Mechanical Design: Theory and Applications

By P.R.N. Childs

Mechanical Design: Theory and Applications, Third Edition introduces the design and selection of common mechanical engineering components and machine elements, hence providing the foundational "building blocks" engineers needs to practice their art. In this book, readers will learn how to develop detailed mechanical design skills in the areas of bearings, shafts, gears, seals, belt and chain drives, clutches and brakes, and springs and fasteners. Where standard components are available from manufacturers, the steps necessary for their specification and selection are thoroughly developed.

Descriptive and illustrative information is used to introduce principles, individual components, and the detailed methods and calculations that are necessary to specify and design or select a component. As well as thorough descriptions of methodologies, this book also provides a wealth of valuable reference information on codes and regulations.

• Presents new material on key topics, including actuators for robotics, alternative design methodologies, and practical engineering tolerancing
• Clearly explains best practice for design decision-making
• Provides end-of-chapter case studies that tie theory and methods together
• Includes up-to-date references on all standards relevant to mechanical design, including ASNI, ASME, BSI, AGMA, DIN and ISO

• Language: English
• Publisher: Elsevier Science
• Release date: Jun 29, 2021
• ISBN: 9780128211038

P.R.N. Childs

Brief Author Bio: Peter Childs is Head of the Dyson School of Design Engineering and the Professorial Lead in Engineering Design at Imperial College London. His general interests include: creativity tools and innovation; design process and design rationale; fluid flow and heat transfer, particularly rotating flow; sustainable energy component, concept and system design; robotics. Prior to his current post at Imperial he was director of the Rolls-Royce supported University Technology Centre for Aero-Thermal Systems, director of InQbate and professor at the University of Sussex. He has contributed to over 180 refereed journal and conference papers, and several books including the Handbook on Mechanical Design Engineering (Elsevier, 2013, 2019) as well as co-authoring books on rural urban migration, inclusive sports and sports technology. He has been principal or co-investigator on contracts totalling over £80 million. His roles at Imperial include joint course director for the Innovation Design Engineering double master degree run jointly by Imperial and the Royal College of Art, and Design Lead for the Manufacturing Futures Lab. He is Editor of the Journal of Power and Energy and Founder Director and Chief Scientific Officer at QBot Ltd.

Book preview

Mechanical Design

Theory and Applications

Third Edition

P.R.N. Childs

Dyson School of Design Engineering, Imperial College London, London, United Kingdom

