ME F241:

Design of Machine Elements

Semester II, 2013-14

Introduction to

Mechanical Engineering Design

Instructor-In-Charge:

Dr. Srinivasa Prakash Regalla

What is Mechanical Engineering Design?

To design is to either formulate a plan for the

satisfaction of a specified need or to solve a

specific problem

Design is an innovative and highly iterative process

Design is a communication-intensive activity

Design

CREATIVENESS

COMMUNICATIVE ABILITY

PROBLEM SOLVING SKILL

KNOWLEDGE OF TECHNOLOGY

Engineering Tools (mechanics of solids and fluids, mass and momenturm

transport, manufacturing processes, mathematics, statistics, electrical

and information theory, engineering graphics, computers, Languages)

PLAN

A PRODUCT

that is:

Functional,

safe,

Reliable,

Competitive,

Usable,

Manufacturable,

and

Marketable

Implementation

The Mechanical Design Process: Different phases and interactions

DESIGN CONSIDERATIONS

READ: DESIGN FOR

DESIGN TOOLS AND RESOURCES

CAD (ProEngineer, AutoCAD, Unigraphics,

Solidworks) and Rapid Prototyping (SLA, LOM,

SLS, FDM, LENS, Objet-Polyjet)

CAE: Finite Element Analysis (Algor, Ansys,

MSC/Nastran, LS-DYNA, Hyperworks,

COMSOL, ABAQUS)

Non-engineering-specific computer tools: word

processing, spreadsheets (WORD, Excel, Lotus,

OpenOffice)

Mathematical Solvers (MathCad, Maple,

MATLAB, Mathematica, TKSolver)

DESIGN ENGINEERS RESPONSIBILITY

PLEDGE (From National Society of Professional Engineers, USA):

STANDARDS AND CODES

ECONOMICS: cost of manufacturing, as can be affected at the design stage

Standard sizes

Preferred numbers

Large tolerances

Breakeven points

Cost estimation

Safety and Product Liability

The concept of strict liability (used in the USA)

Design engineers should review the

advertisements, make suitable amendments with

warnings, instructions etc.

Stress and Strength

Strength is a property of the material of a

mechanical component

Stress is a state property at a specific point within a

body, which is a function of:

Load

Geometry

Temperature

Manufacturing processing

Uncertainty and Design Factor

Factor of safety, n, is another number calculated

using the same equation but after the preferred

numbers are chosen, hence n may differ from nd

When yielding is treated as failure:

Maximum allowable load = Yield strength/nd

When rupture or fracture is treated as failure:

Maximum allowable load = Ultimate tensile strength/nd

Design Engineers Professional Responsibilities: As important as anything

else in this course

Understand the problem

Identify the knowns

Identify the unknowns and formulate the solution

strategy

State all assumptions and decisions

Analyze the problem

Evaluate your solution

Present your solution

PRESENT YOUR SOLUTION

Your communication skills are important here

Here you are selling your technical abilities

All your work may be misunderstood unless you

skillfully present

Know you audience

The Designers Communication Skills

How to develop?

Keep a neat and clean log book of your activities of

the project/course

Enter frequently in the log book with dates

Separate logbooks should be used for different

projects/courses

Logbook is the only reference you have to explain

when questioned about your decisions

Materials

Material selection for a component while designing

is governed by:

Stress and deflection

Aesthetics

Merely to fill up the space

Corrosion resistance

Temperature concerns

Material Strength and Stiffness

Simple tension test to

determine E, y, u

fracture strain (f) and

percentage elongation.

The point a corresponds to the 0.2 percent permanent

deformation. The strength is called

as proof strength.

True stress-strain diagram

Hardness

The resistance to penetration/indentation is called

as hardness

Hardness Tests: Brinell, Rockwell, Vickers, Knoop

All tests involve indentation but with different

types of indenters with different loads

For steels, minimum Sut for BHN between 200 and

450 is:

Sut = 3.41(BHN) MPa

For cast irons, minimum Sut is:

Sut = 1.58(BHN) 86

Impact Testing for Toughness

Charpy and Izod Tests

Toughness is desirable for taking tensile and

bending loads

Effect of strain rate on mechanical properties

Temperature effect on Strength

Effect of Manufacturing Process on Strength

MATERIAL FAMILIES AND CLASSES

MATERIAL FAMILIES AND CLASSES

YOUNGS MODULUS OF VARIOUS MATERIALS

MATERIAL INDEX

Load and Stress Analysis

Free-body diagram of

component and equillibrium

equation

SHEAR FORCE AND BENDING MOMENT