Water Wheel

A water wheel turns the flow or fall of water into rotation, which gears then pass to millstones, hammers, bellows or pumps. From the third century BCE until the nineteenth century it was the main source of mechanical power after people and animals.

What it is

An undershot wheel: the current pushes the lowest paddles.

A water wheel is a wheel fitted with paddles or buckets that moving water pushes round, and its axle drives machines. For two thousand years it ground grain. From the Middle Ages it also fulled cloth, sawed timber, worked the bellows and hammers of forges and pumped water out of mines.

An undershot wheel dips its paddles into a stream and the current pushes them. A simple traditional design needs no dam and converts only a small share of the available water energy into work. An overshot wheel, the kind on this plate, receives water at the top from a channel and turns under the weight of water in its buckets. It needs a fall at least as high as the wheel. A natural drop with an intake and channel can provide this fall; a dam and pond are optional. A breastshot wheel receives water at axle height. Efficiency depends on the design and water supply; improved undershot wheels can also be efficient.

How it works

Water held in a millpond stores energy because of its height. In the overshot mill the flume (1), a wooden trough, carries it out over the top of the wheel, and a sluice gate lets the miller start the wheel, stop it and control its speed. The water drops into the buckets (2) on the descending side. That side is heavy and the other is empty, so the wheel turns slowly, a few revolutions a minute. The buckets hold their water as long as possible and empty near the bottom into the tailrace.

The available hydraulic power is P = ρgQH: water density ρ multiplied by gravitational acceleration g, volumetric flow Q and fall height H. With water at about 1,000 kg/m³, g = 9.81 m/s², a flow of 100 litres per second (0.1 m³/s) and a fall of four metres give about 3.92 kW before losses. A well-made overshot wheel converts most of this into work on its axle.

In the horizontal mill there is no gearing: the wheel’s shaft is the millstone’s spindle.

Inside the mill, the axle (3) carries the pit wheel (4), a large toothed wheel whose cogs drive the small wallower (5) on a vertical shaft. As in a windmill, the drive turns from horizontal to vertical and speeds up, because the wheel is slow and the upper millstone must turn much faster. That runner stone (6) grinds the grain, fed from a hopper, against the fixed bed stone beneath it.

Some mills have no gears at all. In the horizontal mill, long used in Ireland, Scandinavia and around the Mediterranean, a small wheel lies flat under the floor in a jet of water from a steep chute, and its vertical shaft turns the runner stone directly. Simple traditional designs are inexpensive and convert a relatively small share of the available water energy into work. Improved horizontal wheels can reach high efficiencies.

How to build

This outline is for a small overshot wheel driving a pump or a light machine. Taking water from a stream usually needs permission.

Find the fall. Look for a stream with a drop of two metres or more. Dig a channel, the leat, that leaves the stream upstream on a very gentle slope, so that at the wheel it runs well above the stream bed.

Size the wheel. Make the wheel a little smaller than the fall, so the water drops a short way into the top bucket. Its width depends on the flow. Buckets that are only partly filled spill less early.

Build the axle and arms. Use an oak axle, squared where the arms are fixed, with iron pins at the ends turning in bearings of hardwood or bronze on stone or concrete piers.

Make the shrouds and buckets. Fit the buckets at an angle between two rings of boards, the shrouds, and close the inner side with boards. Soak the wheel before use so the wood swells and the joints seal.

Fit the flume and sluice. Carry the water to the top of the wheel in a wooden trough on trestles, with a sliding gate and a spillway that sends the water past the wheel when it is stopped. Check that the wheel is balanced, then open the sluice gradually.

Safety. A loaded wheel is heavy and does not stop at once: never climb on it or reach into it while it turns. Before maintenance, shut off the water supply, lock the sluice against reopening and divert any leakage away from the wheel. Verify that the wheel has stopped completely. From a safe position, drain retained water and secure the wheel and driven machinery with restraints rated for the remaining loads so stored energy cannot move them. Have a competent person establish and check this isolation. Fast water in a channel can knock over and drown an adult, so fence the tailrace and never work near it alone. Fit guards over all accessible gearing to prevent hands and clothing from reaching it.

History and evolution

The first wheels lifted water. In the third century BCE Philo of Byzantium described a wheel with compartments used for irrigation in Egypt. The historian M. J. T. Lewis argues that grain mills driven by water appeared in the same century, first with a vertical axle, then with a horizontal one. The earliest clear description of a geared water mill comes from Vitruvius around 25 BCE: an undershot wheel drives the millstone through a toothed drum and a larger horizontal wheel.

At Barbegal, near Arles in southern France, the Romans built sixteen overshot wheels in two rows of eight down a hillside, fed by an aqueduct. The complex was built in the second century CE. In China, Du Shi used water power to work the bellows of iron foundries in 31 CE.

A noria: the current turns the wheel, and the wheel lifts part of the river.

In North Africa and the Near East, the noria, a large wheel turned by the current with pots on its rim, raised water into irrigation channels and supplied towns. In the tenth century the physician al-Razi described one in Iraq lifting about 153,000 litres an hour.

Water mills multiplied in medieval Europe. The Domesday survey of 1086 counted about six thousand in England, and wheels came to drive fulling stocks, saws, forge bellows and trip hammers.

In the 1750s the English engineer John Smeaton tested model wheels. At comparable available hydraulic energy, his overshot designs did at least twice the work of his undershot designs. He published the results in 1759. This comparison applies to the tested designs; it is not a fixed ratio for all water wheels. The Laxey Wheel on the Isle of Man, finished in 1854 to pump water out of the Laxey mines, is 22.1 metres across. From 1827 Benoît Fourneyron’s turbine began to replace the wheel, and the turbines of modern hydroelectric plants descend from it.