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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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