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C.U.SHAH COLLEGE OF ENGG. & TECH.

DEPARTMENT OF MECHANICAL ENGINEERING

Name :A.B.Solanki Subject Code : 131902Subject Name : Machine Design and Industrial Drafting

B.E Mechanical Engineering , 3rd

Semester

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1. Design consideration of Machine Parts

Contents:1. Definition and understanding of various types of design2. Morphology of design3. Design procedure4. Selection of materials, Properties and I.S. coding of various

materials,5. Factors of safety6. Stress Concentration and methods of relieving stresses

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

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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 oropportunity the task is no longer one of design.Design therefore deals with the unknown and

gives shape to this unknown future and in thisprocess creates a new vision of the future thatcan be adopted through a rigorous process of evaluation and testing.

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Various Types of Design

Adaptive design : This is based on existing design, forexample, standard products or systems adopted for a newapplication. Conveyor belts, control system of machinesand 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 one butbased on existing scientific principles. No scientific

invention is involved but requires creative thinking to solvea problem. Examples of this type of design may includedesigning a small vehicle for transportation of men andmaterial on board a ship or in a desert. Some researchactivity may be necessary.

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Types of design based on methods

Rational design : This is based on determiningthe stresses and strains of components andthereby deciding their dimensions.

Empirical design : This is based on empiricalformulae which in turn is based on experience andexperiments. For example, when we tighten a nuton a bolt force exerted is P=284d

Industrial design : These are based on industrialconsiderations 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 forProduct and select proper Mechanism

Prepare General Layout of Configuration

Prepare Assembly and Detail Drawingsand Modify Drawings after Testing

Prototype Model

Design Individual Components

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Selection of materials

Selection of material depends on thefollowing criterion.

Properties of materials Cost of materials

Availability of materials Application

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EngineeringMaterials

Metals

Ferrous

Cast Iron

Wrought Iron

Steel

NonFerrous

AluminiumAlloys

Copper Alloys

Y-Alloys

Other Alloys

Non-Metals

Timber

Rubber

Plastic

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 bepresent as free carbon or iron carbide Fe3C. In general the types of cast iron are (a) grey

cast iron and (b) white cast iron (c) malleablecast iron (d) spheroidal or nodular cast iron(e) austenitic cast iron (f) abrasion resistantcast iron.

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Advantages of Cast Iron Available in a wide range of mechanical/physical

properties, 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 thepresence of 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 geometriesand section sizes in a wide range of sizes, fromounces to over 100 tons

Possibility of casting intricate shapes as well as verythin to very thick section sizes

Capability of redesigning and combining two ormore components from other materials into asingle casting, thus reducing assembly cost andtime

Capability of casting with inserts of other materials Variety of casting processes for low, medium or

high production

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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, FG35or FG35Si15.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 castirons are abrasion resistant.

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Malleable cast iron :

These are white cast irons renderedmalleable by annealing. These are tougher than grey cast iron and

they can be twisted or bent withoutfracture. They have excellent machining properties

and are inexpensive. Malleable cast iron are used for making

parts where forging is expensive such ashubs for wagon wheels, brake supports.

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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 firstnumber gives the tensile strength in Mpaand the second number indicatespercentage 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 iron designated, for

example, ASGNi20Cr2. These are alloy cast irons and they contain small

percentages of silicon, manganese, sulphur, phosphorus etc.They may be produced by adding alloying elements viz. nickel,chromium, molybdenum, copper and manganese in sufficient

quantities. These elements give more strength and improvedproperties. They are used for making automobile parts such as cylinders,

pistons, piston rings, brake drums etc.

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Abrasion resistant cast iron :

These are alloy cast iron and the alloyingelements render abrasion resistance. A typical designation is ABR33 Ni4 Cr2

which indicates a tensile strength inkg/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 beforged or welded.

It cannot however take sudden shock. Chains, crane hooks, railway couplings

and such other components may be made

of this iron. 18

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Plain carbon steel : The properties of plain carbon steel

depend mainly on the carbon percentagesand other alloying elements are notusually present in more than 0.5 to 1%

such as 0.5% Si or 1% Mn etc. There is a large variety of plane carbon

steel and they are designated as C01,

C14, C45, C70 and so on where thenumber 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

are added in sufficient quantities to impart desiredproperties, such as wear resistance, corrosionresistance, electric or magnetic properties.

Chief alloying elements arenickel 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 rolledsilicon for high elastic limit,cobalt for hardness and molybdenum for extratensile strength.

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Some examples of alloy steels are35Ni1Cr60, 30Ni4Cr1, 40Cr1Mo28,37Mn2.

Stainless steel is one such alloy steel thatgives good corrosion resistance.

A typical designation of a stainless steel is15Si2Mn2Cr18Ni8 where carbonpercentage is 0.15.

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I.S. coding of various Materials

AISI(American Iron and Steel Ind.)

Steel-SAE1020-0.2% C And 13% t.

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Factors of Safety Older definition : FOS is totally empirical

number used to reduce the ultimatestrength or the yield strength of a materialso as to obtain a value of design stresswhich would prevent the failure of themachine part. It varies with applicatio and

depended on the experience of part failurefor which a clear explanation was notavailable. Hence called

--------- Factor of Ignorance --------- 24

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Factors of Safety Example

Ductile Material FOS = yield stress/designstress

Brittle Material FOS = ultimate stress/design stress

Varying Load FOS = Endurance

Limit/Design stress

Moreover for stress cycles other than completelyreversed stress the design will be base onSoderberg or Goodman criterion 25

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Stress Concentration

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In developing a machine it is impossible to avoidchanges in cross-section, holes,notches, shoulders

etc.

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We see from the stress-distribution that the stress at the point away from thehole is practically uniform and the maximum stress will be induced at the edge of the hole.

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Stress Concentration

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Methods of Relieving Stresses

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we see that stress lines tend to bunch up and cut very close to the sharp re-entrant corner. In order to improve the situation, fillets may be provided, asshown in Fig. ( b) and (c) to give more equally spaced flow lines. It may benoted that it is not practicable to use large radius fillets as in case of ball androller bearing mountings.

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Types of stresses

Compressive Tensile

Shear

Eccentric axial stresses 35

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Types of stresses

Bending Bearing

Crushing Principle stress

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