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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 efficient 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.
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