Watermill

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Watermill of Braine-le-Château, Belgium (12th century)

A watermill is a structure that uses a water wheel or turbine to drive a mechanical process such as flour or lumber production, or metal shaping (rolling, grinding or wire drawing). A watermill that only generates electricity is more usually called a hydroelectric plant.

The technology behind the watermill is somewhat older than that of the windmill. The ancient Greeks and Romans are known to have used the technology; the Romans used both fixed and floating water wheels and introduced water power to other countries of the Roman Empire. So-called 'Greek Mills' used water wheels with a vertically mounted shaft. A "Roman Mill" features a horizontally-mounted shaft. Greek style mills are the older and simpler of the two designs, but only operate well with high water velocities and with small diameter millstones. Roman style mills are more complicated as they require gears to transmit the power from a shaft with a horizontal axis to one with a vertical axis. The Cistercian Order built huge mill complexes all over Western Europe during the medieval period.

1 Operation of a watermill
2 Types of watermills
3 See also
4 Notes
5 External links

[edit] Operation of a watermill

Roblin's Mill, a watermill, at Black Creek Pioneer Village in Toronto

Watermills in Bosnia

Typically, water is diverted from a river or impoundment or mill pond to a turbine or water wheel, along a channel or pipe (variously known as a flume, head race, mill race, leat, leet^[1], or penstock). The force of the water's movement drives the blades of a wheel or turbine, which in turn rotates an axle that drives the mill's other machinery. Water leaving the wheel or turbine is drained through a tail race, but this channel may also be the head race of yet another wheel, turbine or mill. The passage of water is controlled by sluice gates that allow maintenance and some measure of flood control; large mill complexes may have dozens of sluices controlling complicated interconnected races that feed multiple buildings and industrial processes.

The interior of a functional water mill

There are three kinds of water mill: undershot, overshot and horizontal. The oldest of these were horizontal mills in which the force of the water, striking a simple paddle wheel set horizontally in line with the flow turned a runner stone balanced on a shaft leading directly up from the wheel. The problem with this type of mill arose from the lack of gearing; the speed of the water directly set the maximum speed of the runner stone which, in turn, set the rate of milling.

At some point gearing was invented. This allowed the mill designers to employ a large mill wheel set perpendicular to the mill race. (This is the type of water wheel we generally think of when we imagine a water mill.) This large wheel drove a shaft which turned a smaller face wheel set with pegs that, in turn, turned a lantern gear attached to the shaft which drove the runner stone. Additional gearing might allow a single water wheel to drive as many as four stones. Each step in the process increased the gear ratio which increased the maximum speed of the runner stone. Adjusting the sluice gate and thus the speed of the water past the main wheel allowed the miller to finely adjust the speed of his stone(s) depending on the type of grain being milled.

A later innovation was the overshot wheel. With an undershot wheel, in which the main water wheel is simply set into the flow of the mill race there exists an inherent inefficiency stemming from the fact that the wheel itself, entering the water behind the main thrust of the flow driving the wheel, followed by the lift of the wheel out of the water ahead of the main thrust, actually impedes its own operation. The overshot wheel solves this problem by bringing the water flow to the top of the wheel. The water fills buckets built into the wheel, rather than the simple paddle wheel design of undershot wheels. As the buckets fill, the weight of the water starts to turn the wheel. The water spills out of the bucket on the down side into a spillway leading back to river. Since the wheel itself is set above the spillway, the water never impedes the speed of the wheel. This type of mill requires the construction of a dam on the river above the mill and a more elaborate millpond, sluice gate, mill race and spillway or tailrace.^[2]

Toward the end of the 1800s, the invention of the Pelton wheel encouraged some mill owners in at least North America to replace over- and undershot wheels with penstocks and Pelton wheel turbines. By the early 20th century, availability of cheap electrical energy made the water mill obsolete; although in North America, some smaller rural mills continued to operate commercially into the 1960s. A few historic mills (for example, at the Wayside Inn) still operate for demonstration purposes to this day.

A unique type of water mill is the tide mill. This mill might be of any kind, undershot, overshot or horizontal but it does not employ a river for its power source. Instead a mole or causeway is built across the mouth of a small bay. At low tide, gates in the mole are opened allowing the bay to fill with the incoming tide. At high tide the gates are closed, trapping the water inside. At a certain point a sluice gate in the mole can be opened allowing the draining water to drive a mill wheel or wheels. This is particularly effective in places where the tidal differential is very great, such as the Bay of Fundy in Canada where the tides can rise fifty feet, or the now derelict village of Tide Mills in the UK.

Other water mills can be set beneath large bridges where the flow of water between the stanchions is faster. At one point London bridge had so many water wheels beneath it that bargemen complained that passage through the bridge was impaired.

A final, rather elegant, water wheel innovation places the wheel in a boat anchored in midstream. The flow of the river past the boat turns the wheel and drives the millstone.

"Run of the river" schemes do not divert water at all and usually involve undershot wheels, and some types of water wheel (usually overshot steel wheels) mount a toothed annular ring near the outer edge that drives machinery from a spur gear rather than taking power from the central axle. However, the basic mode of operation remains the same; gravity drives machinery through the motion of flowing water.

[edit] Types of watermills

Gristmills grind grains into flour. These were undoubtedly the most common kind of mill.
Fulling mills or Walkmills were used for a finishing process on cloth (see also fulling).
Blade mills were used for sharpening newly made blades.
Sawmills cut timber into lumber
Barking mills stripped bark from trees for use in tanneries
Spoke mills turned lumber into spokes for carriage wheels.
At the beginning of the industrial revolution, cotton mills were usually powered by a water wheel.
Carpet mills for making rugs were sometimes water-powered.
Textile mills for weaving cloth were sometimes water-powered.
Powder mills for making black powder or smokeless powder were sometimes water-powered.
Blast Furnaces, finery forges, slitting mills, and tinplate works were until the introduction of the steam engine invariably water powered and were sometimes called iron mills.
Prior to the introduction of the cupola (a reverberatory furnace), lead was usually smelted in smelt mills.
Paper mills used water not only for motive power but also in large quantities in the manufacturing process.

[edit] See also

[edit] Notes

^ Webster's New Twentieth Century Dictionary of the English Language Unabridged (1952) states: leet, n. A leat; a flume. [Obs.]
^ Dictionary definition of "tailrace".