People think pulleys increase pulling power.

camp4x4

Adventurer
Apparently you missed the Jr High science class..two lines mean nothing . Everyone read and let me know... Spend some time on a ship.
.https://www.explainthatstuff.com/pulleys.html
Randy,
Apparently you didn't read your own reference article...

"Now if you add more wheels, and loop the rope around them, you can reduce the effort you need to lift the weight. Suppose you have two wheels and a rope looped around them, as in the figure below. The 100kg mass (1000 newton weight) is now effectively supported by two sections of the same rope (the two strands on the left) instead of just one (ignoring the loose end of the rope you're pulling with), and this means you can lift it by pulling with a force of just 500 newtons—half as much! That's why we say a pulley with two wheels, and the rope wrapped around it this way, gives a mechanical advantage (ME) of two.

Mechanical advantage is a measurement of how much a simple machine multiples a force. The bigger the mechanical advantage, the less force you need, but the greater the distance you have to use that force. The weight rises 1m, but now we have to pull the loose end of the rope twice as far (2m). How come? To make the weight rise 1m, you have to make the two sections of rope supporting it rise by 1m each. To do that, you have to pull the loose end of the rope 2m. Notice that we can also figure out the mechanical advantage by dividing the distance we have to pull the rope by the distance the weight moves."

Because both ropes are pulling on the vehicle you get a 2:1 mechanical advantage out of the snatch block on a tree. Imagine it like this:

525150

This is the original example. So clearly if the immovable object isn't going anywhere, and your vehicle can roll then the 100 newtons of weight that can be pulled is the vehicle.
525152

You need to check your arrogance and actually examine your argument.
 

Metcalf

Expedition Leader
It doesn't matter if the line goes back to the source of the pull.
The winch doesn't do anything except pull(it's performing all the effort).With one pulley the winch will have to pull a 1000 lbs wherever it's attached.
Now add a pulley at the vehicle, run the line out to the tree pulley, back through the vehicle pullley and attach at the tree, you now cut the effort in half(500lbs).

Nope.

A single pulley will cut your load in half IF the working end of the line is attached back to the vehicle.
This gives you a 2:1 mechanical advantage
 

Essayons

Member
Randy,
Apparently you didn't read your own reference article...

"Now if you add more wheels, and loop the rope around them, you can reduce the effort you need to lift the weight. Suppose you have two wheels and a rope looped around them, as in the figure below. The 100kg mass (1000 newton weight) is now effectively supported by two sections of the same rope (the two strands on the left) instead of just one (ignoring the loose end of the rope you're pulling with), and this means you can lift it by pulling with a force of just 500 newtons—half as much! That's why we say a pulley with two wheels, and the rope wrapped around it this way, gives a mechanical advantage (ME) of two.

Mechanical advantage is a measurement of how much a simple machine multiples a force. The bigger the mechanical advantage, the less force you need, but the greater the distance you have to use that force. The weight rises 1m, but now we have to pull the loose end of the rope twice as far (2m). How come? To make the weight rise 1m, you have to make the two sections of rope supporting it rise by 1m each. To do that, you have to pull the loose end of the rope 2m. Notice that we can also figure out the mechanical advantage by dividing the distance we have to pull the rope by the distance the weight moves."

Because both ropes are pulling on the vehicle you get a 2:1 mechanical advantage out of the snatch block on a tree. Imagine it like this:

View attachment 525150

This is the original example. So clearly if the immovable object isn't going anywhere, and your vehicle can roll then the 100 newtons of weight that can be pulled is the vehicle.
View attachment 525152

You need to check your arrogance and actually examine your argument.
You missed the part "ignoring the part of the rope you are pulling with" . With a single pulley, you don't count it. Reduced effort doesn't begin until after the first pulley,

Let's do this, let's say someone needs to lift a 100lb object in a loft. With the previous poster's premise, I could attach a line on the loft floor, run it down to the object(with the pulley) and back up to me on the loft floor, and when I pull it with be only 50 lbs effort. It won't be, it will still require 100 lbs. effort,
 

Metcalf

Expedition Leader
You missed the part "ignoring the part of the rope you are pulling with" . With a single pulley, you don't count it. Reduced effort doesn't begin until after the first pulley,

Let's do this, let's say someone needs to lift a 100lb object in a loft. With the previous poster's premise, I could attach a line on the loft floor, run it down to the object(with the pulley) and back up to me on the loft floor, and when I pull it with be only 50 lbs effort. It won't be, it will still require 100 lbs. effort,

That would be redirection, not mechanical advantage.

If you are the 100lb object providing the force you gain mechanical advantage.
If you are NOT the 100lb object providing the force you loose your mechanical advantage.

You can easily demonstrate this with a Boatswain Seat and some simple rigging.
 

Essayons

Member
That would be redirection, not mechanical advantage.

If you are the 100lb object providing the force you gain mechanical advantage.
If you are NOT the 100lb object providing the force you loose your mechanical advantage.

You can easily demonstrate this with a Boatswain Seat and some simple rigging.
180 degree direction is not a change in direction? Can uou lift your own weight?Then do this. Obtain a pulley, tie it off on a rafter, run a line through it. Tie a loop on one end and sit in it. Now that your weight is cut in half, pull yourself up. Good luck.
 

Metcalf

Expedition Leader
180 degree direction is not a change in direction? Can uou lift your own weight?Then do this. Obtain a pulley, tie it off on a rafter, run a line through it. Tie a loop on one end and sit in it. Now that your weight is cut in half, pull yourself up. Good luck.

As I said, you can do this with a Boatswain chair and simple rigging to test it.

In simple terms, it is all about force over distance.

