Design of Machine Elements _ Chapter one Introduction.pptx

DESIGN OF MACHINE
ELEMENTS
INTRODUCTION

Engineering/ Machine Design and it’s Classification
Definition: The scientific principles, technical information's and the imaginations for creating the mechanical or
machine system for fulfilling our required needs with appropriate design, max-efficiency and cost effective manner
This definition of machine design contains the following important features:
(i) A designer uses principles of basics and engineering sciences such as physics, mathematics, statics and dynamics,
thermodynamics and heat transfer, vibrations and fluid mechanics for the required components design
Some of the examples of these principles are
(a)Newton’s laws of motion,
(b) D’Alembert’s principle,
(c) Boyle’s and Charles’laws of gases,
(d) Carnot cycle, and
(e) Bernoulli’s principle.

(ii) The designer has technical information of the basic elements of a machine. These elements include fastening
devices, chain, belt and gear drives, bearings, oil seals and gaskets, springs, shafts, keys, couplings, etc..
 https://www.youtube.com/watch?v=JOLtS4VUcvQ
Why Design and Material Study are IMPORTANT ?
The Engineering components to be designed based on the utility, comfortable, attractiveness, luxuries, etc.
The Engineering (mechanical, civil, etc.), Bio-Medical components’ design and it's materials selection, are based on
the applications in such industries, Automobile, Construction, Nuclear power plant, Glass melting plants, etc.
Materials’ properties (Elastic moduli, Poisson’s ratio, Stiffness, Toughness, ductility, Tensile, compressive stress, etc.)
must be studied while designing the Engineering components based on that functions and for ensuring the Safety,
Reliability, Cost-effective, Life extension, etc., of components in various applications.

Basic Design Procedures
A logical sequence of steps to be followed for most of the design projects
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Steps Involved in the Design Procedure
Product specification
The first step consists of preparing a complete list of the requirements of the product. The requirements include the
capacity of the machine, and its service life, cost and reliability. In some cases, the overall dimensions and weight.
Depending upon the type of product, various requirements and it’s functions
Selection of Mechanism
Depending upon the efficiency, comfortness, safety, cost-competitiveness, etc., availability of raw materials and
manufacturing facility, the best possible mechanism is selected for the required product.
For Example: Blanking or Piercing press, (the following mechanisms are possible)
(i) A mechanism involving the crank and connecting rod, converting the rotary motion of the electric motor into the
reciprocating motion of the punching machine.
(ii) A mechanism involving nut and screw, which is a simple and cheap configuration but having poor efficiency; and
(iii) a mechanism consisting of a hydraulic cylinder, piston and valves which is a costly configuration but highly efficient.

Layout of Configuration
To prepare a block diagram of machine system, that shows the general layout of the selected configuration (for example
Automobile Power transmission layout and the layout of an Electrically-operated Overhead Travelling (EOT) crane
Design of individual Components
The design of individual components of a machine is an important step in a design process.
(i) Determine the forces acting on the component.
(ii) Select proper material for the component depending upon the functional requirements.
(iii) Determine the likely mode of failure for the component and depending upon it, select the
criterion of failure, such as yield strength, ultimate tensile strength, endurance limit or permissible deflection and (iv)
Determine the geometric dimensions of the component using a suitable factor of safety.
This stage involves detailed stress and deflection analysis
This subject ‘Machine Design’ or ‘Elements of Machine Design’ cover mainly the design of machine elements or
individual components of the machine.

Preparation of Drawings
To prepare the drawings to assemble the induvial components for making a required mechanical system.
Also, the same drawings, the material of the component, its dimensions, tolerances, surface finish grades and
machining symbols are specified.
The designer prepares two separate lists of components—standard components to be purchased directly from the
market and special components to be machined in the factory. Then the finalized assembly mechanical system to be
tested.