Table of Contents

Cover image

Title page

Copyright

About the Author

Preface

Acknowledgments

1. Design

Abstract

1.1 Introduction

1.2 The Design Process

1.3 Design Models

1.4 Design Optimization

1.5 Design Reviews

1.6 The Technology Base

1.7 Conclusion

References

Standards

Web Sites

Nomenclature

Worksheet

2. Journal Bearings

Abstract

2.1 Introduction

2.2 Sliding Bearings

2.3 Design of Boundary-Lubricated Bearings

2.4 Design of Full-Film Hydrodynamic Bearings

2.5 Conclusion

References

Standards

Web Sites

Nomenclature

Worksheet

Answers

3. Rolling Element Bearings

Abstract

3.1 Introduction

3.2 Bearing Life and Selection

3.3 Bearing Installation

3.4 Conclusion

References

Standards

Web Sites

Nomenclature

Worksheet

Answers

4. Shafts

Abstract

4.1 Introduction

4.2 Shaft–Hub Connection

4.3 Shaft–Shaft Connection—Couplings

4.4 Critical Speeds and Shaft Deflection

4.5 Analysis of Transmission Shafting

4.6 Detailed Design Case Study

4.7 Conclusion

References

Standards

Web Sites

Nomenclature

Worksheet

Answers

5. Gears

Abstract

5.1 Introduction

5.2 Construction of Gear Tooth Profiles

5.3 Gear Trains

5.4 Tooth Systems

5.5 Force Analysis

5.6 Simple Gear Selection Procedure

5.7 Condition Monitoring

5.8 Conclusion

References

Standards

Web Sites

Nomenclature

Worksheet

Answers

6. Spur and Helical Gear Stressing

Abstract

6.1 Introduction

6.2 Failure Due to Contact Stresses

6.3 AGMA Equations for Bending and Contact Stress

6.4 Gear Selection Procedure

6.5 Conclusion

References

Standards

Web Sites

Nomenclature

Worksheet

Answers

7. Belt and Chain Drives

Abstract

7.1 Introduction

7.2 Belt Drives

7.3 Chain drives

7.4 Conclusion

References

Standards

Web Sites

Nomenclature

Worksheet

Answers

8. Clutches and Brakes

Abstract

8.1 Introduction

8.2 Clutches

8.3 Brakes

8.4 Conclusion

References

Standards

Web Sites

Nomenclature

Worksheet

Answers

9. Springs

Abstract

9.1 Introduction

9.2 Helical Compression Springs

9.3 Helical Extension Springs

9.4 Helical Torsion Springs

9.5 Leaf Springs

9.6 Belleville Spring Washers

9.7 Conclusion

References

Standards

Web Sites

Nomenclature

Worksheet

Answers

10. Fastening and Power Screws

Abstract

10.1 Introduction to Permanent and Nonpermanent Fastening

10.2 Threaded Fasteners

10.3 Power Screws

10.4 Rivets

10.5 Adhesives

10.6 Welding

10.7 Snap Fasteners

10.8 Conclusion

References

Standards

Web Sites

Nomenclature

Worksheet

Answers

11. Tolerancing and Precision Engineering

Abstract

11.1 Introduction

11.2 Component Tolerances

11.3 Statistical Tolerancing

11.4 Precision Engineering and Case Studies

11.5 Conclusion

References

Standards

Nomenclature

Worksheet

Answers

Index

Copyright

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Notices

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ISBN: 978-0-12-821102-1

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About the Author

P.R.N. Childs is the Professorial Lead in Engineering Design at Imperial College London. He is a fellow of the Royal Academy of Engineering, the Institution of Mechanical Engineers, and the American Society of Mechanical Engineers. His general interests include creativity tools and innovation, design processes, fluid flow and heat transfer, sustainable energy, and robotics. Prior to his current post at Imperial, he was Director of the Rolls-Royce supported University Technology Centre for Aero-Thermal Systems, Director of InQbate, and a Professor at the University of Sussex.

He has contributed to over 200 refereed journal and conference papers, and several books including the Handbook on Mechanical Design Engineering (Elsevier, 2013, 2019) as well as monographs on rotating flow and temperature measurement. He has been principal or coinvestigator on contracts totaling over £100 million.

His roles at Imperial include Professor at Large for the Innovation Design Engineering double master degree run jointly by Imperial and the Royal College of Art and Enterprise Champion in the Dyson School of Design Engineering. He was the founding head of the Dyson School of Design Engineering at Imperial. He is Editor of the Journal of Power and Energy, Professor of Excellence at MD-H, Berlin, and Founder Director and former Chairman at Q-Bot Ltd. He is Chairman of BladeBUG Ltd.

He is very passionate about responsible business, and the what, how, and why of education.

Preface

Engineering involves the application of technical and mathematical principles in combination with professional and domain knowledge to deliver products, service, and systems to realize a requirement or opportunity. This book aims to present an overview of the design process and to introduce the technology and selection of a number of specific machine elements that are fundamental to a wide range of mechanical engineering design applications.

The first edition of this book was produced in 1998, with the second edition in 2004. The text was expanded to the mechanical design engineering handbook with editions in 2013 and 2018. This edition, which draws on the content of the handbook, focuses on a series of key machine elements relevant to students, including bearings, shafts, gears, belts and chains, springs, and fasteners. These technology elements serve as building blocks for a significant quantity of machine design and provide an excellent basis in mechanical engineering education. Experience from the previous editions has been used to preserve features such as detailed worked examples and flow charts illustrating step-by-step strategies for developing a design for a specific machine element.

The book includes 200 worksheet questions and over 350 images, with line drawings complemented by solid model illustrations to aid understanding of the machine elements and assemblies concerned. The context for engineering and mechanical design is introduced in the first chapter, which also presents a blended design process incorporating principles from systematic and holistic design as well as practical project management. This chapter is followed by nine chapters focusing on specific machine elements, and the book concludes with a chapter on tolerancing relevant to combining machine elements in practical designs.

