The screw is classified as one of six simple machines. It is often described as an inclined plane wrapped around a cylinder. Screws in many forms can be combined with other simple machines such as the lever and pulley to assemble machines as simple as a pair of scissors or as complex as a modern automobile. Machines and devices based on the screw are found everywhere in daily life.
A screw is a simple device that is used to attach two things together. Screws have a round central shaft around which an inclined plane is wrapped and a wedge-shaped point. A screw works by twisting itself into a piece of wood or metal. As the screw turns, the inclined plane pulls the screw into the wood. The design of a screw differs depending on the materials it will attach and the size and weight of the workpieces.
Nuts and Bolts
A bolt and its matching nut are also used to attach or connect pieces. A bolt is a screw with a flat end instead of a pointed wedge on its tip and an inclined plane or “thread” wrapped around the center. The nut has a hollow center with a matching thread that fits on the bolt. As the nut turns relative to the bolt, the inclined plane makes the nut move along the shaft to tighten or loosen the bolt.
Faucets use a screw to open or close, allowing water to run through a pipe or out a spout. The handle of a faucet is the head of a screw, and there is a rubber or plastic washer mounted at the opposite end. When the handle turns, the screw rotates in the body of the faucet and the washer retracts to open a channel for water to come through. Turn the handle the other way, and the rubber washer seals off the opening again.
A hand or power drill bit is a form of screw that has the same diameter all along its length. The bits that bore into wood are cylinders wrapped with sharp-edged inclined planes. As the bit is rotated into the wood by hand or with a power drill, the sharp edges cut away shreds of wood. The design of bits used in power drills also allows the wood shreds or cuttings to move up the bit and collect on the surface.
Although it doesn’t look like a screw at first glance, a boat propeller uses the same principles. The blades of a propeller form an inclined plane that is mounted around a central shaft. As a motor turns the shaft, the blades spin. The inclined plane made by the blades pushes the water away from the boat, which drives the boat forward at the same time. An airplane propeller does the same thing, except that propellers usually pull the plane through the air instead of push it from behind.
The metal or plastic lid on a glass jar is nothing more than a wide screw, very similar to the nut and bolt. The lip of the glass jar has a raised ridge that forms a screw shape, and the metal lid has a matching thread on the inside. Turn the lid one way and it comes off the jar, turn it the other and it tightens on the jar.
An electric fan, whether in a box or mounted on the ceiling, is a form of screw. The fan blades are mounted on a central shaft, and all are slightly tilted. When the fan motor turns the blades, they push air in front of them to create a breeze. Ceiling fans usually have reversible motors to allow the fans to push air toward the floor or pull air toward the ceiling.