A BACKGROUND TO ENGINEERING DESIGN Other Macmillan titles of related interest Introduction to Engineering Materials V. B. John Understanding Phase Diagrams V. B. John Analysis and Presentation of Experimental Results R. H. Leaver and T. R. Thomas Production Engineering Technology, Second Edition J.D. Radford and D. B. Richardson Mechanical Engineering Design, Second Edition G. D. Redford 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