Acknowledgments

I would like to express my special thanks to Dr. Kamyar Hazeri for his assistance in the generation of some of the images for this revised edition and to Dr. Marc Masen for his guidance on the rolling element bearings and gear-stressing chapters. Particularly, I would like to thank Caroline Childs for her patience and regular input on proofreading.

1

Design

Abstract

The aims of this book are to present an overview of the design process, and to introduce the technology and selection of a number of specific machine elements that are fundamental to a wide range of mechanical engineering design applications. This chapter introduces the design process from an inventor’s perspective, the double diamond model, and more formal approaches such as systematic, total, and blended design. The chapter also presents an overview of technologies, which serves as building blocks for machinery and mechanical design.

Keywords

Design; engineering; process; technology; function; total; optimization

1.1 Introduction

The aims of this book are to present an overview of the design process, and to introduce the technology and selection of a number of specific machine elements that are fundamental to a wide range of mechanical engineering design applications. This chapter introduces the design process from an inventor’s perspective, the double diamond model, and more formal approaches such as systematic, total, and blended design. The chapter also presents an overview of technologies, which serves as building blocks for machinery and mechanical design.

The term design is popularly used to refer to an object’s esthetic 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 some classically acclaimed vehicles are given in Figs. 1.1 and 1.2. In these examples, it is both visual impact, appealing to our visual perception, and the concept of function, that the product will fulfill a range of requirements, which are important in defining the so-called good design. In this section, we will consider a number of definitions and explanations relevant to design and engineering. Such definitions can be helpful in understanding the context for the activities associated with design and engineering.

Figure 1.1 Piaggio’s Vespa launched in 1946. 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. Courtesy Audi.

The word design is used as both a noun and a verb and carries a wide range of context-sensitive meanings and associations. We can, for example, refer to a product or machine and say that we like or rate the design. Alternatively, we might refer to an activity and talk about designing a product or machine.

Cox (2005) in his review on creativity in business stated Design is what links creativity and innovation. It shapes ideas to become practical and attractive propositions for users or customers. Design may be described as creativity deployed to a specific end. The word design has its roots in the Latin designare, which means to designate or mark out, and such notions will be familiar to an engineer developing a technical drawing or an architect producing a plan for a building. 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, service, or system. The design generally begins with either a need or requirement or, alternatively, an idea. It can end with a set of drawings or computer representations and other information that enables a product to be manufactured, or a service or system to be realized and utilized. Generically, design can be defined as the transformation of an existing state to a preferred state. While recognizing that there is no widely accepted single definition, to clarify what the term design means, the following statement can provide a basis:

Design is the process of conceiving, developing and realising products, artefacts, processes, systems, services, platforms and experiences with the aim of fulfilling identified or perceived needs or desires typically working within defined or negotiated constraints.

This process may draw upon and synthesize principles, knowledge, and method skills and tools from a broad spectrum of disciplines depending on the nature of the design initiative and activity. Design can also be regarded as the total activity necessary to provide a product or process to meet a market need. This latter 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).

According to a Royal Academy of Engineering document, engineering can be defined as

The discipline, art and profession of acquiring and applying scientific, mathematical, economic, social and practical knowledge to design and build structures, machines, devices, systems, materials and processes that safely realise solutions to the needs of society.

This definition is not attributed to a single source and ABET (2011), the Institution of Mechanical Engineers and the National Academy of Engineering (2004) all have similar definitions for engineering involving the application of scientific and mathematic principles to design. The following statement provides an indication of the scope of engineering:

Engineering is the application of technical and mathematic principles in combination with professional and domain knowledge, in order to deliver products, service and systems to realise a requirement or opportunity.

The terms engineering design and design engineering are often used interchangeably. The inclusion of the word engineering in both suggests that they involve the application of scientific, technical, and mathematical knowledge and principles. It may be useful to think of engineering design sitting alongside engineering science as the strand of engineering that is concerned with application, designing, manufacture, and building. Design engineering suggests a process in which engineering (scientific and mathematical) approaches are applied in the realization of activities that began with a design concept or proposal (Childs and Pennington, 2015). However, such distinctions remain subtle and subject to context. In launching the Dyson School of Design Engineering at Imperial College London, Childs defined design engineering as follows (see Childs, 2019):

Design engineering is the fusion of design thinking, engineering thinking and practice within a culture of innovation and enterprise.

