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

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Design : Aesthetic vs. Engineering

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What is Common Among These ?

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

h bh3 1 I =

12

b

h

1 I =

12 hb3

d Round hollow section

64 I = [(do)4 – (di)

4]

do

di

BH3 - 1

I =

12 bh3 1

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

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

Bas

ic S

ize

Bas

ic S

ize

Lo

wer

Lim

it

Up

per

Lim

it

Up

per

Lim

it

Lo

wer

Lim

it

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.