Mi 291 chapter 1 (introduction)

Engineering Design & Analysis
MI-291
Course Instructors
Dr. P. M. Pathak & Dr. A. Parashar
Department of Mechanical & Industrial Engineering
IIT-Roorkee

INDIAN INSTITUTE OF TECHNOLOGY ROORKEE
Mechanical & Industrial Engineering Department
•
1. Subject Code: MI-291
2. Course Title: Engineering Analysis and Design
3. Contact Hours: L: 3 T: 1 P: 2/2
4 Examination Duration (Hrs.): Theory : 3 hrs Practical 0
5 Relative Weightage: CWS 15 PRS 15 MTE 30 ETE 40
6. Credits: 4 7. Semester: Autumn 8. Subject Area: DCC
9. Pre-requisite: None
10. Objective:
• This course aims to describe the role of analysis in
engineering design and enhance critical thinking and design
skills;

• This course aims to describe the role of analysis in engineering design
• Compare and contrast design alternatives to select the most promising
idea.
• Enhance critical thinking and design skills;
• Reinforce the importance of mathematics and science in engineering
design and analysis;
• Emphasize communication skills, both written and oral;
• Develop teamwork skills;
• Offer experience in hands-on, creative engineering projects;
• Introduce students to professional ethics and the societal context of
engineering practice

CONTENTS
 Chapter I Introduction
 Design
 History of design
 Mechanical engineering design
 Different phases of design
 Chapter II Engineering Analysis
 Role of analysis
 The design spiral
 Computer aided engineering analysis
 Visualization/analysis and redesign
 Statistical consideration
 Safety and reliability

 Chapter III Reverse Engineering
 Introduction
 Applications
 Chapter IV Learning from Failures
 Learning from case studies
 Failure of mechanical components
 Chapter V Engineering Design
 Project for design of mechanical elements
 Chapter VI Aesthetic & Engineering Design
 Written and oral presentation
 Poster presentation

Chapter 1
INTRODUCTION TO
MECHANICAL ENGINEERING
DESIGN

Mechanical Engineering Design
Involves
 Simple journal bearing involves fluid flow, heat transfer, friction,
energy transport, material selection, thermo-mechanical treatments,
statistical descriptions, and so on.
 A building is environmentally controlled. The heating, ventilation,
and air-conditioning considerations are sufficiently specialized that
some speak of heating, ventilating, and air- conditioning design as if
it is separate and distinct from mechanical engineering design.
 Similarly, internal-combustion engine design, turbo machinery
design, and jet-engine design are sometimes considered discrete
entities.

Design
 Design is an innovative and highly iterative process. It is also a
decision-making process.
 Decisions sometimes have to be made with limited information,
occasionally with just the right amount of information, or with an
excess of partially contradictory information.
 Engineers have to communicate effectively and work with people
of many disciplines.
 Engineering tools (such as mathematics, statics, computers,
graphics, and languages) are combined to produce a plan that,
when carried out, produces a product that is functional ,safe,
reliable, competitive, usable, manufacturable, and
marketable, regardless of who builds it or who uses it.

Design : Aesthetic vs. Engineering
www.hostpaperz.com
www.danier.ca

Design : Aesthetic vs. Engineering
www.ferrari.com
www.asiandefensenews.com

Common Considerations ?
 FUNCTION: most of the creations serve multiple
functions.
 SHAPE : All designs have sort of shape/form. Shape is
related with function, material selection, aesthetics.
 MATERIAL : All objects have material.
 COST : Cost consideration.
 OTHER FACTORS : Safety, environment, aesthetic etc.

FUNCTIONS vs COST?
Function 1
Functions 1
2
Functions 1
2
3
Functions 1
2
3
4
5

Wind Turbine Structure
The support structure should be optimized for weight
and stiffness (deflection)
Support
Structure

Ken Youssefi
Lattice structure
Wind Turbine Structure
Hollow tube with guy wire
Hollow tapered tube

Structural Failure
Support structure failure, New York.
Stress at the base of the support
tower exceeding the strength of the
material
Support structure failure, Denmark.
Caused by high wind.

