Editing Screw thread (section)

{{Short description|Helical structure used to convert between rotational and linear movement or force}}
[[File:Dubendorf floodgate.jpg|thumb|Screw thread, used to convert torque into the linear force in the [[flood gate]]. The operator rotates the small vertical bevel gear in the center. Through mechanical advantage this causes the horizontal bevel gears (at far left and far right, with threaded center holes) to rotate. Their rotation raises or lowers the two long vertical threaded shafts - as they are not free to rotate.]]

A '''screw thread''' is a [[helix|helical]] structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a [[cylinder (geometry)|cylinder]] or [[cone (geometry)|cone]] in the form of a helix, with the former being called a ''straight'' thread and the latter called a ''tapered'' thread. A screw thread is the essential feature of the [[screw (simple machine)|screw as a simple machine]] and also as a [[threaded fastener]].

The [[mechanical advantage]] of a screw thread depends on its ''lead'', which is the linear distance the screw travels in one revolution.<ref>{{cite book|url=https://archive.org/details/mathematicsform01burngoog|page=[https://archive.org/details/mathematicsform01burngoog/page/n149 137]|title=Mathematics for Machinists|year=1915|first=Reuben Wesley|last=Burnham|publisher=John Wiley & sons, Incorporated|access-date=4 April 2018|via=Internet Archive}}</ref> In most applications, the lead of a screw thread is chosen so that [[friction]] is sufficient to prevent linear motion being converted to rotary, that is so the screw does not slip even when linear force is applied, as long as no external rotational force is present. This characteristic is essential to the vast majority of its uses. The tightening of a fastener's screw thread is comparable to driving a wedge into a gap until it sticks fast through friction and slight [[Deformation (engineering)#Elastic deformation|elastic deformation]].