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STEMingBoredom 25: engineer a gearbox

Where would we be without gears?

Largely, we’d be at home, or very close to it. A world without gears would be a world mostly without cars, lifts (elevators), foods from abroad such as bananas and much more.

Gears take one input and either magnify it, or reduce it; if one turn of a wheel doesn’t do enough, gears can make that movement much bigger, or they might take a big, unmanageable movement coming in, and reduce it to something manageable.

Let’s look a bicycles (one of our favourite things, actually). Very keen cyclists often refer to a gear as being “52 inches”. Clearly, neither of the cogs that the pedals attach to, nor the cog in the middle of the back wheel is 52 inches, because 52 inches is 4 feet 4 – as big as a 6 year old, so what can they mean? Well, it refers to how much forward movement one turn of the crank (what the pedals attach to) will give you; pedal once round and you move 52 inches forward.

Using gears, often many gears together, means that you can move something without being incredibly strong – they do the work for you. Some modern cars have 7 gears; the first gear will only get you to a maximum of maybe 75 miles per hour, but 7th gear will get you to 155 miles per hour – yet the engine puts out the same power in both gears. It’s pretty amazing.

Here we are going to make a simple gear of two cogs.

How to do it:

The critical point to this is the gears must not be the same size! Deliberately make one bigger than the other!

You’ll need:

Corrugated cardboard
Cocktail sticks

Get two round things, maybe a mug and a saucer and cut the cardboard around them so that you have two circles of different sizes. Next, cut 2.5cm wide strips of the cardboard that are as long as your cardboard. Leave some of the cardboard uncut as a base.

Measuring your cardboard circles from one side to the other, halve the number that you measure and push a cocktail stick through at that point – this is the centre of your cog.

Cut some small strips of sticky tape to have at the ready. Wrapping a strip of the cardboard around the outside of one of the circles that you cut out, cut it so that the two ends of the cardboard meet and then stick them together using the tape. REMEMBER, the tape will have to go into the corrugations...

Do the same for the other circle and then stick the outside sleeves of corrugated cardboard to the circles – it’s probably best to stick on the inside of the sleeve in many places, to stop it slipping – the circle and its sleeve should now move as one thing on the cocktail stick that you are using as an axle.

Place the first cog onto the cardboard base that you left and push the cocktail stick into the base. You’ve just started to make a gearbox!

Getting the second cog, place it so that the corrugations of the second cog fit into those of the first. See the picture and you’ll get the idea!

Push the cocktail stick on the second through to the cardboard base. Hey presto! A gearbox!

Extension task:

Can you make a working seven cog gearbox – like a modern high-performance car?

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