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 number of turns it makes for one turn of the large gear equals the ratio of their teeth.
The science
Two gears meshing together turn in opposite directions. The gear ratio is the number of teeth on the driven gear divided by the number on the driver, and for each turn of the driver the driven gear makes (driver teeth divided by driven teeth) turns. 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 for four discs: two large ones about 9 cm wide and two small ones about 3 cm wide
- About 64 small teeth cut from card strips, each about 1 cm long and 0.6 cm wide
- A bottle cap to use as the turning handle
- Skewers as axles and drinking straws as bearings
- A base board
- Paper strips marked with a colour
- Glue and a pair of compasses
Safety
For this project: An adult must cut the cardboard and skewers. Mind the sharp tips of skewers and cover them with tape.
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.
Step 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.
Use compasses to draw four discs on cardboard, two large ones 9 cm wide and two small ones 3 cm wide, cut them out with an adult and mark the centre of each.
Glue evenly spaced teeth, 24 on each large disc and 8 on each small, so that the gap between teeth is the same (about 1.2 cm) on every disc.
Push skewers through the centres, fix them in the base board with straw holders and glue the bottle cap to the first skewer as a handle.
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.
Fix the second large gear on the same skewer as the small gear and mesh it with the second small gear to make a compound train, then 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.
01What is a gear ratio?
The teeth of the driven gear divided by the teeth of the driver.
02What happens when a big gear drives a small one?
The small one turns faster with less force.
03Why do meshing gears turn in opposite directions?
Their teeth push each other the opposite way.
04Where are gears used?
In bicycles, cars and clocks.
Take it further
Add an idler gear between the two and see that the ratio stays the same but the driven gear now turns the same way as the driver.
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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Physics Working Models Explained
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Mini and Major Project Guide
For diploma and BTech students: choose, plan, build, document and present
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