Engineering Design Principles PDF

Engineering Design Principles by Ken Hurst ISBN: 0340598298 Pub. Date: May 1999 Publisher: Elsevier Science & Technology Books Contents Chapter 1 1[ntroduction to engineering design 1 1.1 Historical perspective 1 1.2 Engineering design definition 4 1.3 The engineering design process 8 1.4 General example 11 1.5 Engineering design interfaces 12 1.6 Principles 14 Chapter 2 ]Problem identification 16 2.1 Introduction 16 2.2 PDS criteria 19 2.3 Content of a PDS 25 2.4 Sample PDS 27 2.5 Principles 29 2.6 Exercises 29 Chapter 3 Creativity 32 3.1 Introduction 32 3.2 Psychological *set' 35 3.3 Inversion 37 3.4 Analogy 39 3.5 Fantasy 40 3.6 Technological advances 41 3.7 Brainstorming 42 3.8 Morphological analysis 43 3.9 Presentation 44 3.10 Answers to problems 47 3.11 Principles 47 3.12 Exercises 48 Chapter 4 Concept selection 49 4.1 Introduction 49 4.2 Subjective decision-making 53 4.3 Criteria ranking 54 4.4 Criteria weighting 55 4.5 Datum Method 56 4.6 EVAD (Design EVAluation) method 59 4.7 Recommended concept selection method 62 4.8 Computer aided decision making 63 vi I Contents 4.9 Principles 64 4.10 Exercises 64 Chapter 5 Embodiment 66 5.1 Introduction 66 5.2 Size and strength 68 5.3 Scheme drawing 68 5.4 Form design 69 5.5 Provisional materials and process determination 72 5.6 Design for assembly and manufacture 73 5.7 Industrial design 75 5.8 Principles 81 Chapter 6 Modelling 82 6.1 Introduction 82 6.2 Mathematical modelling 84 6.3 Optimization 88 6.4 Scale models 93 6.5 Simulation 96 6.6 Principles 97 6.7 Exercises 98 Chapter 7 Detail design 101 7.1 Introduction 101 7.2 Factor of safety 101 7.3 Selection procedure for bought out components 106 7.4 Robust design 109 7.5 Principles 124 7.6 Exercises 124 Chapter 8 Design management 125 8.1 Introduction 125 8.2 Management of design for quality 126 8.3 Project planning and control 129 8.4 Product Design Specification (PDS) 133 8.5 Quality Function Deployment (QFD) 133 8.6 Design review 136 8.7 Value analysis/engineering 137 8.8 Principles 140 Chapter 9 Information gathering 141 9.1 Introduction 141 9.2 Continuous information gathering 141 9.3 Information gathering for a particular PDS 143 9.4 Information sources 145 9.5 Principles 156 9.6 Exercises 158 Contents |vi Chaptei1 0 ]Presentation techniques 159 10.1 Introduction 159 10.2 Concept sketches 160 10.3 Scheme drawing 162 10.4 Design report 164 10.5 Principles 166 Index 167 a Introduction to engineering design In this introductory chapter the engineering design process which is covered in detail later is defined. A historical perspective is taken to explain the need for a formal process and the complexity of current engineering is outlined. A definition is given for both the engineering design process and the duties of an engineering designer. Design is defined as a technology, not a science, and accepted models of the process are presented. Finally the levels of communica- tion necessary for successful engineering design are illustrated. 1.1 Historical perspective The study of history is often very illuminating and two main purposes are served. The mistakes made by previous generations should not be repeated. Also, a vast body of knowledge has been gathered over the centuries, which can be usefully employed in the present day. There are important lessons here for engineering designers, since most new products are not completely new inventions but are new applications and combinations of existing technologies. A study of the history of science and technology serves to enhance our understanding of modem day engineering and helps to prevent the all too real tempta- tion of 're-inventing the wheel'. It is surprising how far back in time this study can usefully delve. As an example consider the invention of the force pump, the earliest description of which was given by Philo of Byzantium in the second century BC. Figure 1.1 is a reproduction of Philo's drawing. There is little evidence of refinement or aesthetic considerations but all the essential principles are presented and the design is unexpectedly complex. Water flows into the partial vacuum created by the upward motion of the piston and on the down stroke, with the valves reversed, the water is forced up the pipe into the tank. When an invention such as the force pump first surfaces it is considered to be a dynamic product. That is to say that conceptually it is a significant advance or step change from anything which has gone before. Also, there remains scope for significant product development in a dynamic product which is not possible in those products defined as static. As is the case with most useful inventions, the force pump was subsequently refined and next appeared in the form illustrated in Fig. 1.2, which is Hero's force pump from the first century AD. At this stage of development the pump would be considered to be conceptually static since the later design follows the previous design. The refinements which are most noticeable include the replacement of the two pipes for conveying water to the tank with one, the single actuation beam pivoting in the centre and the introduction of a nozzle. The nozzle was introduced specifically for fire fighting applications although it was many centuries later that the pump was mounted on a chassis in order to travel to the scene of a fire. Thus the conceptually static force pump could again be considered as dynamic when the idea for mobility was first suggested, many hundreds of years after the initial design. [ 2 1 Engineering Design Principles Figure 1.1 Philo's force pump (Reproduced from Carra de Vaux, Les pneumatics de Philon, p. 217) To prevent the wrong impression being created by this example it is important to realize that not all design work is innovative in nature and many product developments are incremental. In fact the majority of an engineering designer's life is spent making relatively minor improvements to existing products. In those early days, artisans such as Hero and Philo conceived ideas almost completely in their minds and generally worked in isolation before communicating the finished concept to others. Thoughts now, as then, can be verbal but are more often than not visual and three-dimensional, particularly for engineers. Unlike Hero and Philo the modem design engineer must be able to express thoughts clearly and communicate them constantly, throughout the whole design and development process, both within the design team and outside. This communication process inevitably involves a great deal of sketch- ing and some skill in this area is thus essential for a designer. It is very important therefore that the student engineer develops by practice the skill of sketching quickly in 3D. The level of scientific and technical knowledge possessed by Hero and Philo was limited when compared with today's understanding. Nevertheless, study of the two figures Introduction to engineering design J=: Figure 1.2 Hero's force pump with adjustable nozzle (Reproduced from a facsimile of the 1851 Woodcroft edition, introduced by Marie Boas Hall, London 1971, drawing of 'The Fire-Engine', p. 44) showing the force pump indicates that they must have had an intimate knowledge of materials, the engineering sciences and manufacturing processes which were current at the time. These artisans did not openly employ a formal engineering design process. However, with the considerable advances made in materials and manufacturing technology, increased knowledge in engineering science, ever more stringent environmental consider- ations, increasing competition, greater emphasis on energy efficiency and increasing sophistication required of today's products, a formal engineering design process has become essential. They must also have had at least an appreciation