Mechanical Engineering Systems 2008 Part 1 pdf

This motivation for this series of books was born out of the desire ofthe UK’s Engineering Council to increase the number of incorporatedengineers graduating from Higher Education establ

Mechanical Engineering Systems

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

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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 LibraryISBN 0 7506 5213 6

Composition by Genesis Typesetting, Laser Quay, Rochester, KentPrinted and bound in Great Britain

1 Introduction: the basis of engineering 1

Series Preface

‘There is a time for all things: for shouting, for gentle speaking, forsilence; for the washing of pots and the writing of books Let now thepots 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 Andante AllegroModerato.’

Oliver Heaviside was one of the greatest engineers of all time,ranking alongside Faraday and Maxwell in his field As can be seenfrom the above excerpt from a seminal work, he appreciated the need tocommunicate to a wider audience He also offered the advice So berigorous; that will cover a multitude of sins And do not frown.’ Theseries of books that this prefaces takes up Heaviside’s challenge but in

a world which is quite different to that being experienced just a centuryago

With the vast range of books already available covering many of thetopics developed in this series, what is this series offering which isunique? I hope that the next few paragraphs help to answer that;certainly no one involved in this project would give up their time tobring these books to fruition if they had not thought that the series isboth unique and valuable

This motivation for this series of books was born out of the desire ofthe UK’s Engineering Council to increase the number of incorporatedengineers graduating from Higher Education establishments, and theInstitution of Incorporated Engineers’ (IIE) aim to provide enhancedservices 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 – fromFoundation Degrees or Higher Nationals through to first year modulesfor traditional ‘Chartered’ degree courses The reason why these bookswill appeal to such a wide audience is that they present the core subjectareas for engineering studies in a lively, student-centred way, with keytheory delivered in real world contexts, and a pedagogical structure thatsupports 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 andcommunications dominated, technology organisations Both facets have

an equal importance in the health of a nation and the prospects ofindividuals In preparing this series of books, we have tried to strike abalance between traditional engineering and developing technology

The philosophy is to provide a series of complementary texts whichcan be tailored to the actual courses being run – allowing the flexibilityfor course designers to take into account ‘local’ issues, such as areas ofparticular staff expertise and interest, while being able to demonstratethe depth and breadth of course material referenced to a commonframework The series is designed to cover material in the core textswhich approximately corresponds to the first year of study with moduletexts focussing on individual topics to second and final year level Whilethe 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 fordiscussion

Another set of factors which we have taken into account in designingthis series is the reduction in contact hours between staff and students,the evolving responsibilities of both parties and the way in whichadvances 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 thestudent

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 DuffySeries Editor

De Montfort University, Leicester, UK

Further information on the IIE Textbook Series is available frombhmarketing@repp.co.uk

www.bh.com/IEE

Please send book proposals to:

rachel.hudson@repp.co.uk

Other titles currently available in the IIE Textbook Series

Business Skills for Engineers and Technologists

Design Engineering

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 professionalmechanical engineer needs during a career By considering a typical engineering problem it isshown 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 theimportance 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

1.1 Real

engineering

Cast your mind forward a few years; you have graduated successfullyfrom your course, worked for a spell as a design engineer and now youare 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 peoplethink that there is scope to sell a good number of low-slung ride-onlawnmowers to places which use a lot of barriers or fences for crowdcontrol, such as amusement parks Their idea is that the new mowercould be driven under the fences, cutting the grass as it goes, without thetime wasting activity of having to drive to a gateway in order to movefrom one area to another They have produced something they call a

2 Introduction: the basis of engineering

‘concept specification’ which is really a wish list of features that theywould 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 quicklyaround the park

(3) It should dry and collect the grass cuttings as it goes so that thepark 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 becausethis is the part that would support all the other components You have agood understanding of statics, which is the field of engineeringconcerned with supporting loads, and you could design a tubular steelframe without too much of a problem if you know the loads and theirdistribution The trouble, however, is that you do not know the load thatneeds to be carried and you cannot base your design on the company’sexisting products as all their current ride-on mowers are shaped morelike small versions of farm tractors You could calculate the load on thebasis of an average driver weight but you do not yet know how much theengine will weigh because its power, and hence its size, has not beenestablished Furthermore, if the mower is to be driven fast around thepark over bumpy ground then the effective dynamic loads will be muchgreater than the static load It is therefore probably not a good idea tostart 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 aframe that would break easily because it is too flimsy and at the otherextreme a frame that is unnecessarily strong and hence too expensive orheavy

Time to think again!

