Screw

(sk roo.) [From Icel. skrufa, to twist.] (Mech.) One of the six mechanical powers, consisting of a spiral thread or groove cut round a cylinder of wood or metal, so as to bring all the lines drawn on its surface parallel to its axis at the same angle. When the thread is on the outside it is a male or convex screw; when along the inner surface of the cylinder, a female screw, or nut. The screw is reducible to an inclined plane; and its efficiency is heightened by diminishing the distance between the threads, which is equivalent to le ening the height of the plane; or by increasing its diameter, which is the same as increasing the length of the plane. An endle S. is a wheel turned by a screw, which, as it never ceases to be in contact with some of the teeth of the wheel, is practically never-ending; whence its name. Arch i me dean S. (Hydraul.) A spiral tube arranged as a spiral and turned on an axis; -it is employed in raising water.-(Shipbuilding.) See PROPELLER. [s. 807] Screw -jack. (Mech.) A strong screw appl Screw -jack. (Mech.) A strong screw applied in the lifting or sustaining a heavy weight; resting by means ofa large nut on a hollow base or pedestal, upon which it is elevated or depre ed by turning the nut. [s. 807] Screw' -pine. (Bot.) The common name of Screw' -pine. (Bot.) The common name of the gen. Pandanus. See PANDANACEE. [s. 807] Screw - pre . (Mach.) A pre in which Screw - pre . (Mach.) A pre in which the motive power is obtained by means of a screw; as distinguished from a hydraulic-or a lever-pre . [s. 807]