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.