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
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.
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