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Experiment #1
Using a Spur Gear
System in a Crank Fan
Objectives: Understand and describe the transfer of
motion through a spur gear system and investigate the
relationship between gear size, speed of rotation and force.
Materials You Will Need:
built CRANK FAN model
masking tape
paper or notebook
79318
PROCESS:
1. Build the CRANK FAN model by following the step-by-step building instructions.
2. Explore your model. Locate and identify the gears. Watch the gear mechanism in
operation as you turn the crank.
3. Using your notebook, explain how the gear system turns the fan blades. How do
the fan’s gears fit together? Are they in line with each other?
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4. This arrangement is called a Spur Gear System. In this
arrangement, the gears fit together, or mesh, along the same line
or in the same plane. In the Crank Fan, the gears are arranged
one above the other. Turn your model on its side so you can see
how the gears are in line with each other.
fan blades
5. The names of the various parts of this
model are: Crank, driver gear, driven gear,
and fan blades. Using masking tape, label
the different parts of your model.
crank
driven gear
driver gear
6. Using your notebook, respond to the following:
(a) Describe how the moving parts that you labeled above are connected to each
other.
(b) Describe the input motion (the motion you use when you operate the crank).
(c) Describe the motion of the gears.
(d) Draw a diagram of the gear arrangement in your notebook and use arrows to
show the direction each part moves as you operate the fan.
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7. Attach a small piece of masking tape to the edge of one fan blade and select a
reference point so you can keep track of the fan blade as it rotates.
(a) Turn the crank to make one rotation. Continue turning the crank but vary the
speed at which it is turned. How can you make the fan turn faster/slower?
(b) Mark the two gear wheels with either a dot sticker or with a pencil mark. The
marks should be made at the point where the two gears mesh. Now turn the
crank one slow turn. What do you notice?
(c) Note in your notebook the sizes of the two gears - driver and driven – used in
the model.
(d) Could there be a relationship between the size of the gears and your findings
to (b)?
(e) Turn the crank one additional turn, but this time, notice how far the fan blades
travel. Ask a friend, parent, or teacher to count the number of times the blade
with the masking tape passes the selected reference point, and focus on
making just one full turn with the crank.
(f) How easy/hard is it to turn the crank with this gear arrangement?
(g) Summarize your observations concerning the distance the two gears and the
fan blades turn with one rotation of the crank.
8. (a) What do you think will happen if you use:
(i) a large gear wheel to drive a small gear wheel &
(ii) a small gear wheel to drive a large gear wheel.
• Make a note of your responses in your notebook.
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(b) Discover if your predictions were
correct by rebuilding your models
with two gears that are different
in size using these images.
(c) Think of a way to compare the speed that the fan turns, with the speed of the
crank when the big gear is attached to the crank and the small gear is attached
to the fan blades.
(d) How easy/hard is it to turn the crank with this arrangement compared to when
the gears were the same size?
• Make a note of your responses in your notebook.
(e) Compare the speed at which the fan turns with the speed of the crank, when the
small gear is attached to the crank axle and the big gear is attached to the fan
blade’s axle.
(f) How easy/hard is it to turn the crank with this arrangement, compared to when
the gears were (i) the same size and (ii) the large gear was the driver?
• Make a note of your responses in your notebook.
9. (a) Discuss your observations of gear systems using different sized gears.
(b) Do your observations support the prediction that you wrote earlier? Report your
conclusions using evidence from your investigations.
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EXTENSION ACTIVITY: Gear Ratios
1. You used a crude measurement to compare the input and output speeds of the gear
wheels in the experiment you just performed. What you discovered was a simple
Gear Ratio.
2. A more accurate approach is to compare results by counting the number of teeth on
each gear wheel.
Gear Ratio =
Number of teeth on the driven (follower) gear
Number of teeth on the driver gear
For example: 14/84 gives a gear ratio of 1/6 or 1:6
3. A 1:6 gear ratio means that for every complete revolution of the driver gear, the
driven gear makes 6 complete revolutions. Or, said another way: the output speed
is faster than the input speed.
4. (a) Now remove the fan blades from your crank fan and set aside.
(b) First, use the large gear as the driver gear and the small gear is the driven gear.
(c) Second, use the small gear as the driver gear and the large gear is the driven
gear.
NOTE: Do not to
disassemble your fan.
Simply attach the crank to
the appropriate shaft. (See
diagrams below.) In order to
watch the speed of rotation
of the second gear in the
gear chain, you should
attach a yellow connector to
the end of that gear’s axle.
(This connector replaces the
blades that, if used on the
lower axle, will strike the
Set-up one: Crank on upper axle. Put the
yellow connector on the end of the lower axle.
Set-up two: Crank on the lower axle. Put the
yellow connector on the end of the upper axle
where the blades used to be.
tabletop unless the model is
pushed to the very edge.)
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(d) Determine the gear ratio of your crank fan. Write down the gear ratio in your
notebook and describe, in your own words, what the gear ratio means in
reference to their crank fan.
(e) What is gained by using this gear train? Did your fan turn quickly or slowly?
5. Use the table below to record your findings and repeat steps (4c-4e) with the small
gear as the driver gear and the large gear is the driven gear.
GEAR TRAIN FAN SPEED GEAR RATIO INCREASED OUTPUT
SPEED VS. CRANK SPEED (APPROXIMATE) SPEED OR INCREASED
OUTPUT FORCE
Gears are same size
Large driver gear moving
small driven gear
Small driver gear moving
large driven gear
6. Brainstorm how you can change the design of the crank fan so that the crank and
the fan turn in the same direction in your journal. Hint: What can you add to the
mechanism? This will introduce the concept of the idler gear.
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