Basic Requirements of Machine Elements
Each component has motion with respect to some other component, is called a machine element, (for example :
bearings, couplings, slider crank mechanism, etc.).
Machine elements can be classified into two groups—general-purpose and special-purpose machine elements
General purpose machine elements include; shafts, couplings, clutches, bearings, springs, gears and machine frames
Special-purpose machine elements include; pistons, crank shaft, cam shaft, inlet &exhaust manifold, valves or
spindles
There are some technical information's are required while designing the machine components for ensuring the safety
operating conditions, life of the components, etc.
1. Strength 2. Rigidity
3. Minimum dimensions and weight 4. Wear resistance
5. Manufacturability 6. Safety
7. Conformation of standards 8. Reliability
9. Maintainability 10. Minimum life cycle cost

1. Strength: A machine part should not fail under the effect of the forces are acting on it.
2. Rigidity: A machine component should be rigid, that is, it should not deflect or bend too much due to forces or
moments that act on it.
3. Wear resistance : Wear also leads to the loss of accuracy of machine tools (example brakes). There are different
types of wear such as abrasive wear, corrosive wear and pitting. Surface hardening can increase the wear
resistance (gears and cams).
4. Minimum dimensions and weight : A machine part should be sufficiently strong, rigid and wear resistant with
minimum possible dimensions and weight. This will result in minimum material cost.
5. Manufacturability :Ease of fabrication and assembly with less labour cost.
6. Safety: The shape and dimensions of the machine components should ensure the safety of the operator or customer
of the machine.
7. Conformation of standards: Each machine components to be ensured that national or international standards.
8. Reliability: A machine part should be reliable, that is, it should perform its function (under various loading)
satisfactorily over its lifetime.
9. Maintainability (Ex: Brakes)
10.Minimum life cycle cost
Basic Requirements of Machine Elements

In order to ensure the basic requirements of machine elements (size or dimensions of the components), calculations are
carried out to find out the dimensions of the machine elements. These calculations form an integral part of the design of
machine elements.

Traditional Design Methods
There are two traditional methods of design;
Design by craft evolution and
Design by drawing.
Design by craft evolution (Examples; Bullock cart, rowing boat, and musical instruments
are some of the products, which are produced by the craft-evolution process).
• The craftsmen do not prepare dimensioned drawings of their products. They cannot
offer adequate justification for the designs, what they made
• These products are developed by trial and error over many centuries. Any modification in the product is costly,
because the craftsman has to do the experiment with the product itself
• Moreover, only one change at a time can be attempted and complete reorganization of the product is
difficult
• The essential information of the product such as materials, dimensions of parts, manufacturing methods and assembly
techniques is transmitted from place to place and time to time by two ways. First, the product, which basically
remains unchanged, is the main source of information
• The exact memory of the sequence of operations required to make the product is second source of information. There
is no symbolic medium to record the design information of the product

Design by drawing method
• The dimensions of the product are specified in advance of its manufacture
• The complete manufacturing of the product can be subdivided into
separate pieces and also, which can be made by different people. This
kind of work is not possible with craft-evolution.
• When the product is to be developed by trial and error, the
process is carried out on a drawing board instead of shop floor.
• The drawings of the product are modified and developed prior
to manufacture and also redesigned.
• In this method, much of the intellectual or creativity activity is
taken away from the shop floor and assigned to design
engineers

Design Synthesis
Design synthesis is defined as the process of creating or selecting configurations, materials, shapes and dimensions for
a product. It is a decision making process with the main objective of optimization process.
Design analysis: the designer assumes a particular mechanism, a particular material and mode of failure for the
component based on this study, the dimensions of the product is defined.
Design synthesis: The designer is determined the optimum shape and dimensions of the component on the basis of
mathematical analysis, with help of the optimization studies (number of solutions, optimum configuration and
alternative materials).
In design synthesis:
The objective can be minimum cost & weight or volume, maximum reliability or life.
The second step is mathematical formulation of these objectives based on the requirements.
The final step is mathematical analysis for optimization and interpretation of the results

Standards and Codes
The characteristics of a product should be conformed or followed mandatory norms “national or international
standards”.
The characteristics include materials, dimensions and shape of the component, method of testing and method of
marking, packing and storing of the product
Standard: A standard is defined as a set of specifications for parts, materials or processes.
The objective of a standard is to reduce the variety and limit the number of items to a reasonable level, while
manufacturing the components. (Standard size of bolts, nuts and mobile chargers)
Codes: A code is defined as a set of specifications for the analysis, design, manufacture, testing and erection of the
product. For example: the purpose of a code is to achieve a specified level of safety
There are three types of design used in design of components
Company standards: They are used in the particular companies.
National standards: These are the IS (Bureau of Indian Standards), DIN (German), AISI or SAE (USA) and BS (UK)
standards.
International standards These are prepared by the International Standards Organization (ISO)