Structural Failure
Blade failure, Illinois. Failure at
the thin section of the blade
Support structure failure

Leonardo de Ser Piero da Vinci
(1452-1519)
Monalisa
The Last Supper
 A man full of curiosity
& energy for inventive
imaginations.
 Painter, sculptor,
architect, musician,
engineer and writer.

Leonardo & Early Engineering
 Water pumps and machines.
 Sketches were inspired from
the work of Martini (1490).
 His eye and hand coordination
was exemplary, his drawings
shows his grasp of mechanics
and nature.
 Leonardo dreamt of great
things.

Leonardo’s Concept of Friction
 In modern terms, Leonardo
assigns a value of 0.25 to
coefficient of friction.
 He repeatedly employed the
same value of coefficient of
friction.
 Specific value 0.25 for
coefficient of friction for most of
the materials is appropriate.

Design: Flying Machine
 Leonardo was
fascinated with the
idea of flying.
 Leonardo’s design
of parachute and
giant crossbow
were interpreted
and tested.

Sir Isaac Newton
(1642-1727)
 A British physicist and
mathematician.
 Respected as the most influential
scientist of all time.
 “Mathematical Principles of
Natural Philosophy” is considered
as most prominent contribution to
classical mechanics.
 Also made contributions in the
field of optics and calculus.
“Nature and nature’s laws lay
hid in night. God said let
NEWTON be, and all was light”
Epitaph by Pope

Design
• To design is either to formulate a plan for the
satisfaction of a specified need or to solve a
problem.
• If the plan results in the creation of something
having a physical reality then the product
must be functional, safe, reliable, competitive,
usable, manufacturable, and marketable.

• A design must be:
– Functional- fill a need or customer expectation
– Safe- not hazardous to users or bystanders
– Reliable- conditional probability that product will perform
its intended function without failure to a certain age.
– Competitive- contender in the market
– Usable- accommodates human size and strength
– Manufacturable- minimal number of parts and suitable for
production
– Marketable- product can be sold and serviced

Design Process Actions
• Conceive alternative solutions
• Analyze, test, simulate, or predict
performance of alternatives
• Choose the “best” solution
• Implement design

Design is…
• An innovative and iterative process
• A communication intensive activity
• Subject to constraints

Standard Design Process
 The complete design process from
start to finish, is often outlined as in
the figure.
 Begins with an identification of need
and a decision to do something about
it.
 After many iterations, the process
ends with the presentation of the
plans for satisfying the need.
 Several design phases may be
repeated throughout the life of the
product.

Phase of the Design Process
 Identification of need generally starts the design process. The need may
only be a vague discontent, a feeling of uneasiness, or a sense that
something is not right.
 The definition of problem is more specific and must include all the
specifications for the object that is to be designed.
 The synthesis of a scheme connecting possible system elements is
sometimes called the invention of the concept or concept design. This is
the first and most important step in the synthesis task.
 Analyses must be performed to assess whether the system performance is
satisfactory.
 Synthesis, analysis and optimization are intimately and iteratively
related.
 Evaluation is the final proof of a successful design and usually involves the
testing of a prototype in the laboratory.
 Presentation is a selling job.

1. Recognition of need
• The process of designing begins when there is a need.
• Wherever there are people there are problems needing
solutions. In some cases the designer may have to
invent a product. An example might be a game for
blind persons.
• At other times the designer may change an existing
design. (If the handle of a pot becomes too hot to
touch, it must be redesigned.)
• Designers also improve existing products. They make
the product work even better. Could the chair in the
waiting room of a bus or train station be altered so that
waiting seems shorter?

2. The Definition of the problem
• It involves thorough specification of items to be designed.
i.e. what is to be designed.
• Specification includes physical & functional
characteristics, cost, quality, and operating performance.
• The problem definition cannot be vague. Some examples
of need and problem definition are listed below:
• Need: The handle of a pot becomes too hot to hold when
the pot is heated.
• Prob. Definition Design a handle that remains cool when
the pot is heated.
• Need: Travel time in a bus or train station seems too long.
There is nothing to do.
• Prob. Definition Modify the seats so that a small
television can be attached.

