What it shows
Gears change speed and turning force. A small gear driven by a large one spins faster but with less force, and the speed ratio equals the ratio of teeth.
The science
Two gears meshing together turn in opposite directions. The speed ratio is the number of teeth on the driven gear divided by the number on the driver. A 40-tooth gear driving a 10-tooth gear makes the small one turn four times as fast, but with only a quarter of the turning force. Bicycle gears and clocks use the same idea.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Corrugated cardboard
- Four bottle caps or round lids with teeth cut in them
- Cardboard teeth glued around the discs
- Skewers as axles
- A base board
- Paper strips marked with a colour
- A stopwatch
Safety
For this project: An adult must cut the cardboard and skewers. Mind the sharp tips of skewers and cover them with clay.
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
8 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.
Cut four discs from cardboard, two large and two small, and mark the centre.
Glue evenly spaced teeth, 24 on each large disc and 8 on each small.
Push skewers through the centres and fix them in the base board with straw holders.
Mesh a large gear with a small one and put a marker on each.
Turn the large gear once and count how many turns the small gear makes.
Add a second pair on the same shaft as a compound train and count again.
Compare the measured ratio with the ratio of teeth.
Write the result in a table with the percentage error.
What you should see
Measured ratios of 3 to 1 for one pair and 9 to 1 for the compound train, close to the teeth ratios.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is a gear ratio?
The teeth of the driven gear divided by the teeth of the driver.
2What happens when a big gear drives a small one?
The small one turns faster with less force.
3Why do meshing gears turn in opposite directions?
Their teeth push each other the opposite way.
4Where are gears used?
In bicycles, cars and clocks.
Take it further
Add an idler gear and see the direction and ratio stay the same.
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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