Pulley

A pulley is a simple machine made of a wheel with a groove around its rim, with a rope, chain or belt running over the wheel. By pulling one end of the rope, you can lift a load attached to the other end. A single fixed pulley simply changes the direction of the force (pulling down to lift up). Several pulleys combined can dramatically multiply your strength, letting a single person lift hundreds of kilograms. Pulleys have been used for thousands of years, from raising sails on ancient ships to operating modern lifts in skyscrapers.

• What it isWheel with rope or chainRope runs in a groove
• Fixed pulleyChanges direction onlyNo force multiplication
• Movable pulleyDoubles your forcePull twice as far
• Block and tackleMany pulleys combinedForce multiplied by N ropes
• Used sinceAncient times3,000+ years ago
• Modern usesCranes, lifts, blinds, flagpolesPlus ship rigging and gym equipment

How a pulley works

A pulley is essentially a wheel that lets a rope change direction smoothly with very little friction. The grooved rim keeps the rope in place. By running a rope over a pulley, you can:

• Change the direction in which you pull (often more convenient).
• Use your body weight to pull down instead of trying to lift up.
• Multiply your strength when combined with other pulleys.

Types of pulley

Fixed pulley

A fixed pulley is attached to a fixed point (like a ceiling or beam). The rope passes over it. Pulling down on one side lifts the load on the other side. The force you apply is the same as the loads weight: it does not multiply your strength, but it lets you pull down (using your body weight) instead of lifting up. Examples: flagpoles, window blinds.

Movable pulley

A movable pulley is attached directly to the load. One end of the rope is fixed to the ceiling; the other end goes up over the pulley and back down. When you pull on the free end, the load rises by half the distance you pull. Your effort needs to be only half the loads weight, but you have to pull twice as far. Mechanical advantage = 2.

Block and tackle

A block and tackle combines several fixed and movable pulleys to multiply force further. The mechanical advantage equals the number of rope sections supporting the load. A system with 4 supporting rope sections multiplies your strength by 4: you can lift a 200 kg load with just 50 kg of effort, but you have to pull 4 times the distance.

Fact The Greek mathematician Archimedes is said to have demonstrated the power of pulleys around 250 BC by single-handedly pulling a full warship onto land, using a clever system of ropes and pulleys. King Hiero of Syracuse was astonished. The story may be exaggerated, but Archimedes really did make important advances in pulley design for the king of his island.

Pulleys in everyday life

• Flagpole: a single fixed pulley at the top lets you raise and lower the flag from the ground.
• Window blinds: a single pulley turns the cord motion into raising and lowering of the slats.
• Sailing boats: extensive pulley systems (called blocks) let one or two sailors haul up huge sails.
• Cranes: building site cranes use block-and-tackle systems to lift heavy loads with motorised winches.
• Lifts: passenger lifts use heavy steel cables running over big pulleys at the top of the lift shaft, plus a counterweight that balances most of the cabins weight.
• Gym equipment: many resistance machines use pulleys so you can pull a cable at different angles to exercise different muscles.
• Garage doors: counterweights on pulleys make the heavy door easy to lift.
• Theatrical curtains: pulleys above the stage let crew open and close heavy curtains.
• Rock-climbing rescue: rescuers use pulley systems to haul injured climbers up cliffs.

Did you know? A modern passenger lift uses a clever pulley trick: the cabin is connected over a pulley at the top of the lift shaft to a counterweight that weighs about as much as the empty cabin plus half a load. This means the motor only has to lift the imbalance, not the full weight of the cabin. The result is that even a heavy lift can be moved by a relatively small motor.

Trade-offs of using a pulley

Pulleys obey the same rule as all simple machines: you cannot get something for nothing. If a system has a mechanical advantage of 5, then to lift a load by 1 metre you have to pull the rope by 5 metres. Total work done stays the same.

The main benefits are:

• You can use less force at a time, so a single person can do work that would normally take a team.
• You can pull in a more comfortable direction (downwards instead of upwards).
• You can let the rope through slowly, controlling a heavy load as it lowers.

Pulleys vs other simple machines

For lifting heavy loads, pulleys often replace levers and inclined planes. They are particularly useful for:

• Lifting things straight up over long distances (where a lever would need impossibly long arms).
• Lifting in tight spaces (where a ramp would not fit).
• Working from far away (you can stand far from the load and pull on a long rope).

Try this Hang a rope over a sturdy door frame or low tree branch (this acts as a fixed pulley). Tie one end to a bag of books and lift them by pulling down on the other end. The force is the same as the bags weight, but it is much easier than lifting up because you can use your body weight to help. If you can rig two pulleys (one fixed, one attached to the load), the force needed will be just half the bags weight, although you have to pull the rope twice as far.

Deeper dive: how block and tackle made the age of sail possible

From around 1500 to 1850, the worlds oceans were ruled by giant sailing ships. The British, Dutch, Portuguese, French and Spanish empires all relied on fleets of three-masted ships carrying spices, silks, sugar, tobacco, slaves and ideas around the world. None of it would have been possible without one simple machine: the block and tackle.

A large warship like HMS Victory (Nelsons flagship at Trafalgar) had over 40 different sails, totalling about 6,500 square metres of canvas. Each sail had to be raised, set, reefed (rolled up part way), and lowered as the wind changed. The sails and their ropes weighed several tonnes. Without pulleys, just one big sail would have needed dozens of men hauling on a single rope.

Pulleys (sailors called them "blocks") solved the problem. A typical ship of the line had thousands of pulleys built into hundreds of block-and-tackle systems. Each system multiplied the strength of the crew, letting a small team raise, lower or trim a heavy sail.

Sailors had a special vocabulary for it all. A simple block was a single pulley in a wooden case. A "double" or "triple" block contained 2 or 3 pulleys side by side, sometimes giving a mechanical advantage of 6 or more. The whole network of ropes and blocks was called the ships rigging, and it took years for a sailor to learn it.

The era of giant sailing ships came to an end as steam engines (and later diesel engines) took over. But every modern yacht, racing boat and tall ship still uses the basic block-and-tackle principles worked out by ancient sailors. Walk down a marina today and you will see ropes running through pulleys exactly as they did 500 years ago.

For more, see lever and wheel and axle.