Lecture1 ME 131 Introduction-Machine Design

8/17/2019 Lecture1 ME 131 Introduction-Machine Design

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Introduction Machine Desi


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Engineering Design

Engineering design may be defined as-the iterative decision making activity to create a plan or planthe available resources are converted, preferably optimally, insystems, processes or devices to perform the desired functionmeet human needs.

-An iterative decision making process to conceive and implemoptimum systems to solve society’s problems and needs.


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Mechanical Engineering Design

- defined as iterative decision making process to describe a mmechanical system to perform specific function with maximueconomy and efficiency by using scientific principles, technicainformation, and imagination of the designer


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Machine Design

Machine Design or mechanical design is primarily concernedsystems by which the energy is converted into useful mechanand of mechanisms required to convert the output of the mathe desired form. The design may lead to an entirely new maan improvement on an existing one.

Thus machine design is the production or creation of the righcombination of correctly proportioned moving and stationarycomponents so constructed and joined as to enable the liberatransformation, and utilization of energy


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Steps in Machine Design

1.Market Survey2. Define specification of a product

3. Study alternative mechanism for product and select proper mec

4. Compute for the transmitted force ,select proper materials and for allowable stress

5. Prepare general layout of configuration and select joining methbetween individual components of product mechanism

6. Design individual components based on proper dimensions

7. Prepare assembly and detailed drawing and modify drawing aftprototype model mechanism


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1. Market Survey

The chief objective of an enterprise that produces a product is to scustomer. It is essential that you know your customers’ desires befbeginning a product design

2. Define specification of a product

•

a complete list of specifications for the functional requirement oproduct is to be prepared. The requirement may include, for exaOutput capacity; Service life; Cost; Reliability; etc.

• In consumer products, in addition appearance, noiseless operatisimplicity in control are important requirements.

• Depending upon the type of product, various requirements are gweightage and a priority list of specifications is prepared.


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3. Study alternative mechanism for product and select propemechanism

After a careful study of the requirements the designer preparsketches of different possible mechanisms of machine and deupon the cost competitiveness, availability of raw material, anmanufacturing facilities, the possible mechanisms are compaeach other and the designer selects the best possible mechanthe product


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4.Compute for the transmitted force ,select proper materials andfor allowable stress

• Machine is made up of various machine elements on which varioare applied. Calculate the forces acting on each of the element atransmitted by them.

• Select the appropriate materials for each element of the machin

they can sustain all the forces and at the same time they have lepossible cost.

• Considering the various forces acting on the machine elements, material and other factors that affect the strength of the machinthe allowable or design stress for the machine elements.


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5. Prepare general layout of configuration and select joiningbetween individual components of product mechanism

• a block diagram is to be prepared which showing the generathe selected configuration.

• specifies the joining methods, such as riveting, bolting, and to connect the individual components. Rough sketches of shindividual parts are prepared.


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6. Design individual components based on proper dimension

Find out the appropriate dimensions for the machine elemenconsidering the forces acting on it, its material, and design strsize of the machine elements should be such that they should

distort or break when loads are applied


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7. Prepare assembly and detailed drawing and modify drawtesting prototype model mechanism

• The last stage in design process is to prepare the blue printsassembly and individual component after conducting testing

provide modifications, if necessary.• On these drawings, the material of the components, dimens

tolerances, surface finish and machining methods are specif


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Terminologies

Adoptive design• It is the use of existing/known scientific principles and techn

information for development of systems or device with suitamodifications/changes.

• Very often only minor alterations or modifications are made

existing designs (based on the feed back from manufacturinor marketing departments).

• This type of design needs no special knowledge or skill and aby first level designers with ordinary technical training.


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Terminologies

Development DesignThis type of design involve modifying an existing design into a

product with appropriate changes in size, shape, form, matepower range etc. This requires considerable scientific trainindesign ability.


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Terminologies

Creative design• It is development of an unusual or novel solution to meet an

need.

• Very often it results in or needs further scientific understand

• This type of design needs creative thinking, higher technical

and can be attempted by only experienced designers who hpersonal qualities of sufficiently high order.


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Skills Needed in Machine Design

• technical drawing and CAD• properties of materials

• manufacturing processes

• statics, dynamics, and strength of materials

• kinematics and mechanisms


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The Criteria for Evaluating Machine Desi

Decisions

• Safety

• Performance (the degree to which the design meets or exceeds the desi• Reliability (a high probability that the device will reach or exceed its des

• Ease of manufacture

• Ease of service or replacement of components

• Ease of operation

• Low initial cost

• Low operating and maintenance costs

• Small size and low weight

• Low noise and vibration, smooth operation

• Use of readily available materials and purchased components

• Prudent use of both uniquely designed parts and commercially available

•

Appearance that is attractive and appropriate to the application.


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Introduction Stress


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Stress

- Force per unit area


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Stress

Normal Stress =

Tensile Stress- Normal

stress that pulls the

imaginary surface away

from the material

Compressive Stress-

Normal stress that

pushes the imaginary

surface into the

material


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Stress

Maximum normal stress in inclined surfaces


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Stress

Determine the normal stress in each segment


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Stress

FBD

Stresses


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Stress

If the cross sectional area of each member is 900mm2, determnormal stress in members AC and BD


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Stress

FBD of entire truss: Ay = 40 kN, Hy = 60 kN, and Hx = 0

FBD of pin A:


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Stress

Method of section in 1 and :0

E M


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Stress

The cross sectional area of the bars is 800mm2. If the workingfor members AB and AC are 110 ands 120 Mpa, respectivelydetermine the allowable value of the weight W.


