# Lead (engineering)

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thumb|Definition of ''lead'' per ANSI/AGMA 1012-G05
thumb|Definition of ''lead angle'' per ANSI/AGMA 1012-G05
thumb|Depiction of the lead angle of a screw thread. d<sub>m</sub> is the mean diameter of the screw thread.

'''Lead''' is the axial advance of a [helix](/source/helix) or [screw](/source/screw_(simple_machine)) during one complete turn (360°)<ref name="agma">{{cite book|isbn=1-55589-846-7|oclc=65562739|title=Gear Nomenclature, Definition of Terms with Symbols|date=2005 |id=ANSI/AGMA 1012-G05|publisher=[American Gear Manufacturers Association](/source/American_Gear_Manufacturers_Association)|pages=72}}</ref> The lead for a [screw thread](/source/screw_thread) is the axial travel for a single revolution.<ref>{{Cite web | title = Thread form terms | url = http://www.nookindustries.com/acme/AcmeGlossary.cfm | accessdate = 2008-12-22 | archive-date = 2008-07-05 | archive-url = https://web.archive.org/web/20080705035113/http://www.nookindustries.com/acme/AcmeGlossary.cfm | url-status = dead }}</ref>

''Pitch'' is defined as the axial distance between adjacent threads on a helix or screw.  In most screws, called "''single start''" screws, which have a single helical thread along their length, the lead and pitch are equal.  They only differ in "''multiple start''" screws, which have several intertwined threads.  In these screws, the lead is equal to the pitch multiplied by the number of "''starts''". 

Lead angle is the angle between the helix and a [plane of rotation](/source/plane_of_rotation).  It is the complement of the [helix angle](/source/helix_angle), and is used for convenience in worms and hobs.  It is understood to be at the [standard pitch diameter](/source/standard_pitch_diameter) unless otherwise specified.

The lead angle can be expressed as:

:<math>\mbox{Lead angle} = \arctan \left( \frac {l} {\pi d_m} \right)</math>

where
:''l'' is lead of the helix
:''d<sub>m</sub>'' is mean diameter of the helix

In American literature ''λ'' is used to notate the Lead Angle.<ref>Juvinall & Marshek. Fundamentals of machine component design. John Wiley & Sons Asia: 2006. Page 391.</ref>
In European literature, ''Υ'' (Greek letter gamma) may be used.
<br />
== See also ==
* [List of gear nomenclature](/source/List_of_gear_nomenclature)
* [Leadscrew](/source/Leadscrew)

== Notes ==
{{reflist}}

== External links ==

* [http://www.mitsubishicarbide.com/index.php?cID=2884 Calculation of Thread Lead Angle] (online [calculator](/source/calculator) on [Mitsubishi Materials](/source/Mitsubishi_Materials) site)Mitsubishi Materials

Category:Gears
Category:Screws
Category:Metalworking terminology

[de:Gewinde#Gewinde-Kenngrößen](/source/de%3AGewinde)

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