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CHAPTERCHAPTER--11

DESIGN CONSIDERATION

(INTRODUCTION)

-BY CHETAN S. JADAV



CONTENTCONTENT

•Definition and various types of designs

•Morphology of design

•Design procedure

•Selection of material

•Properties and I.S. coding of various materials

•Factors of safety

•Stress Concentration and methods of relievingstresses

•Types of stresses



DefinitionsDefinitions

MACHINEMACHINE

++

DESIGNDESIGN



What’s Machine ?What’s Machine ?




•Machine: “ It is defined as a combination of rigidand resistant bodies having definite motion andcapable of performing some useful work” .

Input

(Source of power)

KinematicArrangement

Oflinks

Output

(useful work)




•Mechanism: it is a simplified model, frequently inthe form of a line diagram, which will reproduceexactly the same motion that takes place in actual

machine. The fundamental objective in case of.

•Structure: It is also a combination of rigid andresistant bodies, but there is no relative motion

between its various parts.The purpose of structureis not to do some useful work, but to supportexternal load.



Engineering DesignEngineering Design

It is The process of applying the various techniquesand scientific principles for the purpose ofdesigning a device, a process, or a system in

sufficient detail to permit its realization



Machine designMachine design

• Machine Design deals with the creation ofmachinery that workssafely, reliably and well

• It is a creation of plans for machine to perform

the desired functions.

process.

It is also a decision-making process.

Design is a communication-intensive activity inwhich both words and pictures are used, andwritten and oral forms are employed.



Machine designMachine design

Engineering tools (such as mathematics, statistics,computers, graphics,and languages) are combinedto produce a plan that, when carried out,produces a product that is functional, safe, reliable,com etitive usable manufacturable an marketable regardless of who builds it or who uses it.



Essence of Machine designEssence of Machine design

• Machine create motion and develop forces.

•Engineer’sTask

•To define and calculate those motions , forces,

and changes in energy in order to determine, ,

of the interrelated parts in machine




•Machine design:

“it is defined as the use of scientific principles,technical information and imagination in the

description of a machine or a mechanical system toper orm spec c unct ons



Ultimate Goal of DesignUltimate Goal of Design



Design procedureDesign procedure

Need or AimSynthesis

OrMechanism

AnalysisOf

Forces

ModificationDesign

OfElements

MaterialSelection

DetailedDrawing Production



Types of designsTypes of designs

•Adaptive design

•Development design

•New design:



Types of designsTypes of designs

•Rational design (maths formulae)

•Emparical design (emparical formulae)

•Industrial design (production aspect)

•Optimum design (best design) • ys em esgn comp ex mec an ca sys em

•Element design (design of part)

•Computer Aided Design (CAD-CAM)



Design considerationsDesign considerations

The design of a component or system may be influenced by a number ofrequirements. If a requirement affects design, it is called a designconsideration.For example, if the ability to carry large loads without failure isimportant, we say that strength is a design consideration. Most product

development projects involvea number of design considerations: - - -

- Distortion/stiffness - Processing requirements - Surface finish

-Wear -Weight - Lubrication

- Corrosion - Life - Marketability

- Safety - Noise - Maintenance

- Reliability - Aesthetic considerations -Volume

- Friction - Shape - Liability

- Usability/utility - Size - Scrapping/recyclability



Selection of materialsSelection of materials

•Availability of materials

•Suitability of the material in the workingconditions

•Cost of the material



PropertiesProperties

• Strength

• Elasticity

• Plasticity

• Malleabilit

• Ductility

• Impact Strength

• Hardness



PropertiesProperties

• Toughness

• Brittleness

• Resilience

• Cree

• Fatigue



STANDARDS & CODESSTANDARDS & CODES

A standard is a set of specifications for parts,materials, or processes intended to achieveuniformity,efficiency,and a specified quality.

It places a limit on the number of items in the

inventory of tooling, sizes, shapes, and varieties.

A code is a set of specifications for the analysis,

design, manufacture, and construction ofsomething.

It achieve a specified degree of safety, efficiency,and performance or quality



I.S. coding of various materialsI.S. coding of various materials

•Aluminum Association (AA)

•American Gear Manufacturers Association (AGMA)

•American Institute of Steel Construction (AISC)

•American Iron and Steel Institute (AISI) • mer can a ona an ar s ns u e

•ASM International6

•American Society of Mechanical Engineers (ASME)

•American Society of Testing and Materials (ASTM)•American Welding Society (AWS)



I.S. coding of various materialsI.S. coding of various materials

•British Standards Institution (BSI)

•Industrial Fasteners Institute (IFI)

•Institution of Mechanical Engineers (I. Mech. E.)