The following standards are used in mechanical engineering design:
Standards for Materials, their Chemical Compositions, Mechanical Properties and Heat
Treatment (Batteries, IC valves, etc.). (IS 1570)
Standards for Shapes and Dimensions of Commonly used Machine Elements (Gears , nuts, etc.) (IS 2494, IS 5129..) .
Standards for Fits, Tolerances and Surface Finish of Component (IS 2709, IS 919,)
Standards for Testing of Products (IS 807 erection and testing of cranes, IS 2825 P V)
Standards for Engineering Drawing of Components
There is a special publication SP46 prepared by Bureau of Indian Standards on ‘Engineering Drawing Practice for
Schools and Colleges’ which covers all standards related to engineering drawings.

Advantages of Standardization
The reduction in types and dimensions of identical components to a rational number makes it possible, for
manufacturing the standard component on a mass scale in a centralized process (like SKF company).
Availability of standard components like bearings, seals, knobs, wheels, roller chains, belts, hydraulic cylinders and
valves have considerably reduced the manufacturing facilities required by the individual organization or customers.
Standard parts are easy to replace when worn out due to interchangeability. The work of servicing and maintenance can
be carried out even at an ordinary service station. These factors reduce the service and maintenance cost of machines.
The standards of specifications and testing procedures of machine elements improve their quality and reliability.
(Standard components like SKF bearings, Dunlop belts or Diamond chains have a long-standing reputation in their
products design & standardization norms for their reliability in engineering industries).

Selection of Preferred Sizes
In engineering design components, the designer has to specify the size of the product.
The ‘size’ of the product is a general term, which includes different parameters “like power transmitting capacity, load
carrying capacity, speed, dimensions of the component such as height, length and width, and volume or weight” of the
product.
These parameters are expressed numerically, e.g., 5 kW, 10 kN or 1000 rpm.
Often, the product is manufactured in different sizes or models; for instance, a company may be manufacturing seven
different models of electric motors ranging from 0.5 to 50 kW to cater to the need of different customers.
Now, the preferred numbers are used to specify the sizes of the products

Consider a manufacturer of lifting tackles who
wants to introduce nine different models of
capacities ranging from about 15 to 100 kN.
Referring to the R10 series, the capacities of
different models of the lifting tackle will be 16,
20, 25, 31.5, 40, 50, 63, 80 and 100 kN.

There are two terms, namely, ‘basic series’ and ‘derived series’, which are frequently used in relation to preferred numbers.
Any series that is formed on the basis of these five basic series that is called derived series

Design of Machine Elements _ Chapter one Introduction.pptx

Machine Element Design Process

A tension test is one of the simplest and basic tests and determines values of number of parameters concerned with
mechanical properties of materials like strength, ductility and toughness.
The following information can be obtained from a tension test:
(i) Proportional limit
(ii) Elastic limit
(iii) Modulus of elasticity
(iv) Yield strength
(v) Ultimate tension strength
(vi) Modulus of resilience
(vii) Modulus of toughness
(viii) Percentage elongation
(ix) Percentage reduction in area
Stress Strain Diagrams

Specimen
‘’Necking’’
Specimen
‘’Fracture’’
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Stress-Strain Diagram: Ductile Materials

The mechanical properties of the metals are those which are associated with the ability of the material to resist mechanical
forces and load.
1. Strength. It is the ability of a material to resist the externally applied forces without breaking or yielding. The internal
resistance offered by a part to an externally applied force is called *stress.
2. Stiffness. It is the ability of a material to resist deformation under stress. The modulus of elasticity is the measure of stiffness.
3. Elasticity. It is the property of a material to regain its original shape after deformation when the external forces are removed.
This property is desirable for materials used in tools and machines.
4. Plasticity. It is property of a material which retains the deformation produced under load permanently. This property of the
material is necessary for forgings, in stamping images on coins and in ornamental work.
5. Ductility. It is the property of a material enabling it to be drawn into wire with the application of a tensile force. A ductile
material must be both strong and plastic. The ductility is usually measured by the terms, percentage elongation and percentage
reduction in area.
The ductile material commonly used in engineering practice (in order of diminishing ductility) are mild steel, copper, aluminum,
nickel, zinc, tin and lead.
Material Properties