of economic constraints since people with buckets could probably perform the same tasks as a force pump for a much lower initial investment! It should be said at this point that it comes as a considerable shock to most young engineers when they first realize that only a very small percentage of decisions made by a design engineer are based on complete knowledge of the engineering sciences. The knowledge used by a design engineer is extremely broad and varied in nature. It is true that part is derived from science but a great deal comes from testing and evaluation and on observations of materials and systems. r4~l Engi neering Design Principles In the days of Hero and Philo engineering effort made a significant impact on people *s lives relatively infrequently and so was regarded as marvellous. Also, the practitioners are remembered. This is not very often the case today, even though engineers have a disproportionate effect on the kind of world we live in. Less than 1% of the population are engineers and yet virtually everything we see if we look around is man-made and has been designed to be that way. 1.2 Engineering design definition In the study of science we seek to develop theories that explain natural phenomena. Scientific theories consist of a statement or set of statements that define some kind of ideal or theoretical system. These scientific principles, which are self-evident in the natural sciences, are also employed in the engineering sciences. Engineering science subjects such as thermodynamics, mechanics and materials science are generally based on established scientific principles like the first and second laws of thermodynamics, Newton's laws, and atomic and molecular theories of matter respectively. Engineering design is quite different since theories and hypotheses cannot be developed or tested by laboratory experiments. Engineering design involves much broader issues including the consideration of people and organizations. It must therefore be regarded as a technology. This is particularly so since no single absolute answer can be found for any problem which involves both design decisions and compromise, since almost inevitably design parameters are contradictory. Having established that engineering design is a technology it is necessary to present a definition. Many attempts at a definition have been made, particularly in the search for a snappy, short definition, but all attempts to date have been defeated. The dictionary definition of design is often *to fashion after a plan', which tells us very little about the way of working that we call engineering design. What follows is an amalgam of definitions for both the process and practitioners taken from the UK based Institution of Engineering Designers and the engineering design lecturer organization, SEED Ltd (Sharing Experience in Engineering Design). Engineering design is the total activity necessary to establish and define solutions to problems not solved before, or new solutions to problems which have previously been solved in a different way. The engineering designer uses intellectual ability to apply scientific knowledge and ensures the product satisfies an agreed market need and product design specification whilst permitting manufacture by the optimum method. The design activity is not complete until the resulting product is in use providing an acceptable level of performance and with clearly identified methods of disposal. In order to increase our understanding of design it is helpful to extend this definition and to identify and highlight the main characteristics of engineering design: Trans-disciplinary Highly complex Iterative. Introduction to engineering design I 5 1 Most engineering design is now a trans-disciplinary team effort and the distinctions between the traditional disciplines, mechanical, electrical, electronic, civil and even chemical engineers are becoming blurred. Relatively new areas of engineering specializa- tion, such as control and software engineering should be added to this list. Consider for example automobiles, which not so very long ago were the sole province of mechanical engineers. Complex engine management systems, anti-lock braking systems, active suspension systems, four-wheel steering, air bags, and automatic seat belt tension- ing are just some of the new developments. These systems are highly complex and require input from many different kinds of engineers for their optimum design. Also, the selection of the appropriate technology for each part of a truly integrated design has become critical to the success of the product. Only engineers with a broad understanding of all potentially useful technologies and all the issues involved can make optimal decisions. As an illustrative example we can usefully consider anti-lock braking systems (ABS). Perhaps because traditionally automobile design was the province of the mechanical engineer, the first anti-lock braking systems introduced were purely mechanical in nature. Although the performance of mechanical units was considered to be adequate in the early stages, their performance has since been surpassed by integrated systems which include software, electronic and mechanical technologies. Purely mechanical systems cannot match the performance levels which can be achieved by integrated designs. An ABS, as the name suggests, improves braking performance by preventing wheel lock. A modem system can be seen in Fig. 1.3 with (a) being the general layout and (b) being the electronic circuit diagram. In the complete description of the system a hydraulic circuit diagram would also be required along with component details. Such systems allow braking to occur without impairing directional control and shorten braking distances considerably. This is accomplished by sensing the speed of each wheel along with wheel acceleration, comparing this to forward speed and modulating brake pressure accordingly. The complexity of the complete system shown in Fig. 1.3 (a) and (b) serves to illustrate the need for inter-disciplinary engineering design teams. The main components in the general layout are: (1) Front wheel sensor (2) Front pulse wheel (3) Hydraulic modulator (4) Control unit (5) Rear wheel sensor (6) Rear pulse wheel (7) Indicator lamp (8) Brake tubes. The induction sensors are used to signal wheel speed information to the control unit (computer). Signals are received by the control unit as sinusoid voltages and converted to digital signals for processing in the logic circuits. The main components in the electrical layout are: (1) Battery (2) Ignition switch (10) Alternator 6 I Engineering Design Principles Figure 1.3(a) General layout of a Volvo ABS system 34h Figure 1.3(b) Electronic circuit diagram for a Volvo ABS system Introduction to engineering design f 7~1 (11/2) Fuse (15) Distribution coil (66) Brake switch (85) Speedometer (105) Charging lamp (107) Indicator lamp (252) Control unit (253) Hydraulic modification (254) Surge protection unit (255) Speedometer converter unit (256) Sensor - left front (257) Sensor - right front (258) ABS Fuse box (270) Sensor - rear wheels. It is not possible to give a complete and detailed account of the design of anti-lock braking systems in this text, nor is it desirable. However, it is important to note that a design such as this is very soon superseded. The detailed information presented regarding ABS is reproduced with the kind permission of Volvo Car Corporation, whose designs have become more sophisticated. For example, the S80 is available with the active chassis system DSTC (Dynamic Stability and Traction Control). This system uses a number of sensors, including a yaw angle sensor, to compare the way the car is handling with the way it ought to be behaving. DSTC then retards the appropriate