3. Synthesis
• In synthesis you must write down all the information you think you
may need. Some thing to consider are the following:
• FUNCTION: A functional object must solve the problem described
in the problem definition. The basic question to ask is : "What,
exactly, is the use of the article?"
• APPEARANCE: How will the object look? The shape, color, and
texture should make the object attractive.
• MATERIALS: What materials are available to you? You should
think about the cost of these materials. Are they affordable? Do they
have the right physical properties, such as strength, rigidity, color,
and durability?
• CONSTRUCTION: Will it be hard to make? Consider what
methods you will need to cut, shape, form, join, and finish the
material.
• SAFETY: The object you design must be safe to use. It should not
cause accidents.

4. Analysis & Optimization
• synthesis and analysis and optimization are
intimately and iteratively related.
• Both analysis and optimization require that we
construct or devise abstract models of the system
that will admit some form of mathematical
analysis.
• We call these models mathematical models.
• In creating them it is our hope that we can find
one that will simulate the real physical system
very well.

Developing alternative solutions
– You should produce a number of solutions. It is very important
that you write or draw every idea on paper as it occurs to you.
This will help you remember and describe them more clearly. It is
also easier to discuss them with other people if you have a
drawing.
– These first sketches do not have to be very detailed or accurate.
They should be made quickly. The important thing is to record all
your ideas. Do not be critical. Try to think of lots of ideas, even
some wild ones. The more ideas you have, the more likely you
are to end up with a good solution.

Choosing a solution
– You may find that you like several of the solutions. Eventually,
you must choose one. Usually, careful comparison with the
original design brief will help you to select the best.
– You must also consider:
• Your own skills.
• The materials available.
• Time needed to build each solution.
• Cost of each solution.
– Deciding among the several possible solutions is not always
easy. Then it helps to summarize the design requirements and
solutions and put the summary in a chart. Which would you
choose? In cases like this, let it be the one you like best.

5. Design Evaluation
– Testing and evaluating answers three basic questions:
• Does it work?
• Does it meet the problem definition?
• Will modifications improve the solution?
– The question "does it work?" is basic to good design. It has
to be answered.
– This same question would be asked by an engineer
designing a bridge, by the designer of a subway car, or by
an architect planning a new school.
– If you were to make a mistake in the final design of the
pencil holder what would happen? The result might simply
be unattractive. At worst, the holder would not work well.
Not so if a designer makes mistakes in a car's seat belt
design. Someone's life may be in danger!

• MODELS AND PROTOTYPES
– A model is a full-size or small-scale simulation of an
object. Architects, engineers, and most designers use
models. Models are one more step in communicating an
idea. It is far easier to understand an idea when seen in
three-dimensional form. A scale model is used when
designing objects that are very large.
– A prototype is the first working version of the designer's
solution. It is generally full-size and often handmade. For a
simple object such as a pencil holder, the designer probably
would not make a model. He or she may go directly to a
prototype.

6. Presentation
• Documentation of design by drawing
• Material specifications
• Assembly list i.e., the creation of design
database.

Application of
computers to
design process
Geometrical
modelling
Engineering
analysis
Evaluation
Design review
& evaluation
Automated
drafting
Analysis &
optimization
Definition of
problem
Recognition
of need
Synthesis
Design
processes
CAD
Presentatio
n

Design Considerations
 Functionality
 Strength/stress
 Distortion/deflection/stiffness
 Wear
 Corrosion
 Safety
 Reliability
 Manufacturability
 Utility
 Cost
 Friction
 Weight
 Life
 Noise
 Styling
 Shape
 Size
 Control
 Thermal properties
 Surface
 Lubrication
 Marketability
 Maintenance
 Volume
 Liability
 Remanufacturing/resource
recovery

Design Engineer’s Responsibilities
 In general, design engineering is required to satisfy the
needs of customers (management, clients, consumers, etc.)
and is expected to do so in a competent, responsible,
ethical, and professional manner.
 Careful attention to the following action steps will help you to
organize your solution processing technique.
 Understand the problem.
 Identify the known.
 Identify the unknown and formulate the solution strategy.
 State all assumption and decision.
 Analyze the problem.
 Evaluate your solution.
 The design engineer’s professional obligations include
conducting activities in an ethical manner.

