screw

Some different types of wood screw. The screw heads vary depending on the nature of the job; for example, countersunk screws are used for work where it is important that the screw head is flush to the surface.

In construction, cylindrical or tapering piece of metal or plastic (or formerly wood) with a helical groove cut into it. Each turn of a screw moves it forward or backwards by a distance equal to the pitch (the spacing between neighbouring threads).

Its mechanical advantage equals 2 r/P, where P is the pitch and r is the radius of the thread. Thus the mechanical advantage of a tapering wood screw, for example, increases as it is rotated into the wood.

The thread is comparable to an inclined plane (wedge) wrapped around a cylinder or cone.