Archimedes' screw

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The Archimedes screw , also known as Archimedes' screw, the Archimedean screw or the screwpump is a machine historically used for transferring water from a low-lying body of water into irrigation ditches. It was one of several inventions and discoveries traditionally attributed to Archimedes in the 3rd century BCE.

Contents

Design

The Archimedes' Screw consists of a screw inside a hollow pipe. The screw is turned usually by a windmill or by manual labor. As the bottom end of the tube turns, it scoops up a volume of water. This amount of water will slide up in the spiral tube as the shaft is turned, until it finally pours out from the top of the tube and feeds the irrigation systems. It was mostly used for draining water out of mines .

The contact surface 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 with respect to the angular frequency and 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 mechanical equilibrium is achieved while using the machine, thus limiting a decrease in mechanical efficiency.

In some designs, the screw is fixed to the casing and they rotate together instead of the screw turning within a stationary casing. A screw could be sealed with pitch resin or some other adhesive to its casing, or, cast as a single piece in bronze, as some researchers have postulated as being the devices used to irrigate the Hanging Gardens of Babylon, one of the Seven Wonders of the Ancient World. Depictions of Greek and Roman water screws show the screws 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.

Uses and History

Along with transferring water to irrigation ditches, this device was also used for reclaiming land from under sea level in the Netherlands and other places in the creation of polders. A part of the sea would be enclosed and the water would be pushed up out of the enclosed area, starting the process of draining the land for use in farming. Depending on the length and diameter of the screws, more than one machine could be used to successively lift the same water.

An Archimedes screw was used by British soils engineer Dr. John Burland in the successful 2001 stabilization of the Leaning Tower of Pisa. Small slivers of subsoil saturated by groundwater were removed from far below the north side of the Tower, and the weight of the tower itself corrected the lean.

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.

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