A gear ratio demonstration project shows how gears can change motion in a simple machine. By building a LEGO or cardboard gear train, students can see that different tooth counts change the speed and turning force at the output. This matters because gears are used in bicycles, clocks, cars, robots, and many other machines.
The project makes an invisible tradeoff visible: more torque usually means less speed, and more speed usually means less torque.
In a gear pair, the input gear is called the driver, and the output gear is called the driven gear. When two gears mesh, they rotate in opposite directions, and their speed ratio depends on the number of teeth on each gear. A small driver turning a large driven gear gives more output torque but fewer output rotations.
A large driver turning a small driven gear gives more output speed but less output torque.
Key Facts
- Gear ratio = number of teeth on driven gear / number of teeth on driver gear
- Output speed = input speed x driver teeth / driven teeth
- Output torque = input torque x driven teeth / driver teeth, ignoring friction
- If a 12 tooth gear drives a 36 tooth gear, the gear ratio is 36 / 12 = 3:1
- Meshed gears rotate in opposite directions unless an idler gear is added
- Power is approximately conserved: input torque x input speed ≈ output torque x output speed, ignoring losses
Vocabulary
- Driver gear
- The gear that receives the input turning motion from a hand crank, motor, or axle.
- Driven gear
- The gear that is turned by the driver gear and provides the output motion.
- Gear ratio
- The comparison of gear tooth counts that tells how speed and torque change from input to output.
- Torque
- A measure of turning force, such as the force that helps a wheel, crank, or axle rotate.
- Idler gear
- A gear placed between the driver and driven gears that changes rotation direction but does not change the overall gear ratio.
Common Mistakes to Avoid
- Reversing the gear ratio, because using driver teeth divided by driven teeth gives the speed multiplier, not the torque multiplier.
- Forgetting that meshed gears spin in opposite directions, which makes rotation arrows incorrect on a diagram or test setup.
- Counting gear diameter instead of teeth, because the tooth count is the reliable value used to calculate the gear ratio.
- Ignoring friction and axle rubbing, because real LEGO or cardboard gear trains lose some energy and will not match ideal calculations exactly.
Practice Questions
- 1 A 10 tooth driver gear turns a 40 tooth driven gear. What is the gear ratio, and how many output rotations occur for every 1 input rotation?
- 2 A 24 tooth driver gear turns an 8 tooth driven gear at 60 rpm. What is the output speed in rpm, ignoring friction?
- 3 A student wants a gear train to lift a heavier load using the same hand crank. Should the output gear have more teeth or fewer teeth than the input gear? Explain the torque and speed tradeoff.