Eccentric (Mechanism)- Definition, Parts, Working and Differences

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Today, I am going to discuss Eccentric(Mechanism) with its definition, Parts, Working, and some difference with Crank down, Here at Engineers Rail.

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Eccentric (Mechanism)-

An eccentric is used to convert the rotary motion of the shaft into a reciprocating motion, but cannot reverse the action.

It is mainly used in steam engines to convert the rotary motion of the crankshaft into the sliding motion of the D-slide valve or piston valve.

It is also used to run general valves, small pump plungers, shaking sheaves and mechanical stokers, etc.

Main Parts of Eccentric-

There are mainly two parts of Eccentric-, Listed and described below-

  1. Sheave
  2. Strap

sheave and strap eccentric mechanism

1. Sheave-

 A sheave is also known as an eccentric pulley or eccentric disc. There is a hole made in the sheave which is some distance away from the center in which the shaft is fitted by means of a key.

The distance between the centers of the sheave and the shaft is called eccentricity or eccentric-throw or eccentric radius.

The sheave is normally fixed at one end of the shaft, but if required, it can be fixed at another place in the length of the shaft.

For this, the sheave is made in two halves and they are joined by a nut-bolt or screw etc. There is a slot or projection at the periphery of the sheave which helps to fix the sleeve in the strap.

A sheave is normally made of Cast iron or steel.

2. Strap-

The strap encloses the sheave. It is made in two parts which are joined by nuts and bolts. The inside surface of the strap carries a slot or projection corresponding to the projection or slot.

The sheave fits into the strap with the help of this slot or projection.

Straps are normally made of cast iron. They can also be made of cast steel, brass, bronze, and wrought iron as required.

When straps and sheaves are made of cast iron they do not require any lining in between their mating surfaces.

But in the case of other materials, there is a need for lining between the mating surfaces of sheave a strap made of soft material like brass or white metal.

They require lubrication to reduce friction between the surfaces.

Working of Eccentric-

It contains an eccentric hole made in the sheave in which the end of a shaft fits by means of a key. Enclosing the sheave, the strap fits in two pieces.

On one piece of strap, an eccentric rod fits with the help of bolts and studs. The sheave is connected to the shaft at its center, whose rotary motion is e convened into reciprocating motion.

The sheave moves in between straps with the rotary motion, thereby imparting reciprocating motion to the eccentric rod. The other end of the eccentric rod is connected to the machine part requiring reciprocating motion.

The part connected moves with the total displacement equal to twice the eccentric throw.


Difference between Eccentric and Crank-

The following differences between eccentric and crank are stated below:

  1. A crank can convert reciprocating motion into rotary motion and rotary motion into reciprocating motion. whereas, an eccentric can only convert the rotary motion of the machine part into reciprocating motion. It is due to the high friction between the sheave and the strap.
  2. During the operation of the crank, if the length of the crank shortens, such that the crank pin crank-shaft circumference, then the crank functions like an eccentric. In this position, the big end of the connecting-rod functions as the strap of eccentric.
  3. In the case of eccentric, the continuous motion of the shaft is not intercepted.
  4.  In the case of eccentric, the crank pin enlarges to the extent that the shaft is fully covered by its motion.

⇒Must Read: What is Crank?- Types, Working, Applications, and Examples


Wrapping Up-

This is it for Eccentric(Mechanism). I hope you gained some value from this short and sweet article and further it will add a few drops to your knowledge tank.

Till then, Take care of yourself and your loved ones.

See you in the next session.

Before you go, here are some suggested articles, you should consider reading further-

What is Mechanical Pin?: Classifications, Types, Uses with Pictures

Coupling: Definitions, Types with Advantages and Limitations, Application and Much More

Locking Devices: Types with Pictures


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