Water wheel
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Berry_Schools'_Old_Mill,_Floyd_County,_Georgia.jpg
A water wheel (also waterwheel, Norse mill, Persian wheel or noria) 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 water wheel was to mill flour, where it was known as the watermill, but other uses included machining and pounding linen for use in paper. The largest waterwheels in the world are located in the Syrian city of Hama.
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. Historic water wheels came in two basic forms – undershot and overshot.
The overshot 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 overshot 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 undershot design, sometimes called 'Vitruvian' after the Roman engineer Vitruvius, 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 cheaper and simpler to build, but is less powerful and can only be used where the flow rate is sufficient to provide torque. A unique advantage is that it can be used on rivers' surfaces, and can even be movable, if it is placed in an anchored boat. The fountains of King Louis XIV's summer residence, the Palace of Versailles were powered by a large permanent installation of undershot wheels.
A more modern version combines features of both: the Banki turbine. In this version the water stream is "dug out" below the wheel, and pushes open-bottomed buckets. It captures energy from both the top, and then the water drains to the bottom of the wheel and pushes the bottom as well. This version captures power from both the flow and the weight, and became the most popular version throughout Europe. It is still a very practical low-technology hydropower design.
Water wheels used shafts driving belts to transmit power from the wheel to machinery. One wheel would usually be used to power many machines, and sometimes even different mills. A fine advantage of the shaft technology was that the shaft could go through a bearing in a load-bearing wall. This both supported the wheel and let most of the machinery be indoors, where it is protected from the elements. Usually the main shaft bearing could be lubricated from inside. The shaft would usually drive a leather belt. The belt could drive any of a set of other shafts. Correctly-sized pulleys adapted the torque and speed of the wheel to the torque and speed needed by each piece of machinery.
Modern installations for "waterwheel" sites tend to use small weatherproof Kaplan turbines. These are purchased from the manufacturer and installed, sometimes in groups. The higher efficiencies permit smaller, less expensive dams, and use with smaller streams or smaller flumes than historic waterwheels. These usually have integral electric generators.
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. Cistercian monasteries, in particular, made extensive use of water wheels to power grist mills, sawmills, and other otherwise labor-intensive tasks. The water wheel remained competitive with the steam engine well into the Industrial Revolution. Water wheel technology was developed extensively in England in the 18th century, with notable figures including John Smeaton and James Brindley, following theoretical calculations and practical experiments in France and elsewhere. Smeaton performed experiments in 1754 that conclusively demonstrated the superiority of the overshot system: Brindley was Smeaton's pupil, and one of his water wheels can be seen at the Brindley Mill in Leek, Staffordshire, England. 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.
See also
External links
- Glossary of water wheel terms (http://www.angelfire.com/journal/millbuilder/terms.html)
- Water wheel history (http://www.waterhistory.org/histories/waterwheels/)
- Esay/audio clip (http://www.uh.edu/engines/epi105.htm)
- Waterwheel Factory, with pictures of water wheels (http://www.waterwheelfactory.com/index.htm)cs:Vodní kolo