Editing Gear (section)

===Straight===
[[File:Spur Gear 12mm, 18t.svg|thumb|upright|Spur gear]]
In a cylindrical ''[[spur gear]]'' or ''straight-cut gear'', the tooth faces are straight along the direction parallel to the axis of rotation.  Any imaginary cylinder with the same axis will cut the teeth along parallel straight lines.  

The teeth can be either internal or external.  Two spur gears mesh together correctly only if fitted to parallel shafts.<ref>{{cite web|url=http://science.howstuffworks.com/transport/engines-equipment/gear2.htm|title=How Gears Work|date=16 November 2000|website=howstuffworks.com|access-date=20 September 2018}}</ref> No axial thrust is created by the tooth loads. Spur gears are excellent at moderate speeds but tend to be noisy at high speeds.<ref>{{Cite book|title=Machinery's Handbook|url=https://archive.org/details/machineryshandbo00ober_611|url-access=limited|publisher=Industrial Press|year=2012|isbn=978-0-8311-2900-2|location=New York|pages=[https://archive.org/details/machineryshandbo00ober_611/page/n2130 2125]}}</ref> 

For arrangements with crossed non-parallel axes, the faces in a straight-cut gear are parts of a [[conical surface|general conical surface]] whose generating lines (''generatrices'') go through the meeting point of the two axes, resulting in a [[bevel gear]].  Such gears are generally used only at speeds below {{cvt|5|m/s|ft/min|-1}}, or, for small gears, 1000 [[rpm]].<ref name="straightbevel">{{harvnb|McGraw-Hill|2007|p=742}}.</ref>
{{clear}}