This book is principally concerned with mechanical engineering design within the context of applications of a mechanical engineering nature, particularly those using a range of machine elements such as bearings, gears, shafts, belts and chains, clutches and brakes, springs, and fasteners. An example of an application showing the use of a range of these machine elements for an automotive transmission is given in Fig. 1.3.

Figure 1.3 A seven-speed sports transmission incorporating a wide range of machine elements considered in this book. Courtesy Daimler AG, release date November 17, 2014.

1.2 The Design Process

Having a defined approach to undertaking design can aid the activity, helping to ensure the process is undertaken to a professional and high standard, with, for example, thorough consideration of what is required and ensuring that due consideration is given to technical, esthetic, social, and economic function. Many design processes have been proposed over the years with consultancies, engineering corporations, and industry bodies as well as academic groups developing their own brand of approaches (e.g., see Clarkson and Eckert, 2005). Commonly cited methods include the educational approach CDIO (conceive, develop, implement, operate), total design, double diamond, concurrent engineering, six sigma, multidisciplinary design optimization (MDO), and gated reviews. Design processes can be broadly categorized as activity-based, involving generation, analysis, and evaluation, and stage-based, involving distinct phases of, for example, task clarification and conceptual design. It is also widely recognized that experienced practitioners approach design in a different manner to novice designers (e.g., see Björklund, 2013), and this has resulted in the use of some approaches in education that are distinct from commercial engineering practice.

Probably from your own experience you will know that design can consist of examining a need or opportunity and working on the problem by means of sketches, models, brainstorming, calculations as necessary, and development of styling as appropriate; making sure the product fits together and can be manufactured; and considering costs. The process of design can be represented schematically to levels of increasing formality and complexity. Fig. 1.4 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 and judgment of the design and reevaluation 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. Fig. 1.5 illustrates the possible results from this process for a bicycle lock.

Figure 1.4 The traditional and familiar inventor’s approach to design.

Figure 1.5 The LiteLok bicycle lock. Courtesy (A) Neil Barron, (B) and (C) LiteLok Ltd.

Fig. 1.6 shows a more prescribed description of a design process that might be associated with engineers operating within a formal company management structure. The various terms used in Fig. 1.6 are described in Table 1.1.

Figure 1.6 The design process illustrating principal phases and some of the iterative steps involved in the process.

Table 1.1

Although Figs. 1.4 and 1.6 at first sight suggest design occurring in a sequential fashion, with one task following another, the design process may actually occur in a step forward, step back fashion. For instance, you may propose a solution to the design need and then perform some calculations or judgments, which indicate that the proposal is inappropriate. A new solution will need to be put forward and further assessments made. This is known as the iterative process of design and forms an essential part of refining and improving the product proposal. The nonlinear nature of design is considered by Hall and Childs (2009).

Note that the flow charts shown in Figs. 1.4 and 1.6 do not represent a method of design but rather a description of what actually occurs within the process of design. The method of design used is often unique to the engineering or design team. Design methodology is not an exact science, and there are indeed no guaranteed methods of design. Some designers work in a progressive fashion, others work on several aspects simultaneously. An example of design following the process identified in Fig. 1.6 is given in the following example in order to introduce some of the typical activities that might occur.

Example 1.1

Following some initial market assessments, the board of a plant machinery company has decided to proceed with the design of a new product for transporting pallets around factories and warehouses. The board has in mind a forklift truck but does not wish to constrain the design team to this concept alone. The process of the design can be viewed in terms of the labels used in Fig. 1.6.

Solution

Recognition of need (or market brief)—The company has identified a potential market for a new pallet-moving device.

Definition of problem—A full specification of the product desired by the company should be written. This allows the design team to identify whether their design proposals meet the original request. Here a list of information needs to be developed and clarified before design can proceed. For example, for the pallet-moving device being explored here, this would likely include aspects for consideration such as

What sizes of pallet are to be moved?

What is the maximum mass on the pallet?

What is the maximum size of the load on the pallet?

What range of materials are to be moved and are they packaged?

What is the maximum height the pallet needs to be lifted?