6. Brittleness. It is the property of a material opposite to ductility. It is the property of breaking of a material with little
permanent distortion when subjected to tensile loads, without giving any sensible elongation. Cast iron is a brittle material.
7. Malleability. It is a special case of ductility which permits materials to be rolled or hammered into thin sheets. A malleable
material should be plastic but it is not essential to be so strong. The malleable materials commonly used in engineering practice
are lead, soft steel, wrought iron, copper and aluminum.
8. Toughness. It is the property of a material to resist fracture due to high impact loads like hammer blows. The toughness of the
material decreases when it is heated. It is measured by the amount of energy that a unit volume of the material has absorbed after
being stressed up to the point of fracture. Desirable in parts subjected to shock and impact loads.
9. Machinability. It is the property of a material which refers to a relative case with which a material can be cut or remove easily,
brass can be easily machined than steel.
10. Resilience. It is the property of a material to absorb energy and to resist shock and impact loads. It is measured by the
amount of energy absorbed per unit volume within elastic limit. This property is essential for spring materials.
Material Properties

11. Creep. When a part is subjected to a constant stress at high temperature for a long period of time, it will undergo a slow and
permanent deformation called creep. This property is considered in designing internal combustion engines, boilers and turbines.
12. Fatigue. When a material is subjected to repeated stresses, it fails at stresses below the yield point stresses. Such type of
failure of a material is known as *fatigue. The failure is caused by means of a progressive crack formation which are usually
fine and of microscopic size. This property is considered in designing shafts, connecting rods, gears, etc.
13. Hardness. It is a very important property of the metals and has a wide variety of meanings. It embraces many different
properties such as resistance to wear, scratching, deformation and machinability etc. It ability of a metal to cut another metal.
Material Properties

Ductile, brittle and polymer comparison

 The knowledge of materials and their properties is of great significance for a design engineer. The machine elements
should be made of such a material which has properties suitable for the conditions of operation.
 In addition to this, a design engineer must be familiar with the effects which the manufacturing processes and heat
treatment have on the properties of the materials
Materials and their classifications
Classification of Engineering Materials
The engineering materials are mainly classified as :
 Metals and their alloys, such as iron, steel, copper, aluminum, etc.
 Non-metals, such as glass, rubber, plastic, etc.
 The metals may be further classified as: (a) Ferrous metals, and (b) Non-ferrous metals.
 The ferrous metals are those which have the iron as their main constituent, such as cast iron, wrought iron and steel.
 The non-ferrous metals are those which have a metal other than iron as their main constituent, such as copper,
aluminum, brass, tin, zinc, etc.

BIS System of Designation of Steels
A large number of varieties of steel are used for machine components. Steels are designated by a group of letters or
numbers indicating any one of the following three properties:
(i) tensile strength;
(ii) carbon content; and (iii) composition of alloying elements
Designation of Steels:
It can be specified by two ways: a symbol Fe followed by the minimum tensile strength in N/mm2
or a symbol FeE
followed by the yield strength in N/mm2
.
For example, Fe 360 indicates a steel with a minimum tensile strength of 360 N/mm^2.
Similarly, FeE 250 indicates a steel with a minimum yield strength of 250 N/mm2
The designation of plain carbon steel consists of the following three quantities:
(i) a figure indicating 100 times the average percentage of carbon;
(ii) a letter C; and (iii) a figure indicating 10 times the average percentage of manganese.
For example, 55C4 indicates a plain carbon steel with 0.55% carbon and 0.4% manganese.
A steel with 0.35–0.45% carbon and 0.7–0.9% manganese is designated as 40C8

The designation of unalloyed free cutting steels based on following quantities
a figure indicating 100 times the average percentage of carbon;
a letter C,
a figure indicating 10 times the average percentage of manganese;
a symbol ‘S’, ‘Se’, ‘Te’ or ‘Pb’ depending upon the element that is present and which makes the steel free cutting;
a figure indicating 100 times the average percentage of the above element that makes the steel free cutting.
As an example, 25C12S14 indicates a free cutting steel with 0.25% carbon, 1.2% manganese and 0.14% Sulphur.
Similarly, a free cutting steel with an average of 0.20% carbon, 1.2% manganese and 0.1 to 0.15% lead is designated as
20C12Pb13.

