Introductory Presentation

Opening Activity
In “Get in Gear,” we changed the gears on our robot to adjust its speed. If we want to change the speed more precisely however, we need to know exactly which gears to use.

Opening Activity
Explain :
• What is the difference between the driving gear and the driven gear?
• What should you do to make the driven gear turn faster or slower?

Opening Activity
What is the difference between the driving gear and the driven gear?
Our driving gear is connected to our motor and is responsible for spinning the driven gear, which is connected to the wheel.

Opening Activity
What should you do to make the driven gear turn faster or slower?
To make our robot move faster, we made our driving gear bigger and/or our driven gear smaller. To slow it down we swapped the two.

Preview: Gear Ratio
The relationship between the sizes of your driving gear and driven gear is called the “Gear Ratio.”

Preview: Gear Ratio
A gear ratio is the number of turns your driving gear must make in order to turn your driven gear once.
Gear
Ratio
Implies…
1:2 or
½
1:1 or 1
The driving gear must make ½ a turn to turn the driven gear once
The driving gear must turn once to turn the driven gear once
3:1 or 3
The driving gear must turn 3 times to turn the driven gear once

Preview: Gear Ratio
Let’s consider the following example for the rest of the presentation. Here we have an 8 tooth driving gear and a 40 tooth driven gear.
Driving Gear
8 Teeth
Driven Gear
40 Teeth

Preview: Gear Ratio
Lets look at each tooth as a “push.” A 1-tooth “push” moves 1 tooth, and a 2tooth “push” moves 2 teeth. If we rotate our 8-tooth gear once, which is an 8-tooth
“push,” we’ll push 8 teeth on the 40-tooth gear.
8-tooth push
8-tooth push

Preview: Gear Ratio
Suppose we want to turn the 40-tooth gear one entire revolution. We have to turn our 8-tooth gear enough times to create a 40tooth “push.” Well, this is easy because:
5 x 8 = 40
So we just have to turn our 8-tooth gear five times to create a 40tooth “push.” This is enough to turn the big gear once.

Preview: Gear Ratio gear must make in order to turn your driven gear once.
our driven gear once.

Preview: Gear Ratio
For our 8-tooth and 40-tooth gears the Gear Ratio is 5. Does this mean our Gear Ratio can also be written 8:40 ?
NO
NOT
8:40 means 8/40, which equals 1/5. This implies that our driving gear has to turn 1/5 of a rotation to rotate our driven gear once. This is obviously wrong from what we know.

Preview: Gear Ratio
We already discovered that our driving gear must turn
5 times to turn our driven gear once. So a Gear Ratio of 1/5 is clearly incorrect.
We can write our Gear Ratio like this:
40:8
Because this means:
40/8 = 5

Preview: Gear Ratio
You can calculate Gear Ratio by dividing the number of teeth on the Driven Gear by the number of teeth on the Driving Gear.
Gear Ratio
# teeth on Driven Gear
# teeth on Driving Gear

Preview: Gear Ratio
Now that we’ve cleared that up, it makes sense that our equations to calculate Gear Ratios would look like this:
Gear Ratio = Driving axle turns
Driven axle turns
Driving Gear
8 Teeth
Gear Ratio = # of Driven Teeth
# of Driving Teeth
40 Teeth
Driven Gear

Preview: Gear Ratio
From our example we get:
Driving Gear
8 Teeth
Gear Ratio = Driving axle turns = 5
Driven axle turns = 1
= 5
Gear Ratio = # of Driven Teeth = 40
# of Driving Teeth = 8
= 5
40 Teeth
Driven Gear

Preview: Gear Ratio
Find the Gear Ratios for these examples:
1.
A 5-tooth gear driving a 45-tooth gear.
Gear Ratio = 9
2.
A 3-tooth gear driving a 12-tooth gear.
Gear Ratio = 4
3.
A 12-tooth gear driving a 3-tooth gear.
Gear Ratio = 1/4

Now you have the necessary knowledge to get started in the Gears and Speed Activity.