Eccentric (mechanism)

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Eccentric sheave, with strap and eccentric rod fitted.

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 fulfils 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[edit]

Eccentric bottom bracket 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 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.


See also[edit]


  1. ^ Norton, Robert L. (2004). Design of machinery (3rd ed.). McGraw-Hill. ISBN 0-07-247046-1. 
  2. ^ Brown, Sheldon. "Sheldon Brown's Bicycle Glossary E-F: Eccentric". Sheldon Brown. Retrieved 2007-06-09. 
  3. ^ "Cannondale 1FG Easy Chain Tensioning". Retrieved 2007-10-19.