Water screw

Water screw (also known as the Egyptian screw, Archimedes' screw or Archimedean screw) is a simple machine historically used for transferring water from a low-lying body of water, mostly into irrigation ditches. A screw, in this case, is simply an inclined plane (another simple machine) wrapped around a cylinder.

History

The screw pump is the oldest positive displacement pump. This device was reportedly 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) appears to describe casting water screws in bronze some 350 years earlier. This is consistent with Greek historian Strabo, who describes the Hanging Gardens as irrigated by screws.

The first clear records of a water screw, or screw pump, date back to Egypt before the 3rd century BC. 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 or sheets of metal closely covering the surfaces between the grooves.

The screw pump was later introduced from Egypt to Greece. It was described by Archimedes, on the occasion of his visit to Egypt, circa 234 BC. This tradition may reflect only that the apparatus was unknown to the Greeks before Hellenistic times. Archimedes never claimed credit for its invention, but it was attributed to him 200 years later by Diodorus, who believed that Archimedes invented the screw pump in Egypt.

Archimedes' screw

Old design

This machine consists of a screw inside a hollow pipe.

How it works

The lower end of the device is put in the water, and the screw is then turned (usually by a windmill or by animal or human labor). As the bottom end of the tube turns, it scoops up an amount of water. This puddle of water will slide up in the spiral tube as the shaft is turned, until finally it falls out from the top of the spiral tube and feeds the irrigation system.

Reqirement

The interface between the screw and the pipe does not need to be perfectly water-tight because of the relatively large amount of water being scooped at each turn in respect to the angular speed of the screw. Also, water leaking from the top section of the screw leaks into the previous one and so on, so a sort of equilibrium is achieved while using the machine, thus reducing the decrease in efficiency.

Early uses

Along with transferring water to irrigation ditches, this device was also used for "stealing" land (in early days) from under sea level in Netherlands.

Since the primary objective in this case is to lift water to a given height rather than simply move it from a river to the irrigation ditches, more than one machine was used to successively lift the same volume of water , due to the limitations of this machine.

The mechanism is also used in coal fired power stations to move slurries and in some injection moulding industries to move the paste to the moulding dies.