wheel or wheels in order to stabilize the car. Volvo describe DSTC as an invisible hand which keeps the car on the road, even in extremely slippery conditions! The purpose in using ABS as an example is purely to reinforce the stated definition of engineering design and to illustrate the technical and human interface complexities which are encountered in modem systems design. Along with this definition of the design process it is illuminating to consider the job description of an engineering designer. Although this can vary in detail, in general an engineering designer must be capable of dealing with the following: The production of practical design solutions starting from a limited definition of require- ments taking into account many factors. The production of design schemes, analysis, manufacturing drawings and related docu- mentation within defined timescales. The assessment of the design requirements of a particular component, system, assembly or installation in consultation with other departments. The production of designs which will favourably influence the cost and functional quality of the product and improve profitability and/or the company's reputation with customers. The undertaking of feasibility studies for future projects. Negotiations with vendors on aspects of bought out components and equipment, and with subcontractors or partner firms on interfaces. The assessment of the work of others. The personal characteristics, derived from these responsibilities, which a design engineer must possess are: Engineering Design Principles ability to identify problems ability to simplify problems creative skills sound technical knowledge sense of urgency analytical skills sound judgement decisiveness open mindedness ability to communicate negotiating skills supervisory skills. These abilities and skills are possessed by everyone to a greater or lesser extent. They are developed in engineering designers over a period of time, mainly by the practice of engineering design and by exposure to the design process. 1.3 The engineering design process The cost of a product, particularly in international markets, is only one factor which has a bearing on success. Reliability, fitness for purpose, delivery, ease of maintenance and many other factors have a significant influence and many of these are determined by design. Good design is therefore critical for success both in national and export markets and can only be ensured by adherence to a formal design process. The engineering design process in its simplest form is a general problem solving process which can be applied to any number of classes of problem, not just engineering design. It must be remembered that the design process as outlined will not produce any design solutions. The aim in recommending a design process to adopt is to support the designer by providing a framework or methodology. Without such a process there is the very real danger that when faced with a design problem and a blank sheet of paper the young engineer will not know how to begin. The rigorous adherence to the process as outlined later will free the mind, which can become extremely cluttered during a project, so that more inventive and better reasoned solutions emerge. A systematic approach permits a clear and logical record of the development of a design. This is useful if the product undergoes development and redesign. Also, the disturbing trend of law suits against companies and individuals often means that the designer must be able to prove that best practices were employed. This can best be established by reference to comprehensive supporting documentation, such as records of the decisions made and reasons why they were made. If we accept the need for a systematic approach, how and in what order should we consider the influencing factors? There are several suggested systems which vary in detail but are basically similar. Figure 1.4 illustrates the design process as presented by Pahl and Beitz (G. Pahl and W. Beitz (1984) Engineering Design, London, Design Council) and Fig. 1.5 is that recommended by SEED. Study of the two figures reveals an under- lying similarity with the basic process being to identify the problem, generate potential solutions, select from the solutions, refine and analyse the selected concept, carry out detail design and produce product descriptions which will enable manufacture. Quite Introduction to engineering design r Task j 1 o Clarify the task n Elaborate the specification B t I Specification CO < Identify essential problems \ — > I CL Establish function structures cd c Search for solution principles 0 O (D Combine and firm up into concept variants Evaluate against technical and economic criteria 'o c o T Concept Q. < > o 11 Develop preliminary layouts and form designs Select best preliminary layouts t Refine and evaluate against technical and economic criteria T3 o> c I Preliminary layout J) CO < > * iE o N UJ 1 Optimize and complete form designs "5. Check for errors and cost effectiveness O Prepare the preliminary parts list and production documents Definitive layout <. > T c Finalize details w Complete detail drawings and production documents Check all documents Q Documentation > 1 ( Solution ) Figure 1.4 Pahl and Beitz's model of the design process 101 Engineering Design Principles Design core bounded by product specification Information _ Technique Total > Planned "^ < Organized Figure 1.5 Pugh's model of the design process Introduction to engineering design ! 11 obviously for both models to be complete they must be extended to include use and recycling or disposal. The SEED (Pugh) model is the one we will be following throughout the text. As indi- cated by the return lines, design is an iterative process involving much back tracking and parallel activity. This is normal. The principle of iteration is the fundamental principle of the design process. Designing something new is like a voyage of discovery. As the design progresses, more and more information is discovered and more knowledge gained. If the designer does not iterate the new information, concepts emerging would not be acted upon. The systematic approach is not a series of instructions to be followed blindly. There is never a unique solution. Some words of caution. Engineering design is not always a sequential process, nor can it be neatly divided into discrete activities each of which must be fully completed before the next is begun. This is why feedback loops are always included on any diagram of the design process. However, the reader should be aware that even this does not do justice to the necessary continual iteration and that all steps in the design process are often going on simultaneously. Also, an engineering designer is rarely completely satisfied with the solu- tion arrived at. This is partly due to the principle of time. If a company is to maximize its profits from the labours of a design team then the shortest possible time must be taken in getting the product launched. It is inevitable therefore that with a second look the product could be improved. This lack of perfection often causes dissatisfaction and must be accepted as a consequence of working as an engineering designer. The first and most important stage in the design process as outlined in Fig. 1.5 is the formulation of a Product Design Specification (PDS). This is especially important as international trade becomes simpler and competitiveness becomes harder to achieve. Companies must use a logical and comprehensive approach to design if they are to profit from their labours. Therefore an all encompassing problem definition which is used to audit and guide the remainder of the design process is essential. The process of design is always the same and is not dependent on the size or complexity of the problem. However, it is almost always subject to unforeseen complications and a flexible design management approach is essential. 