What it shows
Gearboxes in bicycles, clocks and winches trade speed for torque. A model gear train made from salvaged toy gears or laser-cut MDF lets a student check that the speed ratio equals the inverse of the teeth ratio and measure backlash and losses.
The science
When two gears mesh, their teeth move at the same linear speed, so the speed ratio equals the ratio of teeth in reverse: N2/N1 = T1/T2. In a compound train the ratios multiply. Output torque rises by the same ratio as the speed falls, less friction losses, so a small motor can lift a heavy load. Backlash is the small gap between teeth that allows play.
Materials
Cost: under Rs 1,500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A set of plastic gears with 12, 24, 36 and 60 teeth (from old toys or hobby gear kits)
- A wooden or MDF base with 4 mm shafts and bearings made of straws
- A 12 V small motor and a pulley to lift weights
- Cord and 50 g weights
- A stopwatch or tachometer
- Marker paint for the gear teeth reference
Safety
For this project: Keep fingers away from the meshing gears. An adult must cut MDF and drill. Use a battery or sealed 12 V adapter for the motor.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
10 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Count the teeth of each gear and write down the ratios you can create, for example 12 to 36 gives 3 to 1.
Fix two shafts at a spacing equal to half the sum of the pitch diameters and mount the first pair of gears.
Turn the input shaft by hand by a marked number of revolutions and count the output revolutions; compare with the teeth ratio.
Add a second pair on a compound shaft to make a train with total ratio near 9 to 1 and test again.
Mount the motor on the input and a pulley with a cord on the output, and measure the largest weight the train can lift.
Measure the time for the weight to rise 30 cm for different loads.
Calculate output power as weight x height divided by time, and compare with the motor input power to find efficiency.
Measure backlash by rotating the input until the output just moves, and note the input angle lost.
Test with and without a drop of oil on the shafts to see how efficiency changes.
Write the report with calculations, tables, drawings and applications.
What you should see
The measured speed ratios match the teeth ratios exactly when counting revolutions. The train lifts a load that the bare motor cannot, and efficiency is typically 40 to 70 percent, increasing slightly with oiled shafts.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does a larger ratio give more torque?
Power stays nearly constant, so if speed falls by a factor, torque rises by about the same factor.
2What is an idler gear?
A gear between input and output that reverses direction without changing the ratio.
3What does backlash cause?
It causes play and positioning errors in robots and machines when direction changes.
4Why does a bicycle have different sprockets?
To give different speed ratios for climbing or fast riding.
Take it further
Build a three-speed selector with sliding gears and compare the measured top speed for each ratio.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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