This lesson introduces students to the concept of gearing up and gearing down to trade off speed and power (torque). Students learn to calculate a gear ratio using multiplication of fractions. An advanced lesson which introduces compound gearing is also available.
Walk student through the Tribot Gearing PPT. [1]:
If your class completed the "Introduction to Gear Trains" lesson, have students recall what they remember about gear trains (the bicycle example is an effective tool).
Students should have their kits with them so they can construct the gearing examples as the lesson proceeds
Slide 1
 We will be working with the example of a twogear great train
 The gear attached to the motor is the "Driving" gear and the gear attached to the Wheel is the "Driven"

The gears' teeth mesh together, causing the two gears to rotate in opposite directions
 Have students construct a gear train using a crank on one axle (driving gear) and a small wheel on the other axle (driven gear)
 The gear ratio is always calculated with (# of teeth on driving gear)/(# of teeth on driven gear)
Slide 2

Have students work through these three gear ratios examples
 Construct each example to help reinforce concept

Why might you want a gear ratio =1
 Change direction of rotation without changing speed
 Move the center of rotation from the motor to another position
Slide 3

A gear ratios less than one is called gearing down
 Speed (rotations per second) of wheel (vs motor) goes down
 Power (or torque  rotational power) will go up
 Every gear pair causes some loss of energy due to friction
 So, a gear ratio of 1/2 will less than double the power
 Useful moving a heavier load, going up a ramp, accelerating faster, etc.

A gear ratios greater than one is called gearing up
 Speed of wheel (vs motor) goes up
 Power (or torque) will go down
 Every gear pair causes some loss of energy due to friction
 Useful for going faster (but, at the expense of accelerating slower)
Slide 4
 A tribot will need structural support for a gear train to be added
 The illustration shows a modified tribot structure with a gear ratio of one
 With this same structure (axle spacing) the 12 and 20 tooth gears shown could be used to gear up or gear down the tribot
 How could the other gears in the NXT kit be used to gear up/down?
Note: There is no lessonlevel summative assessment for this basic gearing lesson (although the extended lesson, below, does have an included challenge). The assessment for this learning is intended to be demonstrated in the subsequent STEM Robotics 101 "Engineering Challenge #1" lesson.
The Advanced Gearing PPT [2]takes gearing to the next level by introducing Complex and Compound Gearing. This presentation also includes a compound gearing challenge.