eccentric (mechanism) on:
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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
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
In mechanical engineering
Mechanical engineering is the study of physical machines that may involve force and movement. It is an engineering branch that combines engineering physics and mathematics principles with materials science, to design, analyze, manufacture, ...
, an eccentric is a circular disk (''eccentric sheave'') solidly fixed to a rotating axle
An axle or axletree is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In the former case, beari ...
with its centre offset from that of the axle (hence the word " eccentric", out of the center).
It is used most often in steam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be ...
s, and used to convert rotary motion into linear reciprocating motion to drive a sliding valve
A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fitting ...
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
In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by ...
and deceleration, an eccentric or return crank can only impart an approximation of simple harmonic motion.
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
The bottom bracket on a bicycle connects the crankset (chainset) to the bicycle and allows the crankset to rotate freely. It contains a spindle to which the crankset attaches, and the bearings that allow the spindle and crankset to rotate. The ...
to properly tension the chain.
They may be held in place by a built-in wedge, set screw
In American English, a set screw is a screw that is used to secure an object, by pressure and/or friction, within or against another object, such as fixing a pulley or gear to a shaft. A set screw is normally used without a nut (which distin ...
s threaded into the bottom bracket shell, or pinch bolts that tighten a split bottom bracket shell. As a standard sized bottom bracket threads into the eccentric, an oversized bottom bracket shell is required to accommodate the eccentric.
Gallery
See also
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{{Reflist Mechanisms (engineering)