<P> A screw is a mechanism that converts rotational motion to linear motion, and a torque (rotational force) to a linear force . It is one of the six classical simple machines . The most common form consists of a cylindrical shaft with helical grooves or ridges called threads around the outside . The screw passes through a hole in another object or medium, with threads on the inside of the hole that mesh with the screw's threads . When the shaft of the screw is rotated relative to the stationary threads, the screw moves along its axis relative to the medium surrounding it; for example rotating a wood screw forces it into wood . In screw mechanisms, either the screw shaft can rotate through a threaded hole in a stationary object, or a threaded collar such as a nut can rotate around a stationary screw shaft . Geometrically, a screw can be viewed as a narrow inclined plane wrapped around a cylinder . </P> <P> Like the other simple machines a screw can amplify force; a small rotational force (torque) on the shaft can exert a large axial force on a load . The smaller the pitch, the distance between the screw's threads, the greater the mechanical advantage, the ratio of output to input force . Screws are widely used in threaded fasteners to hold objects together, and in devices such as screw tops for containers, vises, screw jacks and screw presses . </P>

What is the spiral part of a screw called