A water-wheel is a hydropower system, a system for extracting power from a flow of water. It was a widely used system in the middle ages, powering most industry in Europe, along with the windmill. The most common use of the waterwheel was to mill flour, where it was known as the watermill, but other uses included machining and pounding linen for use in paper.

A water-wheel consists of a large wheel, typically wooden, with a number of blades or buckets arranged on the outside rim forming the driving surface. The wheel is mounted vertically on a horizontal axle that is used as a power take-off. Water-wheels come in two basic forms – under-shot and over-shot.

The over-shot wheel has the water channeled to the wheel at the top and slightly to one side in the direction of rotation. The water collects in the buckets on that side of the wheel, making it heavier than the other "empty" side. The weight turns the wheel, and the water flows out into the tail-water when the wheel rotates enough to invert the buckets. The over-shot design uses almost all of the water flow for power (unless there is a leak) and does not require rapid flow. The overshot wheel is a far more powerful and effiecietn design, but because it required constructing a dam and a pond it was far more capital intensive.

The under-shot design places the wheel over a fast-flowing body of water. Here it is the flow of the water directly against the buckets (or paddles) that turns the wheel, not the weight. It has the advantage of being more powerful, but can only be used where the flow rate is sufficient to provide torque.

A more modern design of the under-shot system combines the features of the over-shot as well. In this version the water stream is "dug out" below the wheel, so the water has to flow against the buckets, as well as fill them and drain out as in the over-shot design. This version captures power from both the flow and the weight, and became the most popular version throughout Europe.

Water wheels used belts to transmit power from the wheel to machinery. One wheel would usually be used to power many machines, and often even different mills.

The water wheel was a long known technology but it was not put into widespread use until the European Dark Ages when an acute shortage of labour made machines such as the water wheel cost effective. The water wheel remained competitive with the steam engine well into the industrial revolution. The main difficulty of water wheels was their inseperability from water. This meant that mills often needed to be located far from population centres and away from natural resources. Water mills were still in commercial use well into the twentieth century, however.

Modern Hydro-electric dams can be viewed as the descendants of the water wheel as they too take advantage of the movement of water downhill.

External link