SCREW. This mechanical power gene rally consists of two parts, one of which is a solid cylinder of wood or metal, on whose convex surface is formed a projecting rib or fillet, frequently called a thread, which passes spirally round in such a manner as constantly to make equal angles with lines parallel to the axis of the cylinder. The other is a cylindrical perforation through a block of some material, the surface of the perforation having on it a spiral groove corresponding to', the projecting rib or fillet on • the solid cylin der. The first of these parts is called a con vex screw, and the other a concave screw. When the two parts are in action, the convex screw, being turned round in the other by a power applied at its surface, moves at the same time rectilinearly in the direction of its axis : occasionally however the convex screw is fixed, and then the other being turned about, it acquires at the same time :like rec tilinear motion.
As a mechanical power, the screw possesses the property of an inclined plane. ' In practice, a lever or wheel is always fixed perpendicu larly to the axis, and the moving or sustaining power is applied near the outer extremity of the lever, or at the circumference of the wheel.
An Endless Screw consists of two or more spiral fillets or threads on a rod which is capable of being turned nu its axis by a power applied to the handle of a winch, or to a string passing over the circumference of a pulley attached to the rod. The threads work between teeth on the circumference of a wheel, so that, while the revolution of the rod con tinues, the wheel turns on its axis. A
Double-Threaded Screw is one in which a fillet is formed upon each of two continuous spiral lines on the surface of the cylinder; the two making equal angles with lines parallel to the axis. With apparently an equal distance between the threads on both screws, the power of the single-threaded screw is double that of the other. From the high ratio which the resistance bears to the moving power in the screw, the use of this machine for moving or compressing bodies is very great; it is also extensively employed in the construction of philosophical instruments for measuring small angles or distances. [Mieno XETE11.3 A Differential $crew consists of one convex screw which world in the interior of another convex screw. The hitter works in a concave screw, which is fixed ; and the former is capable of moving in a rectilinear direction only, being prevented from turning on its axis with the rotation of the exterior screw. Also the number of threads in an inch on the convex surface of this last is less by one than the number in an inch on the convex surface of the other. Thus, if the exterior screw have 100 threads in an inch, and the interior screw 101 threads, then one turn of the machine will cause the latter to move through the very minute extent of inch, and this space may be further subdivided by means of a micrometer head applied to the exterior screw.