Editing Screw thread (section)

===Handedness===
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[[File:Screw thread handedness.png|thumb|upright|Right- and left-handed screw threads]]
[[File:Right-hand grip rule.svg|thumb|upright|The right-hand rule of screw threads]]

The helix of a thread can twist in two possible directions, which is known as ''handedness''. Most threads are oriented so that the threaded item, when seen from a point of view on the axis through the center of the helix, moves away from the viewer when it is turned in a [[clockwise]] direction, and moves towards the viewer when it is turned counterclockwise. This is known as a ''right-handed'' (''RH'') thread, because it follows the [[right hand grip rule|right-hand grip rule]]. Threads oriented in the opposite direction are known as ''left-handed'' (''LH'').

By common convention, right-handedness is the default handedness for screw threads. Therefore, most threaded parts and fasteners have right-handed threads. Left-handed thread applications include:
* Where the rotation of a shaft would cause a conventional right-handed nut to loosen rather than to tighten due to applied torque or to [[Precession (mechanical)|fretting induced precession]]. Examples include:
** The [[bicycle pedal|left foot pedal]] on a [[bicycle]]<ref>{{cite web | url = http://www.sheldonbrown.com/gloss_p.html#pedal | title=Bicycle Glossary: Pedal
| access-date=2010-10-19|last=Brown |first=Sheldon |author-link=Sheldon Brown (bicycle mechanic) |publisher=Sheldon Brown}}</ref>
** The left grinding wheel on a [[bench grinder]]
** The axle nuts, or less commonly, [[lug nut]]s on the left side of some automobiles
** The securing nut on some circular saw blades – the large [[torque]] at startup should tend to tighten the nut
** The spindle on [[brushcutter (garden tool)|brushcutter]] and [[line trimmer]] heads, so that the torque tends to tighten rather than loosen the connection
** The hand-tightened nut holding the fan blade to the motor spindle in many designs of oscillating [[table fan]]s and floor standing fans
* In combination with right-hand threads in [[turnbuckle]]s and clamping studs<ref>{{cite web|url=http://www.directindustry.com/prod/s-w-manufacturing-co-inc/clamping-studs-57807-574221.html|title=Threaded stud / steel / clamping – S&W Manufacturing Co., Inc.|website=www.directindustry.com|access-date=4 April 2018}}</ref>
* In some gas supply connections to prevent dangerous misconnections, for example:
** In gas welding the flammable gas supply uses left-handed threads, while the oxygen supply if there is one has a conventional thread
** The [[POL valve]] for [[LPG cylinder]]s
* In a situation where neither threaded pipe end can be rotated to tighten or loosen the joint (e.g. in traditional heating pipes running through several rooms in a building). In such a case, the [[Coupling (piping)|coupling]] will have one right-handed and one left-handed thread.
* In some instances, for example early [[ballpoint pen]]s, to provide a "secret" method of disassembly
* In artillery projectiles, anything that screws into the projectile must be given consideration as to what will happen when the projectile is fired, e.g., anything that screws into the base from the bottom of the projectile must be left hand threaded
* In mechanisms to give a more intuitive action as:
** The leadscrew of the cross slide of a [[Lathe (metal)|lathe]] to cause the cross slide to move away from the operator when the leadscrew is turned clockwise
** The depth of cut screw of a "Bailey" (or "Stanley-Bailey") type metal [[plane (tool)]] for the blade to move in the direction of a regulating right hand finger
* Some [[Edison base]] lamps and fittings (such as those formerly used on the [[New York City Subway]] or the pre-[[World War I]] [[Sprague-Thomson]] rolling stock of the [[Paris Metro]]) have a left-hand thread to deter theft, because they cannot be used in other light fixtures