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Screw Definition #2
This simple machine is a modification of the wedge designed to yield a very large mechanical advantage in minimum space. The screw is essentially a transfer device of motion and/or force.
Around 200 BC, a Greek mathematician ( Apollonius of Perga ) worked out the geometry of the spiral-helix and laid the groundwork for the screw!
In one sense, a screw is not a "simple" machine at all since it depends upon another machine ( the lever ) for its operation. It can be looked at as a twisted wedge that derives its power not from percussion but from being turned by a lever. In other words, it is a cylinder with an inclined plane wrapped around it. The most famous screws of antiquity were those of Archimedes...one of them designed to raise water and another enabled him to drag a fully loaded three masted ship onto dry land!
The screw can function in two principle ways: It can raise weights and it can press or fasten objects! In the former role, it converts rotary motion into straight line motion. It was first used a fastener by ancient goldsmiths for locking bracelets. A screw finds its mechanical advantage in the ratio of two dimensions: the length of the lever that turns it and the distance between threads ( or its pitch ).
The old press was used to squeeze excess sizing out of paper. A modern version of Archimedes' famous water screw is the meat grinder in a ship's propeller...the three screw form blades act like oars thrusting against the water behind them while being drawn forward by the lower pressure ahead of them. A screw and nut are opposed twins! The hollow nut is an inversion of the solid screw in that the spiral helix is cut into the inner surface of a cylindrical cavity rather than raised in relief on the external surface of a solid cylinder.
