Table Of ContentConceptual
Design
for Engineers
Second Edition
M. J. French MA MSc FIMechE
Professor of Engineering Design
University of Lancaster
I
DETS!UI.G N
Springer-Verlag Berlin Heidelberg GmbH ...
COUNCIl
Conceptual Design For Engineers
Second Edition
© Springer-Verlag Berlin Heidelberg 1985
Originally published by Springer-Verlag Berlin Heidelberg New York Tokyo in 1985
Softcover reprint of the hardcover 2nd edition 1985
First edition published 1971
as Engineering Design: The Conceptllal Stage
by Heinemann Educational Books Ud, London
AII rights reserved. No part of this publication
may be reproduced, stored in a retrieval
system, or transmitted, in any form or by any
means, electronic, mechanical, photocopying,
recording or otherwise, without the prior
permission ofThe Design Council.
Typeset by Sunrise Setting, Torquay
British Library CIP Data
French, Michael
Conceptual design for engineers
1. Engineering design
r. Title
620' .00425 TA 174
ISBN 978-3-662-11366-0 ISBN 978-3-662-11364-6 (eBook)
DOI 10.1007/978-3-662-11364-6
Contents
Preface to Second Edition Vll
Preface to First Edition V111
Units and symbols IX
Questions IX
1 Introduction
1.1 Design: conceptual design: schemes
1.2 The anatomy of design
1.3 The scope and nature of design methods 4
1.4 Insight 5
1.5 Diversification of approach 6
1.6 Reduction of step size 7
1.7 Prompting of inventive steps 7
1.8 Generation of design philosophies 8
1.9 Increasing the level of abstraction 8
1.10 Design and the computer 10
2 Combinative ideas
2.1 Introduction 13
2.2 Construction of tahles of options: functional analysis 14
2.3 Functional analysis: axial flow compressor rotor 15
2.4 Parametric mapping of viable options 17
2.5 Liquid natural gas tanker: alternative configurations 23
2.6 Further examples of combinative treatments 25
2.7 Elimination procedures for tables of options 28
2.8 Otherwaysofreduction 30
2.9 Synthesis from tables of options: kernel tables 32
2.10 Evolutionary techniques: hybrids 33
2.11 An example: wave energy converters 34
2.12 Evolutionary techniques: redistribution of functions 40
2.13 Repechage 43
2.14 Combinative ideas: general remarks 44
Questions 45
Notes 45
11\
3 Optimisation
3.1 Introduction 49
3.2 Making capital and running costs commensurate 49
3.3 Optimum speed of a tanker 50
3.4 The optimisation of the sag:span ratio of a suspension bridge 52
3.5 Optimisation with more than one degree of freedom: heat exchanger 55
3.6 Putting a price on heat-exchanger performance 57
3.7 Variation of costs with application 59
3.8 Further aspects of heat-exchanger optimisation 59
3.9 An elementary programming problem 60
3.10 Classification of optimisation problems and methods of solution 62
3.11 The design of rotating discs: an optimum structure 66
3.12 Hubdesign 73
3.13 Summary 73
Questions 73
Answers 74
4 Insight
4.1 Introduction 76
4.2 Rough calculations 76
4.3 Optimisation of compressor shaft diameter 83
4.4 The optimum virtual shaft: a digression 85
4.5 Useful measures and concepts 87
4.6 Bounds and limits 91
4.7 Scale effects 94
4.8 Dimensional analysis and scaling 98
4.9 Proportion 99
4.10 Change of viewpoint 100
Questions 102
Answers 104
5 Matching
5.1 Matching: the windlass 107
5.2 An extended example of matching: ship propulsion 107
5.3 Matching within a single machine III
5.4 Further aspects of ship propulsion 112
5.5 Specific speeds: degrees of freedom 113
5.6 Matching of a spring to its task liS
5.7 Matching in thermodynamic processes 117
5.8 Two old cases of matching 121
5.9 Carhandbrake 123
5.10 General remarks on matching problems 126
Questions 126
Answers 129
IV
6 Disposition
6.1 Problems of disposition 132
6.2 Dovetail fixing 132
6.3 Splined shaft 133
6.4 Disc brake caliper: breaking logical chains 134
6.5 Alternator rotor 136
6.6 Structures: the feather 137
6.7 Structures: form design 139
6.8 Joints 141
6.9 The design of gear teeth 142
6.10 Disposition of allowable stress 146
6.11 Electromagnets 146
Questions 147
Answers 149
7 Kinematic and elastic design
7.1 Introduction 153
7.2 Spatial degrees offreedom: examples of kinematic design 153
7.3 Elastic design and elastic pairs 160
7.4 Comparison of kinematic and elastic means 166
