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,
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
mechanical engineering
Mechanical engineering is the study of physical machines and mechanism (engineering), mechanisms that may involve force and movement. It is an engineering branch that combines engineering physics and engineering mathematics, mathematics principl ...
, an eccentric is a circular disk (''eccentric sheave'') solidly fixed to a rotating axle
An axle or axletree is a central shaft for a rotation, rotating wheel and axle, 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 ...
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 Work (physics), 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 (locomotive), cyl ...
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 Slurry, slurries) by opening, closing, or partially obstructing various passageways. Valves are technically Pip ...
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
Cam or CAM may refer to:
Science and technology
* Cam (mechanism), a mechanical linkage which translates motion
* Camshaft, a shaft with a cam
* Camera or webcam, a device that records images or video
In computing
* Computer-aided manufacturin ...
, which also converts rotary into linear motion at almost any rate of acceleration
In mechanics, acceleration is the Rate (mathematics), rate of change of the velocity of an object with respect to time. Acceleration is one of several components of kinematics, the study of motion. Accelerations are Euclidean vector, vector ...
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 bicycle
A single-speed bicycle is a type of bicycle with a single gear ratio. These bicycles are without derailleur gears, hub gearing or other methods for varying the gear ratio of the bicycle.
There are many types of modern single speed bicycles; BMX b ...
s with a rear disc brake
A disc brake is a type of brake that uses the #Calipers, calipers to squeeze pairs of #Brake pads, pads against a disc (sometimes called a rakerotor) to create friction. There are two basic types of brake pad friction mechanisms: abrasive f ...
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 disting ...
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
* * * * * *References
{{Reflist Mechanisms (engineering)