Screw
in mechanics, a cylinder surrounded by a spiral ridge or groove, every part of which forms an equal angle with the axis of the cylinder, so that if developed on a plane surface it would be an inclined plane. The screw is considered as one of the six mechanical powers, but is really only a modification of the inclined plane. A convex screw is known as the external or male screw, a concave or hollow screw (generally termed a nut) is an internal or female screw. The mechanical effect of a screw is increased by lessening the distance between the threads, or by making them finer, or by lengthening the lever to which the power is applied; this law is, however, greatly modified by the friction, which is very great. The parts of a screw are the head, barrel or stem, thread, and point. The head has a slit, nick, or square. In number screws vary, as single, double, triple; the numbers representing the individual threads, and those above single being known as multi- SCRIBE plex-threaded. A right and left screw is one in which the threads on the opposite ends run in different directions.