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Mechanical Design This book is dedicated to Hazel,Tabitha and Angus Mechanical Design Second edition Peter R. N. Childs BSc (Hons), DPhil, CEng, FIMechE, MIED, ILTM, Mem ASME University of Sussex, UK AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK • OXFORD PARIS • SAN DIEGO • SAN FRANCISCO • SI...

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Mechanical Design

This book is dedicated to Hazel,Tabitha and Angus

Mechanical Design Second edition

Peter R. N. Childs BSc (Hons), DPhil, CEng, FIMechE, MIED, ILTM, Mem ASME University of Sussex, UK

AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK • OXFORD PARIS • SAN DIEGO • SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO

Elsevier Butterworth-Heinemann Linacre House, Jordan Hill, Oxford OX2 8DP 200 Wheeler Road, Burlington, MA 01803 First published 1998 by Arnold Second edition 2004 Copyright © 2004, Elsevier Ltd.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 W1T 4LP.Applications for the copyright holder’s written permission to reproduce any part of this publication should be addressed to the publisher Permissions may be sought directly from Elsevier’s Science and Technology Rights Department in Oxford, UK; phone: (⫹44) (0) 1865 843830; fax: (⫹44) (0) 1865 853333; e-mail: [email protected] may also complete your request on-line via the Elsevier homepage (http://www.elsevier.com), by selecting ‘Customer Support’ and then ‘Obtaining Permissions’ British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of Congress ISBN 0 7506 5771 5

For information on all Elsevier Butterworth-Heinemann publications visit our website at http://books.elsevier.com

Disclaimer While the content of this book has been obtained from a variety of reliable sources and has been checked carefully, the author and the publisher accept no responsibility for any loss, damage or underachievement arising out of the use of the material presented. Typeset by Charon Tec Pvt. Ltd, Chennai Printed and bound by Great Britain

CONTENTS Preface

ix

About the author

xi

Acknowledgements

xiii

1

Design 1.1 Introduction 1.2 The design process 1.3 Total design 1.4 Product design specification 1.5 Conceptual design 1.6 The technology base 1.7 Conclusions References and sources of information Nomenclature Worksheet

1 1 2 5 9 12 16 18 18 19 19

2

Mechanical engineering 2.1 Introduction 2.2 Thermodynamics 2.3 Mechanics 2.4 Materials 2.5 Conclusions References and sources of information Nomenclature

22 22 25 27 27 29 29 29

3

Machine elements 3.1 Introduction 3.2 Tribology 3.3 Bearings 3.4 Gears, belts and chains 3.5 Seals 3.6 Clutches and brakes 3.7 Springs 3.8 Fasteners 3.9 Enclosures 3.10 Conclusions References Nomenclature

30 30 31 32 33 34 35 36 36 37 38 38 38

Contents vi

4

Bearings 4.1 Introduction 4.2 Sliding bearings 4.3 Rolling contact bearings 4.4 Conclusions References and sources of information Nomenclature Worksheet

5

Shafts 5.1 Introduction 5.2 Shaft–hub connection 5.3 Shaft–shaft connection – couplings 5.4 Critical speeds and shaft deflection 5.5 ASME design code for transmission shafting 5.6 Conclusions References and sources of information Nomenclature Worksheet

79 79 82 84 85 94 101 102 102 103

6

Gears 6.1 Introduction 6.2 Construction of gear tooth profiles 6.3 Gear trains 6.4 Tooth systems 6.5 Force analysis 6.6 Simple gear selection procedure 6.7 Conclusions References and sources of information Nomenclature Worksheet

107 107 113 116 122 122 124 133 133 134 134

7

Detailed gear stressing 7.1 Introduction 7.2 Wear failure 7.3 AGMA equations for bending and contact stress 7.4 Gear selection procedure 7.5 Conclusions References and sources of information Nomenclature Worksheet

