The water screw, popularly known as the Archimedes' screw and also known as the screw pump, Archimedean screw, or Egyptian screw,^[1] is a machine used for transferring water from a low-lying body of water into irrigation ditches. Water is pumped by turning a screw-shaped surface inside a pipe. Archimedes screws are also used for materials such as powders and grains. Although commonly attributed to Archimedes, the device had been used in Ancient Egypt long before his time.

The screw pump is the oldest positive displacement pump.^[1] The first records of a water screw, or screw pump, dates back to Ancient Egypt before the 3rd century BC.^[1][2] The Egyptian screw, used to lift water from the Nile, was composed of tubes wound round a cylinder; as the entire unit rotates, water is lifted within the spiral tube to the higher elevation. A later screw pump design from Egypt had a spiral groove cut on the outside of a solid wooden cylinder and then the cylinder was covered by boards ot sheets of metal closely covering the surfaces between the grooves.^[1]

Some researchers have postulated this as being the device used to irrigate the Hanging Gardens of Babylon, one of the Seven Wonders of the Ancient World. A cuneiform inscription of Assyrian King Sennacherib (704–681 BC) has been interpreted by Stephanie Dalley^[3] to describe casting water screws in bronze some 350 years earlier. This is consistent with classical author Strabo, who describes the Hanging Gardens as irrigated by screws.^[4]

The screw pump was later introduced from Egypt to Greece.^[1] It was described by Archimedes,^[5] on the occasion of his visit to Egypt, circa 234 BC.^[6] This tradition may reflect only that the apparatus was unknown to the Greeks before Hellenistic times.^[5] Archimedes never claimed credit for its invention, but was attributed to him 200 years later by Diodorus, who believed that Archimedes invented the screw pump in Egypt.^[1] Depictions of Greek and Roman water screws show them being powered by a human treading on the outer casing to turn the entire apparatus as one piece, which would require that the casing be rigidly attached to the screw.

German engineer Konrad Kyeser equipped the Archimedes screw with a crank mechanism in his Bellifortis (1405). This mechanism quickly replaced the ancient practice of working the pipe by treading.^[7]

The Archimedes screw consists of a screw (a helical surface surrounding a central cylindrical shaft) inside a hollow pipe. The screw is usually turned by windmill, manual labor, cattle, or by modern means, such as a motor. As the shaft turns the bottom end scoops up a volume of water. This water is then pushed up the tube by the rotating helicoid until it pours out from the top of the tube.

The contact surface between the screw and the pipe does not need to be perfectly watertight, as long as the amount of water being scooped with each turn is large compared to the amount of water leaking out of each section of the screw per turn. If water from one section leaks into the next lower one, it will be transferred upwards by the next segment of the screw.

In some designs, the screw is fused to the casing and they both rotate together, instead of the screw turning within a stationary casing. The screw could be sealed to the casing with pitch resin or other adhesive, or the screw and casing could be cast together as a single piece in bronze.

The screw was used predominately for the transport of water to irrigation systems and for dewatering mines or other low-lying areas. It was used for draining land that was underneath the sea in the Netherlands and other places in the creation of polders.

Archimedes screws are used in sewage treatment plants because they cope well with varying rates of flow and with suspended solids. An auger in a snow blower or grain elevator is essentially an Archimedes screw. Many forms of axial flow pump basically contain an Archimedes screw.

The principle is also found in pescalators, which are Archimedes screws designed to lift fish safely from ponds and transport them to another location. This technology is used primarily at fish hatcheries, where it is desirable to minimize the physical handling of fish.

An Archimedes screw was used in the successful 2001 stabilization of the Leaning Tower of Pisa. Small amounts of subsoil saturated by groundwater removed from far below the north side of the tower, and the weight of the tower itself corrected the lean. Archimedes screws are also used in chocolate fountains.

A screw conveyor is an Archimedes screw contained within a tube and turned by a motor so as to deliver material from one end of the conveyor to the other. It is particularly suitable for transport of granular materials such as plastic granules used in injection molding, and cereal grains. It may also be used to transport liquids. In industrial control applications the conveyor may be used as a rotary feeder or variable rate feeder to deliver a measured rate or quantity of material into a process.

A variant of the Archimedes screw can also be found in some injection molding machines, die casting machines and extrusion of plastics, which employ a screw of decreasing pitch to compress and melt the material. It is also used in a rotary-screw air compressor. On a much larger scale, Archimedes's screws of decreasing pitch are used for the compaction of waste material.

If water is fed into the top of an Archimedes screw, it will force the screw to rotate. The rotating shaft can then be used to drive an electric generator. Such an installation has the same benefits as using the screw for pumping: the ability to handle very dirty water and widely varying rates of flow at high efficiency. Settle Hydro and Torrs Hydro are two reverse screw micro hydro schemes operating in England. The screw works well as a generator at low heads, commonly found in English rivers, including the Thames, powering Windsor Castle.^[8]

In 2017, the first reverse screw hydropower in the United States opened in Meriden, Connecticut.^[9][10] The Meriden project was built and is operated by New England Hydropower having a nameplate capacity of 193 kW and a capacity factor of approximately 55% over a 5-year running period.