net Mechanical Engineering Systems www.net Mechanical Engineering Systems Richard Gentle Peter Edwards Bill Bolton OXFORD AUCKLAND BOSTON JOHANNESBURG MELBOURNE NEW DELHI www.net Butterworth-Heinemann Linacre House, Jordan Hill, Oxford OX2 8DP 225 Wildwood Avenue, Woburn, MA 01801-2041 A division of Reed Educational and Professional Publishing Ltd A member of the Reed Elsevier plc group First published 2001 © Richard Gentle, Peter Edwards and Bill Bolton 2001 All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1P 0LP. Applications for the copyright holder’s written permission to reproduce any part of this publication should be addressed to the publishers While every effort has been made to trace the copyright holders and obtain permission for the use of all illustrations and tables reproduced from other sources in this book we would be grateful for further information on any omissions in our acknowledgements so that these can be amended in future printings. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0 7506 5213 6 Composition by Genesis Typesetting, Laser Quay, Rochester, Kent Printed and bound in Great Britain www.net Contents Series Preface vii 1 Introduction: the basis of engineering 1 1.3 Units used in this book 5 2 Thermodynamics 7 2.2 Perfect gases, gas laws, gas processes 13 2.3 Work done and heat energy supplied 24 2.4 Internal combustion engines 33 2.5 The steady flow energy equation 54 2.8 Heat transfer 101 3 Fluid mechanics 112 3.1 Hydrostatics – fluids at rest 113 3.2 Hydrodynamics – fluids in motion 135 4 Dynamics 169 4.1 Introduction to kinematics 170 4.2 Dynamics – analysis of motion due to forces 183 5 Statics 204 5.3 Stress and strain 235 5.8 Case study: bridging gaps 292 Solutions to problems 295 Index 307 www.net Series Preface ‘There is a time for all things: for shouting, for gentle speaking, for silence; for the washing of pots and the writing of books. Let now the pots go black, and set to work. It is hard to make a beginning, but it must be done’ – Oliver Heaviside, Electromagnetic Theory, Vol 3 (1912), Ch 9, ‘Waves from moving sources – Adagio.’ Oliver Heaviside was one of the greatest engineers of all time, ranking alongside Faraday and Maxwell in his field. As can be seen from the above excerpt from a seminal work, he appreciated the need to communicate to a wider audience. He also offered the advice So be rigorous; that will cover a multitude of sins. And do not frown.’ The series of books that this prefaces takes up Heaviside’s challenge but in a world which is quite different to that being experienced just a century ago. With the vast range of books already available covering many of the topics developed in this series, what is this series offering which is unique? I hope that the next few paragraphs help to answer that; certainly no one involved in this project would give up their time to bring these books to fruition if they had not thought that the series is both unique and valuable. This motivation for this series of books was born out of the desire of the UK’s Engineering Council to increase the number of incorporated engineers graduating from Higher Education establishments, and the Institution of Incorporated Engineers’ (IIE) aim to provide enhanced services to those delivering Incorporated Engineering Courses. How- ever, what has emerged from the project should prove of great value to a very wide range of courses within the UK and internationally – from Foundation Degrees or Higher Nationals through to first year modules for traditional ‘Chartered’ degree courses. The reason why these books will appeal to such a wide audience is that they present the core subject areas for engineering studies in a lively, student-centred way, with key theory delivered in real world contexts, and a pedagogical structure that supports independent learning and classroom use. Despite the apparent waxing of ‘new’ technologies and the waning of ‘old’ technologies, engineering is still fundamental to wealth creation. Sitting alongside these are the new business focused, information and communications dominated, technology organisations. Both facets have an equal importance in the health of a nation and the prospects of individuals. In preparing this series of books, we have tried to strike a balance between traditional engineering and developing technology.net The philosophy is to provide a series of complementary texts which can be tailored to the actual courses being run – allowing the flexibility for course designers to take into account ‘local’ issues, such as areas of particular staff expertise and interest, while being able to demonstrate the depth and breadth of course material referenced to a common framework. The series is designed to cover material in the core texts which approximately corresponds to the first year of study with module texts focussing on individual topics to second and final year level. While the general structure of each of the texts is common, the styles are quite different, reflecting best practice in their areas. For example Mechanical Engineering Systems adopts a ‘tell – show – do’ approach, allowing students to work independently as well as in class, whereas Business Skills for Engineers and Technologists adopts a ‘framework’ approach, setting the context and boundaries and providing opportunities for discussion. Another set of factors which we have taken into account in designing this series is the reduction in contact hours between staff and students, the evolving responsibilities of both parties and the way in which advances in technology are changing the way study can be, and is, undertaken. As a result, the lecturers’ support material which accom- panies these texts, is paramount to delivering maximum benefit to the student. It is with these thoughts of Voltaire that I leave the reader to embark on the rigours of study: ‘Work banishes those three great evils: boredom, vice and poverty.’ Alistair Duffy Series Editor De Montfort University, Leicester, UK Further information on the IIE Textbook Series is available from bhmarketing@repp.com/IEE Please send book proposals to: rachel.uk Other titles currently available in the IIE Textbook Series Business Skills for Engineers and Technologists Design Engineering www.net 1 Introduction: the basis of engineering Summary The aim of this chapter is to set the scene for the rest of the book by showing how the content of the remaining chapters will form the basis of the technical knowledge that a professional mechanical engineer needs during a career. By considering a typical engineering problem it is shown that the four main subjects that make up this text are really all parts of a continuous body of knowledge that will need to be used in an integrated manner. The chapter concludes by looking at the units that are used in engineering and showing the importance of keeping to a strict system of units. Objectives By the end of this chapter the reader should be able to: understand the seamless nature of basic engineering subjects; appreciate the way in which real engineering problems are tackled; recognize the correct use of SI units. Cast your mind forward a few years; you have graduated successfully 1.1 Real from your course, worked for a spell as a design engineer and now you engineering are responsibile for a team which is being given a new project. Your job is to lead that team in designing a new ride-on lawnmower to fill a gap in the market that has been identified by the sales team. The sales people think that there is scope to sell a good number of low-slung ride-on lawnmowers to places which use a lot of barriers or fences for crowd control, such as amusement parks. Their idea is that the new mower could be driven under the fences, cutting the grass as it goes, without the time wasting activity of having to drive to a gateway in order to move from one area to another. They have produced something they call a www.net 2 Introduction: the basis of engineering ‘concept specification’ which is really a wish list of features that they would like the new lawnmower to have. (1) It must be very low, like a go-kart, to go under the barriers. (2) It must be fast when not mowing so that it can be driven quickly around the park. (3) It should dry and collect the grass cuttings as it goes so that the park customers do not get their shoes covered in wet grass. Now comes the worst part of any engineering design problem – ‘Where do you start?’ Perhaps you should start with the framework of the mower because this is the part that would support all the other components. You have a good understanding of statics, which is the field of engineering concerned with supporting loads, and you could design a tubular steel frame without too much of a problem if you know the loads and their distribution. The trouble, however, is that you do not know the load that needs to be carried and you cannot base your design on the company’s existing products as all their current ride-on mowers are shaped more like small versions of farm tractors. You could calculate the load on the basis of an average driver weight but you do not yet know how much the engine will weigh because its power, and hence its size, has not been established. Furthermore, if the mower is to be driven fast around the park over bumpy ground then the effective dynamic loads will be much greater than the static load. It is therefore probably not a good idea to start with the frame design unless you are willing to involve a great deal of guesswork. This would run the risk of producing at one extreme a frame that would break easily because it is too flimsy and at the other extreme a frame that is unnecessarily strong and hence too expensive or heavy. Time to think again! Perhaps you should start the design by selecting a suitable engine so that the total static weight of the mower could be calculated. You have a good basic knowledge of thermodynamics and you understand how an internal combustion engine works. The trouble here, however, is that you cannot easily specify the power required from the engine. So far you have not determined the maximum speed required of the mower, the maximum angle of slope it must be able to climb or the speed at which it can cut grass, let alone considered the question of whether the exhaust heat can dry the grass. In fact this last feature might be a good place to start because the whole point of a mower is that it cuts grass. First of all you could decide on the diameter of the rotating blades by specifying that they must not protrude to the side of the mower beyond the wheels. This would give you the width of the cut. A few measurements in a field would then allow you to work out the volume and mass of grass that is cut for every metre that the mower moves forward. Lastly you could find the forward speed of your company’s other ride-on mowers when cutting in order to calculate the mass of grass which is cut per second. From this you can eventually work out two more pieces of key information. Using your knowledge of fluid mechanics you could calculate the flow rate of air which is needed to sweep the grass cuttings into the collection bag or hopper as fast as they are being produced.