Perhaps you should start the design by selecting a suitable engine sothat the total static weight of the mower could be calculated You have

a good basic knowledge of thermodynamics and you understand how aninternal combustion engine works The trouble here, however, is thatyou cannot easily specify the power required from the engine So faryou have not determined the maximum speed required of the mower, themaximum 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 exhaustheat can dry the grass In fact this last feature might be a good place tostart 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 byspecifying that they must not protrude to the side of the mower beyondthe wheels This would give you the width of the cut A fewmeasurements in a field would then allow you to work out the volumeand mass of grass that is cut for every metre that the mower movesforward Lastly you could find the forward speed of your company’sother 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 theflow rate of air which is needed to sweep the grass cuttings into thecollection bag or hopper as fast as they are being produced

Introduction: the basis of engineering 3

 Using your knowledge of thermodynamics you could calculate therate at which heat must be supplied to the wet grass to evaporatemost of the surface water from the cuttings by the time they reachthe hopper

At last you are starting to get somewhere because the first point will allowyou to calculate the size of fan that is required and the power that isneeded to drive it The second point will allow you to calculate the rate atwhich waste heat from the engine must be supplied to the wet grass.Knowing the waste power and the typical efficiency of this type of engineyou can then calculate the overall power that is needed if the engine is tomeet this specification to dry the grass cuttings as they are produced.Once you have the overall power of the engine and the portion of thatpower that it will take to drive the fan you can calculate the power that

is available for the mowing process and for driving the mower’s wheels.These two facts will allow you to use your knowledge of dynamics toestimate the performance of the mower as a vehicle: the accelerationwith and without the blades cutting, the maximum speed up an inclineand the maximum driving speed Of course this relies on being able toestimate the overall mass of the mower and driver, which brings us back

to the starting point where we did not know either of these two things

It is time to put the thinking cap back on, and perhaps leave it on,because this apparently straightforward design problem is turning out to

be a sort of closed loop that is difficult to break into

What can we learn from this brief look into the future? There arecertainly two important conclusions to be drawn

 The engineering design process, which is what most engineering isall about, can be very convoluted While it relies heavily on calcula-tion, there is often a need to make educated guesses to start thecalculations To crack problems like the one above of the new moweryou will need to combine technical knowledge with practical experi-ence, a flair for creativity and the confidence to make those educatedguesses The engineering courses that this textbook supports musttherefore be seen as only the start of a much longer-term learningprocess that will continue throughout your professional career

 A good engineer needs to think of all the subjects that are studied on

an undergraduate course in modular chunks as being part of a singlebody of technical knowledge that will form the foundation on which

a career can be built At the introductory level of this book it is best

to keep the distinction between the various topics otherwise it canbecome confusing to the student; it is difficult enough coming toterms with some of the concepts and equations in each topic withouttrying to master them all at the same time The lawn mowerexample, however, shows that you must be able to understand andintegrate all the topics, even though you may not have to become anexpert in all of them, if you want to be a proficient engineer

1.2 Units The introduction is now over and it is almost time to plunge into thedetailed treatment of the individual topics Before we do that, however,

we must look at the subject of the units that are used, not only in thisbook but also throughout the vast majority of the world’s engineeringindustry

4 Introduction: the basis of engineering

Every engineering student is familiar with the fact that it is not goodenough to calculate something like the diameter of a steel support rod andjust give the answer as a number The full answer must include the unitsthat have been used in the calculation, such as millimetres or metres,otherwise there could be enormous confusion when somebody else usedthe answer in the next step of a large calculation or actually went aheadand built the support rod However, there is much more to the question ofunits than simply remembering to quote them along with the numericalpart of the answer The really important thing to remember is to base allcalculations on units which fit together in a single system The system that

is used in this book and throughout engineering is the International

System of Units, more correctly known by its French name of Syst`eme

Internationale which is abbreviated to SI.

SI units developed from an earlier system based on the metre, thekilogram and the second and hence is known as the MKS system fromthe initial letters of those three units These three still form the basis ofthe SI because length, mass and time are the most importantfundamental measurements that need to be specified in order to definemost of the system Most of the other units in the system for quantitiessuch as force, energy and power can be derived from just these threebuilding blocks The exceptions are the units for temperature, electricalcurrent and light intensity, which were developed much later andrepresent the major difference between the SI and the MKS system One

of these exceptions that is of concern to us for this book is the unit for

measuring temperature, the kelvin (K) This is named after Lord Kelvin,

a Scottish scientist and engineer, who spent most of his career studyingtemperature and heat in some form or another The Kelvin is actually

equal to the more familiar degree Celsius (°C), but the scale starts at what is called absolute zero rather than with the zero at the freezing

point of water The connection is that 0°C = 273K

The SI is therefore based on the second, a unit which goes back to

early Middle East civilization and has been universally adopted for

centuries, plus two French units, the metre and the kilogram, which are

much more recent and have their origin in the French Revolution Thatwas a time of great upheaval and terror for many people It was also,however, a time when there were great advances in science andengineering because the revolutionaries’ idealistic principles were based

on the rule of reason rather than on inheritance and privilege One of thegood things to come out of the period, especially under the guidance ofNapoleon, was that the old system of measurements was scrapped Up

to that time all countries had systems of measurement for length,volume and mass that were based on some famous ruler In England itwas the length between a king’s nose and the fingertip of his

outstretched arm that served as the standard measure of the yard, which could then be subdivided into three feet This was all very arbitrary and

would soon have caused a great deal of trouble as the nations of WesternEurope were poised to start supplying the world with their manufacturedgoods Napoleon’s great contribution was to do away with any unit thatwas based on royalty and get his scientists to look for a logicalalternative What they chose was to base their unit of length on thedistance from one of the earth’s poles to the equator This was a distancewhich could be calculated by astronomers in any country around theworld and so it could serve as a universal standard They then split this

distance into ten million subdivisions called measures In French this is