Instruction Guide: Getting in Gear

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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.:

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 two-gear 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 lesson-level 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 takes gearing to the next level by introducing Complex and Compound Gearing. This presentation also includes a compound gearing challenge.