Once the working end of the line is attached back to the force generator it isn't redirection anymore, it becomes mechanical advantage.
It is no longer a 1:1 ratio. You will have to pull 2x as far to move the load the same distance, this cuts the force in exactly half.
 

camp4x4

Adventurer
You missed the part "ignoring the part of the rope you are pulling with" . With a single pulley, you don't count it. Reduced effort doesn't begin until after the first pulley,

Let's do this, let's say someone needs to lift a 100lb object in a loft. With the previous poster's premise, I could attach a line on the loft floor, run it down to the object(with the pulley) and back up to me on the loft floor, and when I pull it with be only 50 lbs effort. It won't be, it will still require 100 lbs. effort,

Dude, seriously, slow down and pay attention. "... the part of the rope you are pulling with..." is the one going off to the side, not either of the ones connected to the mass or the vehicle. This is literally as clear as I can make it.


525181

This is not what we're talking about. This is the redirected end of the rope being attached to an immovable object, NOT back to the vehicle.

525183

525184
 

762X39

Explorer
Wow, now I know why I never let anyone else rig for me. I had a hard time in physics back in the early 70's but the basic concepts were clear. This thread, not so much...
 

camp4x4

Adventurer
Wow, now I know why I never let anyone else rig for me. I had a hard time in physics back in the early 70's but the basic concepts were clear. This thread, not so much...

I definitely understand why people get confused; there's a significant difference base on the seemingly small act of attaching the end point back to the vehicle vs attaching it to something else. One doubles the pull, one doesn't. People seem to have a hard time getting their heads around that.
 

ChinaLakePW

Member
Most of the diagrams don't show the relationship between the winch and the pulley. The "moveable pulley" diagrams reflect what is taking place:
525200

Assuming the pulley is anchored to a tree, a winch pulling with 5N of force exerts 10N on the tree. This is where people underestimate the requirements (WLL) of the shackle attached to that pulley.
 
Only lines attached to the load or anchor point are included in the calculations. i.e. in a self recovery off of a tree, there is one line involved -1/1.

If you run that line through a snatch block back to the winch, there's 2 lines between load and anchor - 2/1.

Now I run from winch/vehicle #1 to a snatch block attached to a tree, to vehicle #2, there is only 1 line from the load (Vehicle #2) and the anchor (the tree) - 1/1.

If you ran the line back to the tree from Vehicle #2 you would have 2 lines between load and anchor - 2/1. As many lines through pulleys as you have between load and anchor divides that total load. The line going off to the winch or whatever would be equal to one of the load lines.

 

67cj5

Man On a Mission
All this is all very well but the major benefit of using one or more pulleys/Snatch Blocks Is the it Low gears the speed of the load being moved which allows for a slower more refined recovery, The down side is that it does not give you 3X the pulling Power for a few simple reasons, Using more pulleys is more about precision placement of the load that needs to be moved

1) A 9000Lbs winch and cable is still a 9000Lbs rig So if you add 2 pulleys that is not going to give you 27000Lbs of pulling power because the winch doing the pulling is rated at 9000Lbs and it is Pulling a Single Line and that line goes through those 2 pulleys from the winch to a single Anchor Point from a single cable.

2) the Winch is a single 9000Lbs of pulling power and the Anchor Point is a single 9000Lbs Fixed Point. At this point the weakest links are the 9000Lb Winch and the cable at it's Anchor point.

If a person needs to move a 27000Lb Load then they still need a winch that is rated at 27000Lbs to 30000Lbs or bigger, You can't go putting a 27000Lb Load on a 9000Lb Winch, It does not matter if you use 10 pulleys because it is a 9000Lb winch doing the pulling on a single line to the winch as is the single line Anchor Point the winch cable of a 9000Lb winch is not rated to pull 27000Lbs, By using pulleys all you are doing is low gearing the winch / Travel Speed,

Just because a pulley reduces the speed/Load ratio does not mean you can exceed the Rating of the Winch, this is why the Winch companies tell you to always buy a Winch 2 and a half times bigger that the load that needs to be moved, IE your 6000Lb Truck needs a 15000Lb to 17500Lb (18000) Winch,

IE My Truck/SUV weighs 3990Lbs kerb weight (Empty)
Loaded it weighs 5533Lbs
With a Trailer hooked up fully loaded it can legally weigh 11927Lbs,

So if I am Towing a Camper Trailer etc then my 9500Lb Winch is useless because with just the Truck '5533Lbs @ 9500Lbs minus 5533lbs = 3967Lbs spare, So the correct smallest winch I should be using is 13832Lbs @ 2.5 times the weight of my Truck and the closest winch with that kind of pulling power is a 15000Lbs and if I needed a Winch that can handle the gross vehicle Combination weight then @ 2.5 times that weight then I would need a Winch that is rated for 29820Lbs just to keep within the Winch companies recommendations, which is totally nuts because a winch that powerful would tear the truck in half,

Winching and pulleys etc are a science all by them selves and Pulleys do not half the load all they do is Half the speed of the line pull because the Winch and the Anchor Point still have to deal with the full weight of the Load.
 

luthj

Engineer In Residence
Wow, it's absolutely amazing. Pulleys seem so obvious that folks have no idea how little they actually know...

You could spend 30 minutes educating yourself. Nah... I'll just be a keyboard cowboy...

 

67cj5

Man On a Mission
Oh No !!!
Not another winch dyslexic... Will it never end ??
Keep ya insults to your self,

The point is no matter how many pulleys you use there is still "ONE" Single cable coming off the Winch and that One Single Cable is attached to a Single Anchor Point and it is those 2 single points which are the weak points, and Regardless of how many pulleys you use because that ONE SINGLE CABLE is still only rated at 9000Lbs.
 

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