1.4 General example As a simplified illustration of the design process consider the problem of building an extension to a court house. The brief stated that the whole of the works must be carried out with the existing court in full operation and, due to national terrorist activity, that bomb blast protection be provided. The requirement was: A new courtroom Offices, stores and amenities for ushers and clerks A new boiler room. Specification In order to develop a full and detailed specification of the problem many initial investigations were carried out. Drawings of the existing building were obtained but, as is normal, no original strength calculations were available and depth of foundation piles was not known. A site survey was undertaken and a geotechnical investigation revealed Engineering Design Principles that the ground was poor down to depth of 21 m. Samples extracted from bore holes were laboratory tested enabling moisture content, chemical composition and particle sizes to be obtained. Concept generation Having defined the specification, including the relevant British Standards for foundations (BS 8004), structural use of concrete (BS 8110) and notes on blast resilient designs, the next stage was to consider alternative concepts. After initial brainstorming and the consideration of many concepts only three were considered worthy of investigation. These were: (1) add an additional storey; (2) infill at ground floor level and extend at ground floor level; (3) an additional two storey building linked with corridors. Concept selection Concept 1 was ruled out since on investigation the existing piles were found to be fully loaded and the cost of strengthening the structure would be prohibitive. Concept 3 was ruled out because it was impossible to allow for sufficient daylight reaching existing windows. Concept 2 was selected because it presented the optimum solution, when compared with the specification, including customer requirements. Detail design Having made this overall decision, further detailed investigation was necessary, with much engineering science and materials knowledge being employed. Decisions such as whether to use driven or angered piles and use ground beams or slabs had to be made. Above foundation level masonry stability, vertical, shear and bending loading capacities needed to be calculated. Manufacture Once this detail design stage had been completed the construction phase could begin. This phase can be likened to prototype manufacture prior to mass production of products. The project reported was very complex and was completed slightly ahead of schedule within the customer's budget. Many engineering projects do not go so well which, in most cases, can be traced back to a lack of adherence to the iterative step-by-step approach being advocated. In this case the design process was followed rigidly. Effective lines of commu- nication established at the outset of the project were just as instrumental in the success of this project as the engineering expertise employed. 1.5 Engineering design interfaces As outlined earlier it is essential that a design engineer has good communication skills. This can be further reinforced by study of Fig. 1.6 which gives an indication of the most frequently used lines of communication, both within the design department and outside. As explained earlier the design process begins with a design brief or Product Design Specification. This then is the major trigger which causes the design department to act. Two broad types of communication can be identified, internal and external. Those internal communications with the design department may include defining input parameters for computation, discussion and information transfer with other relevant design groups, informing the drawing office by such means as scheme drawings and materials Introduction t o engineering design EXTERNAL INTERFACES INTERNAL INTERFACES! Sales Computation Purchasing ENGINEERING Other design groups DESIGN Analysis/Specialists Drawing office Manufacturing Design originator Commissioning Design reviewers Maintenance Development Subcontract I OUTPUT Engineering drawings Specifications Design intent Figure 1.6 C o m p a n y - w i d e engineering design interfaces specifications, gaining approval for proposals from the originator and answering the questions of reviewers at design audit meetings. External communications both with other departments and outside the company are easier to define than internal communications and are probably of greater importance. Those identified in Fig. 1.6 are the main lines of communication although many others exist. In detail the types of communication with other departments and outside are: Sales There is continuous two way communication between the design and sales depart- ments. The sales department supply customer requirements and the design department supplies technical descriptions, performance data and predictions. Purchasing This is generally one way communication with the design department supplying the technical information essential for the purchasing department to buy in components. Analysis/Specialists Within companies there are many specialists who are often consulted by the design team. These may include standards, materials and stress analysis experts, amongst many others. ri4 1 Engineering Design Principles Manufacturing Although this is indicated as a one way communication process in Figs 1.4 and 1.5, with design supplying working drawings to manufacturing, many other links exist. During all design review meetings at least one representative of the manu- facturing department will be present to ensure optimum manufacturing methods are being specified by the design team. This is part of what has become known as concurrent engineering, which serves to shorten the time taken from initial concept to the produc- tion of the first products for sale. Also, manufacturing departments provide feedback to design and request design changes which ease manufacture. Commissioning and Maintenance This is generally a one way process with informa- tion being fed back to the design department when problems are encountered. Development In smaller companies design and development form one department which is an indication of how closely linked the two departments are. In the develop- ment department tests are carried out on particular aspects of design concepts generally by manufacturing the design and performing accelerated tests or by simulations. The results of these tests are fed back to the design team. Subcontract There are very few companies which have the facilities to manufacture fully everything they sell. Also, it is often cheaper, due mainly to economies of scale, for components to be bought in. It is necessary for the design team, in conjunction with the purchasing department, to communicate with potential subcontractors and to use their expertise. As already discussed, not only do design engineers need to communicate with many different people in many different departments they also require inter-disciplinary skills encompassing the many different fields of engineering. However, the required breadth of knowledge cannot be gained in the classical academic way. As illustrated in Fig. 1.7 industry requires engineers who have complete knowledge in all disciplines. By contrast traditional academic courses tend towards producing people who know everything about very narrow subject areas. There is also a very real danger that engineering designers will not develop the required level of detailed knowledge to complement the required breadth. Thus, in the final illustration in Fig. 1.7 the design engineer is shown as having broad knowledge complemented by 'ears' of detailed knowledge. 