The term ‘alloy’ steel is used for low and medium alloy steels containing total alloying elements not exceeding 10%.
The designation of alloy steels consists of the following quantities:
(i) a figure indicating 100 times the average percentage of carbon.
(ii) chemical symbols for alloying elements each followed by the figure for its
average percentage content multiplied by a factor. The multiplying factor
depends upon the alloying element
As an example, 25Cr4Mo2
is an alloy steel having average 0.25% of carbon, 1% chromium and 0.2% molybdenum
40Ni8Cr8V2
is an alloy steel containing average 0.4% of carbon, 2% nickel, 2% chromium and 0.2% vanadium.

Low Carbon Steel
Low carbon steel contains less than 0.3% carbon. It is popular as ‘mild steel’. Low carbon steels are soft and very ductile.
They can be easily machined and easily welded.
Medium Carbon Steel
Medium carbon steel has a carbon content in the range of 0.3% to 0.5%.
It is popular as machinery steel. Medium carbon steel is easily hardened by heat treatment. Medium carbon steels are
stronger and tougher as compared with low carbon steels.
High Carbon Steel
High carbon steel contains more than 0.5% carbon. They are called hard steels or tool steels. High carbon steels have very
high strength combined with hardness. They do not have much ductility as compared with low and medium carbon steels.
High carbon steels are difficult to weld. Excessive hardness is often accompanied by excessive brittleness.

Materials Properties Selection
Selection of Materials for Engineering applications:
 The selection of a proper material, for engineering purposes, is one of the most difficult problem or task for the designer.
While selecting the material :
 Availability of the materials,
 Suitability of the materials for the working conditions in service,
 The cost of the materials
We discuss the important properties, which determine the utility of the material are physical, chemical and mechanical
properties

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Important considerations in Materials Selection
The Engineers (mechanical, civil, chemical, electrical, mechatronics, etc.) must select the appropriate materials based on
their application in land/space and it’s working scenario/environmental conditions
Selection of Materials based on the working scenario/environments
• To select the type/quality/size/geometric of materials based on it’s operation either in static or dynamic (light or heavy
loads).
• To select the type/quality/size/quantity of materials based on construction field (power plants, flyovers, rail tracks, home..)
• In electronic industries, materials’ type/quality/size is important factors for making micro chips, mobiles, insulator, …
• To consider the environmental conditions for materials selection (temp, corrosion, surrounding medium(gas, liquid, etc.)
• The material properties are with in safe limits (chemical, physical, electrical and mechanical properties, etc.).
• Safety, comfortness, user friendly, reliability, life extension and cost-effective parameters are very important
considerations while the selecting materials as well as designing the engineering/bio-medical components

46
Manufacturing Considerations
in Design

 Product design, selection of materials and processing the materials into finished
components are closely related to one each another.
 Manufacturing can be considered as processing the available material, then make into useful components of the
product, (e.g., converting a mild steel sheet into car body, converting a billet of cast iron into a machine tool bed or
converting a steel bar into a transmission shaft).
Selection of Manufacturing Method
Casting Processes In these processes, molten metals such as cast iron, copper, aluminum or non-metals like plastic are
poured into the mould and solidified into the desired shape, e.g., housing of gear box, flywheel with rim and spokes,
machine beds
Deformation Processes In these processes, a metal, either hot or cold, is plastically deformed into the desired shape.
Forging, rolling, extrusion, press working are the examples of deformation processes. The products include connecting
rods, crankshafts
Material Removal or Cutting Processes In these processes, the material is removed by means of sharp cutting tools.
Turning, milling, drilling, shaping, planning, grinding., etc. The examples of material removal processes. For eg.,
transmission shafts, keys, bolts and nuts

In addition, there are joining processes like bolting, welding and riveting
• Complex parts, which are difficult to machine, are made by the casting process using sand mould technique
• Cast components are stable, rigid and strong compared with machined or forged parts. (Typical examples of cast
components are machine tool beds and structures, cylinder blocks of internal combustion engines, pumps and gear box
housings)
• Poor shaping of a cast iron component can adversely affect its strength more than the composition of the material

