(auger; worm). A simple machine employing the principle of the inclined plane, invented by the Greek scientist Archimedes. It consists of a central shaft around which winds a spiral of ribs (called “flights”) that are integral with the shaft. The distance between the flights is the pitch, or angle of inclination of the screw. This distance may be uniform throughout the length of the screw or it may vary from one point to another, depending on use requirements. The shorter the distance between flights, the more pressure the screw will deliver. Screws have a number of important applications in industrial operations. (1) Extrusion of rubber, plastics, and food products: the screw rotates in a chamber or barrel, the product being introduced through a port where the flights are farthest apart; it is forced through a die at the opposite end of the barrel, which molds it to the form desired. (2) Mixing of solids: continuous mixing is possible with screws having a wide variety of pitches and contours that impart a back-and-forth motion to the product being mixed without moving it to the discharge end until it is pushed along by added material. Two screws may operate in parallel, their pitches opposing each other in such a way as to effect maximum mixing. Some screws are cored for circulation of cooling water. (3) Conveying of solids: for this purpose screws with uniform and fairly wide pitch are used; as the screw turns the solids are passed along from one flight to the next. These are used for conveying wet and dry solids, wood chips, and similar particulates. (4) An engineering application of the screw is the mechanism known as a worm gear, in which the flights of the screw engage corresponding indentations or notches in a shaft or wheel, causing it to turn.