7.5 Elastic design and structural economy 169
Questions 171
Answers 174
8 Costs
8.1 Design and costs: tasks, values and assets 178
8.2 Tasks and values in gearing 180
8.3 Gears for contra-rotating propellers 183
8.4 Large pressure vessels for gas-cooled reactors 184
8.5 Other cases 185
9 Various principles and approaches
9.1 Introduction 187
9.2 Avoiding arbitrary decisions: degrees of freedom of choice 187
9.3 Mathematical models 189
9.4 The search for alternatives 192
9.5 Logical chains 193
9.6 Past practice and changed circumstances 195
9.7 Brainstorming 196
9.8 Use of solid models 196
9.9 Some maxims for designers 197
Ouestions 200
Answers 201
v
10 Conclusion
10.1 Resume 203
10.2 Checking and evaluation 203
Ouest ions 205
Answers 210
References 219
Index 222
vi
Preface to the Second Edition
At the time this book first appeared the importance of design in engineering
education was not widely accepted: above all, it was often seen as a thing apart, to
be taught without close links with the analytical parts of the course. Attitudes have
since changed, and it is now recognised that design should have a central and
integrating role: as it was put in the Design Council report on Engineering Design
Education (The Moulton Report), 'Engineering should be taught in the context of
design, so that design is a continuous thread running through the teaching of
undergraduate engineering.' This view has since been widely endorsed, most
recently in the Report of the Working Party of the Science and Engineering
Research Council on Engineering Design (The Lickley Report).
It is this climate of opinion above all that makes it seem timely to re-issue this
book, which should be an ideal text for second and third year mechanical
engineering students. Showing as it does how relatively simple engineering
science, along with commonsense considerations, dominates the early or
conceptual stages of design, it can help them achieve the kind of thinking, synthetic
rather than analytical, that distinguishes the engineer. Staff, too, will find the book
a source of ideas for mind-stretching exercises which are, however, still within the
scope of students with little practical experience. The numerical or algebraic nature
of many of the examples helps to dispel the idea that design is lacking in rigorous
lines of thought.
The opportunity has been taken in this new edition to make various changes,
chiefly the substitution for old examples of ones which are more recent, of wider
interest or simply easier to follow. Some material of doubtful value has been
dropped, and a few new ideas have been added, particularly some maxims, which
encapsulate much of what is said elsewhere.
M. J. French
Lancaster July 1984
vii
Preface to the First Edition
This book is about the early or conceptual stages of design-more specifically
engineering design, hut many of the ideas put forward are equally applicable to all
kinds of functional design and so will be of interest to industrial designers and
others.
Much of the book requires little or no engineering knowledge, and all of it is
within the grasp of a second-year engineering student. The demands made on the
reader's knowledge are small, but those made upon his intelligence are less so.
It is hoped that reading this book and wrestling with the problems in it will he of
great benefit to young engineers, not only those who later hecome designers but
also those who will be involved in creative engineering in other roles, such as
research and development, or who will supply specialist functions like stress
analysis in support of design. It should also help those who, as managers, will have
to make decisions about design policy.
Further, it is felt that the practising designer, to whom most of the ideas will be at
least somewhat familiar, will find this book of interest and perhaps of value in
strengthening and refining his approach. And if one such, reading it, says here, 'I
know a better example than this,' and there, 'But hasn't he realised you can look at
such problems so?,' and sits down and writes a better hook, that will be so much the
more justification for this one.
