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Compound
Machines Science
Projects
Using Pulleys
If you need to do a science project on simple
machines or compound machines, you can use pulleys to construct variety of
working models.
| Pulley is a simple machine
consisting essentially of a wheel with a grooved rim in which a pulled
rope or chain can run to change the direction of the pull and thereby lift
a load.
A wheel turned by or driving a belt is
also called pulley.
The plural of pulley is pulleys. |
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A Simple Science Fair Project for
Compound Machines.
We can't say enough about the
educational value of this pulley set. Its
prize-winning design enables your children to learn mechanics and
spatial perception. Watch as they are inspired to effortlessly incorporate
these concepts into their creative ideas.
MiniScience Part# PP54_S2
In your compound machine project you
may show how a set of pulleys may be used to lift heavy objects
effortlessly. Show how pulleys are used to lift trucks, other heavy
objects and people.
Optionally use a spring
scale to show how pulleys reduce the force needed to lift heavy
objects.
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This pulley set increases the force by about 4
times. For example you may show that a 4 kilogram weight can be lifted with
about 1 kilogram force.
| Force and mechanical advantage
Pulleys are clever devices that allow you to lift large weights with
much smaller forces. The length of the string used to lift the pulley determines
how much force is needed. For example three string lengths allow us to
lift a heavy object with a force equal to 1/3rd of its weight.
Do some experiments and measure the force it takes to lift the pulley
by attaching a spring scale to the end of the string. |
Pulleys can be used to simply
change the direction of an applied force or to provide a force/distance
tradeoff in addition to a directional change.
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Pulleys are very flexible
because they use ropes to transfer force rather than a rigid object such
as a board or a rod. Ropes can be routed through virtually any path. They
are able to abruptly change directions in three-dimensions without
consequence. Ropes can be wrapped around a motor's shaft
and either wound up or let out as the motor turns.
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Ropes also have the advantage that their
performance is not affected by length.
A compound pulley `trades' force for
distance through an action/reaction force pair. In a double pulley, as the
rope passes over the pulley the force is transmitted entirely but the
direction has changed. The effort is now pulling up on the left side of
the bottom pulley.
The disadvantages of pulleys, in contrast
to machines that use rigid objects to transfer force, are slipping and
stretching. A rope will permanently stretch under tension, which may
affect the future performance of a device. If a line becomes slack, then
the operation of a machine may change entirely. Also, ropes will slip and
stick along pulley wheels just like belts. One solution to the problems
associated with rope is to use chain. Chain is pliable like rope, and is
able to transfer force through many direction changes, but the chain links
are inflexible in tension, so that the chain will not stretch. Chains may
also be made to fit on gears so that slipping is not a problem.
Lifting Heavy Objects
A common way to lift heavy objects is with
a block-and-tackle, a schematic diagram of which is below
(usually an actual block-and-tackle has all the pulleys on the the same
axle, but that's hard to draw). Suppose there is a heavy crate (mass 100
kg) attached to the lower pulleys. What force must be applied to rope R
to lift the crate off the ground? Assume all the pulleys are
frictionless and massless. 
Among benefits of pulleys:
While searching the Internet
about uses of pulleys, I came across a website
that describes how homes and communities may be modified to adapt
the needs of disabled
village children. This seemed to be a very good science
project idea for students who need to make a compound machine or
show the applications of pulleys. With the help of pulleys a
person can lift himself to an elevated area such as a three or
second floor of a village house. A model of house may be made
using wood sticks and cardboards. Pulleys are available at
MiniScience.com and other science suppliers.
The purpose of this project is to
show that a system of ropes and pulleys may be the best way for a
person with strong arms to lift herself without help to a 'house
on stilts'.
The 'lift' can be made with a
platform so that the whole wheelchair can be lifted. But if the
house is small and people cook and eat at floor level, it may be
best to leave the wheelchair outside.
For people who do not have strong
arms, you may use pulley blocks with 2 or more pulleys. For
example a pair of pulley blocks with 3 pulleys in each will
increase the force by 6.
Available Pulley sets for science fair projects:
| Two, single plastic pulleys
can be combined to form a fixed/moveable block and tackle pulley system.
These pulleys have sheaves with deep V-grooves with diameters of 50 mm.
MiniScience Part # PP5_S2
In addition to the pulleys you will need
a wooden frame or structure to hang your fixed pulley. You will also need
Nylon Mason line #18 or any similar nylon string (Available from Hardware
stores). This set will reduce the lifting force by about 50%. |
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| Two, double pulley blocks can
be combined to form a fixed/moveable block and tackle pulley system. Each
pulley block has 2 plastic sheaves (mounted side by side) with deep
V-grooves with diameters of 50 mm.
MiniScience Part # PP55_S2
In addition to the pulley blocks you will
need a wooden frame or structure to hang your fixed pulley. You will also
need Nylon Mason line #18 or any similar nylon string (Available from
Hardware stores). This set will reduce the lifting force by about 75%. |
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| Two, quad pulley blocks can be
combined to form a fixed/moveable block and tackle pulley system. Each
pulley block has 2 plastic sheaves (mounted side by side) with deep
V-grooves with diameters of 50 mm.
MiniScience Part # PP5555_S2
In addition to the pulley blocks you will
need a wooden frame or structure to hang your fixed pulley. You will also
need Nylon Mason line #18 or any similar nylon string (Available from
Hardware stores). This set will reduce the lifting force by about 87%.
Pairs of triple parallel pulley blocks
are also available:
MiniScience Part # PP555_S2 |
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Possibilities are unlimited. You can design
your own compound machine using pulleys.
| The diagram in the right shows
a pulley system using 2 single pulleys and 2 sheaves. The end of the
string is connected to a spring
scale in order to measure the force required to lift the heavy block.
In this design the upper block (fixed
block) is made of sheaves. |
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| Sheaves
may be used in fixed blocks or may be used to construct pulleys or pulley
blocks. In the middle of each sheave is a hole that can be used to nail it
to a board or block. |
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More advanced compound machines:
| The picture on the right shows
an elevator for lifting heavy loads. This is one of the best compound
machines suggested in ScienceProject.com.
In this design pulleys are used to reduce the lifting force as well as
changing the direction of the force. This design also incorporate a wheel
and axle. |
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| This elevator project in the
right shows how pulley blocks may be made using sheaves. This simple
design reduces the lifting force by 75%.
Compound machines are generally
classified as engineering projects. If you need to do a compound machine
project as an experimental project, you can use your elevator to lift
different masses from 1 Lb up to 5 Lbs and use a 2000-gram spring scale to
measure the lifting force. Your results table will show the lifting force
for different actual weights. You can then use your results table to draw
a graph. |
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We have the largest
collection of pulleys for science fair projects and educational activities.
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