Standards and Codes
1. A standard is a set of specifications for parts, materials, or processes
intended to achieve uniformity, efficiency, and a specified quality.
1. A code is a set of specifications for the analysis, design, manufacture,
and construction of something.
1. All of the organizations and societies listed below have established
specifications for standards and safety or design codes.
 American Welding Society (AWS)
International Bureau of Weights
and Measures (BIPM)
 International Standards
Organization (ISO)
 National Institute for Standards
and Technology (NIST)
 Society of Automotive Engineers
(SAE)
 American Bearing Manufactures
Association (ABMA)
 British Standards Institute (BSI)
 Industrial Fasteners Institute (IFI)
 Institution of Mechanical
Engineers (I.Mech.E)

Economics
 The consideration of cost plays an
important role in the design decision
process.
 The use of standard or stock sizes is a
first principle of cost reduction.
 Among the effects of design
specifications on costs, tolerances are
perhaps most significant.
 When two or more design
approaches are compared for cost,
there occurs a point corresponding to
equal cost, which is called the
breakeven point.

Stress and Strength
 The survival of many products depends on how the
designer adjusts the maximum stresses in a component to
be less than the component’s strength at specific locations
of interest.
 Strength is a property of a material or of a mechanical
element. The strength of an element depends on the choice,
the processing of the material.
 Stress is a state property at a specific point within a body,
which is a function of load, geometry, temperature, and
manufacturing processing.
 We shall use the capital letter S to denote strength, the
Greek letters σ (sigma) and τ (tau) to designate normal and
shear stresses, respectively.

Spring Stiffness
F
F
Δx
where
k = spring constant
Δ x = spring stretch
F = applied force
F = k (Δx)
Compression
spring
Tension
spring

Stiffness : Solid Bar
 k (stiffness) = F/δ
 E = Stress/Strain =(FLo)/(Aδ)
 k = (AE)/Lo
E (steel) = 30 x 106
psi
E (Al) = 10 x 106
psi
E (concrete) = 3.4 x 103
psi
E (Kevlar, plastic) = 19 x 103
psi
E (rubber) = 100 psi
Tensile
Load (F)
Fixed
End
Initial Length (Lo)
δ
Final Length (Lf)

Concept of Area Moment of Inertia
 The Area Moment of Inertia is an important parameter in
determine the state of stress in a part (component, structure),
the resistance to buckling, and the amount of deflection in a
beam.
 The area moment of inertia allows you to tell how stiff a
structure is.
 The Area Moment of Inertia, I, is a term used to describe the
capacity of a cross-section (profile) to resist bending. It is always
considered with respect to a reference axis, in the X or Y
direction. It is a mathematical property of a section concerned
with a surface area and how that area is distributed about the
reference axis. The reference axis is usually a centroidal axis.

Mathematical Equation for Area
Moment of Inertia
Ixx = ∑ (Ai) (yi)2
= A1(y1)2
+ A2(y2)2
+ …..An(yn)2
A (total area) = A1 + A2 + ……..An
X X
Area, A
A1
A2
y1
y2

Moment of Inertia Equations for
Selected Profiles
 (d)4
64
I =
Round solid section
Rectangular solid section
b
hbh31
I = 12
b
h
1
I = 12
hb3
d Round hollow section

64
I = [(do)4
– (di)4
]
do
di
BH3
-
1
I = 12
bh31
12
Rectangular hollow section
H
B
h
b

Example – Optimization for Weight & Stiffness
Consider a solid rectangular section 2.0 inch wide by 1.0 high
I = (1/12)bh3
= (1/12)(2)(1)3
= .1667 , Area = 2
.
(.1995 - .1667)/(.1667) x 100= .20 = 20% less deflection
(2 - .8125)/(2) = .6 = 60% lighter
Compare the weight of the two parts (same material and length), so
only the cross sectional areas need to be compared.
I = (1/12)bh3
= (1/12)(2.25)(1.25)3
– (1/12)(2)(1)3
= .3662 -.1667 = .1995
Area = 2.25x1.25 – 2x1 = .8125
So, for a slightly larger outside dimension section, 2.25x1.25 instead of 2 x 1,
you can design a beam that is 20% stiffer and 60 % lighter
2.0
1.0
Now, consider a hollow rectangular section 2.25 inch wide by 1.25 high by
.125 thick.
H
B
h
b
B = 2.25, H = 1.25
b = 2.0, h = 1.0

