Water wheel

'Noria' redirects here. For the singer, see Noria Shiraishi.

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 foundry work and 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.

Overshot wheel

A vertically mounted water wheel that is rotated by falling water striking paddles, blades or buckets near the top of the wheel is said to be overshot. Overshot wheels are the most efficient type; an overshot steel wheel that is also backshot can be more efficient than all but the most advanced and well-constructed turbines. In some situations an overshot wheel is vastly preferable to any turbine.

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 requires constructing a dam and a pond it is far more capital intensive.

Unlike undershot wheels, overshot wheels gain a double advantage from gravity. Not only is the force of the flowing water partially transferred to the wheel, the weight of the water descending in the wheel's buckets also imparts additional energy. The mechanical power derived from an overshot wheel is determined by the wheel's physical size and the available head, so they are ideally suited to hilly or mountainous country.

Although traditionally water wheels have been made mostly from wood, the use of steel in overshot wheels allows faster rotation (possibly reducing the need for gearing) without extreme reductions in available torque. A wooden wheel with a wooden axle that can easily turn low-speed, high-torque loads such as a run of millstones cannot necessarily sustain high speeds such as are needed for hydroelectric power generation.

Other types of vertically mounted water wheels include Under Shot and Breast Shot. Horizontally mounted water wheels are usually called tub wheels or Norse wheels.

Undershot wheel

A vertically mounted water wheel that is rotated by water striking paddles or blades at the bottom of the wheel is said to be undershot. With the exception of very advanced implementations such as the Poncelot wheel (which is basically a turbine mounted sideways) undershot wheels are the least efficient type, with efficiencies of 15% or less.

Undershot wheels gain no advantage from head and are never backshot. They are most suited to shallow streams in flat country.

Undershot wheels are also well suited to installation on floating platforms, and the Romans mounted them immediately downstream from bridges where the flow restriction of arched bridge supports increased the speed of the current.

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 Château de Marly were powered by a large permanent installation of undershot wheels.

Most overshot water wheels turn in the opposite direction of the water's flow. This is because the water flows over the top of the wheel, and not under it as in an undershot or breastshot design.

Backshot wheel

An overshot wheel is backshot by introducing the intake water from the same direction as the flow of the output water, sometimes by the introduction of a 180 degree turn just past the wheel itself, and some times through a lengthy flume or penstock configuration dictated by the surrounding terrain.

A backshot wheel continues to function until the water in the wheel pit rises well above the height of the axle, when any other overshot wheel will be stopped or even destroyed. This makes the technique particularly applicable in streams that experience extreme seasonal variations in flow, and reduces the need for complex sluice and tail race configurations. A backshot wheel may also gain power from the water's current past the bottom of the wheel, and not just the weight of the water falling in the wheel's buckets.

Water wheels that are both overshot and backshot are often referred to as pitchback wheels or banki turbines.

Breast wheel

A vertically mounted water wheel that is rotated by falling water striking buckets near the center of the wheel's edge, or just above it, is said to be breastshot. Breastshot wheels are the most common type in the United States of America and are said to have powered the American industrial revolution.

Breastshot wheels are less efficient than overshot wheels, more efficient than undershot wheels, and are not backshot. The individual blades of a breastshot wheel are actually buckets, as are those of most overshot wheels, and not simple paddles like those of most undershot wheels (the Poncelot design being a notable exception). A breastshot wheel requires a good trash rake and typically has a masonry "apron" closely conforming to the wheel face, which helps contain the water in the buckets as they progress downwards.

Unlike undershot wheels, breastshot wheels gain a double advantage from gravity. Not only is the force of the flowing water partially transferred to the wheel, the weight of the water descending in the wheel's buckets also imparts additional energy. Breastshot wheels are preferred for steady, high-volume flows such as are found on the fall line of the North American East Coast.

Modern versions

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 could 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 axle bearing could be lubricated from inside. The shaft would usually drive a pulley which in turn would drive a leather belt. The belt could drive any of a set of other shafts (axles). Correctly-sized pulleys adapted the torque and speed of the wheel to the torque and speed needed by each piece of machinery. In other cases, the machinery was driven directly from the wheel shaft, sometimes via gearing.

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.

Water wheel technology

The technology of the water wheel had long been known, but it was not put into widespread use until the Middle 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 watermills of many kinds. An early example of a very large waterwheel is the still extant wheel at the early 13th century Real Monasterio de Nuestra Senora de Rueda, a Cistercian monastery in the Aragon region of Spain. Grist mills (for corn) were undoubtedly the most common, but there were also sawmills, fulling mills and mills to fulfil many other 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: one of Brindley's water wheels can be seen at the Brindley Mill in Leek, Staffordshire, England. In the 19th century, Jean-Victor Poncelet worked on improving the efficiency of the undershot design using modern hydraulic physics for the first time.

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.

Overshot & pitchback waterwheels are suitable where there is a small stream with a height difference of more than 2 metres, often in association with a small reservoir. Breastshot and undershot wheels can be used on rivers or high volume flows with large reservoirs.

The most powerful waterwheel built in the United Kingdom was the 100 hp Quarry Bank Waterwheel near Manchester. A high breastshot design, it was retired in 1904 and replaced with several turbines. It has now been restored and is a musium open to the public.

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 links

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