What terrain must the pallet-moving device operate on?

What range is required for the pallet-moving device?

Is a particular energy source/fuel to be used?

What lifetime is required?

Are there manufacturing constraints to be considered?

What is the target sales price?

How many units can the market sustain?

Is the device to be automatic or manned?

What legal constraints need to be considered?

This list is not exhaustive and would require further consideration. The next step is to quantify each of the criteria. For instance, the specification may yield that standard size pallets, see Fig. 1.7, are involved, the maximum load to be moved is 1000 kg, the maximum volume of load is 2 m³, the reach must be up to 3 m, use is on factory floor and asphalt surfaces, the device must be capable of moving a single pallet 100 m and must be able to repeat this task at least 100 times before refueling or recharging as necessary, the design life for the product is 7 years, production is in a European country, the target selling price is 20,000 Euros, the production run is 3000 units per year, the device is to be operated by a person, and the design must be compliant to ISO (International Organization for Standardization) and target country national standards (e.g., see BS ISO 509, BS ISO 6780, BS EN ISO 445, BS EN 13545, BS ISO 18334, BS 5639-1, and BS ISO 2330).

Figure 1.7 Pallet dimensions and terminology (see BS ISO 509).

Synthesis—This is often identified as the formative and creative stage of design. Some initial ideas must be proposed or generated in order for them to be assessed and improved. Concepts can be generated by imagination, experience, or by the use of design techniques such as morphological charts. Some evaluation should be made at this stage to reduce the number of concepts requiring further work. Various techniques are available for this, including merit and adequacy assessments.

Analysis—Once a concept has been proposed, it can then be analyzed to determine whether constituent components can meet the demands placed on them in terms of performance, manufacture, cost, and any other specified criteria. Alternatively, analysis techniques can be used to determine what size components need to be to meet the required functions.

Optimization—Inevitably, there are conflicts between requirements. In the case of the forklift truck, size, maneuverability, cost, esthetic appeal, ease of use, stability, and speed are not necessarily all in accordance with each other. Cost minimization may call for compromises on material usage and manufacturing methods. These considerations form part of the optimization of the product producing the best or most acceptable compromise between the desired criteria. Optimization is considered further in Section 1.4.

Evaluation—Once a concept has been proposed and selected, and the details of component sizes, materials, manufacture, costs and performance worked out, it is then necessary to evaluate it. Does the proposed design fulfill the specification? If it appears to, then further evaluation by potential customers and use of prototype demonstrators may be appropriate to confirm the functionality of the design, judge customer reaction, and provide information of whether any aspects of the design need to be reworked or refined.

1.3 Design Models

The process of design has been the focus of research and development for many years, and a number of design models and methodologies have been formalized. Design methodology is a framework within which the designer can practice with thoroughness. Taking a standard approach to undertaking design can be useful in helping to ensure important aspects are addressed and leverage prior experience. Following a formalized approach or model is not necessarily going to mean that a high-quality design outcome is guaranteed. Nevertheless, a model or formal approach can aid management of the activity. Various approaches to the design process are introduced here, including systematic design, double diamond, CDIO, and total design. In the section on Total and blended design an updated model is presented. blending aspects of project management such as contemporary topics of need and opportunity analysis, virtual realization, sustainability, and responsible business.

Systematic design

A systematic approach to design has been developed and proposed by Pahl and Beitz (1996) who divide their model into four phases:

1. product planning and clarifying the task

2. conceptual design

3. embodiment design

4. detail design

The activities associated with systematic design are outlined in Table 1.2. Although the activities take place in phases, iteration does occur between each of the activities. For example, information arising from work on the requirements list can serve to help clarify the task and opportunity defined in the original requirement that gave the initial impetus for the project concerned. Similarly, work toward a concept will result in new information that can help clarify the requirement list. Similar iterations with an activity informing and resulting in flows of information and clarification of what actually needs to be done, can occur between each of the activities defined in Table 1.2. The intended outcome is for the arising insights and information to enable optimization of the requirement, layout, and production.

Table 1.2

The approach taken in systematic design acknowledges that due to the complex nature of modern technology, it is now rarely possible for a single person to undertake the design and development of a major project on their own. Instead, a large team will be involved in the activity, and this introduces the challenges of organization and communication within a larger network. The aim is to provide a comprehensive, consistent, and clear approach to systematic design.