1.6 Principles Throughout the book engineering design principles which are identified are presented at the end of each chapter. Introductory principles Iteration Progress towards a solution should involve all the stages identified in order, but much backtracking is essential. This is the nature of engineering design. Compromise A perfect or single solution rarely emerges and the best that can be achieved is an optimum solution. That is a design which best satisfies the customer. Introduction to engineering design [ 1 ^ lit Classical aim of academic education What industry wants r^x^n Danger - no detail knowledge Aim - design engineer Figure 1.7 Graduate profiles Complexity Engineering is a technology, not a science, so along with the engineering science knowledge used the importance of communication, teamwork, project manage- ment and ergonomics cannot be underestimated. Responsibility There is the potential for many failures to occur due to negligence or oversight and the ultimate responsibility for safe and correct 'products' rests squarely on the shoulders of the professional engineering designer. Simplification In general the simplest solution is the best and all professional engineers seek elegant and simple solutions. As a final thought consider the alternative, humorous though cynical, design process suggested by Dr Glockenspiel: (1) Euphoria (2) Disenchantment (3) Search for the guilty (4) Punishment of the innocent (5) Distinction for the uninvolved. Problem identification Here the processes necessary for the definition of a Product Design Specification (PDS) are detailed. As a prelude to writing the PDS much research must be carried out and much information gathered. This is a continual process and is described in Chapter 9. In this chapter the required contents of a PDS are explained and the format of a PDS illustrated by example. The writing of a PDS is the essential first step in every design project. 2.1 Introduction If you were asked to design a corkscrew could you do it? Reference to Fig. 2.1, which illustrates many different types of corkscrew, probably convinces us that the answer to the question is yes. However, why are there so many fundamentally different types? How is it possible for different design teams to set out to design a corkscrew and end up with completely different devices? In a little more detail the corkscrews illustrated in Fig. 2.1 are the plain corkscrew, with from left to right a double helix, lazy-tongs, the waiter's friend, a lever system and a screw pull. The double helix uses both left and right hand screws. One is inserted in the cork and the other forces the corkscrew against the neck of the bottle and removes the cork. The lazy-tongs illustrated provide a 4:1 mechanical advantage. Once the screw is inserted in the Figure 2.1 Corkscrews Problem identification I 17l cork the handle is pulled and travels four times the distance that the cork travels thus reducing the force required. The waiter's friend provides a mechanical leverage which is dependent on the length of the handle. When the screw is inserted in the lever system the levers rise. As they are pressed down the cork is extracted by pushing against the neck of the bottle. In the final device the screw is simply inserted in the cork and the turning continued. The cork *climbs' the screw. All of the devices described rely on the screw to be inserted in the cork. They differ in the mechanical advantage provided, in appearance, in complexity and in production cost. In order that a satisfactory product is designed the market need must be thoroughly researched and a technical specification reflecting customer requirements developed. In the case of a corkscrew constraints such as the mechanical advantage required, the appearance and the production (ex-works) cost must be specified. In the solution of any design problem the design process begins with the defining of the boundaries within which a solution must be found. The project brief as presented to the design team is often incomplete. Hence, research often needs to be conducted and infor- mation sought before a full Product Design Specification (PDS) can be produced. Even if a full PDS is provided it is the duty of the designer to question the validity of that PDS. This questioning approach can often make a customer alter their requirements. As an example consider the problem which was set as one of designing a corkscrew. If the original problem statement had been to design a device for removing a cork from a bottle then many more solutions are possible. Figure 2.2 illustrates two devices for removing corks which do not use a screw, the wiggle and twist extractor and an air pump. In application the two prongs of the wiggle and twist extractor are inserted between the walls of the bottle and the cork. By careful combination of pulling and twisting the cork is removed. The air pump employs a hollow needle which is pushed through the cork. Subsequent pumping action increases the pressure behind the cork and the cork is pushed out. Both of these valid devices were ruled out by the thoughtless problem statement which dictated that a screw be used. As a final thought on this problem it is interesting to consider the problem statement as to remove wine from a bottle. More importantly, the new problem statement is as intended from the outset. If this is the intention then removing the cork may only be one category of solution! As further emphasis of the importance of a clear problem statement consider two wonderful engineering achievements. The two photographs. Figs 2.3 and 2.4 show what was until April 1998 the longest single span suspension bridge in the world, the Humber bridge, and Concorde (the only super- sonic airliner in the world) respectively. Both are elegant and simple in form and each is a Figure 2.2 Cork extractors ri8 Engineering Design Principles »< v:- ^^vsSV> '^^>'.,'^.: \ i Figure 2.3 Humber Bridge (Reproduced by Icind permission of Tlie Hunnber Bridge Board) Figure 2.4 British Airways Concorde, the flagship of the world's civil aviation fleet (Reproduced with kind permission of British Airways) Problem identification magnificent feat of engineering which must be seen to be believed. However, neither has made a profit for their owners! A first draft of a PDS must be developed before any attempt is made at generating solutions to a problem. This is an important discipline since so much time, effort and money can be wasted by providing a solution to the wrong problem. Whilst it is desirable that a fully defined PDS be written before the design process starts it must be recognized that for many projects this proves impossible. The design process is iterative and the PDS must be regarded as a fluid document which will develop along with the design. This is indicated in Fig. 1.5 by means of the return arrows. The PDS is questioned at all stages and reference made to the customer as and when changes are suggested by the design team. However, the aim at the outset is to define the PDS as fully as possible. It is extremely important that prospective customers are identified and that the language used in the PDS can be readily understood. Even within engineering each discipline, mechanical, electrical, electronic, civil and chemical, has evolved a specialist code not readily understood by other engineers. The customer may be involved in a totally different profession and yet must be able to understand fully the PDS. It is the duty of the design team to verify that every function and constraint specified is relevant, correct and realistic. Consequently, it is essential that a thorough investigation of the problem is made by the designer before a solution is sought. For large, complex and diverse problems it is generally worthwhile breaking the project down into smaller, more manageable, sections. In general there are two main tasks which have to be completed if a thorough identifica- tion of the problem is to be achieved: (1) definition of the problem area; (2) formulation of the exact problem. The exact formulation of the problem involves the writing of a comprehensive PDS defining all the required functions which the solution must provide and all the constraints within which the solution must work. The information necessary for addressing these two tasks may be known or could be determined by calculation, by testing and by information search. Wherever possible a questioning approach should be employed and questions should be phrased in such a manner that a specific or numerate response is demanded. The information gathering process, which is a continual process, is explained in Chapter 9 and illustrated in Fig. 9.1. The information inputs required for the PDS are illustrated in Fig. 9.2. 