mdid introduction

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INDUS INSTITUTE OFTECHNOLOGY & ENGINEERING

DEPARTMENT OF

MECHANICAL ENGINEERING

Name : Keval Patel

Subject Code : 131902

Subject Name : Machine Design and Industrial Drafting

B.E Mechanical Engineering , 3rd Semester

1PREPARED BY: KEVAL PATELMECHANICAL ENGG. DEPT.


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1. Design consideration of Machine Parts

Contents:

1. Definition and understanding of various types of design

2. Morphology of design

3. Design procedure4. Selection of materials, Properties and I.S. coding of

various materials,

5. Factors of safety

6. Stress Concentration and methods of relieving stresses

7. Types of stresses - tensile, compressive, shear,bending, bearing, crushing, Eccentric axial stresses,principle stress Residual stresses.

PREPARED BY: KEVAL PATEL

MECHANICAL ENGG. DEPT.

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Definition and Basic Understanding

WHAT IS MACHINE?

WHAT IS DESIGN?

WHAT IS MACHINE DESIGN?

If everything were known about the problem or

opportunity the task is no longer one of design. Design

therefore deals with the unknown and gives shape to this

unknown future and in this process creates a new visionof the future that can be adopted through a rigorous

process of evaluation and testing.

PREPARED BY: KEVAL PATELMECHANICAL ENGG. DEPT.

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Various Types of Design

Adaptive design : This is based on existing design, forexample, standard products or systems adopted for anew application. Conveyor belts, control system ofmachines and mechanisms

Developmental design : Here we start with an existing

design but finally a modified design is obtained. A newmodel of a car is a typical example of a developmentaldesign .

New design : This type of design is an entirely new onebut based on existing scientific principles. No scientificinvention is involved but requires creative thinking to

solve a problem. Examples of this type of design mayinclude designing a small vehicle for transportation ofmen and material on board a ship or in a desert. Someresearch activity may be necessary.


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Types of design based on methods

Rational design : This is based on determining the

stresses and strains of components and thereby

deciding their dimensions.

Empirical design : This is based on empirical formulaewhich in turn is based on experience and experiments.

For example, when we tighten a nut on a bolt force

exerted is P=284d

Industrial design : These are based on industrial

considerations and norms viz. market survey, external

look, production facilities, low cost, use of existing

standard products.


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Morphology of Design

The morphology of design refers to the study of the

chronological structure of design


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Basic Procedure of Machine Design


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Market Survey

Define Specifications of Product

Study Alternative Mechanisms for

Product and select proper Mechanism

Prepare General Layout of

Configuration

Prepare Assembly and Detail Drawings

and Modify Drawings after Testing

Prototype Model

Design Individual Components


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Selection of materials

Selection of material depends on the following criterion.

Properties of materials

Cost of materials

Availability of materials

Application


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Engineeringteri l

et l

Ferrou

C t Iron

Wrought Iron

Steel

NonFerrou

AluminiumAlloy

Copper Alloy

Y-Alloy

Other Alloy

Non- et l

Timber

Rubber

Pl tic

Leather


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Properties of materials

Cast iron-

It is an alloy of iron, carbon and silicon and it is hard and

brittle. Carbon content may be within 1.7% to 3% and

carbon may be present as free carbon or iron carbideFe3C.

In general the types of cast iron are (a) grey cast iron

and (b) white cast iron (c) malleable cast iron (d)

spheroidal or nodular cast iron (e) austenitic cast iron (f)

abrasion resistant cast iron.


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Advantages of Cast Iron

Available in a wide range of mechanical/physicalproperties, i.e. tensile strength from 20 KSI to over 200KSI, hardness from 120 to about 300 Brinell in standardgrades and up to about 600 Brinell in special abrasionresistant grades

Good strength to weight ratio Typically lower cost than competing materials and

relatively low cost per unit of strength than othermaterials

Lower density and higher thermal conductivity than

steels at comparable tensile strength levels Excellent machinability, allowing for high speeds and

feeds and reduced (minimal) energy due to the presenceof free graphite


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Excellent damping capacity, especially in Gray Irons

Rapid transition from design to finished product Capability of producing highly complex geometries and

section sizes in a wide range of sizes, from ounces toover100 tons

Possibility of casting intricate shapes as well as very thin

to very thick section sizes Capability of redesigning and combining two or more

components from other materials into a single casting,thus reducing assembly cost and time

Capability of casting with inserts of other materials

Variety of casting processes for low, medium or highproduction


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Grey cast iron : Carbon here is mainly in the form of

graphite. Inexpensive, high compressive strength

Graphite is an excellent solid lubricant and this makes it

easily machinable but brittle.

