Because there is no slippage, the efficiency of the pump is very high and stays high even at low discharge. The rollers carry most of the radial load. The lower bearing (a self-aligning set of rollers mounted above the highwater level) is easily accessible. If the pump is stopped, the water is retained between the flights. Because the flights are welded to the outer cylinder, there is no slippage (backflow). Except for the following differences, the enclosed screw pump is similar to the open type: The enclosed screw, in which both the screw and an enclosing cylinder rotate.The limiting speed ranges from 20 rev/min in large pumps to 75 rev/min in small ones and cannot be exceeded without water spilling over the top of the screw. It consists of a torque tube to which spiral flightsare attached, a lower submerged bearing, an upper radial and thrust bearing, a gear reducer (typically driven by an electric motor), and a trough in which the screw rotates at a constant speed. The open screw, which rotates in a trough.There are two types of the Archimedes Screw: Besides being of simple construction, it has the additional advantage of being able to move water that contains mud, sand, or gravel. The Archimedes Screw has been widely used over the centuries, especially to raise irrigation water and for land drainage, often being powered by people or animals. In such cases, it is perhaps the most efficient of the ancient methods, because two men can raise an enormous volume of water in less than an hour. It can be used where the water needs to be raised less than 1.5 meters (5 feet). Successive revolutions will raise the water thread-by-thread until it emerges at the top of the cylinder. When the open lower end of the cylinder is placed in water and the screw is turned, water trapped between the cylinder and the threads of the screw rises. It operates with the cylinder somewhat tilted from vertical. The Archimedes Screw is basically a large helix, open at both ends and encased within a watertight cylinder. However, archaeological evidence has led others to posit its earlier invention in Assyria (modern day Iraq) or Egypt Archimedes simply improved upon an earlier design. The Archimedes Screw (named after a Greek mathematician) is though to have been around since 250 B.C.E. Photo: Helen and Frank Schreider of the National Geographic staff. An Egyptian farmer turning an Archimedes screw by hand to irrigate a field.
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