2.2 PDS criteria The main headings and criteria listed here and illustrated in Fig. 2.5 are intended to assist in the writing of the PDS. They are not to be regarded as an all embracing check-list which if followed blindly will completely define any PDS. Design projects are by their nature diverse and substantially different criteria are required from one project to the next. Nevertheless, the check-list will provide a good foundation upon which you, the student engineer can build. Once the project is begun you will find that many of the important criteria will suggest themselves. However, it is true that there is no substitute for experience and you should always be prepared, at any stage of the design process, to ask for help and guidance from experts such as component suppliers. Engineering Design Principles MANUFACTURE REQUIREMENTS Processes Materials Assembly PERFORMANCE Packing Quantity REQUIREMENTS Delivery date ACCEPTANCE Function(s) STANDARDS Appearance Inspection Reliability Testing Environment Standards Ex-works cost Patents Ergonomics Quality PRODUCT DESIGN Weight Noise SPECIFICATION OPERATION DISPOSAL REQUIREMENTS Standards Installation Legislation Use Company policy Maintenance Hazards Safety Figure 2.5 PDS criteria The five main headings on Fig. 2.5, performance requirements, manufacture requirements, acceptance standards, disposal and operation requirements are now considered in detail. Performance requirements Function(s) There may only be a single main function which is to be provided by the product to be designed but this is unusual. More often than not multiple functions can be identified which can be divided into primary and secondary functions. These can vary in nature from mechanical, electrical, optical, thermal, magnetic and acoustic functions to name but a few. The primary function of an engine in a vehicle is to drive the wheels. Secondary functions, such as providing heating inside the vehicle and supporting alternators must also be listed. Loading Loading can be divided into primary and consequential loading. Primary loads are due directly to the required function being provided. Shocks and vibration are generally consequent on the situation in which the product is used. Consequential loading is often very difficult to quantify without empirical data. Specified performance requirements should generally be met comfortably, with some performance to spare. Aesthetics In some instances this is not important, particularly where the device or structure is not seen. However, for many consumer products or structures a pleasing elegant design is required and colour, shape, form and texture should be specified. All visible aspects must be in accordance with the nature of the product and reflect the corporate image of the company. Any statement in a specification which relates to the way a product will look is inevitably more qualitative than quantitative and should include analogy to qualities found in existing products or natural objects. It is possible to use techniques like golden section, which indicates that for aesthetic beauty any shape should be divided into two thirds and one third. Problem identification I 21 | Reliabitity The required design life, taking due account of routine maintenance, must be specified. This is usually done by specifying the number of operating cycles rather than in units of time. Within this number of cycles an acceptable level (%) of random failures or breakdowns is also specified. Where high levels of life expectancy of components exist and it is known that those components will be employed in a controlled environment, such as in electronic circuits, it is common practice to specify the MTTF (Mean Time To Failure) and the MTBF (Mean Time Between Failures). Where reliability is critical, redundancy, either active or stand-by, should be specified. Reliability is inextricably linked with maintenance, even if a maintenance free product is envisaged. Environmental conditions These include the temperature range, humidity range, pressure range, magnetic and chemical environmental conditions to which the product will be exposed. It is important to consider manufacture, store and transport environmental conditions along with the more obvious operating conditions. Also, any physical size restrictions should be specified. This is mainly dictated by the area available to the product when working but is often determined by considering transport and erection. The simplest form of expression for this constraint can be a diagram which forms an integral part of the PDS. EX'WOrlcs cost Companies sell products for the maximum price the market will stand which often bears little relation to the cost of producing that product. Hence, the maximum cost specified in the PDS and which the design team must work to, should be the production (ex-works) cost and not the selling price. Ergonomics (Human factors) If a product is intended for human use then account must be taken of the characteristics of those users. The design of the product and the tasks required of the product and the users must reflect their respective capabilities. The person/product interface, as identified in Fig. 2.6, must be carefully specified. Decisions are based on those functions which can be carried out by products and will vary as capabilities of machines increase. The functions carried out by the user are generally to sense a display, interpret it and make a decision and perform a controlling action. The environment in which the product is to be operated should be specified carefully. For example, if noise levels are high then audible signals to which a user must respond may not be heard. Anthropometrics is the branch of ergonomics which deals with body measurements and it is normal to specify a user population who fall between the 5th and 95th percentile sizes in any particular respect. Any controls must operate in a logical or expected manner. Controls should be placed in easy reach of the operator. Quality The quality of the product should meet market requirements and the quality of all components should be consistent. All workmanship must be in accordance with the best commercial practices. Robust design practices should be used where possible. All materials and components shall be new and free from defects. Weight In some industries, such as aerospace, this is the most critical constraint. However, this is not always the case and weight is not always required to be a minimum. Generally in any product involving motion reduced weight is an advantage whereas a product where stability is critical may require weight to be a maximum. Minimum weight generally means less material which leads to reduced production costs and economic advantages. Engineering