Editing Machine (section)

== Mechanisms ==
The ''mechanism'' of a mechanical system is assembled from components called ''[[machine element]]s''. These elements provide structure for the system and control its movement.

The structural components are, generally, the frame members, bearings, splines, springs, seals, [[fastener]]s and covers. The shape, texture and color of covers provide a [[industrial design|styling and operational interface]] between the mechanical system and its users.

The assemblies that control movement are also called "[[mechanism (engineering)|mechanisms]]."<ref name="Reuleaux1876" /><ref name="Uicker2003"/> Mechanisms are generally classified as [[gear]]s and [[gear train]]s, which includes [[belt drive]]s and [[chain drive]]s, [[Cam (mechanism)|cam]] and [[cam follower|follower]] mechanisms, and [[linkage (mechanical)|linkages]], though there are other special mechanisms such as clamping linkages, [[Geneva drive|indexing mechanisms]], [[escapement]]s and friction devices such as [[brake]]s and [[clutch]]es.

The number of degrees of freedom of a mechanism, or its mobility, depends on the number of links and joints and the types of joints used to construct the mechanism. The general mobility of a mechanism is the difference between the unconstrained freedom of the links and the number of constraints imposed by the joints. It is described by the [[Chebychev–Grübler–Kutzbach criterion]].

=== Gears and gear trains ===
[[File:Antikythera Fragment A (Front).webp|thumb|The [[Antikythera mechanism]] (main fragment)]]
The transmission of rotation between contacting toothed wheels can be traced back to the [[Antikythera mechanism]] of Greece and the [[south-pointing chariot]] of [[China]]. Illustrations by the renaissance scientist [[Georgius Agricola]] show gear trains with cylindrical teeth. The implementation of the [[involute gear|involute tooth]] yielded a standard gear design that provides a constant speed ratio. Some important features of gears and gear trains are:
* The ratio of the pitch circles of mating gears defines the [[gear ratio|speed ratio]] and the [[mechanical advantage]] of the gear set.
*  A [[epicyclic gearing|planetary gear train]] provides high gear reduction in a compact package.
*  It is possible to design gear teeth for gears that are [[non-circular gear|non-circular]], yet still transmit torque smoothly.
* The speed ratios of [[chain drive|chain]] and [[belt (mechanical)|belt drives]] are computed in the same way as gear ratios. See [[bicycle gearing]].

=== Cam and follower mechanisms ===
A [[Cam (mechanism)|cam]] and [[cam follower|follower]] is formed by the direct contact of two specially shaped links. The driving link is called the cam (also see [[camshaft|cam shaft]]) and the link that is driven through the direct contact of their surfaces is called the follower. The shape of the contacting surfaces of the [[Cam (mechanism)|cam]] and [[cam follower|follower]] determines the movement of the mechanism.

=== Linkages ===
[[File:Landing gear schematic.svg|thumb|right|Schematic of the actuator and four-bar linkage that position an aircraft landing gear]]
A [[linkage (mechanical)|linkage]] is a collection of links connected by joints. Generally, the links are the structural elements and the joints allow movement. Perhaps the single most useful example is the planar [[four-bar linkage]]. However, there are many more special linkages:
* [[Watt's linkage]] is a four-bar linkage that generates an approximate straight line. It was critical to the operation of his design for the steam engine. This linkage also appears in vehicle suspensions to prevent side-to-side movement of the body relative to the wheels. Also see the article [[Parallel motion]].
* The success of Watt's linkage lead to the design of similar approximate straight-line linkages, such as [[Hoekens linkage|Hoeken's linkage]] and [[Chebyshev linkage|Chebyshev's linkage]].
* The [[Peaucellier-Lipkin linkage|Peaucellier linkage]] generates a true straight-line output from a rotary input.
* The [[Sarrus linkage]] is a spatial linkage that generates straight-line movement from a rotary input.
* The [[Klann linkage]] and the [[Theo Jansen|Jansen linkage]] are recent inventions that provide interesting walking movements. They are respectively a six-bar and an eight-bar linkage.

===Planar mechanism===
A planar mechanism is a mechanical system that is constrained so the trajectories of points in all the bodies of the system lie on planes parallel to a ground plane. The rotational axes of hinged joints that connect the bodies in the system are perpendicular to this ground plane.

===Spherical mechanism===
A '''spherical mechanism''' is a mechanical system in which the bodies move in a way that the trajectories of points in the system lie on concentric spheres. The rotational axes of hinged joints that connect the bodies in the system pass through the center of these circle.

===Spatial mechanism===
A '''spatial mechanism''' is a mechanical system that has at least one body that moves in a way that its point trajectories are general space curves. The rotational axes of hinged joints that connect the bodies in the system form lines in space that do not intersect and have distinct common normals.

=== Flexure mechanisms ===
A flexure mechanism consists of a series of rigid bodies connected by compliant elements (also known as flexure joints) that is designed to produce a geometrically well-defined motion upon application of a force.