Waterwheel

Waterwheel, simple mechanical device used to convert water power into rotary motion. A waterwheel works on the principle that when water undergoes a controlled change in elevation, the falling water is a source of power and will turn the axle of the waterwheel. The three general types of waterwheels, which are sometimes called gravity wheels, are the undershot, the overshot, and the breast wheel.

The oldest variety is the undershot wheel. This waterwheel was used by the early Egyptians and Persians to drive water-lifting devices, called norias, which were used in irrigating fields. The simplest form of an undershot waterwheel is a paddle wheel, which is placed with the lower paddles in a stream of water. The water current exerts a force against the paddles and turns the wheel. In order to improve the power obtained from a given source, the paddles may be operated in a trough that prevents water from passing around the paddles without doing work. The waterwheel was subsequently adapted to turn millstones for grinding corn and grain. See also Waterpower.

When a waterwheel is constructed in a hilly region where there is an appreciable difference between upstream and downstream water elevations, an undershot wheel with a regulating gate or a breast waterwheel may be used. A breast waterwheel is located at the base of a waterfall; the water catches and turns the paddles as the water flows over the falls. Various forms of the undershot wheel were used exclusively until the time of the Renaissance. The early water wheels were constructed of wood; use of iron parts became common during the Renaissance.

In areas where the difference in water elevation is about equal to the wheel diameter or slightly larger, the more efficient overshot wheel is installed. In the overshot wheel, which came into common use during the Renaissance, the stream of water is directed to flow over the waterwheel paddles, and additional energy is gained from the falling water.

With the introduction of hydraulic turbines in the early part of the 19th century, the importance of waterwheels decreased. Today they are limited to small power plants. The best modern overshot waterwheels attain efficiencies of 85 percent or greater, breast wheels about 75 percent, and undershot wheels about 35 percent.