### Gear ratios

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To understand and calculate gear ratios
To understand and demonstrate the relationship
of torque and rotational speed
To understand and properly express the
relationship between the driving and driven
gears
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How do you get 4000 lbs of wheels and steel
moving from a stop and keep it going without
over revving the engine?
Gears! They are used to keep the engine in its
ideal RPM range for any given condition
What can be done to get a race car to the end of
the ¼ mile faster?
Use a set of low gears (numerically higher ratios)
What would you do to a vehicle to achieve a
higher top speed?
Use a set of high gears (numerically lower ratios)
 If you have 20 apples and 10 oranges,
what is the ratio of oranges to apples ?
 10/20=2:1
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Write a sentence
to the left.
The graph has a
positive return.
When you put
money in, you get
more money out.
What is the ratio
of output to
input? (Show
Work!)
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Write a
sentence
graph to the
left.
What is the
ratio of
output to
input? (Show
Work!)
Torque Output (ft/lb)
250
200
150
100
50
0
0
20
40
60
80
Torque
Input
(ft/lb)
Write a sentence about the blue and red line? What is the gear ratio
of the blue and red line?
100
120
Under Drive
Direct Drive
3000
2500
2000
1500
1000
Write a sentence about the blue and red line?
What is the gear ratio of the blue and red line?
500
0
0
1000
2000
3000
4000
5000
Under Drive
Direct Drive
6000
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Always divide the driven
gear by the drive gear
2 divided by 1 = 2:1 ratio
This ratio is used to
calculate the output
torque
25 ft-lb x 2= 50 ft-lb
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Divide the input rpm by the gear
ratio
1000 rpm divided by 2= 500 rpm
1. Calculate the ratio of the first set of gears by
dividing the driven (output) gear by the drive
(input) gear
2. Do the same for the second set of gears
3. Multiply the answer from the first calculation
with the answer from the second calculation
Divide the driven by the drive gear
44 divided by 11= 4:1
•How many times will the drive gear turn to
rotate the driven gear once with a ratio of
4:1?
• 4 times
• 100 ft-lb of torque is input to a gear ratio of
4:1, how much is output?
•400 ft-lb
• 1000 rpm is input to a gear ratio of 4:1, what
is the output?
•250 rpm
300
Torque Output (ft/lbs)
250
200
150
100
Write a sentence about the blue and red line?
What is the gear ratio of the blue and red line?
50
0
0
50
100
150
200
250
300
Torque Input (ft/lbs)
350
400
Overdrive
Direct Drive
450
9000
Speed Output (RPM)
8000
7000
6000
5000
4000
3000
2000
1000
0
0
1000
2000
3000
4000
5000
Speed Input (RPM)
Write a sentence about the blue and red line?
What is the gear ratio of the blue and red line?
Overdrive
Direct Drive
6000
•If the input torque is 150 ft-lbs and the output torque
is 450 ft-lbs, is this is under, direct, or over drive?
What is the ratio
•Under drive
•3:1
•If the input rpm is 1000 and the output rpm is 400, is
this under, direct, or overdrive? What is the gear
ratio?
•Under drive
•2.5:1
Two or more sets of gears in mesh- calculate the first
set, then the second and multiply together for final
ratio
3.63 x 3.52=12.78:1
Driven gear (A) divided by Drive gear (A)
multiplied by
Driven gear (B) divided by Drive gear (B)
equals
Final transmission gear ratio
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Gear set A-drive gear has 20 teeth, driven has 40,
Gear set B drive gear has 30 teeth, driven gear
has 40. What is the drive ratio?
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2:1 x 1.33:1=2.66:1
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