Theory of Machines 3(2+1)

Lesson 1. Introduction of Theory of Machine

1.1.1 THE MACHINE

A machine is a combination of components which can transmit power in a controlled manner and which is capable of performing useful work. A machine consists of a number of kinematically related links.

A machine is a combination of resistant bodies (links or elements) with successfully constrained relative motions, which is used for transmitting other forms of energy into mechanical energy or transmitting and modifying available energy to do some particular kind of work.

INPUT	MACHINE	OUTPUT
Mechanical Electrical Hydraulic Chemical or Nuclear	Kinematic Arrangement of Links         - Rigid         - Rigid-Hydraulic         - Rigid-Pneumatic	Mechanical Electrical Hydraulic or Thermal
Force  Velocity Energy/Time Power	A machine has moving parts, which must be constrained and controlled.	Force  Velocity Energy/Time Power

Machine Arrangement

Every machine will be found to consist of a system of parts (links or elements) connected together in such a manner that, if one be made to move, they all receive a motion, the relation of which to that of the first depends upon the nature of connections (i.e. joints).

The links may be rigid, rigid-hydraulic, or rigid-pneumatic. The power input may be mechanical, electrical, hydraulic, chemical, or nuclear. The power output may be mechanical, electrical hydraulic or thermal.

Examples of machines:

Heat engine- Receives heat energy and transformers it into mechanical energy.

Electric motor- Changes electric energy into mechanical energy.

A pump- Input electric power and output hydraulic power.

The majority of machines receives mechanical energy, and modify it so that the energy can be used for doing some specific task, for which it is designed, common examples of such machines being hoist, lathe, screw jack, etc.

Note:-It should be noted that machine must be capable of doing useful work. A series of kinematically related links put into motion with no output link, and which simply converts input energy to friction heat, is not a machine, unless the original purpose was only to generate heat. 

1.1.2 CLASSIFICATION OF MACHINES

1. Machines for generating mechanical energy

    - Converts other forms of energy into mechanical work

    Examples: Steam engines, Steam turbines, I. C. engines, gas turbines, water turbines etc

2. Machines for transmitting mechanical energy into other form of energy

    - Known as converting machines

    Examples: Electric generators, air or hydraulic pumps, etc.

3. Machines for utilizing mechanical energy in the performance of useful work.

Examples: Lathe, and other machine tools, etc.  

The transmission and modification of energy within the machine require the inclusion of a number of parts (links or elements), which are so selected that they will produce the desired motion and carry with safety the forces to which they are subjected so that the machine can perform its task successfully.

The study of relative motion between the various parts of a machine, and the forces which act on them, is covered under they field of “Theory of machines”, or “The Theory of Machines may be defined as that branch of engineering science which deals with the study of relative motion between various elements of a machine and the forces which act on them.

1.2 DIFFERENCE BETWEEN MACHINE AND MECHANISM        

In kinematics, a mechanism is a mean of transmitting, controlling, or constraining relative movement. The central theme for mechanisms is rigid bodies connected together by joints. It can also be defined as a combination of resistant bodies that are shaped and connected in such a way that they move with definite relative motion with respect to each other.

A machine is a combination of rigid or resistant bodies, formed and connected in such a way that they move with definite relative motions with each other and transmit force also. A machine has two functions: transmitting definite relative motion and transmitting force. The term mechanism is applied to the combination of geometrical bodies which constitute a machine or part of a machine.

MACHINE	MECHANISM
It is a combination of links having relative motion and has the capability of modifying available energy in a suitable form. A machine can consist of one or more than one mechanism. e.g. lathe machine has several mechanisms  Its objective is to transmit mechanical energy. It is a combination of links which transmit and transform its motion only.	A mechanism is a single system to transmit motion.       Its objective is to transmit motion only.

 

Example: A simple example of machine and mechanism is IC engine and slider crank mechanism. A slider crank mechanism converts rotary motion of crank into sliding motion of slider. Where as, in the IC engine the same mechanism is used to convert available mechanical energy at the piston into the required torque at the crank shaft.

1.3 KINEMATIC ANALYSIS OF MECHANSIM

Various mechanisms have its own set of outputs when they are put in motion. The analysis of the mechanism is done by calculating the position, velocity and acceleration at various points on the mechanisms. For the analysis of velocity & acceleration at any point on the mechanism we don’t need to calculate forces & stresses acting in the parts of the mechanism. In other means, in analysis of motion of a particular mechanism we don’t need to consider the cross section area or strength of the parts in that mechanism. Also, it does not matter whether the parts are made of cast iron or wood or anything else to study it motion analysis.

1.3.1 GRAPHICAL AND ANALYTICAL METHOD: Analysis of the Mechanism can be done by two types of methods, generally known as graphical and analytical methods. Each method has its own advantages and disadvantages. The graphical method is easy to follow and gives the visual image of the working of mechanism which can be applied in some simple problems. But for more complex problems analytical methods are more suitable. It is up to us by which method we want to solve the problem in hand. With the advent of high speed computing, analytical methods has very useful tool for solving complex problems. In this course will concentrate our study to graphical methods due its ease and simplicity.

1.4 SYNTHESIS OF DESIGN

In the design of a mechanism, we will consider stress analysis & others design parameters like bending, fatigue etc. to find the dimensions of the parts. The synthesis of a mechanism can be done by following two approaches. In the first approach the dimensions of the parts in a mechanism is found by considering load, stress & bending etc. in the different parts of the mechanism. In the second approach, the dimensions of the parts are assumed first and then the analysis is done to check its strength. The second method of synthesis is preferred by most of the engineers.

1.5 KINEMATICS OF MECHANISM   

It involves the study of the relative motions of various parts of a mechanism without considering the forces producing the motion in the parts. It is the study from the geometric point of view by which we can know the displacement, velocity and acceleration at the various points on the parts of a mechanism.

1.6 DYNAMICS OF MECHANISM

It involves the calculations of the forces impressed upon various parts of a mechanism. The forces impressed on a mechanism can be divided into static & kinetics. In static, the study of forces is done when all the parts of the mechanisms are in equilibrium. Where in kinetics the study of inertia forces are done which may occur due to the combination of mass and motion of the parts.