Oct 30, 2020

# Everyday Engineering - Kinematics

In this blog series we are going to discuss the mechanisms that are used in almost in every aspect in the mechanical engineering the major mechanisms used in the day to day life like:

- The working of engine
- The working of crusher
- The working of wending machine
- The working of the lathe
- The working slotter machine
- Motion of arms, legs
- Working of crane
- Working of lever mechanisms and their uses
- Like this there are wide applications in our life these machines makes our works easier and less time consuming so in this series of blogs lets discuss the basic working of the mechanisms , how they can be applied in machines , and also we shall discuss where and why these mechanisms are used in our day today life.

You all might have seen these types of foldable mirrors in our daily life but if we go in deep about its working it’s the series of scissor linkages which make the motion of the mirror like this we will be using much more objects like scissors, staplers, punching machine still many which in turn works on the simple working models and the engineer identifies it as the mechanism..

What exactly is **Mechanism**…

It can be defined as **if the members of the bodies are assembled in such a way that the motion of one cause constrained and predictable motion to the others this is known as mechanism The mechanism transmits and modifies a motion…**

Then if mechanism is this than what is a machine????

Is this question rising in your mind if so, the machine can be defined as **it’s the mechanism itself or the combination of mechanisms which imparts the definite motions to the parts and also transmits and modifies the available mechanical energy into the desired work.**

Before diving into the mechanisms we shall tell you about the basic terms which are the basic concepts which will help us to understand the mechanisms more easily in the engineering terms….

**Kinematic pair** : It is the connection between between the two physical objects that impose constraints on their relative motion this was first introduced by *Franz Rouleaux*

Further these kinematic links are divided according to the nature of the relative motion as

**Sliding pair****Turning pair****Rolling pair****Helical pair****Spherical pair**

**Degree of freedom** : The number of independent relative motions both translation and rotational this number is known as the degree of freedom.

**Revolute pair** : It is the one-degree degree-of-freedom kinematic pair this is known as the revolute pair.

**Link **: A resistant body or group of resistant bodies with the rigid connections preventing their relative movement is known as a link.

- The primary link type of rigid link is the binary link which means a link with two revolute pairs
- The link with the three revolute pairs is known as the ternary link and with the four pairs is known as the quaternary link .

These are the basic building blocks of every mechanisms

**Structure** : If one of the links of a redundant cha in is fixed its known as the structure or the **Locked** system and if the degree of freedom is zero then the link is the super structure…

So, let’s begin journey with the simple link we can imagine is with the four links why only four links why are we not considering the three links mechanism any ideas?????

So, lets discuss about this in our next blog till than we want you people to think and share your views about this that is there a three links mechanism let us know in the comments. For this blog let’s discuss about the four link mechanism

The four bar mechanism is the one of the simplest mechanism and widely used mechanism in this particular mechanism there are four bars which make a quadrilateral as below

As per the diagram the links are

LINK 1: — AD of length d

LINK 2: — AB of length a

LINK 3: — BC of length b

LINK 4: -CD of length c

The length of these links are not just randomly chosen we can choose the length according to the relation which we get doing some geometry and some math which is

In words we can tell **the sum of the shortest and the longest link is less than that of the sum of the other two links** or we can express as

**: d+b< a+c :**

Also, it must obey the **Grasshoff’s law** i.e. **for at least one link to be capable of making a full revolution, the sum of the lengths of the shortest link and the largest link is less than or equal to the sum of the lengths of the other two links**

So hence we can get the mechanisms of the 4 bars linkage first we make the link 1 as the fixed link so that we fix the point A and D to the rigid body or to the ground so that we get the link 2 as the crank

**Crank** : It is that link which make the complete circular motion about the point this is known as the crank.

We can name this as the first inversion of the 4 link mechanism in the engineering terms. The first inversion is obtained by making one of the revolute pair and the prismatic pair as a fixed link. The link can be shown as shown in the figure below

The basic moto of the first inversion is **to convert the rotary motion or the circular motion of the crank into the translation motion as the output and vice versa**.

The working of the 1st inversion is the very simple to understand. The link 1 that is shown with the grey coloured link which is fixed to the rigid body or made rigid. Further, the link AB which is link 2 is connected to the rigid or to the ground at point A, we see a circle is drawn making the point A as the center with length of AB as a radius this is because the link AB is made to move than the point B makes the complete circle so we can refer the link AB as the crank in our mechanism.

Further the link BC is connected to the crank AB which connects the crank to the output link. This Is generally known as the connecting rod. Going ahead this connecting link BC is connected to the output link 4 which is along the axis of the link 1. The output link is based on the desired work if the rotation is the desired work then the link AB which is the crank in the mechanism is the output, If the translation is the desired work output than the link 4 acts as the output link this is all based on the area of application.

The working can be shown using the motion picture as of the below :

**The hood**: the opening and closing of the hood**Parallelogram mechanism****Slider crank mechanism**: the basic working of the engine and the compressor in which the crank is the output for the engine and the translation motion is the desired output for the compressor.**The block feeding mechanism**

5.** Motion of the train wheel **: Here the translation motion of the link is connected to the engine and the crank link is fixed to the wheel of the train.

So we shall meet you all again with one more informative blog of the mechanical field and till then be informative and gain the continuous knowledge in one or the other form… see you all with a new blog….

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