What it shows
Gears pass motion from one shaft to the next. A small gear driving a large one turns it more slowly but with more turning force, and neighbouring gears turn in opposite directions.
The science
The teeth of one gear lock between the teeth of the next so that they turn together without slipping. In one turn, a gear with 12 teeth moves 12 teeth past the contact point. A gear with 24 teeth meshed with it moves only half a turn. So the gear ratio, the number of teeth on the driven gear divided by that on the driver, is 2, and the larger gear turns at half the speed. Because of energy conservation, the larger gear gives twice the turning force, called torque. Gears that touch turn in opposite directions.
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.
- Thick corrugated cardboard, 3 sheets
- Compass cutter or a pair of scissors
- A ruler and a protractor
- Wooden skewers or pencils as axles, 4 pieces
- Bottle caps for bearings
- A cardboard base, 30 cm by 30 cm
- Glue
- Coloured stickers
Safety
For this project: An adult must supervise cutting of the gear teeth and the skewers, and cover the pointed ends 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.
Draw three circles on cardboard of diameter 4 cm, 8 cm and 12 cm. Cut them out.
Mark 12 equal teeth on the small circle, 24 on the medium one and 36 on the large one, using a protractor to space them. Teeth should be 4 mm wide.
Cut out the teeth carefully with scissors, making them rectangular and the same size on all gears.
Stick the gears on skewers through their centres and fix the skewers to the base with bottle caps as supports, so that the teeth of the gears mesh.
Stick a coloured sticker on the edge of each gear to count turns.
Turn the small gear through exactly 10 turns and count the turns of the middle and the big gear.
Calculate the gear ratio from the numbers of teeth and compare it with the turns you counted.
Add a fourth gear to make a train of three and note the direction of each gear and the final speed.
What you should see
The medium gear turns half as fast as the small one and the big gear turns one third as fast, matching the tooth ratio, and neighbouring gears turn in opposite directions.
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 the gear ratio?
The number of teeth on the driven gear divided by the number on the driver gear.
2Why does a large gear driven by a small one turn slowly?
It has more teeth, so it needs more of the small gear’s turns to make one turn.
3Why do two meshing gears turn in opposite directions?
Their teeth push against each other where they meet.
4Where are gears used in a bicycle?
In the pedal and rear wheel sprockets, which let the rider choose speed and force.
Take it further
Make a gear train that lifts a small weight with a thread and see how the gear ratio changes the force needed.
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.
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