Design models and methodologies encourage us to undertake careful marketing and specification. Because of their sequential presentation, design starts with a need or design starts with an idea, they inherently encourage us to undertake tasks sequentially. This is not necessarily the intention of the models, and indeed this approach is countered within the descriptions and instructions given by the proponents of the model, who instead encourage an iterative feedback working methodology.

A criticism of the systematic and other design models is that they tend to be encyclopedic with consideration of everything possible. As such, their use can be viewed as a checklist against which a personal model can be verified. A further criticism of design models is that they are overly serialistic as opposed to holistic and that because of the serious manner in which the models are portrayed and documented, they can have a tendency to put the intuitive and impulsive designer off!

Double diamond

The Design Council (2007, 2019) reported a study of the design process in 11 leading companies and identified a four-step design process called the double diamond design process model, involving phases of discovery, definition, development, and delivery. The process involves progression from identification of an initial problem toward concepts and solutions via two phases of divergent and convergent activity as illustrated in Fig. 1.8.

Figure 1.8 Schematic illustrating the double diamond design process. Courtesy the Design Council. (2019). What is the framework for innovation? Design Council’s evolved Double Diamond (last accessed October 13, 2020).

In the discovery divergent phase, many different ideas and aspects can be encouraged to emerge and be considered, for example, using various types of brainstorming. In a divergent phase, the emphasis is sometimes on the quantity of ideas in order to have many items from which to make a selection and to enable consideration of what the competition might consider. In a convergent phase, the emphasis is on selection and refinement of an idea and its embodiment and definition with consideration of details. The development phase also involves divergent activity with simultaneous consideration of different options to fulfill the functional requirements of the product, service, or system concerned, be it technical, esthetic, social, or economic or some combination. In the divergent development phase, use can be made of modeling and analysis of different options. In the final delivery convergent phase, the emphasis is on refinement and detailing of each aspect in order to provide the final outcome for the product, service, or system concerned.

Conceive, design, implement, operate

The conceive, design, implement, operate (CDIO) framework is widely used in design and engineering education and was developed in recognition of a divergence between academic culture and practical engineering requirements. The framework explicitly recognizes the importance of holistic considerations for effective design outcomes with the application of both engineering practice skills such as design, manufacture, personal, professional, interpersonnel, and business in combination with disciplinary knowledge from the sciences and mathematics as well as the humanities (see Crawley, 2001).

In the CDIO framework, attention is given to each of the principal phases and the development of skills needed in order to address the holistic requirements for the product, service, or system concerned. Each of the phases relates and feeds into each other. In the conceive stage, consideration is given to customer’s needs, technology availability, regulations, and the business requirements in order to develop an outline concept. In the design phase, attention is given to the production of detailed plans, technical drawings, and algorithms as appropriate to the specific challenge. In the implement phase, production issues are addressed with attention given to manufacturing practicalities and planning, coding and testing, and validation. In the operate phase, attention is given to delivery of the product, service, or system and the realization of the expected value from the investment, along with consideration of maintenance and servicing, and future evolutions for the product and associated business.

Total and blended design

The total design model was originally proposed by the SEED program (1985) and Pugh (1990), comprising core activities of design: marketing, specification, conceptual design, detailed design, and marketing/selling. This model was developed from extensive industrial consultation and experience, and the phases associated with total design are presented in Table 1.3. Since its inception, the importance of various aspects not explicit in the original formulation has emerged such as opportunity and need analysis, virtual modeling, and digital twins as well as sustainability.

Table 1.3

An important aspect in the development of any product, service, system, or indeed almost any activity is project management. One of the major tools used in modern project management is the V model, enabling consideration and definition of who has to do what and when in a project. The V model along with other project management tools such as PRINCE2 (Projects IN Controlled Environments) places emphasis on verification of what you are doing in one activity against the relevant related activity or activities. A version of the V model is illustrated in Fig. 1.9 illustrating principal phases and how they progress as a function of time. Further information on the V model can be found in the General Directive 250, Software Development Standard for the German Federal Armed Forces, V-Model (Software Life Cycle Process Model), 1992, and Bröhl (1993). Tasks in the V model are linked to a relevant activity to help ensure that the project delivers against the intent. For example, system specification is linked with acceptance testing. The form of the V model provides an impression that activities progress and flow neatly from one phase to another. It should be noted that practical project management frequently diverts from this apparent ideal with major differences in timescales and attention to multiple aspects of a project in order to address issues that arise.

