Pulleys
The single pulley Double pulley Multiple pulleys The pulley rule
Mechanical advantage and pulley systems Transferring movement between pulleys

Click for larger image Pulleys are used to make doing work easier and are found everywhere; in the home, in lifts, in motor mechanics' workshops and on yachts for a start. Pulleys are simple machines using wheels and rope, or cable, or even chain, where one or more pulley wheels can be combined to reduce the effort force required in lifting or moving an object.

Pulleys are also used to transfer movement from one wheel to another as you can find under the bonnet of your car, or in a tape player for example.

The single pulley
Click for larger image
In this case the effort required is equal to the weight of the object to be moved, but the change of direction means that a load can be lifted to a height, lowered from a depth or moved to a point away from where the effort is exerted.





Double pulley

Click for larger image In this pulley system the effort required is only half the load, but the rope has to travel twice as far so the same amount of work is done. Double pulleys are useful for lifting objects whose mass might be too much for one person on a single pulley, or even if the mass to be lifted exceeds that of the person.


Multiple pulleys
Click for larger image Systems can be assembled with any number of pulleys, for example a block and tackle used to lift and lower car engines often has three pulleys, meaning that the effort is only one quarter of the load, in this case the engine. It also means that the chain has to move four times the distance.







The pulley rule

Calculations of effort and distance are easily established. You will have seen from the above examples that there is a simple relationship between number of pulleys, relative effort and distance the rope travels compared to the distance the load moves. For one pulley each is the same. For a double pulley system the effort halves while the distance the effort moves doubles, and for a quadruple system containing three pulleys, it is one quarter the effort and four times the distance.

Mechanical advantage and pulley systems
It is not as simple as counting the pulleys to determine the mechanical advantage (MA) a system will provide. What is important is the distance the rope or chain moves in relation to the input force and output lift. If the user pulls in 4 metres of rope to lift a weight 1 metre, then the MA is 4. Different systems will deliver different MA values.

Transferring movement between pulleys
Click for larger image Under the bonnet of your car you will find one or more rubber belts between pulley wheels of different sizes. If the drive pulley was a greater diameter than the driven pulley then the driven pulley turns faster, the reverse is also true. If the drive pulley has a diameter of 8 cm and the driven pulley a diameter of 2 cm then the driven pulley will rotate at 8/2 = four times the speed. It's also possible to change direction by using belts and pulleys - in this case if the belt crosses then the driven pulley will turn in the opposite direction. This can clearly be seen on steam-engine powered harvesters for example.

Copyright owned by the State of Victoria (Department of Education and Early Childhood Development). Used with Permission.

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Effort Mass