Namas ChandraIntroduction to Mechanical engineering Chapter 4-1

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CHAPTER 5

Engineering Design Tools

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Objectives

Sections

5.1 Estimation

5.2 Significant Figures, Dimensioning, and Tolerance

5.3 Prototyping

5.4 Reverse Engineering

Examine the role of computer in engineering designLearn when and when not to use the computer.Examples of computer use for analysis, data collection and real-time control

5.5 Computer Analyses

5.7 Spreadsheets

5.8 Solid Modeling and CAD

(in conjunction with ME Tools)

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5.1 Estimation

A simple ‘back of the envelope’ calculation is importance to validate the soundness of design ideas.

Anytime you change design, you need to do this-of course full calculations are needed at later stages.

Method 1:

Weight of the Vehicle

-

N =(0.9 kg)(10 )=9Nkg

Energy required=

= (9 N)(0.9 M)

=8 Joules

y

y

F mg

F D

E F y

-1

Method 2:

Angle of the Slope

Height 90cm = cos = =

Path length 150cm

=0.6

θ = cos 0.6 53

Potential Energy

E = (9 ) (1.5 ) (cos 53 )

= 8 Joules

F S N M

Namas ChandraIntroduction to Mechanical engineering Chapter 4-4

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5.1 Estimation-2

mech

elec

Mechanical Power

E 8J P 1.1watts

t 7S Electrical Power

P = V I

P 1.1w I = 120mA

V 9vIt is preferable to have two 9v batteries since each supplies only 100 mA

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5.2 Significant Figures, Dimensioning and Tolerance

Accuracy of a number is specified by significant figures. Definition of significant digits

1. All non-zero digits (1-9) are significant2. Zeroes that have ANY non-zero digit to the LEFT are significant

zeroes3. All other zeroes not covered by rule 2 are NOT significant.

Example: 22.302 has 5 significant figures

0.004000 has 4 significant figures (zeroes to the left of 4 are not significant

1.0000E06 has 5 significant figures. (use scientific notations) Product of a sequence of numbers will have the same

significant figures as the least of the factors.Example: 1.23*22.45*0.1375= not 3.79685625 =3.80

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5.2 Adding and Subtracting -2

Precision of a number is given by the number of significant digits after the decimal.

The rule expresses that the answer should have so many digits to the RIGHT of the decimal instead of focusing in on the number of significant digits as the other rule did.

The focus is upon the number of positions to the RIGHT of the decimal

THE SUM OR DIFFERENCE CAN BE NO MORE PRECISE THAN THE LEAST PRECISE NUMBER INVOLVED IN THE MATHEMATICAL OPERATION.

Example: 160.45 + 6.732 =167.18 (2 positions to the right of decimal)

45.621 + 4.3 - 6.41=43.5

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5.2 Accuracy

Accuracy depends on the number of digits beyond the decimal point.

Number

128.7 0.1

0.40 0.01

5.2 0.1

Accuracy

Tolerance shown in the figure indicates to the machinist the accuracy to which the various dimensions need to be maintained.

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5.3 Prototyping

After many initial revisions and computer simulations, you arrive at a final configuration.

At this stage, you are ready to construct a prototype. Prototype is a mock-up of the final product

that contains all the salient features that omits all the non-essential elements

Rapid Prototyping for testing/verification

A model of a foot for analysis

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5.4 Reverse Engineering

In reverse engineering, engineer dissects a working product to examine the details.

Allows to study the functions of each of the component as built and used.

Very useful in failure analysis of products The detailed study is used

to replicate a competitor’s product with modification understand the working for improvement to analyze and avoid patent violation

Documentation greatly helps reverse engineering

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5.4 Computer analysis

At this stage, you do detailed analysis of the prototype and simulate the system

You can use general engineering programs like Pro-Engineer Abaqus Solid Works Ansys

Or mathematical programs like Matlab Mathcad

Or, use direct programming using C, C++, JAVA or Fortran to build models.

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5.7 Spreadsheets in Design

Excel and Lotus are examples of spreadsheets useful in design. If the system is complex (as most of them are!!) spreadsheets can

be used to study the effect of one variable on another. For example if we are interested in locating the center of gravity

of our vehicle very close to the center, when we are still choosing the various components, we can use spreadsheets.

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5.7 Spreadsheets in Design Since the weights and locations are known then we can use

simple formulae to locate the C.G. With this table now, you

can alter the location or/and add/delete parts still to suit your goal of C.G.

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CHAPTER 5…concludes

Engineering Design Tools