What do gears do... and how do they do it?

Gears are used for transmitting power from one part of a machine to another. In abicycle, for example, it's gears (with the help of a chain) that take power from the pedals to the back wheel. Similarly, in a car,gears transmit power from the crankshaft (the rotating axle that takes power from the engine ) to the driveshaft running under the car that ultim...ately powers the wheels.

You can have any number of gears connected together and they can be in different shapes and sizes. Each time you pass power from one gear wheel to another, you can do one of three things:

*. Increase speed : If you connect two gears together and the first one has more teeth than the second one (generally thatmeans it's a bigger-sized wheel), the second one has to turn round much fasterto keep up. So this arrangement means the second wheel turns faster than the first one but with less force. Looking at our diagram on the right (top), turning the red wheel (with 24 teeth) would make the blue wheel (with 12 teeth) go twice as fast but with half as much force.

*. Increase force : If the second wheel in a pair of gears has more teeth than the first one (that is, if it's a larger wheel), it turns slower than the first one but with more force. (Turn the blue wheel and the red wheel goes slower but has more force.)

*. Change direction : When two gears mesh together, the second one always turns in the opposite direction. So if the first one turns clockwise, the second one must turn counterclockwise. You can also use specially shaped gears to make the power of a machine turn through an angle. In a car, for example, the differential (a gearbox in the middle of the rear axle of a rear-wheel drive car) uses a cone-shaped bevel gear to turn the driveshaft's power through 90 degrees and turn the back wheels.

How do they do it?

Gears sound like magic, but they're simply science in action! Look at the diagram on the right and you'll see exactly how they work. The pair of gear wheels (top) works in exactly the same way as an ordinary pair of wheels the same size that are touching (middle); the only difference is that the gears have teeth cut around the edge to stop them slipping. But a wheel is really just a lever, so a pair of wheels that touch is like a pair of levers that touch (bottom).

Thinking of gears as levers shows exactly how they work. Suppose you turn the axle at point (1). The bar connecting points (1) and (2) moves faster and with less force at point (2) because it's working as a lever. If you can't see this, suppose the red bar were a spanner and you pushed at point (2) to undo a nut at point (1) in the center. Then point (1) would turn with less speed and more force. If you turn at point (1) instead, the opposite is true: you get more speed and less force at point (2). That's the red bar, which is just touching the blue bar.As the two bars touch, they must be going at the same speed. Now the blue bar is alsoa lever, but it's working the other way: like a spanner. So if we apply a force at point (2), it's magnified by the leverage of the blue bar and we get more force (and less speed) at point (3).

Putting everything together, what do we get? We apply a certain force and speed at point (1). The red bar might give us four times the speed and a quarter of the force at point (2). But the blue bar will work the other way and maybe halve the speed and double the force. So when we get to point (3), we have twice the speed and half the force that we had at point (1). That's what we'd expect from a pair of gear wheels where one (red) is twice the size and has twice as many teeth as the other (blue).

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