Gears

Last updated: May 16, 2008.

Have you ever tried pedalling a bicycle up a really steep hill? It's pretty much impossible unless you use the right gear to increase your climbing force. Once you're back on the straight, it's a different story. Flick to a different gear and you can go incredibly fast: you can magically make the wheels turn round much faster than you're pedalling. Gears are helpful in machines of all kinds, not just cars and cycles. They're a simple way to generate more speed or power or send the power of a machine off in another direction. In science, we say gears are simple machines.

Photo: Typical machine gears. Photo by courtesy of NASA Glenn Research Center (NASA-GRC).

What do gears do?

Photo: Unlike in a car, the gears on a bicycle don't link by meshing together directly. Instead, a lubricated chain connects together the gears (sprockets) on the pedal with those on the back wheel. That's simply because the pedal and the back wheel are some distance apart and a chain is the easiest way to link them together.

Gears are used for transmitting power from one part of a machine to another. In a bike, 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 (the central axle running under the car that ultimately powers the wheels).

You can have any number of gears connected together and you can make them in various 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 that means it's a bigger-sized wheel), the second one has to turn round much faster to keep up. So this arrangement means the second wheel turns faster than the first one but with less force.
• Increase force: If the second wheel in a pair of gears has fewer teeth than the first one (that is, if it's a smaller wheel), it turns slower than the first one but with 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.

Why cars need gears

A car has a whole box full of gears—the gearbox—sitting between the crankshaft and the driveshaft. But what do they actually do?

A car engine makes power in a fairly violent way by harnessing the energy locked in gasoline. It works efficiently only when the pistons in the cylinders are pumping up and down at high speeds—about 10-20 times a minute. Even when the car is simply idling by the roadside, the pistons still need to push up and down roughly 1000 times a minute or the engine will cut out. In other words, the engine has a minimum speed at which it works best of about 1000 rpm. But that creates an immediate problem because if the engine were connected directly to the wheels, they'd have a minimum speed of 1000rpm as well—which corresponds to roughly 120km/h or 75mph. Put it another way, if you switched on the ignition in a car like this, your wheels would instantly turn at 75mph! Suppose you put your foot down until the rev counter reached 7000 rpm. Now the wheels should be turning round about seven times faster and you'd be going at 840 km/h or about 525 mph!

It's sounds brilliant, but there's a snag. It takes a massive amount of force to get a car moving from a standstill and an engine that tries to go at top speed, right from the word go, won't generate enough force to do it. That's why cars need gearboxes. To begin with, a car needs a hugh amount of force and very little speed to get it moving, so the driver uses a low gear. In effect, the gearbox is reducing the speed of the engine greatly but increasing its force in the same proportion to get the car moving. Once the car's going, the driver switches to a higher gear. More of the engine's power switches to making speed—and the car goes faster.

Changing gears is about using the engine's power in different ways to match changing driving conditions. The driver uses the gearshift to make the engine generate more force or more speed depending on whether hill-climbing power, acceleration from a standstill, or pure speed is needed.

What's the catch?

Photo: The gears on the crawler transporter that moves the Space Shuttle out to its launchpad. Photo by courtesy of NASA Kennedy Space Center (NASA-KSC).

You might think gears are brilliantly helpful, but there's a catch. If a gear gives you more force, it must give you less speed at the same time. If it gives you more speed, it has to give you less force. That's why, when you're going up hill in a low gear, you have to pedal much faster to go the same distance. When you're going along the straight, gears give you more speed but they reduce the force you're producing with the pedals in the same proportion.

Whenever you gain something from a gear you must lose something else at the same time to make up for it. If that weren't the case, you could use gears to create energy and make what scientists call a perpetual motion machine—and that's absolutely forbidden by a law of physics called the conservation of energy. Formally stated, it says that you can't create or destroy energy, only convert it from one form into another. To put it more informally, as my old physics teacher used to say: "You don't get 'owt for nowt" or "There's no gain without pain"!