Editing Gear (section)

== Special gear trains ==

===Rack and pinion===
[[File:Rack and pinion animation.gif|frame|left|Rack and pinion gearing]]
{{Main|Rack and pinion}}

A ''rack'' is a toothed bar or rod that can be thought of as a sector gear with an infinitely large [[radius of curvature]]. [[Torque]] can be converted to linear force by meshing a rack with a round gear called a ''pinion'': the pinion turns, while the rack moves in a straight line. Such a mechanism is used in the steering of automobiles to convert the rotation of the [[steering wheel]] into the left-to-right motion of the [[tie rod]](s) that are attached to the front wheels.

Racks also feature in the theory of gear geometry, where, for instance, the tooth shape of an interchangeable set of gears may be specified for the rack (infinite radius), and the tooth shapes for gears of particular actual radii are then derived from that. The rack and pinion gear type is also used in a [[rack railway]].
{{clear}}

===Epicyclic gear train===
[[File:Epicyclic gear ratios.png|thumb|left|Epicyclic gearing]]
{{Main|Epicyclic gearing}}

In epicyclic gearing, one or more of the gear [[Axis of rotation|axes]] moves. Examples are [[sun and planet gear]]ing (see below), [[cycloidal drive]], [[automatic transmission]]s, and [[differential (mechanical device)|mechanical differentials]].
{{clear}}

====Sun and planet====
[[File:Sun and planet gears.gif|thumb|left|Sun (yellow) and planet (red) gearing]]
{{Main|Sun and planet gear}}

[[Sun and planet gear]]ing is a method of converting [[reciprocating motion]] into [[rotary motion]] that was used in [[steam engine]]s. [[James Watt]] used it on his early steam engines to get around the patent on the [[Crank (mechanism)|crank]], but it also provided the advantage of increasing the flywheel speed so Watt could use a lighter flywheel.

In the illustration, the sun is yellow, the planet red, the reciprocating arm is blue, the [[flywheel]] is green and the [[driveshaft]] is gray.

{{clear}}