Figure 1.9 The V model.

As indicated in the discussion in this chapter, there are issues associated with models for design be it their encyclopedic nature, variations according to the context, or the need for substantial experience and expertise in order to adequately address the various activities. In order to address a mismatch between models for design and the practical management of design, principles associated with total design have been blended with aspects of project management along with consideration of opportunity and needs analysis, virtual modeling, sustainability, and responsible business, in a blended design process model as shown in Figs. 1.10 and 1.11. The blended design process model functions with phases of opportunity or need and requirements analyses, specification, concept, component and system design, virtual realization and design verification, prototyping, acceptance testing, production release planning, and opportunity realization. The skewed V is symbolic of the progress of time associated with an activity, with typically more time associated with the implementation activities on the right-hand side of the V. Allocation of additional resources can alter the gradient of the V or indeed the time associated with individual activity. The horizontal arrows are symbolic of a validation check between the corresponding activities.

Figure 1.10 Blended design process model.

Figure 1.11 Blended design process model showing iteration between the various activities.

As shown in Figs. 1.4 and 1.6, the iterative nature of design is accounted for, where work on a design results in the need to go back and redo previous work in order to produce a better overall design to meet the requirements. Indeed, it is sometimes necessary to go back a few or several levels. An example might be the discovery at manufacture that an item cannot be made as envisaged and a new concept altogether is required. Ideally, such a discovery should not occur, as every other level of the design process illustrated in Fig. 1.10 should be considered at each stage. Iteration between activities is illustrated symbolically in Fig. 1.11. Each of the design activities illustrated in Fig. 1.10 is described in more detail in the remainder of this section. As it is the same fundamental process being described, these descriptions are similar to those dealt with in Fig. 1.6.

The need/opportunity analysis or marketing phase refers to the assessment of sales opportunities or perceived need to update an existing product, service, system, or platform resulting in a statement sometimes called the market brief, design brief, brief, or statement of need. Any realization of a need or opportunity warrants scrutiny in order to establish whether there is a market opportunity. The potential to undertake a detailed requirements analysis will depend on the nature of the market. In an established market, information can be gathered on the total market size and potential addressable and realizable market. For a brand new or virgin market, it may be much more difficult to establish the potential realizable market in the absence of having refined prototypes for potential customers to react to. Nevertheless, it is important to undertake an analysis of what the product, service, or system needs to address.

Specification involves the formal statement of the required functions, features, and performance of the product or process to be designed. Recommended practice from the outset of design work is to produce a product design specification (PDS) that should be formulated from the brief and requirements analysis or statement of need. The PDS is the formal specification of the product to be designed. Typically, a PDS will include, where relevant, consideration of various aspects of performance, ergonomics, esthetics, costs, timescale, market, materials, size, weight, transportation, packaging, production, sustainability and recycling, maintenance, intellectual property, standards, legal constraints, health and safety, and documentation. The specification acts as the control for the total design activity because it sets the boundaries for the subsequent design. The use can be made of a proforma table to aid in defining each aspect of the specification. Examples of such proforma tables are given in Tables 1.4 and 1.5 where a series of prompts are given to help ensure a rationale is considered for each aspect. An example of a specification using the proforma of Table 1.5 is given in Table 1.6.

Table 1.4

Table 1.5

Table 1.6

CAD, Computer aided design.

The early stages of design where the major decisions are to be made is sometimes called conceptual design or just concept design. During this phase, a rough idea is developed as to how a product will function technically and what it will look like. The process of conceptual design can also be described as the definition of the product’s morphology, how it is made up, and its layout. It involves the generation of solutions to meet specified requirements. Conceptual design can represent the sum of all subsystems and component parts that go on to make up the whole system. Ion and Smith (1996) describe conceptual design as an iterative process comprising a series of generative and evaluative stages that converge to the preferred solution. At each stage of iteration, the concepts are defined in greater detail,