How a block and tackle works

How a block and tackle works

A block and tackle is an example of a class of tools called ‘force multipliers’. If you’ve ever removed a nut from a bolt with a wrench that you could not twist off with your fingers, you’ve used a force multiplier.

Imagine you have a very heavy object, say a hundred-pound crate that you need to lift a certain height off the ground, to load on a truck or ship. To do so, you will have to apply a hundred pounds of force upward against gravity. A strong person could lift a small hundred-pound crate, but if the size of the crate were too large, even the strong person would need help.

The first improvement you might make would be to suspend a pulley from a crane, the warehouse or a stout tree limb, and attach a rope to the crate, threaded through the pulley. Then pulling down on the rope raises the crate. The advantage you gain is that most people find it easier to apply a hundred pounds of force to pull a rope that to lift up against gravity, but in terms of the force needed, you haven’t gained anything. You will still need to apply a hundred pounds of force to lift the weight.

A further refinement is to add a second pulley, attached to the crate itself. Tie the rope securely to a hook in the ceiling or crane, thread through the pulley on the crate, and again through the original pulley overhead, and pull down on the rope. Two ropes, or rather, two pieces of the same rope, are now pulling upward on the crate, so you can lift it with only fifty pounds of force on the pulling end of the rope. You will have to pull in twice as much rope as the distance the crate will raise.

Adding more paired pulleys to the system, one on the weight side and the other on the stationary side (the ceiling or other immovable object), will decrease the force needed to raise the weight, while increasing the amount of rope you will have to pull in to raise it. Unfortunately, you can’t simply add pulleys to each side indefinitely and be able to lift a hundred pounds with your little finger. Friction of the rope against the pulley subtracts slightly from the force that is available to lift the weight, and with each additional pulley, the component of force that is wasted to friction increases.

Incredibly, the great pyramids of Egypt were apparently built without the benefit of cranes and block and tackle techniques, which were unknown to the Egyptian builders and architects of the time. The identity of the originator of the block and tackle is lost to history, although the great inventor Archimedes was known to use them.

The wooden block with two pulleys set side-by-side within it is what most people think of when they hear the phrase ‘block and tackle”. Used in pairs, they are invaluable on sailing ships, where large sails must be moved against the force of strong winds.

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