Pulley – A wheel that usually has a groove around the outside edge. This groove is for a rope or belt to move around the pulley. Pulling down on the rope can lift an object attached to the rope. Work is made easier because pulling down on the rope is made easier due to gravity. Examples: Flag Pole, Crane, Mini-Blinds
A pulley |Pulley Usually, two or more pulleys are used together. When pulleys are used together in this way, they reduce the amount of force needed to lift a load. A crane uses pulleys to help it lift heavy loads. Pulley is one of the six simple machines. more…
pulley (animation) – The amount of increase in the force depends on how many times the rope wraps round the pulleys. By wrapping the rope several times around the pulleys it is easily possible to lift your own weight off the ground!
Mechanical advantage on the high seas
- pictured: A double tackle has two pulleys in both the fixed and moving blocks with four rope parts dividing the load of 100N. The mechanical advantage is 4, requiring a force of only 25N to lift the load.
Long before there were motors, huge ships with big heavy sails crosses the oceans powered by wind. They were controlled by men using ropes and pulleys, like those in the picture. Figuring out how to move these great heavy sails took at lot of thought following the steps in the [/Engineering%20Design%20Process Engineering Design Process].
- Ask – Just pulling on ropes attached to the sails limited the amount of sail that could be used. There needed to be a way to lift more weight by the same number of people. What materials were available? What shapes could be made from these materials?
- Imagine – Once they figured out how to lift twice as much weight using a single pulley, they worked out that it might be possible to do even better. A set of pulleys assembled so they rotate independently on the same axle form a block. Two blocks with a rope attached to one of the blocks and threaded through the two sets of pulleys form a block and tackle. If one set of ropes and pulleys was an improvement, how could more pulleys be used?
- Plan – By working out how to make the pulleys and attach them to the sails or objects to be lifted, even with simple material like wood, they were able to make the pulleys. How could the pulleys be combined to provide the same function? How did the wheel in the pulley guide the rope? How did they make sure the rope ran smoothly?
- Create – The blocks or pulleys in the picture are made of wood. There is another loop of rope, so there is even more mechanical advantage than in the illustration. Today pulleys can be made of metal or plastic, depending on how the pulley will be used. What are some of the materials used for pulleys today?
- Improve – Sailors still use pulleys to control sails, though most are not made of wood any longer. There have been other improvements in pulley design and construction over the years. Reducing friction in the pulleys reduced the amount of effort required to lift a given load. Rope made from plant fibers have been replaced with man-made fibers that are hundreds of times stronger. What are other limits on the effectiveness of a rope and pulley system?
- gravity, weight and mass – we say things fall because the Earth’s gravity pulls on them. Weight changes when the pull of gravity changes. The Moon is much smaller and the pull of gravity on the Moon is about 1/6th that of Earth. So any object on the Moon weighs 1/6th of its weight on Earth. What does not change is the amount of matter in an object. That is called its mass. On Earth, mass and weight are the same, for practical purposes.
Engineering vocabulary, concepts
- wheel, gravity, lift, load, mechanical advantage, mass, weight
Here are some challenges for you to work on…
- find other examples of pulleys being used to lift heavy objects.
- design a pulley system that you can build with “found” objects – things that have been thrown away or are not being used for their original purpose.
- Pulley’ing Your Own Weight – Pulleys enable us to get big items up to the roof without any trouble at all. But, what route do we take using the pulleys?
- Play with pulleys – A single fixed pulley lets you pull in one direction to move a thing in the other direction.