system carries 1/2 our weight, so the tie in point experiences our full weight, even though we only pull half of it. We are applying 2:1 MA on the tie in point.
 
Here's some examples of compound pulley systems, what Bounce is calling complex MA. I stole this out of ON ROPE.
 

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Back to simple systems.

Try this: a moving pulley requires twice as much length of rope to be pulled for the distance the load moves, right?

This is just an example of leverage. Another example is a lever, and most can see how those work...the closer the fulcrum is to the load, the easier it is to use the lever to lift the load.

Take a 6 foot bar, place it's tip under a load and a fulcrum under the bar 3 feet from the load...push down on your end 2 inches and the load lifts 2 inches...no MA. But place the fulcrum 1.5 feet from the load and push down on the bar 2 inches...the load lifts only 1 inch, you have attained 2:1 MA. Move the fulcrum closer yet, and the MA is increased.

Moving more rope to move an object less distance works the same. More pulleys equals moving the fulcrum closer to the load.
 
True, Burnham, but pulling oneself up in a DRT system is much easier with a friction saver versus a natural crotch, which is why I mentioned the reduced friction as a factor. (I thought it was clear that the pulleys allowed more legs of rope to come into play.) No, the elimination of friction is not what enables pulleys to compound pulling power. But I think a careful study will reveal that the pulleys do come nearer yielding true ratios (2:1, 3:1, etc) than a z-rig set up around a tree as Jed mentioned. Without the pulleys, the theoretical ratio would still be 3:1, but the resistance on the rope running over a trunk and back through a loop in itself would result in you having to pull harder to yield the same result.

Sorry if it sounded like simply reducing friction meant compounded force. If it does, I'll start spraying a little WD40 on my ropes.:D
 
Fair point, but...

Reducing friction is a factor only in the real world...in a theoretical sense, which is the only way we'd ever actually achieve a true 2:1 or whatever MA you want to talk about, it is not considered. And it only confuses the issue when trying to explain the concepts of MA.
 
I agree - we're confusing two different things.

1. The way in which a friction saver reduces the force required to body thrust into a tree has nothing to do with mechanical advantage. When you pull down on one end of the line in order to hoist yourself up on the other end of the line, you pull down against your body weight PLUS a certain amount of friction. By reducing that friction, you don't have to pull quite as hard.

2. The way in which MA systems compound pulling power is by pulling on multiple strands of line at once, all of which are connected to the load. When you pull on 3 strands of line with 100 lbs of force on each one, the tree actually feels a cumulative 300 lbs of force. This is a 3:1 system.
 
I was simply saying that the reduction in friction was part of "spreading" the load. I suppose that in the real world, that is not a true definition. That said, I will withdraw to an observation-only stance and commence banging my head against the wall.:banghead:
 
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