How does a Block and tackle lift heavy things?


I’ve always seen them on TV, and in professional garages and factories. I know they allow you to lift things far heavier than you normally could. But, how do they actually do that? I have seen that you pull one chain alot and it lifts it a little, so I’m sure that has something to do with it.

In: Engineering

It’s really hard to describe simply but the most coherent I can make it is by saying that for each line the forces are distributed against the pulley. If you were to lift a 100 lb object with just a pulley you would need to exert 100 lbs of force. However with a block and tackle wrapped once there is now another “rope” taking half the weight of the object so now you only need to exert 50 lbs to lift it (the system doubles your effort.) With more repetitions in the block the greater the force is multiplied.

Look on YouTube for SmarterEveryDay’s snatch block video. He’ll describe it better than any one is going to be able to do in text.

But basically, the rope is all at the same tension, and the pulleys transmit all that tension to the block. So if you pull with 50 units of force, and there’s 6 ropes going to the mobile block, then you’ll have 300 units of force pulling on the block, but have to move the rope 6x the distance that the block will move.

Edit: example ignores friction. So real world block and tackle systems will fall short of the ideal.

Compounded, Newtonian directional forces. As you pull on one end of the rope, it is pulling one of the other supporting pulleys up, and that is pulling another. And as that is happening, you are providing another directional support of the “slack” in the rope that generated as a byproduct, so that one pulley is just not free climbing without providing pull on the rest of the system.

Newtonian directional forces are described in physics as, g/kg/lb/T then, N 👈👉👆👇(cant find arrow symbol on my phone keyboard)

Only thing I remember from my waste of money in college, before I jumped ship, became a free man, and decided to teach myself what I need to know for a specific project.

Also see the all about pulleys episode with Dustin @ Smarter Every Day:

You pull the rope a foot but the weight moves only an inch. So it feels like it’s 1/12 the weight. Or it could be 5:1 or 100:1 or whatever, depending on how you set it up.

Because the rope is supported by pullies which are free to rotate, the tension on the rope is basically the same along it’s whole length.

This means you may be able to supply a downwards force of 350N (78lbs) on the free end of the rope. If the block and tackle system creates 6 returns of rope between the load and the support or anchoring structure, then the system has a Force Multiplication factor of 6. The block and tackle will produce a force of 2100N or or 468lbs.

The force on the load is equal to the number of returns of rope between the load and anchor, multiplied by the tension on the free end.

Nothing’s for free, right? If a block and tackle allows you to lift much heavier weights than you could normally, there has to be a trade-off, right? Right.

Here’s the insight I just recently gained that helped me understand. The trade-off you’re making with a block and tackle is that you have to pull the chain/rope a lot farther, to raise something than you would if you were doing it without the block and tackle.. I mean, if you were just pulling straight on a rope to hoist something, you need to pull ten feet of rope to lift the object ten feet. With a block and tackle, you have to pull a lot more than just ten feet through the block in order to lift an object just ten feet. That’s the cost. The benefit is that you can use the technique to lift much heavier things.

Basically, you’re sacrificing the distance the weight moves in return for moving a bigger weight. Half the distance, double the weight, and so on.

Say you’ve got a weight dangling on a rope. And, conveniently, it’s the heaviest weight you can lift this way. You pull the end of the rope upwards two feet (say). The whole weight moves up two feet. That was hard.

Now you double the rope with a block and tackle (down from a fixed point, through a pulley, and back up to you). You pull the rope end up two feet again. The rope is doubled, though, those two feet are equally spread either side of the pulley, and the weight only rises by one foot. You’ve moved the rope the same amount, but with only half the result – so you only did half as much work. Which, unsurprisingly, felt way easier.

OK – now double the weight as well, and try again. You’re going to have to do twice as much work as the previous time – so that’s the same work as the very first time. And we know you can, just, pull that hard. The weight still only moves a foot, because the rope is doubled – but you’ve just lifted something that’s twice as heavy as the heaviest thing you can lift with a single rope. Just, not as far.

Friction will have its say, clearly – but basically, the more times you pass the rope back and forward, the bigger the weight you’ll be able to move. But you’ll need to pull the rope further and further each time to get the same result.