# Strength and pulleys

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How does it work metal wires, and pulleys?

Had a quick chat with a crane operator, using a wire for lifting. Asked what the max load he could lift. He said X amount, but if he uses a pulley, he could lift almost 2 times X.

Is this true? Is there a max to how many pulleys you can use/weight you can lift
Is the lifting curve linear or exponential?

TYIA

In: 20

It’s all about mechanical advantage. There is a practical limit to it, as every time you introduce a pulley you have to reduce the systems efficiency by about 10% due to friction.

You can think of pulleys as levers. Loop a rope through a pulley hanging from the ceiling, loop it through a loose one sitting on the floor, then connect the rope end to the ceiling. When you pull on the loose end, the loose pulley will rise up half as far as you pull, because the amount of rope you’ve pulled is split between the segment going from the pulley on the ceiling to the loose pulley, and the segment from the loose pulley to the end connected to the ceiling. Just like a lever, you’ve doubled the amount you can lift, but halved the distance you can move it for a given amount of effort.

In a frictionless world, there would be no limit to how many pulleys you can use (with a long enough lever, I could move the world) But apart from friction (and a finite amount of rope on earth), there isn’t much else stopping you from using all the pulleys you’d like.

Yes, you can add pulleys to increase the force exerted at the lifting hook of the crane, but you can’t make the crane heavier, or the boom stronger. The limits on what the crane can lift have more to do with not tipping the crane over and not buckling the rigid structure. There is no point in installing a pulley system that will lift more than what would tip the crane over.

The max will eventually be the physical limitations of the equipment. A wire rope can only take but so much load before it breaks. A crane can only lift but so much weight before it topples over. But if you ignore the basic physics of it there is no limit. I have heard of trucks using multiple snatch blocks and literally pulling the winch off the frame of their vehicles. Force multiplication is powerful.

If you want to lift something up, you’ve got to use your muscles. We call the amount of strength you use “force”. You’re only so strong, so there’s a limit to how much force you can use.

Moving a heavy box a short distance isn’t as hard as moving it a long distance. It’s more work. Work is force x distance. It’s how much effort or energy you need to move something from one place to another before you’re exhausted (out of energy).

If you want to lift something, it’s work. What if it’s heavy, though? Maybe you’re not strong enough to force it to move? What if you wanted to cut the amount of force you need (because you’re only so strong) to do that work?

Work = force x distance

You can halve the force if you double the distance to do the same work. If you have a lever where on end moves 1’ while you push the other end 2’, the force on the side moving 1’ is double the force moving 2’ but the work stays the same. If you have two gears, one with twice as many teeth as the other, the small gear turns with double the force of the other – again, 2x distance x ½ force.

In the case of a pulley, the cable loops from the top and back up again so that every 2’ you reel in of cable the pulley moves 1’. By doubling the distance that you apply the force, you halve the force needed to do the work.

Wires and ropes work under *tension*. You can’t push a rope. Pulling a rope under tension effectively shortens its length. A pulley redirects a length of rope, which can redirect the forces of that rope.

If you tie one end of a rope to something solid (immovable), run it through a pulley *that’s connected to the load*, and pull on the other end, there end up being *two* sides of the same rope pulling on the load. Your pulling force on the load is effectively doubled. The trade off is that you’d have to pull twice as much rope to move it the same distance as without a pulley.

Max number of pulleys is limited by how much rope you have and how much distance the load needs to travel. Then it’s limited by the tensile strength of your rope/wire.

SmarterEveryDay [did a pretty good video on pulleys](https://youtu.be/M2w3NZzPwOM). I also really like [this video of someone lifting 88kg with a LEGO hoist](https://youtu.be/owv6cOmLGgo).

With X pulleys, you need X times less strength to pull an object than with 1 pulley or directly (you would need the same strength, just not in the same direction).

Theoretically, you could use an enormous amount of pulleys and lift anything with one finger. But practically, you need your pulleys to be strong enough not to break, to not have too much friction…

you can lift more, but over a shorter distance (you need more wire/rope), because you’re essentially doubling up the wire, so for each pulley you can pull (nearly) twice as much as before, i.e. you’re using 2 wires, each with its own capacity. Think of a pulley as a “cut” of the wire and then each wire that hangs down from the pulley as its own wire – then it makes more sense intuitively that it doubles the capacity.

