The study of pulleys and tackles leads naturally to that of the wheel and axle, which consists of a wheel or crank attached to an axle. The weight is lifted or moved by means of a rope, belt, or chain running over the axle. The force is applied to the rim of the wheel. In previous problems the pulleys have all been of equal diameters, and operated by cords or ropes, but the wheel and axle may be considered as fixed pulleys of different diameters fastened on a shaft, the larger pulley being the wheel, and the smaller pulley the axle.

The principle of the wheel and axle is very important, since a great many machines, such as derricks, cranes, elevators, steam shovels, etc., are constructed on this plan.

Figure 23 shows the simplest form of wheel and axle, in which A is the wheel and B the axle or drum. If a weight P is hung from a cord wound on A it will wind up a certain weight W on drum B.