The wheel & axle has been used for thousands of years. Ancient Egyptians, Greeks and Romans used the wheel & axle in their chariot designs.
The wheel & axle device is similar to the lever simple machine. The wheel acts as the lever and the axle acts as a fulcrum. The lever applies force to the fulcrum, causing something to move.
There are two basic features to the wheel & axle. The axle goes through the center of the wheel and when it is spun, the wheel spins also.
The function of the wheel & axle is to do work. The mechanical advantage of the wheel & axle is equal to the ratio of the wheel & axle's diameter.
The wheel & axle simple machine is used in every automobile ever made. This simple machine is also used in bicycle wheels, doorknobs and ferris wheels.