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.