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Archimedes screw

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# The open screw, which rotates in a trough. 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 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.

# The enclosed screw, in which both the screw and an enclosing cylinder rotate. Except for the following differences, the enclosed screw pump is similar to the open type:

** Because the flights are welded to the outer cylinder, there is no slippage (backflow). If the pump is stopped, the water is retained between the flights. The lower bearing (a self-aligning set of rollers mounted above the highwater level) is easily accessible. The rollers carry most of the radial load.** Because there is no slippage, the efficiency of the pump is very high and stays high even at low discharge.** A massive concrete structure is not needed.

====Modern uses====

Winona

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