Wednesday, 13 November 2013

Session 4: The Archimedes screw

A screw is basically a slope or inclined plane wrapped around a cylinder in a spiral shape. You can think of it as being like a road up a mountain. If you wanted to drive up a mountain, you would find the mountain too steep to drive in a straight line from the bottom to the top. But if you took a longer, less steep route, spiralling around the mountain and gradually working your way up to the top it would be a lot easier.

Archimedes' screw is a machine, based on a screw design, that is used to move liquids. It was named after the famous ancient Greek mathematician, inventor and engineer, Archimedes of Syraceuse.

Although the machine took his name, Archimedes may not have been the original inventor. It is likely that a similar design had been in use for some years before Archimedes wrote about it. Whether Archimedes was the influence or not there is evidence that the screw design was used in Ancient Egypt to pump water from the Nile to irrigate crops in fields. There are also suggestions that the design may have originated in Assyria, (an area that would now include parts of Turkey, Iraq, Iran and Syria). It is quite possible that several ancient cultures arrived at the same design independently.

Archimedes' screw traditionally consists of spiralling blades inside a hollow cylinder or tray. As the screw is turned the liquid is trapped in the spaces left by the inclined blades and gradually moves up the cylinder.

This demonstration below shows the basic principle (using plastic tubing on the outside of a cylinder). The video below it shows an enormous Archimedes screw in action.

Here is an entertaining use for an archimedes screw design!

The spiral/screw shape can be seen in nature too. This video shows how a long shell could be used in the same manner as an archimedes screw.

Archimedes screw is primarily designed for moving liquids. The design was later developed into the "screw conveyor" which moves dry materials (powders, pellets,grains etc)using the pushing action of its rotating blades. This LEGO model is probably the best illustration of how the screw conveyor works and how it could be used in industry.

The screw design can also be used 'in reverse'. Instead of using power to turn the screw in order to carry liquids upwards, a liquid poured into the top of a screw can turn the screw and produce energy. The video below demonstrates how using a large screw and a flowing water source can generate electricity. (It's an unnecessarily long video (!), but the first minute or so will give you a good overview of how it works).

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