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Stress

FBD of pt. A:


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Stress

Shear Stress =

ℎ


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Stress

Maximum shear stress in inclined surfaces


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Stress

Determine the largest axial force P that can be carried safely

panel if the working stress for the wood is 1120 psi, and the

and shear stresses in the glue are limited to 700 psi and 450

respectively.


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Stress

Design for Working Stress in Wood:

Design for Normal Stress in Glue


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Stress

Bearing Stress)(( riveof diameter plateof thickness

forceShearing

bearing


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Stress

The lap joint shown is fastened by four rivets of 3/4-in. diamthe maximum load P that can be applied if the working stres

14 ksi for shear in the rivet and 18 ksi for bearing in the platethat the applied load is distributed evenly among the four rivneglect friction between the plates.


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Stress


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Stress

Bending Stress inertiaof Moment radius Moment

B

)(


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Moment of Inertia


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Moment of Inertia


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Moment of Inertia


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Moment of Inertia


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Stress

If the moment in the beam described by the cross sectional a

is 4000 ibf-in, detemine the bending stress of the beam.


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Stress

If the moment in the beam is 100kN-m, detemine the bendin

the beam described by each cross sectional area below

a. b.


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Stress

Torsional Stress

= ()


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Stress

A steel propeller shaft is to transmit 4.5 MW at 3 Hz without

a shearing stress of 50 MPa or twisting through more than 1length of 26 diameters. Compute the proper diameter if G =


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Stress

For the bronze segment AB, the maximum shearing stress is l

8000 psi and for the steel segment BC, it is limited to 12 ksi.Determine the diameters of each segment so that each matbe simultaneously stressed to its permissible limit when a to12 kip·ft is applied. For bronze, G = 6 × 106 psi and for steel, 106 psi.


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Strain

Simple Strain,

L

E

L

AE

PL


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Strain

The composite bar below is firmly attached to unyielding sup

Compute the stress in each material caused by the applicatiaxial load P = 50 kips.


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Strain

Stress- Strain Diagram

Elastic Limit

The elastic limit is the limit beyond whichthe material will no longer go back to itsoriginal shape when the load is removed,or it is the maximum stress that may edeveloped such that there is no

permanent or residual deformation whenthe load is entirely removed.

E


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Strain


Elastic and Plastic Ranges

The region in stress-strain diagram from Oto P is called the elastic range. The regionfrom P to R is called the plastic range


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Strain


Yield Point

Yield point is the point at which thematerial will have an appreciableelongation or yielding without anyincrease in load.

Ultimate StrengthThe maximum ordinate in the stress-strain diagram is the ultimate strength ortensile strength.

S i


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StrainStress- Strain Diagram

Rupture Strength

Rupture strength is the strength of thematerial at rupture. This is also known as thebreaking strength.

Modulus of ResilienceModulus of resilience is the work done on aunit volume of material as the force isgradually increased from O to P, in N·m/m3.This may be calculated as the area under thestress-strain curve from the origin O to up tothe elastic limit E (the shaded area in thefigure). The resilience of the material is itsability to absorb energy without creating a

permanent distortion

S i


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StrainStress- Strain Diagram

Modulus of ToughnessModulus of toughness is the work done on a unit volume of

material as the force is gradually increased from O to R, inN·m/m3. This may be calculated as the area under theentire stress-strain curve (from O to R). The toughness of amaterial is its ability to absorb energy without causing it tobreak.

Working Stress, Allowable Stress, and Factor of SafetyWorking stress is defined as the actual stress of a materialunder a given loading. The maximum safe stress that amaterial can carry is termed as the allowable stress. Theallowable stress should be limited to values not exceedingthe proportional limit. However, since proportional limit isdifficult to determine accurately, the allowable tress istaken as either the yield point or ultimate strength dividedby a factor of safety.

The ratio of this strength (ultimate or yield strength) toallowable strength is called the factor of safety.

St i


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Strain

A metal bar which is part of a frame is 50mm diameter and 30

long. It has a tensile force on it of 40kN which tends to stretmodulus of elasticity is 205 Gpa. Calculate the stress and strbar and the amount it stretches.

St i


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Strain

Shear Strain,

L

s

AG

VL

s

G

St i


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Strain

Possion’s Ratio,

Biaxial Deformation

x

z

x

y

21

E y x x

E E

y X

x

E E

x y

y

y

Strain


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Strain

A material has stresses of 2 Mpa in the x-direction and 3 Mpa

direction. Given the elastic contants E=205 Gpa and v=0.27,the strains in both directions.

Strain


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Strain

Possion’s Ratio,

Triaxial Deformation

x

z

x

y

)(1 z y x x E

)(1 z x y y E

)(1 y x z z E

Strain


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StrainA rectangular steel block is 3 inches long

in the x direction, 2 inches long in the ydirection, and 4 inches long in the zdirection. The block is subjected to atriaxial loading of three uniformlydistributed forces as follows: 48 kipstension in the x direction, 60 kipscompression in the y direction, and 54kips tension in the z direction. If ν = 0.30

and E = 29 × 106 psi, determine thesingle uniformly distributed load in the xdirection that would produce the samedeformation in the y direction as theoriginal loading.

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Lecture1 ME 131 Introduction-Machine Design

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