This simple machine involves two circular objects — a larger disc and a smaller cylinder both joined at the center. The larger disc is called the wheel. The smaller cylindrical object or rod is referred to as the axle. Sometimes, there may be two wheels attached to both ends of the axle. A wheel alone or an axle alone is not a simple machine. They need to be joined to be called a simple machine.

If you look closely at how a wheel and axle work, you will notice that it is a kind of class one lever. Here, an action on the axle (turning the axle) will cause output at the other end (wheel turns too). The fulcrum is where the axle meets the wheel.

The Wheel and Axle work in two ways:

Force applied to wheel:

Let us take a screwdriver for instance. If you apply a force to the wheel (the handle), the wheel spins and multiplies the effort to make the output force of the axle (shaft) greater.

A simple doorknob is another example of a wheel and axle. The locking mechanism of the doorknob is inside of the door and can only be controlled by the knob. Since it will be difficult turning the axle to open the door, we can turn the wheel instead and that does that job for us.

Force applied to axle:

Now let us also consider a windmill. If you apply a force to the axle, it will multiply the force to the wheel (blades) and result in a greater distance covered. It is because the wheel is larger than the axle and covers more area. A ceiling fan works similarly. As the axle turns, it powers the larger wheel (fan blades) to cause the desired output.
The Wheel and axle are perfect for turning turbines and fans; They are also used in automobiles. For example, when you turn the steering wheel of a car, your effort is multiplied by the axle and results in more turns of the car wheels.

Gears

A gear is simply a special wheel with teeth called threads on the outside. There are a couple of different gears that do different things, for example, The Spur Gear and the Worm Gear. (CLICK TO SEE WHAT THEY LOOK LIKE)