Editing Gear (section)

===Skew===
[[File:Sprocket35b.jpg|thumb|left|Hypoid gear]]

The gears in a matching pair are said to be ''skew'' if their axes of rotation are [[skew line]]s -- neither parallel nor intersecting.

In this case, the best shape for each pitch surface is neither cylindrical nor conical but a portion of  a [[hyperboloid]] of revolution.<ref>{{citation|chapter-url=http://gemini.tntech.edu/~slc3675/me361/lecture/grnts4.html |chapter=Gear Types |first=Stephen |last=Canfield |title=Dynamics of Machinery |publisher=Tennessee Tech University, Department of Mechanical Engineering, ME 362 lecture notes |year=1997 |postscript=. |url-status=dead |archive-url=https://web.archive.org/web/20080829124537/http://gemini.tntech.edu/~slc3675/me361/lecture/grnts4.html |archive-date=29 August 2008 }}</ref><ref>{{Citation | last1=Hilbert | first1=David | author1-link=David Hilbert | last2=Cohn-Vossen | first2=Stephan | author2-link=Stephan Cohn-Vossen | title=Geometry and the Imagination | publisher=Chelsea | location=New York | edition=2nd | isbn=978-0-8284-1087-8 | year=1952 | page=287|postscript=.}}</ref> Such gears are called ''[[hypoid gear|hypoid]]'' for short. Hypoid gears are most commonly found with shafts at 90 degrees. 

Contact between hypoid gear teeth may be even smoother and more gradual than with spiral bevel gear teeth, but also have a sliding action along the meshing teeth as it rotates and therefore usually require some of the most viscous types of gear oil to avoid it being extruded from the mating tooth faces, the oil is normally designated HP (for hypoid) followed by a number denoting the viscosity. Also, the [[pinion]] can be designed with fewer teeth than a spiral bevel pinion, with the result that gear ratios of 60:1 and higher are feasible using a single set of hypoid gears.<ref name="hypoidgears">{{harvnb|McGraw-Hill|2007|p=743}}.</ref> This style of gear is most common in motor vehicle drive trains, in concert with a [[differential (mechanical device)|differential]]. Whereas a regular (nonhypoid) ring-and-pinion gear set is suitable for many applications, it is not ideal for vehicle drive trains because it generates more noise and vibration than a hypoid does. Bringing hypoid gears to market for mass-production applications was an engineering improvement of the 1920s.