Pulleys
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
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
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
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
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.
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