Screw thread
helical part of screw / From Wikipedia, the free encyclopedia
A screw thread, often shortened to thread, is a helical structure. It is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread.
![Thumb image](http://upload.wikimedia.org/wikipedia/commons/thumb/6/64/Acme_thread.jpg/640px-Acme_thread.jpg)
The tightening of a nut to a bolt is like driving a wedge into a gap until it sticks through friction and plastic deformation (how the surfaces of the two objects slightly squash together).
A screw thread is one of the six simple machines which give mechanical advantage.
The mechanical advantage of a screw thread depends on its lead, which is the linear distance the screw travels in one turn or revolution.[1]
![Thumb image](http://upload.wikimedia.org/wikipedia/commons/thumb/0/00/Lead_and_pitch.png/220px-Lead_and_pitch.png)