A screw can be considered to be an inclined plane that has been wrapped around some central axis. You can see this relationship by making an inclined plane out of paper and then wrapping the paper around a pencil. The spiral shaped form that you make is a screw.
Screws can be used in two major ways. First, they can be used to hold things together. Some simple examples include wood and metal screws and the screws on jars and bottles and their tops. Screws can also be used to apply force on objects. The screws found in vises, presses, clamps, monkey wrenches, brace and bits, and corkscrews are some examples of this application.
The screw acts as a simple machine when an effort force is applied to the larger circumference of the screw. For example, a person might apply the effort force to a wood screw by turning a screwdriver. That force is then transmitted down the spiral part of the screw called the thread to the tip of the screw. The movement of the screw tip into the wood is the resistance force in this machine. Each complete turn of the screwdriver produces a movement of only one thread of the screw tip into the wood. This distance between two adjacent threads is called the pitch.
The mechanical advantage of a screw can be found by dividing the circumference of the screw by its pitch. For example, suppose that a carpenter is working with screws whose heads have a circumference of 1 in (2.54 cm) and a pitch of 1/8 in (.33 cm). Then the mechanical advantage of these screws is 1 divided by 1/8, or 8. The carpenter magnifies his or her efforts by a factor of 8 in driving the screw into a piece of wood.
Science EncyclopediaScience & Philosophy: Linear expansivity to Macrocosm and microcosmSimple Machines - Levers, Mechanical Advantage, Pulleys, Wheel And Axle, Inclined Planes, Screws, Compound Machines - Wedges