•International Bureau of Weights and Measures (BIPM) • n erna ona an ar s rgan za on

•National Institute for Standards and Technology (NIST)8

•Society of Automotive Engineers (SAE)



Stress & StrainStress & Strain

• Stress – a force per unit area

– Since it’s difficult to directly observe stress, geologists study the

effects of past stress when bed rock is exposed after uplift and

erosion at the Earth’s surface

– The principal directions of stress can be determined by our

• Strain – the change in size (volume) and/or shape, in

response to stress



Stress & StrainStress & Strain



Factors of safetyFactors of safety

Structural members or machinesmust be designed such that theworking stresses are less than

the ultimate strength of the

Factor of safety considerations:

• uncertainty in material properties

• uncertainty of loadings

• uncertainty of analyses

stressallowable

stressultimate

safetyof Factor

all

u

FS

FS

.• number of loading cycle

• types of failure

• maintenance requirements anddeterioration effects

• importance of member tostructures integrity

• risk to life and property

• influence on machine function



Young’s ModulusYoung’s Modulus

plastic

Very stiff material

fracture

(typical ceramic)

s = E e

strain

stress

elastic

yield

Very ductile,less stiff material

(typical metal)

(typical polymer)



Stress ConcentrationStress Concentration

In almost engineering components and machine have toincorporate design features which introduce changes inthin cross-section.

Changes in cross section causes localized stress

concentrations and severity depends on the geometry of.

Stress concentration factor, Kt = Smax /Sav

Smax, maximum stress at discontinuity

Sav, nominal stress.

Kt, value depends only on geometry of the part.



Methods of relieving stressesMethods of relieving stresses

Guidelines for design.

Abrupt changes in cross-section should be avoided.

Fillet radii or stress-relieving groove should be provided. (see Fig.11.3(d))

Slot and grooves should be provided with generous run-out radii andwith fillet radii in all corners. (see Fig. 11.3(b))

Stress relieving grooves or undercut should be provided at the endof threads and splines. (see Fig. 11.3(c))

Sharp internal corners and external edges should be avoided

Weakening features like bolt and oil holes, identification marks, and

part number should not be located in highly stressed areas.Weakening features should be staggered to avoid the addition oftheir stress concentration effects, (see Fig. 11.3(d))





Types of stressesTypes of stresses

•Tensile stress

•Compressive stress

•Shear stress

•Bendin stress

•Bearing stress

•Crushing stress

•Eccentric axial stresses•Principle stresses

•Residual stresses



Tensile stressTensile stress

A tensional stress is caused by forces pulling

away from one another in opposite directions.• Tensional stress is produced at divergent plate boundaries and results in a

stretching or extensional strain.



Tensile stressTensile stress

• Axial forces on a two forcemember result in only normalstresses on a plane cutperpendicular to the memberaxis.

• Transverse forces on bolts andins result in onl shear stresses

• Will show that either axial ortransverse forces may produce

both normal and shear stresseswith respect to a plane otherthan one cut perpendicular tothe member axis.

on the plane perpendicular tobolt or pin axis.



Compressive StressCompressive Stress

– A compressive stress is caused by forces pushing

together, or squeezing from opposite directions.

• Compressive stress is common along convergent plate

boundaries

• Typically results in rocks being deformed by a shortenings ra n; e er y en ng an or o ng.



shear stressshear stress

A shear stress is due to forces parallel to one another

by in opposite directions along a discrete surface,

such as a fault.

• A shear stress results in a shear strain parallel to the

direction of the stresses.

• Shear stresses are notable alon transform late boundaries

and actively moving faults.



shear stressshear stress

Single Shear Double Shear

A

F

A

Pave

A

F

A

P

2ave



Bearing stressBearing stress

• Bolts, rivets, and pinscreate stresses on thepoints of contact orbearing surfaces of themembers they connect.

d t

P

A

Pb

• The resultant of the forcedistribution on the surfaceis equal and opposite tothe force exerted on thepin.



DESIGN ASPECTDESIGN ASPECT––Loading ConditionLoading Condition

Static loading – load is applied gradually and remainsapplied throughout part’s life.

Repeated loading – applied and removed several t imes(repetitive) during life. Fail by fatigue at stress lower than

yield strength. Higher design factor is needed.

Impact – require large design factor. (i) sudden loadcauses stresses much higher than computed. (ii) requirepart to absorb energy of the impact.

Static loading but at high T – consider creep,microstructural changes, oxidation & corrosion &influence of method of fabrication on creep.



Designing For Static StrengthDesigning For Static Strength

Static strengthAbility to resist short-term steady load at moderate T.

Measured in terms as yield strength, UTS,compressive strength & hardness.

Aimed at avoiding yielding of the component and / orfracture.Component must be strong enough to support the load& may require stiffness to ensure deflections do notexceed certain limits.

