Eccentric (mechanism)
{{short description|Circular disk rigidly fixed to a rotating axle with its center offset from that of the axle}}
In mechanical engineering, an eccentric is a circular disk (eccentric sheave) solidly fixed to a rotating axle with its centre offset from that of the axle (hence the word "eccentric", out of the center).{{cite book| last=Norton| first=Robert L.| title=Design of machinery| publisher=McGraw-Hill| year=2004| edition=3rd| isbn=0-07-247046-1}}
It is used most often in steam engines, and used to convert rotary motion into linear reciprocating motion to drive a sliding valve or pump ram. To do so, an eccentric usually has a groove at its circumference closely fitted a circular collar (eccentric strap). An attached eccentric rod is suspended in such a way that its other end can impart the required reciprocating motion. A return crank fulfills the same function except that it can only work at the end of an axle or on the outside of a wheel whereas an eccentric can also be fitted to the body of the axle between the wheels. Unlike a cam, which also converts rotary into linear motion at almost any rate of acceleration and deceleration, an eccentric or return crank can only impart an approximation of simple harmonic motion.
On bicycles
File:Eccentric bottom bracket.jpg on a Burley tandem bicycle held in place with two set screws]]
The term is also used to refer to the device often used on tandem bicycles with timing chains, single-speed bicycles with a rear disc brake or an internal-geared hub, or any bicycle with vertical dropouts and no derailleur, to allow slight repositioning, fore and aft, of a bottom bracket to properly tension the chain.{{cite web
| url = https://www.sheldonbrown.com/gloss_e-f.html#eccentric
| title = Sheldon Brown's Bicycle Glossary E-F: Eccentric
| access-date = 2007-06-09|last=Brown |first=Sheldon |author-link=Sheldon Brown (bicycle mechanic) |publisher=Sheldon Brown}}
They may be held in place by a built-in wedge, set screws threaded into the bottom bracket shell, or pinch bolts that tighten a split bottom bracket shell.{{cite web
|url = http://www.cannondale.com/bikes/03/ce/model-3mrss.html
|title = Cannondale 1FG Easy Chain Tensioning
|access-date = 2007-10-19
|url-status = dead
|archive-url = https://web.archive.org/web/20071123103811/http://www.cannondale.com/bikes/03/ce/model-3MRSS.html
|archive-date = 2007-11-23
}} As a standard sized bottom bracket threads into the eccentric, an oversized bottom bracket shell is required to accommodate the eccentric.
Gallery
File:Eccentric animation.gif|An animation of an eccentric sheave, with strap and eccentric rod fitted
File:CSD 310.093 eccentric.jpg|Example of using eccentrics on a steam locomotive
See also
{{Commons category|Eccentrics (mechanism)|Eccentrics}}
- {{annotated link|Balance shaft}}
- {{annotated link|Cam (mechanism)}}
- {{annotated link|Crank (mechanism)}}
- {{annotated link|Concentric objects}}
- {{annotated link|Crankshaft}}
- {{annotated link|Linkage (mechanical)}}
References
{{Reflist}}