137 137 138 139 148 150 150 151 151

8

Belts and chain drives 8.1 Introduction 8.2 Belt drives

154 154 155

39 39 40 63 71 72 73 74

Contents vii

8.3 Chain drives 8.4 Conclusions References and sources of information Nomenclature Worksheet

166 173 173 175 175

9 Seals 9.1 Introduction 9.2 Static seals 9.3 Dynamics seals 9.4 Conclusions References and sources of information Nomenclature Worksheet

177 177 178 182 188 188 190 190

10 Clutches and brakes 10.1 Introduction 10.2 Clutches 10.3 Brakes 10.4 Conclusions References and sources of information Nomenclature Worksheet

192 192 194 203 220 220 221 222

11 Springs 11.1 Introduction 11.2 Helical compression springs 11.3 Helical extension springs 11.4 Helical torsion springs 11.5 Leaf springs 11.6 Belleville spring washers 11.7 Conclusions References and sources of information Nomenclature Worksheet

225 225 229 239 241 242 244 247 247 249 249

12 Fastening and power screws 12.1 Introduction to permanent and non-permanent fastening 12.2 Threaded fasteners 12.3 Power screws 12.4 Rivets 12.5 Adhesives 12.6 Welding 12.7 Snap fasteners

251 251 251 258 261 267 270 270

Contents viii

12.8 Conclusions References and sources of information Nomenclature

272 272 273

13 Frames, casings and enclosures 13.1 Introduction 13.2 Designing to resist bending 13.3 Designing to resist torsion 13.4 Designing to provide adequate ventilation 13.5 Safety 13.6 Conclusions References and sources of information Nomenclature

275 275 276 278 279 281 281 281 281

14 Sensors and actuators 14.1 Introduction 14.2 Sensors 14.3 Actuators 14.4 Conclusions References and sources of information Nomenclature

283 283 284 291 297 297 297

15 Engineering tolerancing 15.1 Introduction 15.2 Component tolerances 15.3 Statistical tolerancing 15.4 Conclusions References and sources of information Nomenclature Worksheet

299 299 299 307 319 319 319 320

16 Design management and case study 16.1 Introduction 16.2 Management of design 16.3 Costing 16.4 A guide to design literature and sources of information 16.5 Case study 16.6 Conclusions References and sources of information Nomenclature Worksheet

322 322 326 329 335 336 346 346 347 348

Index

351

PREFACE

The overall aims of this book are to introduce the subject of total design and the design and selection of various common mechanical engineering components and machine elements. These provide ‘building blocks’ with which the designer and engineer can practise their art. The approach adopted for defining design follows that developed by the SEED (Sharing Experience in Engineering Design) programme where design is viewed as the ‘the total activity necessary to provide a product or process to meet a market need’. Within this framework the text is concentrated on developing detailed mechanical design skills in the areas of bearings, shafts, gears, seals, belt and chain drives, clutches and brakes, springs and fasteners.Where standard components are available from manufacturers,the steps necessary for their specification and selection are developed. The framework used within the text has been to provide descriptive and illustrative information to introduce principles and individual components and to expose the reader to the detailed methods and calculations necessary to specify and design or select a component. To provide the reader with sufficient information to develop the necessary skills to repeat calculations and selection processes, detailed examples and worked solutions are supplied throughout the text. This book is principally a year/level 1 and 2 undergraduate text. Prerequisite skills include some year 1 undergraduate mathematics, fluid mechanics and heat transfer, principles of materials, statics and dynamics. However, as the subjects are introduced in a descriptive and illustrative format and as full worked solutions are provided, it is possible for readers without this formal level of education to benefit from this book.The text is specifically