Design Principles Figure 2.6 Person/product task division Noise The upper limits of noise levels which can be emitted by the type of product being designed should be specified. Regulations differ from one country to the next so either the standard which applies in a particular country or the lowest maximum limit amongst those countries targeted for export must be specified. These standards represent the maximum level of noise which is acceptable but lower levels could be specified, for example, to gain competitive advantage. Manufacture requirements Processes The in-house manufacturing and forming facilities and the criteria under which external resources are sought should be specified. The required reliability of any source of supply and the required quality should be specified. Any special finishing processes which may be required should also be specified. Materials Materials for both the product and its packaging must be considered and the criteria governing the selection of materials specified without constraining the design team unnecessarily. The many criteria which must be considered are corrosion and wear Problem identification I 23 resistance, flammability, density, hardness, texture, colour, aesthetics and recyclability. There are also many regulations governing the use of hazardous materials which must be included in the specification if relevant. Assembly The method of assembly should be specified; automatic, manual or assembly line. The rate of feed of components for assembly and time allowed are also important parameters. The specification should also contain statements with regard to the ease of disassembly. Packing and shipment The maximum size and weight for convenient transportation must be specified. Shape can also be important since stacking products together can reduce transport costs substantially. Provision of suitable packing, lifting points and locking or clamping of delicate assemblies should be specified to prevent damage during transport. It may also be important to ensure large products can be disassembled and reassembled easily for transport. The cost of packing and shipment must be added to the ex-works cost to ensure that the product remains competitive wherever it is used. Quantity The projected quantity of a product which will be sold can have a profound effect on the manufacturing methods and materials used. This must be specified as carefully as possible at the outset. This particularly influences the appropriate levels of toohng, with large quantities justifying expensive tooling. Delivery date It is important that realistic timescales are set for each stage of the design and production process. This is particularly important when a delivery date has been agreed with a customer and costly penalties for late delivery are built into the contract. Hence, the date by which each stage of the process is to be completed must be specified at the outset. The PDS of a single complex system which is to be designed and produced to an agreed contract will state dates by which the design, manufacture, erection, testing, commissioning and hand over of the fully working installation are to be completed. Acceptance standards Inspection The degree of conformance to standards must be specified in accordance with relevant legislation and the objectives set in the PDS. The degree of conformance required to tolerances as stated within the rest of the specification must also be specified. Testing The methods of verification for the product should be specified along with the timescales for carrying out the necessary tests. It is usual on completion that acceptance tests are carried out in the presence of the customer. Tests often include safety interlocks, load capabilities such as speed and power consumption and reliability. Specified means and forms of testing should comply with standards where they exist. The PDS should contain a policy statement on the level of testing, such as every product to be tested or an agreed level of sample testing. Standards These may include national, international and company standards. There may also be many other rules, regulations and codes of practice which must be followed. [ 24 I Engineering Design Principles Patents Following a patent search it is important to state, and subsequently to ensure, that the design must not infringe any patents identified as being relevant. Patents are useful sources of information, particularly when you are beginning a new project with no previous experience in the particular field. Disposal Standards Individual country or international standards for disposal of products and materials must be listed in the PDS. The main implications should be stated. For example, most plastic materials used now must be identified during moulding of the component so that recycling and more importantly, reuse is made possible. Legislation Any legislation governing the disposal of a product must be specified. Many governments are tightening their legislation with a view to ensuring recycling takes precedence over other methods of disposal, that manufacturers are responsible for accepting products from their last owners and that ease of dismantling and disposal are specified from the start. Also, legislation dictates that all materials used can be easily identified for subsequent recycling or disposal at the end of the life of the product. This must be specified. Company policy Products which make less impact on the environment than similar products will have an increasing marketing advantage. They also afford a company significant advertising opportunities, which will also improve their competitive position. There are many ways of specifying this and only one is to specify increased life. Hazards Any potential hazards that may cause difficulties at the end of a product's life should be identified and specified. Operation requirements Installation Where installation of a product is complex it should be specified. This is particularly important when small numbers of large devices are designed. The constraints should include construction, assembly, the time taken, provision of instructions and the skill levels required for installation. Use The cost of ownership of a product, which should be minimized, is, in some cases, more important than the cost of initial purchase. Factors which influence this, such as the number of operators required, the skill level required from these operators, the cost of spares and the maximum tolerable energy consumption should be specified. Continuous, 24 hour a day, operation or the number of stop/starts in a relevant timescale should be specified. An alternative to dividing costs into separate categories is to specify a whole-life cost. The power sources available should also be specified. These may include manual, gravitational, environmental, electrical, gas, water and internal combustion engines. Each should be specified exactly. For example, electrical power may be three-phase and 380-420 volts. Maintenance A policy to minimize down time, simplify maintenance, ensure correct reassembly, provide easy access and provide interchangeable parts must be developed at the outset and specified. If there is to be any routine maintenance, service or overhauls Problem identification 0 50 SR3 Figure 2.7 Test finger IV. From British Standard 3042:1971 (Extracts from BS 3042:1971 are reproduced with the permission of BSI under licence no. PD\1998 1956. Complete editions of the standards can be obtained by post from BSI Customer Services, 389 Chiswick High Road, London, W4 4AL) the intervals and complexity of these should be specified. In order to simplify the maintenance procedure provision of special purpose tools and disassembly features should be specified if appropriate. The required skill levels of maintenance staff should also be specified. Guards should be easily removed. Levels of lubrication should be specified. An operation and maintenance manual must be supplied. Automatic lubrication should be considered. Safety There are many standards, a great deal of legislation and codes of practice which refer to all safety aspects of products. These should be listed in the PDS. As an example consider Fig. 2.7, which is extracted from British Standard 3042 and shows test finger IV. This is one of a series of probing devices for checking protection against mechanical, electrical and thermal hazards. Where standards do not exist it is normal to specify fail safe design with no sharp edges and that electrical panel isolators must be interlocked with the door, for example. Where headroom over walkways is less than 2 m suitable warning notices and head shock absorbers should be provided. Guards should be specified to eliminate danger to individuals or equipment. 