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Gears and SpeedIntroductory Presentation
Opening ActivityIn “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
• 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?
Explain :
Opening Activity
Our driving gear is connected to our motor and is responsible for spinning the driven gear, which is connected to the wheel.
What is the difference between the driving gear and the driven gear?
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.
Opening ActivityWhat should you do to make the driven gear turn faster or slower?
Preview: Gear Ratio
The relationship between the sizes of your driving gear and driven gear is called the “Gear Ratio.”
Preview: Gear RatioA 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
The driving gear must make ½ a turn to turn the driven gear once
1:1 or 1The driving gear must turn once to turn the driven gear once
3:1 or 3The driving gear must turn 3 times to turn the driven gear once
Preview: Gear RatioLet’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.
8 Teeth
40 TeethDriven Gear
Driving Gear
Preview: Gear RatioLets look at each tooth as a “push.” A 1-tooth “push” moves 1 tooth, and a 2-tooth “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 RatioSuppose we want to turn the 40-tooth gear one entire revolution. We have to turn our 8-tooth gear enough times to create a 40-tooth “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 40-tooth “push.” This is enough to turn the big gear once.
Preview: Gear Ratio
What’s our definition of 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 = 5
The Gear Ratio for the example was 5, so this means that our driving gear must turn 5 times to turn our driven gear once.
What is our example Gear Ratio?
Verbally explain what our example Gear Ratio implies.
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 ?
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.
NO it does NOT!
Preview: Gear RatioWe 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.
40:8
Because this means:
40/8 = 5
We can write our Gear Ratio like this:
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.
# teeth on Driven Gear
# teeth on Driving GearGear Ratio =
Preview: Gear RatioNow 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
Gear Ratio = # of Driven Teeth
# of Driving Teeth
8 Teeth
40 Teeth
Driven Gear
Driving Gear
Preview: Gear Ratio
From our example we get:
Gear Ratio = # of Driven Teeth = 40
# of Driving Teeth = 8
Gear Ratio = Driving axle turns = 5
Driven axle turns = 1= 5
= 5
8 Teeth
40 Teeth
Driven Gear
Driving Gear
Preview: Gear Ratio
Find the Gear Ratios for these examples:
1. A 5-tooth gear driving a 45-tooth gear.
2. A 3-tooth gear driving a 12-tooth gear.
3. A 12-tooth gear driving a 3-tooth gear.
Gear Ratio = 9
Gear Ratio = 4
Gear Ratio = 1/4
Now you have the necessary knowledge to get started in the Gears and Speed Activity.
Good Luck!
