Wheel and axle

Wheels are circular objects that spin around a central shaft, called an axle. Unlike levers or inclined planes, wheels and axles use spinning or rotary motion. Wheels and axles can work as either speed or force multipliers.

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Wheels as speed multipliers

In some cases, wheels are speed multipliers. If a large force is applied to the axle of the wheel, the edge (or rim) of the wheel will move very quickly as it covers a greater distance with less force.

An example of a wheel that is a speed multiplier is a wheel on a car. A great amount of turning force is applied to the axle by the car's engine, which is transferred into high speed at the rim of the wheel. This causes the car to move forward very quickly.

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A fan is another example of a wheel that is a speed multiplier. A motor turns the axle with great force at the centre of the fan. The edges of the fan blades, however, are moving very quickly.

Wheels as force multipliers

Wheels can also be force multipliers. If a small force is applied to the rim of a wheel to make it move a great distance, it is transformed into a larger force at the axle to move a smaller distance.

An example of a wheel used as a force multiplier is a windmill. Windmills are turned by the force of the wind. This force is multiplied at the axle. The force is then used to Windmills use wheels as force multipliers.

Another example of a wheel that is a force multiplier is a doorknob. When you turn a doorknob, it turns a shaft called the spindle. It would require a great force to turn the spindle with your bare hands. When you use a doorknob, however, you can apply a smaller force to turn the spindle.

Gears

Another type of wheel is called a gear. Gears are special wheels that have teeth, or pieces that stick out, on them. These teeth can interlock. When the teeth are interlocked, if one gear turns, the other turns as well. A gear that is turned by an axle is called the driving gear, while the gear that is turned by the other gear is called the driven gear. The driven gear moves in the opposite direction as the driving gear.

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Gears are used to transfer motion from one wheel to another. This can transfer motion from one part of a complex machine to another. In addition, gears can be speed or force multipliers. When a larger driving gear is engaged with a smaller driven one, for example, the driven gear moves more quickly than the driving gear. This is called gearing up. When a smaller driving gear is engaged with a larger one, however, the larger (driven) gear moves more slowly. This is called gearing down. When gearing down, the driven gear moves with more force than when gearing up.

Gears are used in many complex machines such as clocks, drills, kitchen mixers, bicycles and cars. Gearing up is common in machines where speed is important, like drills and mixers. Gearing down is helpful for machines that need a lot of force, such as cars and bicycles.