aimed at automotive and mechanical engineering degree programmes and would be of value for modules in design, mechanical engineering design, design and manufacture, design studies, automotive power-train and transmission and tribology, as well as modules and project work incorporating a design element requiring knowledge about any of the content described. The aims and objectives described are achieved by short introductory chapters on total design, mechanical engineering and machine elements followed by nine chapters on machine elements covering: bearings, shafts, gears, seals, chain and belt drives, clutches and brakes, springs and fasteners. Chapters 13 and 14 introduce casings and enclosures and sensors and actuators, key features of most forms of mechanical technology.The subject of tolerancing from a component to a process level is introduced in Chapter 15.The last chapter serves to present the subject of design management and an integrated design using the detailed design aspects covered within the book. The design methods where appropriate are developed to national and international standards. The first edition of this text served to introduce a variety of machine elements as building blocks with which design of mechanical devices can be undertaken. The material covered by this first edition is still valid and the approach adopted of introducing and explaining the aspects of technology by means of text, photographs, diagrams and step-by-step procedures has been well received. A number of important machine elements were omitted in the first edition such as fasteners, springs, sensors and actuators. They are included here. Reworking of aspects of the original text includes the introductory chapters covering total

Preface x

design, the scope of mechanical engineering and machine elements. A new chapter is included on casings and enclosures and the final chapter has been rewritten to provide an integrated approach. The use of multiple worked examples

and completed solutions has been maintained as these proved to be both useful and popular in the first edition. Peter R. N. Childs

ABOUT THE AUTHOR Peter Childs is a Reader in Mechanical Engineering at the University of Sussex and lectures in creative styling, engineering design, fluid mechanics and engine testing. His research interests include design, air-breathing engines and instrumentation. He is actively involved in research and development with projects including work for Ford,

Rolls-Royce, Siemens, Alstom, DaimlerChrysler and Volvo. He is a fellow of the Institution of Mechanical Engineers and in 1999 was the winner of the American Society of Mechanical Engineers–International Gas Turbine Institute John P. Davies award for exceptional contribution to the literature of gas turbine technology.

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ACKNOWLEDGEMENTS The author is grateful to the following for permission to use their material: Figure 1.13c from Case Studies In Engineering Design (1998) by Matthews, C. Reprinted by permission of Elsevier Ltd Figure 2.5 is reprinted with permission of RollsRoyce plc Figure 3.12 from Total Vehicle Technology, How Do We Get the Innovation back into Vehicle Design (2002) by Stobart R.K. and Childs P.R.N. (eds) Reprinted by permission of Professional Engineering Publishing Ltd Figures 6.23, 6.24, 10.16, 10.17, 10.18, 10.32 from Vehicle and Engine Technology, 2nd edn (1999) by Heisler, H. Reprinted by permission of Elsevier Ltd

Figure 4.10 from Plain Bearing Design Handbook (1983) by Welsh, R.J. Reprinted by permission of Elsevier Ltd Figures 4.3, 10.6, 10.7, 10.8, 10.9, 10.10, 10.11, 10.12 from The Tribology Handbook (1995) by Neale, N.J. Reprinted by permission of Elsevier Ltd Figures 11.1 and 11.2 from Materials Selection for Mechanical Design, 2nd end (1999) by Ashby, M.F. Reprinted by permission of Elsevier Ltd Figure 12.20 from Process Selection (1997) by Swift K.G. and Booker J.D. Reprinted by permission of Elsevier Ltd In addition, the author wishes to express his gratitude to Prof. Roy Simons OBE for his assistance in developing some of the design management material presented in Chapter 16.

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d 1

1 DESIGN

The aims of this book are to present an overview of the design process and design methodology, and to introduce the technology and selection of a number of specific machine elements that are fundamental to a wide range of engineering. This chapter introduces the design process from an inventor’s perspective to a more formal model called ‘total design’. LEARNING OBJECTIVES At the end of this chapter you should:

• • • • •

be able to recognize the iterative steps associated with the design process; be aware of the process involved in developing a product design specification from a market brief; be able to develop a morphological design chart to aid the development of conceptual solutions; be able to participate in brainstorming activities; recognize the scope of available technologies to the designer.

1.1 Introduction The term ‘design’ is popularly used to refer to an object’s aesthetic appearance with specific reference to its form or outward appearance, as well as its function. For example, we often refer to designer clothes, design icons and beautiful cars, examples of which are given in Figures 1.1 and 1.2. In these examples it is both visual impact, appealing to our

Figure 1.1 Piaggio’s Vespa launched in 1946 (Piaggio Museo). The Vespa was an early example of monocoque construction where the skin and frame are combined as a single construction to provide appropriate rigidity and mounting for the vehicle’s components and riders.

