You can imagine the screw as an inclined plane wrapped around a metal rod. This can be shown by wrapping a triangle of paper round a pen and noting how the paper edge travels along the pen. Attaching a nut to a bolt demonstrates that the nut moves up the bolt by following a continuous path cut into the bolt. A wood screw operates in a similar manner by pulling itself into a block of wood as it turns. Because a nut on a bolt travels a greater distance around the bolt, compared with the distance it moves along the bolt, there is a great mechanical advantage and a great force can be applied to the bolt with little effort.

Using the screw
The mechanical advantage of a screw can be put to good use in many applications, one being a car jack. There are basically two types of car jack, both of which use the screw mechanism. One type is the scissor jack and the second is a bottle jack. In the scissor jack the screw is used to pull together a diamond shape with hinges top and bottom. As the screw is tightened the hinged sections move apart.

The cork screw works by inserting the sharp point of the screw into the soft cork, forcing the screw into the cork by turning and removing the cork by pushing down on the levers with the screw embedded in the cork.

Archimedes' screw
Archimedes was a Greek inventor, mathematician and experimenter. He experimented with simple machines like the lever and the pulley making moving heavy objects so much easier. He used the principle of the screw in his invention to raise water from a lower level to a higher level.

The Archimedes Screw collects water at the lower level and it flows from one spiral to the next finally emerging at the top of the screw, depositing water at a higher level.