Each pulley slightly decreases the total capacity because the wire is bending, but not by much, so if you have a long enough wire and a supporting structure that can handle it, you can get quite a few pulleys in there, but that’s more academic – for real situations, I’ve seen 6 or 7 pulley block and tackles. There’s probably bigger ones.

If one wire can hold X tension, then two wires can hold 2X tension, yes obviously. Also, how much wires can hold is a completely separate issue from how much the crane can hold, you may not exceed maximum capacity of that.

Where’s Destin we you need him?

0 views

How does it work metal wires, and pulleys?

Had a quick chat with a crane operator, using a wire for lifting. Asked what the max load he could lift. He said X amount, but if he uses a pulley, he could lift almost 2 times X.

Is this true? Is there a max to how many pulleys you can use/weight you can lift
Is the lifting curve linear or exponential?

TYIA

In: 20

It’s all about mechanical advantage. There is a practical limit to it, as every time you introduce a pulley you have to reduce the systems efficiency by about 10% due to friction.

You can think of pulleys as levers. Loop a rope through a pulley hanging from the ceiling, loop it through a loose one sitting on the floor, then connect the rope end to the ceiling. When you pull on the loose end, the loose pulley will rise up half as far as you pull, because the amount of rope you’ve pulled is split between the segment going from the pulley on the ceiling to the loose pulley, and the segment from the loose pulley to the end connected to the ceiling. Just like a lever, you’ve doubled the amount you can lift, but halved the distance you can move it for a given amount of effort.

In a frictionless world, there would be no limit to how many pulleys you can use (with a long enough lever, I could move the world) But apart from friction (and a finite amount of rope on earth), there isn’t much else stopping you from using all the pulleys you’d like.

Yes, you can add pulleys to increase the force exerted at the lifting hook of the crane, but you can’t make the crane heavier, or the boom stronger. The limits on what the crane can lift have more to do with not tipping the crane over and not buckling the rigid structure. There is no point in installing a pulley system that will lift more than what would tip the crane over.

The max will eventually be the physical limitations of the equipment. A wire rope can only take but so much load before it breaks. A crane can only lift but so much weight before it topples over. But if you ignore the basic physics of it there is no limit. I have heard of trucks using multiple snatch blocks and literally pulling the winch off the frame of their vehicles. Force multiplication is powerful.

If you want to lift something up, you’ve got to use your muscles. We call the amount of strength you use “force”. You’re only so strong, so there’s a limit to how much force you can use.

Moving a heavy box a short distance isn’t as hard as moving it a long distance. It’s more work. Work is force x distance. It’s how much effort or energy you need to move something from one place to another before you’re exhausted (out of energy).

If you want to lift something, it’s work. What if it’s heavy, though? Maybe you’re not strong enough to force it to move? What if you wanted to cut the amount of force you need (because you’re only so strong) to do that work?

Work = force x distance

You can halve the force if you double the distance to do the same work. If you have a lever where on end moves 1’ while you push the other end 2’, the force on the side moving 1’ is double the force moving 2’ but the work stays the same. If you have two gears, one with twice as many teeth as the other, the small gear turns with double the force of the other – again, 2x distance x ½ force.

In the case of a pulley, the cable loops from the top and back up again so that every 2’ you reel in of cable the pulley moves 1’. By doubling the distance that you apply the force, you halve the force needed to do the work.

Wires and ropes work under *tension*. You can’t push a rope. Pulling a rope under tension effectively shortens its length. A pulley redirects a length of rope, which can redirect the forces of that rope.

If you tie one end of a rope to something solid (immovable), run it through a pulley *that’s connected to the load*, and pull on the other end, there end up being *two* sides of the same rope pulling on the load. Your pulling force on the load is effectively doubled. The trade off is that you’d have to pull twice as much rope to move it the same distance as without a pulley.

Max number of pulleys is limited by how much rope you have and how much distance the load needs to travel. Then it’s limited by the tensile strength of your rope/wire.

SmarterEveryDay [did a pretty good video on pulleys](https://youtu.be/M2w3NZzPwOM). I also really like [this video of someone lifting 88kg with a LEGO hoist](https://youtu.be/owv6cOmLGgo).