Forged components are widely used in automotive and aircraft industries. They are usually made of steels and non-
ferrous metals. They can be as small as a gudgeon pin and as large as a crankshaft
Forged components are used under the following circumstances
• Moving components requiring light weight to reduce inertia forces, e.g., connecting rod of IC engines
• Components subjected to excessive stresses, e.g., aircraft structures
• Components requiring pressure tightness where the part must be free from internal cracks, e.g., valve bodies
• Components whose failure would cause injury and expensive damage are forged for safety.
While designing a forging, advantage should be taken of the direction of
fiber lines. The grain structure of a crankshaft manufactured by the three
principal methods, viz., casting, machining and forging
 Parallel fiber lines good for tensile force
 Perpendicular lines good for shear force

Advantages of hot working
(i) Hot working reduces strain hardening.
(ii)Hot rolled components have higher toughness and ductility.
(iii)They have better resistance to shocks and vibrations, and increases the
strength of metal by forged parts.
(iv)Hot working reduces residual stresses in the component.
Limitations of Hot working:
(v)Hot working results in rapid oxidation of the surface due to high
temperature and hot rolled components in poor surface.
Advantages of Cold working processes:
(vi) Cold rolled components have higher hardness and strength, better surface
finish parts dimensions are very accurate
Hot working and cold working process

The machine components are usually made from ferrous and non-ferrous metals
(i) Components requiring precision and high dimensional accuracy.
(ii) Components requiring flatness, roundness, parallelism or circularity for their proper functioning.
(iii) Components of interchangeable in the assembly and (iv) Components, which are in relative motion
Avoid Machining: Machining operations may increase cost of the component. Components made by casting or forming
methods are usually cheaper.
Specify Liberal Tolerances: The machining operations costly. Hence, depending upon the functional requirement of
component, the designer should specify the liberal dimensional and geometric tolerances. Closer the tolerance, higher is
the cost.
Avoid Sharp Corners: Sharp corners result in stress concentration. The designer should be avoided sharp corner in their
design.
Use Stock Dimensions: Raw material like bars are available in standard sizes. Using stock dimensions eliminates
machining operations. For example, a hexagonal bar can be used for preparing the nut and bolt head easily.
Design Rigid Parts: During machining (turning or shaping) induces cutting forces on the components. The component
should be rigid enough to withstand these forces (Ex Tool Post).
Avoid Hard Materials: Try to avoid hard materials due to difficult to machining

Welding is the most important method of joining the parts into a complex
assembly. The guidelines must be followed during welding.
Select the Material with High Weldability
Use Minimum Number of Welds
Use Standard Components
Avoid Straps, Laps and Stiffeners
Select Proper Location for the Weld
Prescribe Correct Sequence of Welding
Design Considerations of Welded Assemblies

The design effort makes up only about 5% of the total cost of a product.
However, it usually determines more than 70% of the manufacturing cost of
the product.
Therefore, at best only 30% of the product’s cost can be changed once the
design is finalized and drawings are prepared.
Reduce the Parts Count
Use Modular Designs
Optimize Part Handling
Assemble in the Open
Design for Part Identity
Eliminate Fasteners
Design Parts for Simple Assembly
Reduce, Simplify and Optimize Manufacturing Process
Design for Manufacture and Assembly (DFMA)

Due to the inaccuracy of manufacturing methods. The
components are so manufactured that their dimensions lie
between two limits-maximum and minimum.
The basic dimension is called the normal or basic size,
while the difference between the two limits is called
permissible tolerance.
Tolerance is defined as permissible variation in the
dimensions of the component.
 There are two systems of specification for tolerances, namely, unilateral and bilateral. In the unilateral system, one
tolerance is zero, while the other takes care of all permissible variation in basic size.
 In case of bilateral tolerances, the variations are given in both directions from normal size.
 The upper limit in this case is the basic size plus non-zero positive tolerance, and the lower limit is the basic size plus
non-zero negative tolerance.
Tolerances

Design of Machine Elements _ Chapter one Introduction.pptx

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