Editing Gear (section)

== Relative axis position ==

One criterion for classifying gears is the relative position and direction of the axes or rotation of the gears that are to be meshed together.

===Parallel===
In the most common configuration, the axes of rotation of the two gears are parallel, and usually their sizes are such that they contact near a point between the two axes.  In this configuration, the two gears turn in opposite senses.

Occasionally the axes are parallel but one gear is nested inside the other.  In this configuration, both gears turn in the same sense.

If the two gears are cut by an imaginary plane perpendicular to the axes, each section of one gear will interact only with the corresponding section of the other gear.  Thus the three-dimensional gear train can be understood as a stack of gears that are flat and infinitesimally thin — that is, essentially two-dimensional.

===Crossed===
[[File:Engranaje cónico, Nymphenburg, Múnich, Alemania4.JPG|thumb|left|Bevel gear operating a [[lock gate]] ]]
In a ''crossed'' arrangement, the axes of rotation of the two gears are not parallel but cross at an arbitrary angle except zero or 180 degrees.

For best operation, each wheel then must be a [[bevel gear]], whose overall shape is like a slice ([[frustum]]) of a [[cone]] whose apex is the meeting point of the two axes.

Bevel gears with equal numbers of teeth and shaft axes at 90 degrees are called ''miter'' (US) or ''mitre'' (UK) gears.

Independently of the angle between the axes, the larger of two unequal matching bevel gears may be internal or external, depending the desired relative sense of rotation.<ref name="ansiagma"/>

If the two gears are sliced by an imaginary sphere whose center is the point where the two axes cross, each section will remain on the surface of that sphere as the gear rotates, and the section of one gear will interact only with the corresponding section of the other gear.  In this way, a pair of meshed 3D gears can be understood as a stack of nested infinitely thin cup-like gears.

===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.