It is not the purpose of this book to add to the reader's stock of theory or useful
facts; its purpose is to cultivate an attitude of mind, a discipline ofthought, and a set
of approaches to design problems. To this end, many peripheral topics, such as the
management of design and the relationship of design to other functions, have been
left out. Many books, ostensihly on the same subject, deal almost entirely with
these fringe activities, consigning the central issue to a hlackbox mysteriously
inscribed 'devise solutions: or something of the kind: this fault at least has heen
avoided.
While not quite a textbook, this book should make good hackground reading for
engineering students, and engineering teachers will find it contains much matter on
which exercises for design courses can be based. There are over 70 problems with
sketch solutions and notes, on a very wide range of topics.
To economise in background explanations, the same example has often been
used several times to illustrate different points. No apology should be needed for
this device, since it reflects the value in design, as in other fields, of looking at the
same thing from several points of view.
M.J.F.
Lancaster 1971
VIII
U nits and symbols
Units
Mostly SI units are used. In a few cases British units or both kinds are given. For
reference, unit abbreviations which may be unfamiliar are listed here.
A ampere N newton ( = kg m/s2)
ft foot s second
in inch W watt(=joule/s)
J joule (= newton-metre) Wb weber
K degrees Kelvin p.s.i. pounds force per square inch
kg kilogram p.s.i.a., p.s.i.g. pounds per square
kWh kilowatt hour inch absolute and gauge
Ibf pound force SHP shaft horsepower
m metre
For the convenience of those not yet familiar with the newton, 1N = 0.2248 Ibf.
Pressures and stresses are given mostly in newtons per square millimetre and
IN mm-2 = 145.0 p.s.i.
Symbols
Because of the large variety of topics and the brevity of their treatment, notation
has been introduced and explained in the text. Customary usage has been followed
in appropriate cases.
Questions
A number of questions (Q) are given at the end of each chapter, except Chapters 1
and 8, together with outline answers (A) or notes (N) on their solution. The
average standard is rather high, and it is not expected that the reader will be able to
solve many of them without the help provided. In some cases the question number
is followed by a number between 1 and 5 intended to indicate difficulty; e.g.
Q .4.5(3) indicates the fifth problem on Chapter 4 and that it is of the third order of
difficulty.
The questions are mostly intended as worked examples forming an extension of
the text, throwing new light on the subject matter and indicating ways of applying
the methods and ideas advanced, so that the reader who wishes to go more deeply
into the subject should certainly study them. The questions of Chapter 10 cover
matter from throughout the book.
ix
1 Introduction
1.1 Design: conceptual design: schemes
In this book 'design' is taken to mean all the process of conception, invention,
visualisation, calculation, marshalling, refinement, and specifying of details which
determines the form of an engineering product.
Design generally begins with a need. This need may be met already by existing
designs; in such cases the designer hopes he can meet the need better (i.e. generally
more cheaply). It ends with a set of drawings and other information to enable the
thing designed to be made. Ideally the intervening stages should be of successively
increasing precision, of gradual crystallisation or hardening. The early stages differ
in character somewhat from the later ones, largely because of the greater fluidity of
the situation. These early stages, when there are still major decisions to be made,
are called 'conceptual design' [1]. The products of the conceptual design stages will
be called 'schemes'.
By a scheme is meant an outline solution to a design problem, carried to a point
where the means of performing each major function has been fixed, as have the
spatial and structural relationships of the principal components. A scheme should
be sufficiently worked out in detail for it to be possible to supply approximate costs,
weights, and overall dimensions, and the feasibility should have been assured as far
as circumstances allow. A scheme should be relatively explicit about special
features or components but need not go into much detail over established practice.
1.2 The anatomy of design
Figure 1.1 is a block diagram showing the design process according to the writer.
The circles represent stages reached, and the rectangles represent work in
progress.
Constructing block diagrams is a fashionable pastime, especially in fields like
design where boundaries are imprecise and interactions legion, so that any ten
experts will produce ten (or a hundred). They will all be different, and all valid
'there are nine and sixty ways of constructing tribal lays, and every single one of
them is right.' They express only truisms, and yet they have a value for all that.
In this particular block diagram things that are important but outside the scope of
this book have been omitted-for example, relationships with other activities like
