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]
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It is used most often in steam engines, and used to convert rotary 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 simple harmonic motion.
On bicycles
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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.[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.
Gallery
- An animation of an eccentric sheave, with strap and eccentric rod fitted
- Example of using eccentrics on a steam locomotive
See also
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- Balance shaft
- Cam – Rotating or sliding component that transmits variable motion to a follower
- Crank (mechanism) – Simple machine transferring motion to or from a rotaing shaft at a distance from the centreline
- Concentric objects
- Crankshaft – Linear-to-rotary motion converter
- Linkage (mechanical)
References
- Norton, Robert L. (2004). Design of machinery (3rd ed.). McGraw-Hill. ISBN 0-07-247046-1.
- Brown, Sheldon. "Sheldon Brown's Bicycle Glossary E-F: Eccentric". Sheldon Brown. Retrieved 2007-06-09.
- "Cannondale 1FG Easy Chain Tensioning". Archived from the original on 2007-11-23. Retrieved 2007-10-19.