Figure 1.2 The Audi TT, originally launched in 1998, and a contender for the most attractive sports car of the 20th century. (Courtesy of Audi.)

visual perception, and the concept of function, that the product will fulfil a range of requirements, which are important in defining so-called ‘good design’.

Design 2

The word ‘design’ comes from the Latin ‘designare’, which means to designate or mark out. Design can be taken to mean all the processes of conception, invention, visualization, calculation, refinement and specification of details that determine the form of a product. Design generally begins with either a need or requirement or, alternatively, an idea. It ends with a set of drawings or computer representations and other information that enables a product to be manufactured and utilized. In this book, design is defined as ‘the total activity necessary to provide a product or process to meet a market need’. This definition comes from the SEED (Sharing Experience in Engineering Design, now DESIG the Design Education Special Interest Group of the Design Society) model, see Pugh (1990).

Idea Influencing factors

Sketches and calculations Iteration Evaluation

Final solution

Figure 1.3 The traditional and familiar ‘inventor’s’ approach to design.

1.2 The design process Probably from your own experience you will know that design can consist of examining a design need and working on the problem by means of sketches, models, brain-storming, calculations as necessary, development of styling as appropriate, making sure the product fits together and can be manufactured, and calculation of the costs.The process of design can be represented schematically to levels of increasing formality and complexity. Figure 1.3 represents the traditional approach associated with lone inventors. This model comprises the generation of the ‘bright idea’, drawings and calculations giving form or shape to the idea, judgement of the design and re-evaluation if necessary, resulting in the generation of the end product.The process of evaluation and reworking an idea is common in design and is represented in the model by the iteration arrow taking the design activity back a step so that the design can be improved. Figure 1.4 illustrates the possible results from this process for a helmet providing peripheral and reverse vision. Figure 1.5 shows a more formal description of the design process that might be associated with

engineers operating within a formal company management structure.The various terms used in Figure 1.5 are described in the following text.

•

•

•

Recognition of need. Often design begins when an individual or company recognizes a need, or identifies a potential market, for a product, device or process.Alternatively ‘need’ can be defined as when a company decides to re-engineer one of its existing products (for example, producing a new car model). The statement of need is sometimes referred to as the brief or market brief. Definition of problem.This involves all the specifications of the product or process to be designed. For example, this could include inputs and outputs, characteristics, dimensions and limitations on quantities. Synthesis. This is the process of combining the ideas developed into a form or concept, which offers a potential solution to the design requirement. The term synthesis may be familiar from its use in chemistry where it is used to describe the process of producing a

Chapter 1 3

(a)

(b)

(c)

(d)

Figure 1.4 Panoramic helmet by Alberto Meda and Denis Santachiara (Manzini, 1989). (a) The need: to be able to view behind you. (b) The idea: an optical link using fibre optics and lenses. (c) & (d) Practical sketches showing the concept.

•

compound by a series of reactions of other substances. Analysis.This involves the application of engineering science: subjects explored extensively in traditional engineering courses, such as statics and dynamics, mechanics of materials, fluid flow and heat transfer. These engineering ‘tools’ and techniques can be used to examine the design to give quantitative information such as whether it is strong enough or will operate at an acceptable temperature. Analysis and synthesis invariably go together. Synthesis means putting something together and analysis means resolving something into its constituent parts or taking it to pieces. Designers have to synthesize something before it can be analysed. The famous chicken and the egg scenario!

• •

When a product is analysed some kind of deficiency or inadequacy may be identified requiring the synthesis of a new solution prior to re-analysis and repetition of the process until an adequate solution is obtained. Optimization. This is the process of repetitively refining a set of often-conflicting criteria to achieve the best compromise. Evaluation. This is the process of identifying whether the design satisfies the original requirements. It may involve assessment of the analysis, prototype testing and market research.

Although Figures 1.3 and 1.5 at first sight show design occurring ...