Table Of ContentA BACKGROUND TO ENGINEERING DESIGN
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A BACKGROUND TO
ENGINEERING DESIGN
PETER POLAK
Department ofM echanical Engineering,
University of Sheffield
M
©Peter Polak 1976
All rights reserved. No part of this publication
may be reproduced or transmitted, in any form
or by any means, without permission.
First published 1976 by
THE MACMILLAN PRESS LTD
London and Basingstoke
Associated companies in New York Dublin
Melbourne Johannesburg and Madras
SBN 333 18771 7
ISBN 978-0-333-18771-5 ISBN 978-1-349-02707-1 (eBook)
DOI 10.1007/978-1-349-02707-1
Text set in 10/11 pt. IBM Press Roman,
printed by Photolithography
and bound in Great Britain at
The Pitman Press, Bath
This book is sold subject to the standard
conditions of the Net Book Agreement.
The paperback edition of this book is sold subject to the
condition that it shall not, by way of trade or otherwise, be
lent, re-sold, hired out, or otherwise circulated without the
publisher's prior consent in any form of binding or cover other
than that in which it is published and without a similar condition
including this condition being imposed on the subsequent purchaser.
Contents
Preface vii
1. Introduction and some Ergonomics 1.
Outlines the scheme of thoughts which constitute the design of a machine
or product; particular attention is given to ergonomic aspects.
2. Loads and Structures 7
Discusses the optimum shape of a structure in relation to the loads it has to
bear.
3. How Strength can Fail 14
Goes into detailed considerations of strength, referring particularly to
buckling and fatigue problems; lessons are drawn from some recent failures.
4. Stable and Unstable Systems 25
Describes inherent stability and some useful unstable systems; a control
system is described qualitatively.
5. Some Motions 29
Refers to kinematics, linkages and cams as commonly used in engineering,
particularly in vehicles.
6. Materials in Outline 40
Discusses materials in general terms and describes the properties of some
widely used ones; attention is drawn to brittle fracture of steels and to
corrosion.
7. Static Joints 49
Concentrates on welded, riveted and other static joints, with attention to
static and fatigue strengths, permissible stress levels, discontinuity effects,
kink plates, etc.
8. Bearings and Seals 65
Deals mainly with rolling and sliding bearings (hydrodynamic and hydro
static types) and fluid seals.
9. Damping, Mountings and Vibration 79
Discusses mountings, vibrations, damping, etc., and briefly refers to noise.
10. Some Points on Manufacture and Appearance 86
Considers design aspects of manufacturing methods, briefly describing some
of the less familiar ones and concludes with a short section on visual design
aspects.
Appendix 100
list of symbols; Twist-bend buckling; Unsymmetrical sections;
Behaviour of bolted joints; Bellows expansion joints; Some useful
theorems.
References 111
Index 115
Preface
'Design' is a popular expression with varying implications: wallpaper design
differs from dress design, 'industrial design' differs from engineering design. Some
journals go so far as to use the word design for the external aspects of a machine,
calling the insides, a little airily, the 'engineering'; for example, a teleprinter was
described as designed by a designer particularly well known for elegant table
ware, the makers of the works were mentioned a long way further down in the
'credits'. Engineers iri turn insist that the word should refer almost entirely to the
works, or in simple cases to the stressing.
This semantic difference is best resolved by those engineers who themselves
display a strong sense for the appearance of a product and are prepared to recog
nise that those trained mainly in the visual arts and relatively free from mechanical
habits can have something refreshing to offer.
The present book is concerned almost entirely with functional aspects; indeed
the author feels that not only in machinery should the externals generally take
their place with all the other considerations; for instance in light-fittings, efficiency
and styling can be at loggerheads. Only where the function is simple and sufficient
strength easily provided, as in furniture, can external design take precedence.
Occasionally the function and ease of making actually suggest a happy shape.
Engineering design at its most restricted is finding the right thickness for a part
when shape, function, loading and material are pre-decided. As will be seen later,
even this is not always easy. Higher levels of elaboration are reached in 'shopping
list' design, finding the most economical and/or versatile process plant or produc
tion line consisting of standard but expensive items of equipment. The most
creative design activity, starting from basics, is also the most demanding if it is
not to consist of repeating old mistakes along with inventing a few new ones.
This book does not supersede any established manuals but brings together key
points from the past and more recent data. The references include items dating
back seventy years but the majority are only a few years old.
To avoid items like 'this is a bolt, this is a clamp, this is a keyway', familiarity
with the names and uses of basic mechanical components is assumed, as is a know
ledge of basic stress and strain relations appropriate to first- or second-year under
graduates in engineering.
To reduce tediousness, extensive explanations have been avoided; it is felt that
the intended reader is better served by erring on the side of brevity, leaving room
for thought yet, it is hoped, no room for misunderstanding.
Standard drawing conventions have been varied slightly in the interests of
clarity; each figure should be considered independently. Generous use of shading,
though unnecessary for many, should help to minimise uncertainties for those
students who are unfamiliar with machinery details.
To avoid irritating brackets, SI units are used generally, though occasionally
Imperial units are shown. For stresses, a convenient set of figures to relate the
more common units together is the ultimate strength of a low-carbon steel,
28 tons/in.2 ::::::::400 MN/m2 :::::::: 60 000 p.s.i.(= lbf/in.2).
Apologies for digressions, approximations, simplification of facts and verbal
short-cuts are tendered here and now, in bulk.
The author gratefully acknowledges his debt to all those who taught him or
provided opportunities for learning and experience.
Thanks are due to Sheffield University for the help received from the Applied
Science library, the photographic section, the workshops and laboratories of the
Mechanical Engineering Department and for a certain amount of clerical assistance;
also to Sheffield City Library which was found to form a most useful complement
to the University library.
1
Introduction and some
Ergonomics
Design is not quite the same activity as inventing. New design elements can come
from scientific discoveries, from patient experimentation, from chance observa
tions relevant to a current need. Examples of each are the transistor, the electric
light bulb, the Bessemer converter and the Mannesmann tube-piercing method.
In the case of the electric light bulb the design concept came first, derived from
logical use of scientific principles, followed by tests on likely filament materials
to find which had the most suitable combination of properties.
Bessemer's converter came about by the opposite sequence. When melting
down some pig-iron Bessemer noticed an unmelted pig at the edge of the hearth;
prodding it with an iron rod he found it was hollow. From his knowledge of the
melting points of pig iron and steel he suspected that the outer skin of this piece
had been converted to steel by the flames preferentially removing the carbon, as
in the older Osmond-iron process. Further investigations enabled him to devise
an apparatus to carry this out in an organised manner.
Mannesmann's method is said to have started over dinner; while waiting for his
soup to cool and rolling a piece of bread between his fingers he unexpectedly pro
duced a hole, the high shear gradient at the centre of the oval producing a fracture.
He had the imagination to suppose that the same might apply to steel and to
devise (invent and design) the crossed-roll system which imitates the finger action
on a continuous basis.
We cannot all be great inventors, nor do we need to be; the last two hundred
years have provided a vast reservoir ofideas freely available for the betterment of
mankind, leaving us a task less glorious but no less important and no less difficult
of selecting elements for our purpose. This purpose, need it be said, is to serve
the public; it usually should also include profit for our employer. Good design
achieves both.
The basic parts of a design can be summed up as strength, function and
economy. These, taken in a broad sense, embrace all the features discussed in
1
