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 Sheave without a rope A pulley is a wheel on an axle or shaft enabling a taut cable or belt passing over the wheel to move and change direction, or transfer power between itself and a shaft. A pulley may have a groove or grooves between flan ...

'') 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 In mechanics, acceleration is the 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 vector quantities (in that they have magnit ...

, an eccentric or return crank can only impart an approximation of

simple harmonic motion In mechanics and physics, simple harmonic motion (sometimes abbreviated as ) is a special type of periodic motion an object experiences by means of a restoring force whose magnitude is directly proportional to the distance of the object from ...

On bicycles

The term is also used to refer to the device often used on

tandem bicycle A tandem bicycle or twin is a bicycle (occasionally a tricycle) designed to be ridden by more than one person. The term tandem refers to the seating arrangement (fore to aft, not side by side), not the number of riders. Patents related to ...

s 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

rake Rake may refer to: Common meanings * Rake (tool), a horticultural implement, a long-handled tool with tines * Rake (stock character), a man habituated to immoral conduct * Rake (poker), the commission taken by the house when hosting a poker game ...

rotor) 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 Shimano 600 front derailleur (1980) A derailleur () is a variable-ratio bicycle gearing system consisting of a chain, multiple sprockets of different sizes, and a mechanism to move the chain from one sprocket to another. Modern front and rea ...

, 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 A wedge is a triangle, triangular shaped tool, a portable inclined plane, and one of the six simple machines. It can be used to separate two objects or portions of an object, lift up an object, or hold an object in place. It functions by conver ...

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

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

References

{{Reflist Mechanisms (engineering)

On bicycles

Gallery

See also

References