Some examples of this type of cast iron are FG20, FG35orFG35Si15.

White cast iron : In these cast irons carbon is present in

the form of iron carbide (Fe3C) which is hard and brittle.

The presence of iron carbide increases hardness andmakes it difficult to machine. Consequently these cast

irons are abrasion resistant.


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Malleable cast iron :

T

hese are white cast irons rendered malleable byannealing.

These are tougher than grey cast iron and they can be

twisted or bent without fracture.

They have excellent machining properties and are

inexpensive.

Malleable cast iron are used for making parts where

forging is expensive such as hubs for wagon wheels,

brake supports.

Depending on the method of processing they may bedesignated as black heart BM32, BM30 or white heart

WM42, WM35 etc.


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Spheroidal or nodular graphite cast iron:

In these cast irons graphite is present in the form of

spheres or nodules. They have high tensile strength and good elongation

properties.

They are designated as, for example, SG50/7, SG80/2

etc where the first number gives the tensile strength inMpa and the second number indicates percentage

elongation.


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Austenitic cast iron :

Depending on the form of graphite present these cast

iron can be classified broadly under two headings: Austenitic flake graphite iron designated, for example,

AFGNi16Cu7Cr2

Austenitic spheroidal or nodular graphite irondesignated, for example,

ASGNi20Cr2. These are alloy cast irons and theycontain small percentages of silicon, manganese,sulphur, phosphorus etc. They may be produced byadding alloying elements viz. nickel, chromium,molybdenum, copper and manganese in sufficientquantities. These elements give more strength andimproved properties.

They are used for making automobile parts such ascylinders, pistons, piston rings, brake drums etc.


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Abrasion resistant cast iron :

These are alloy cast iron and the alloying elements

render abrasion resistance.

A typical designation is ABR33 Ni4 Cr2 which indicates a

tensile strength in kg/mm2 with 4% nickel and 2%

chromium.


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Wrought iron :

This is a very pure iron where the iron content is of the

order of 99.5%. It is produced by re-melting pig iron and some small

amount of silicon, sulphur, or phosphorus may be

present. It is tough, malleable and ductile and can easily

be forged or welded.

It cannot however take sudden shock.

Chains, crane hooks, railway couplings and such other

components may be made of this iron.


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Plain carbon steel:

The properties of plain carbon steel depend mainly on

the carbon percentages and other alloying elements arenot usually present in more than 0.5 to 1% such as 0.5%

Si or1% Mn etc.

There is a large variety of plane carbon steel and they

are designated as C01

, C1

4, C45, C70 and so on wherethe number indicates the carbon percentage.


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Following categorization of these steels is sometimes made

for convenience:

Dead mild steel- up to 0.15% C

Low carbon steel or mild steel- 0.15 to 0.46% C

Medium carbon steel- 0.45 to 0.8% C. High carbon steel- 0.8 to 1.5% C

Detailed properties of these steels may be found in any

standard handbook but in general higher carbon

percentage indicates higher strength.


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Alloy steel :

these are steels in which elements other than carbon areadded in sufficient quantities to impart desiredproperties, such as wear resistance, corrosionresistance, electric or magnetic properties.

Chief alloying elements are

nickel for strength and toughness,chromium for hardness and strength,

tungsten for hardness at elevated temperature,

vanadium for tensile strength,

manganese for high strength in hot rolled

silicon for high elastic limit,

cobalt for hardness and molybdenum for extra tensilestrength.


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Some examples of alloy steels are 35Ni1Cr60,

30Ni4Cr1, 40Cr1Mo28, 37Mn2.

Stainless steel is one such alloy steel that gives good

corrosion resistance.

A typical designation of a stainless steel is

15Si2Mn2Cr18Ni8 where carbon percentage is 0.15.


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I.S. coding of various Materials


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Factors of Safety

Older definition : FOS is totally empirical number used to

reduce the ultimate strength or the yield strength of a

material so as to obtain a value of design stress which

would prevent the failure of the machine part. It varies

with applicatio and depended on the experience of part

failure for which a clear explanation was not available.

Hence called

--------- Factor of Ignorance -------------

New Definition : FOS is the amount by which the designstress is kept below the limit stress.


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Factors of Safety

Example

Ductile Material FOS = yield stress/design stress

Brittle Material FOS = ultimate stress/ design stress

Varying Load FOS = Endurance Limit/Design stress

Moreover for stress cycles other than completely reversed

stress the design will be base on Soderberg or Goodman

criterion


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Stress Concentration


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Methods of Relieving Stresses


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Compressive

Tensile

Shear

Eccentric axial stresses

Types of stresses


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Types of stresses

Bending

Bearing

Crushing

Principle stress


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