Stiffness for Different sections
Rectangular
Horizontal
Square
Box
Rectangular
Vertical

Material Strength
Standard Tensile Test
Standard Specimen
Ductile Steel (low carbon)
Sy – yield strength
Su – fracture strength
σ (stress) = Load / Area
ε (strain) = (change in length) / (original length)
1 ksi=6.895 MPa

Mi 291 chapter 1 (introduction)

Mechanical Properties
 Yield strength (SY): stress level
upto which no permanent
deformation on unloading.
 Ultimate stress (SU): Fracture
stress.
 Modulus of elasticity (E): Slope
of stress/strain curve in elastic
region.
 Ductility : plastic deformation before fracture.
% Elongation (ductility) = (Lfrcature – Linitial)/Linitial
 Resilience : Capacity of a material to absorb energy within the elastic region
(Elastic area).
 Toughness : Capacity of a material to absorb energy without fracture (Total
area).

Uncertainty
1. Examples of uncertainties concerning stress and strength
include
Composition of material and the effect of variation on properties.
Variations in properties from place to place within a bar of stock.
Effect of processing locally, or nearby, on properties.
Effect of nearby assemblies such as weldments and shrink fits on stress conditions.
Effect of thermomechanical treatment on properties.
Intensity and distribution of loading.
Validity of stress concentrations.
Influence of time on strength and geometry.
Effect of corrosion.
Effect of wear.
Uncertainty as to the length of any list of uncertainties.
2. Engineers must accommodate uncertainty.
The design factor nd is defined as

• Since stress may not vary linearly with load
using load as the loss-of-function parameter
may not be acceptable. It is more common
then to express the design factor in terms of a
stress and a relevant strength.

Dimensions and Tolerances
 Normal size : size used for general description
 Limits. The stated maximum and minimum dimensions for proper
functioning of the component.
 Tolerance : is the allowable deviation for any given size to achieve a
proper function.
Unilateral tolerance. The basic dimension is taken as one of the limits,
and variation is permitted in only one direction
Bilateral tolerance. The variation in both directions from the basic
dimension, i.e. 25 ± 0.05mm
BasicSize
BasicSize
LowerLimit
UpperLimit
UpperLimit
LowerLimit

Fits
Fit : An assembly condition between hole and shaft
Clearance fit : largest permitted shaft diameter is less than the smallest hole
diameter, so that shaft can rotate or slide.
Interference fit : negative clearance exists between the sizes of holes and shaft.
Minimum permitted diameter of the shaft is larger than the maximum allowable
diameter of the holes. Members are intended to be permanently attached.
Transition fit : diameter of the largest allowable hole is greater than the
smallest shaft, but the smallest hole is smaller than the largest shaft, such that a
small positive or negative clearance exists between the shaft and hole.

Units
 In the symbolic units equation for Newton’s second
law, F=ma. Units chosen for any three of these
quantities are called base units.
 The International System of Units (SI) is an absolute
system. The base units are the meter, the kilogram (for
mass), and the second.

Significant Figures
 The number of significant figures is usually inferred by
the number of figures given (except for leading zeros).
For example, 706, 3.14, and 0.00219 are assumed to be
numbers with three significant figures.
 Computers and calculators display calculations to many
significant figures. However, you should never report a
number of significant figures of a calculation any greater
than the smallest number of significant figures of the
numbers used for the calculation.
 For example, determine the circumference of a solid
shaft with a diameter of d=11mm. The circumference is
given by C=πd. Since d is given with two significant
figures, C should be reported with only two significant
figures.

Mi 291 chapter 1 (introduction)

More Related Content

What's hot (13)

Viewers also liked (6)

Similar to Mi 291 chapter 1 (introduction) (20)

More from varun teja G.V.V (11)

Mi 291 chapter 1 (introduction)