Eccentric (mechanism)
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 centre).[1]
It is most often employed in steam engines and used to convert rotary into linear reciprocating motion in order to drive a sliding valve or a pump ram. In order to do so an eccentric usually has a groove at its circumference around which is closely fitted a circular collar (eccentric strap) attached to which an 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 simple harmonic motion.
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[edit] On bicycles
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 in order to properly tension the chain.[2]
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.[3]As a standard sized bottom bracket threads into the eccentric, an oversized bottom bracket shell is required to accommodate the eccentric.
[edit] Gallery
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An animation of an eccentric sheave, with strap and eccentric rod fitted
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Example of using eccentrics on a steam locomotive
[edit] See also
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Wikimedia Commons has media related to: Eccentrics |
[edit] References
- ^ Norton, Robert L. (2004). Design of machinery (3rd ed.). McGraw-Hill. ISBN 0072470461.
- ^ Brown, Sheldon. "Sheldon Brown's Bicycle Glossary E-F: Eccentric". Sheldon Brown. http://www.sheldonbrown.com/gloss_e-f.html. Retrieved 2007-06-09.
- ^ "Cannondale 1FG Easy Chain Tensioning". http://www.cannondale.com/bikes/03/ce/model-3mrss.html. Retrieved 2007-10-19.