Stiffness , important to avoid misalignment and

maintain dimensional accuracy.Elasticity (Young’s M)

important when designing struts, columns & thin-walled cylinders subjected to compressive axialloading where failure can take place by buckling.



Designing for simple axial loadingDesigning for simple axial loading

Component and structure made from ductile materialare usually designed, so that no yield take place understatic loading conditionBut, when the component is subjected to uniaxial stress,yielding take place

When local stress reaches the yield strength of the

Critical cross-sectional area, A ;

A = KtnsL

YS

Kt = stress concentration factor

L = applied load

ns = factor of safety

YS = yield strength




Factor of safety, ns

Is applied in designing component to ensure it willsatisfactory perform its intended function

To get the strength of material at allowable stress.The definition, strength of material depends on the typeof material and loading condition

The factor of safety, ns.ns = S / Sa

S = Nominal strength, Sa = Allowable strength / Designstrength

It is important to define which type of service conditionwill the material work on before calculating the ns.i. Normal working conditionii. Limit working condition




Given – (1) Magnitude & type of loading & (2) materialcondition.

Determine yield & ultimate strength & % elongation ofmaterial. Decide ductile or brittle.

Specify design factor (factor of safety).

Compute design stress.

Write equation for expected max stress. For directnormal stress,δmax = F/A

Set δmax = δd & solve for required cross-sectional area.Determine minimum required dimension.



Designing forDesigning for TorsionalTorsional LoadingLoading

Torsional – loading of a component / part that tends tocause it to rotate or twist.

When torque is applied, shearing stress is developed &torsional deformation occurs, resulting in an angle oftwist of one end of part relative to the other.

perform properly in service.

Torque = T = F x d where F = applied force & d =distance from action of force to axis of the part.

Power = torque x rotational speed (n in rad/s).Torsional shear stress,ζmax = Td / 2Ip where T = appliedtorque, d = diameter & Ip = polar moment of inertiaof the cross section.




Critical cross sectional area can be calculated,

for circular shaft, 2Ip = KtnsT

d ζmax

where Kt = stress concentration factor ns = actor o saety

Moment, Ip = πd4 / 32 for solid circular shaft &

Ip = π(d04 – di

4) / 32 for hollow circular shaft of inner di &

outer d0.ASTME code of practice ; allowable value of shear stressof 0.3 yield or 0.18 UTS.

For ductile material, design shear stress = yield / 2N(steady torsion, N = 2, so ζd = yield / 4)




Torsional rigidity of component is usually measured bythe angle of twist,θ, per unit length. For circular shaft,θ

is given by,

θ = T / GIp where G = modulus of elasticity in

shear. G = E / (2(1 + ν)) where ν = Poisson’s ratio.

Usual practice is limit the angular deflection in shafts toabout 1 degree, i.e π /180 rad, in length of 20 times the

diameter.Stiffness of part differ depending on shape of the cross-section – circular section has higher rigidity compared toother structural shapes, i.e I-beams,wide-flange beams



Designing forDesigning for BendingBending

Beam – component that carries load transversely, that is,perpendicular to its long axis.

Loading – normal concentrated load, inclinedconcentrated load, uniformly distributed load, varying

distributed load & concentrated moments. Moment – an action that tends to cause rotation of an

object. Can be produced by a pair of parallel forces actingin opposite directions, called couple .

Beam types ; simple, overhanging, cantilever, compound &continuous.

Bending moments – internal moments cause bending.




Relation between bending moment, max allowable stress& dimensions given by ;

Z = nsM

YS= =

c = distance from center of gravity of cross section to theoutermost fiber/beam.

I = moment of inertia of cross section with respect toneutral axis normal to direction of load.

M = bending moment & YS = max allowable stress.

ns = factor of safety.




When load is placed on a beam, the beam isbent and every por tion of it is moved in adirection parallel to the direction of the load.

The distance that a point on the beam moves/

i. Its position in the beam

ii. Type of beam

iii. Type of support



Principal StressPrincipal Stress

“At any point in a strained material, there are threeplanes, mutually perpendicular to each otherwhich carry direct stresses only and no shearstress. - These perpendicular planes which have

no shear stress are known as principal planesand the direct stresses along these planes areknown as Principal Stress.”

Out of these three direct stresses, one will be

maximum and the other will be minimum.



Residual StressResidual Stress

• Residual stresses or locked-in stresses can bedefined as those stresses existing within a body

in the absence of external loading or thermalgradients.

• Residual stresses ma be due to the

technological process used to make thecomponent.

• Manufacturing processes like casting, welding,

machining, molding, heat treatment, plastic• deformation during bending, rolling or forging

are the most common causes of residual stress.



Thank youThank you

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