Pulleys
Pulleys work in a similar way to gears, except
they are not directly joined but linked by a belt
made from elastic bands, tubular springs or
some other flexible but strong material.
A common example is the fan belt in a car
that links a number of pulleys.
To stop the pulley belt slipping off, pulleys
have grooved rims. This also keeps the belt
running in a straight line.
Pulleys that use some form of belt drive are
referred to as
Friction Drive mechanisms.
A drive mechanism such as gears which
physically make contact, is referred to as
Positive Drive.
Pulleys have several advantages over gears,
but also some disadvantages.
The main advantage is the fact that they are
simple to make and can be used at a distance
from each other, unlike gears that need to
touch in order to work. The disadvantage is
that they work by friction and so can slip.
You can get a toothed pulley and belt which
eliminates any slipping or timing problems.
Many cars have a cam belt that works on this
principle. Some model suppliers sell special
toothed pulleys but they tend to be very
expensive.
The belt can also be substituted by a chain.
A bicycle is a good example of this system.
Model suppliers often stock small plastic
toothed pulleys or chain sets which are
reasonably priced.
Like gears, you can use the pulley to either
step up or
step down the drive. But instead of
counting teeth as with gears, you simply
make the diameter of the pulley wheels
larger or smaller.
By dividing the input diameter by that of the
output, you can workout a final ratio. In the
above example the ratio is 2:1. This means
for every two revolutions of the input pulley
the output turns one full revolution. You could
reverse the input and output pulleys.
You can see that pulleys rotate in the same
direction (unlike gears which do the opposite).
Pulleys are useful for getting the drive action
to happen in awkward places. You can use the
drive pulley to transmit it's motion to the output
pulley which may be some distance away.
You can also use pulleys to reverse the action
by putting a twist into the belt. This makes the
output pulley move in the opposite direction.
This can be very useful.
Constructing Pulleys
Laminating is one of the easiest ways to
make a pulley wheel. Use several circles of
card making two of them slightly bigger
(around 5mm). Glue them together,
sandwiching the smaller ones in the middle.
Make sure that the pulley is a little wider than
your belt, and try to keep the centers lined up.
When the glue has set you can drill the center
out to the size of the drive shaft.
Fitting the elastic band is often the final part
of the assembly process before gluing
everything. Should you find that your elastic
band breaks, you could substitute it for a
flexible steel spring driving belt. These come
open ended and can be cut shorter or
extended by joining another spring. They can
be purchased in most model shops and offer
superior performance over elastic bands,
which are prone to stretching and can
disintegrate if left in direct sunlight.
Finally, make sure everything is strong
enough to take the tension of the pulley.
Remember you are dealing with a friction
drive, so the tighter the belt the less chance
you have of it slipping. Make sure that any
axles are strong enough to take the load.
