Lead (engineering)

Image:Leads.jpg

Image:Lead angle.jpg

File:Lead angle.svg

Lead is the axial advance of a helix or screw during one complete turn (360°){{cite book|isbn=1-55589-846-7|oclc=65562739|title=Gear Nomenclature, Definition of Terms with Symbols|id=ANSI/AGMA 1012-G05|publisher=American Gear Manufacturers Association|pages=72}} The lead for a screw thread is the axial travel for a single revolution.{{Cite web | title = Thread form terms | url = http://www.nookindustries.com/acme/AcmeGlossary.cfm | accessdate = 2008-12-22}}

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. It is the complement of the helix angle, and is used for convenience in worms and hobs. It is understood to be at the standard pitch diameter unless otherwise specified.

The lead angle can be expressed as:

:$\backslash mbox\{Lead\; angle\}\; =\; \backslash arctan\; \backslash left(\; \backslash frac\; \{l\}\; \{\backslash pi\; d\_m\}\; \backslash right)$

where

:l is lead of the helix

:d_m is mean diameter of the helix

In American literature λ is used to notate the Lead Angle.Juvinall & Marshek. Fundamentals of machine component design. John Wiley & Sons Asia: 2006. Page 391.

In European literature, Υ (Greek letter gamma) may be used.