2.3 Content of a PDS As described, much work is required before an agreed or final draft of a PDS is produced. The content of each PDS will differ from any other but the way in which the information is ordered should always be the same. The essential information gathering which must precede the definition of the PDS is detailed in Chapter 9. Assuming the necessary Engineering Design Principles information is available, including the identification of customers and any similar previous specifications, the complete format of the specification should be as follows: (a) Identification: Title, designation, authority, date (b) Issue number: Publication history, previous related specifications (b) Figure 2.8 (a) and (b) Excavator loader (Reproduced with kind permission of JCB) Problem identification I 27 (c) Contents list: Guide to layout (d) Foreword: Reason for and circumstances under which the PDS is prepared (e) Introduction: Statement of objectives (f) Scope: Inclusions, exclusions, ranges and limits (g) Definitions: Special terms used (h) Body of PDS: Performance requirements, manufacture requirements, acceptance standards, disposal and operation requirements (i) Appendices: Examples (j) Index: Cross references (k) References: To national, international, or internal specifications. Not all of the sections which represent the full format are necessary in every case. For example, a foreword should only be included where it would assist the understanding of the PDS. Also, students will find the identification of issue numbers and authority fatuous on many occasions since they are only relevant when working within a company environment. However, just as it is important with detail drawings and components to identify unique identifying numbers so it is with specifications. You should therefore identify a PDS as fully as is possible in the circumstances. 2.4 Sample PDS There are many specifications which run to multiple volumes and include such things as contractual and warranty agreements. It is not possible, nor desirable, to produce such a PDS here, so the example presented contains the level of detail considered appropriate for you as students to produce during your engineering course. The PDS is for a suspension mechanism to isolate the vibrations of an excavator loader, such as the one in the photographs in Fig. 2.8 (a) and (b), from the operator. ISSUE: 3 PRODUCT DESIGN SPECIFICATION for REFERENCE NO.: PDS014 DATE: 17:06:98 UNDER SEAT SUSPENSION UNIT RELATED SPECMCATIONS: ISSUING AUTHORITY: CONTENTS: FOREWORD: The photograph in Fig. 2.8 shows an excavator loader. Historically, prior to 1975, machines of this type did not have any suspension, other than that provided by the pneumatic tyres. Investigations into injuries sustained by operators of farm tractors and off highway con- struction vehicles, like the excavator loader, have revealed a high incidence of back trouble. These injuries are more marked in operators of farm tractors because of the need to observe attachments being towed at the rear of the machine. This means that the spinal column is twisted and the induced stresses are consequently higher. INTRODUCTION: The objectives are: To design a suspension mechanism for the operator's seat. To design the mechanism in such a way that the position and orientation of the seat is fully adjustable. To design the mechanism so that driver vibrations are damped to within acceptable limits. SCOPE: All machines, whether with a fully enclosed cab or a simple canopy, are to be provided with such a suspension mechanism. 28 I Engineering Design Principles DEHNITIONS: PERFORMANCE REQUIREMENTS: The mechanism must allow full adjustment of the seat position. To comply with ISO 4253 these adjustments are rotate through 180 degrees in either direction, 80 mm up and down in the vertical plane and 150 mm front and back in the horizontal plane. Increments of adjustment must be less than 30 degrees and 25 mm respectively. The natural frequency of vibration of the combined seat and operator must be r 1 Select suitable unit or 1 component which provides 1 the best match | > r 1 Seek advice from the [ manufacturer Figure 7.4 Unit or component selection procedure Engineering Design Principles generalized procedure for selecting the elements of a system. It shows the necessity for thorough information gathering regarding the application before selection can take place. It is essential at the outset to define the boundaries within which the element must perform as comprehensively as possible. The information gathered relates to the purpose for which the element is required and to the criteria of life, performance, cost and operating environment with which it must comply. The information is needed in order to understand the total system so that the selection is consistent with the rest of the system. The tempta- tion of the design engineer to consider only functionality should be resisted and time and effort expended at this stage is invariably rewarded when an optimum design is selected. Factors which influence the selection of units and components must now be identified. The most important and common factors governing selection are often performance, appli- cation, geometry, environment, safety and commercial. Not all these factors are important in every design so careful study of the system is required to ensure that those considered are actually relevant. Reference should be made to the Product Design Specification for the system. Each factor should be defined in terms which are as objective as possible. Thus, where appropriate and possible, numerical information should be given, terms must be explained and vagueness avoided. Following this the boundaries of satisfaction must be defined for each of the chosen factors. This assists the design engineer in making a selection which meets the stated requirements in every respect. Subjective judgements cannot always be avoided and in such cases a means of comparison must be established. Manufacturers' data should be collated and arranged in a suitable format. There is a finite number of particular types of component available from manufacturers and the selection process is heavily constrained by the form and content of the information presented by them and the range of catalogues available to the design engineer when the need arises. There is a good case for maintaining a ^rolling' catalogue library, or data on microfilm/computer, since gathering such information can be very time consuming, particularly if a unique set of data is collected on each occasion. Data on size, cost and performance can often be noted in numerical form, giving a range where appropriate. In the case of less objective data a rating may be shown based on advice or opinion gathered. Optimizing the choice is now a matter of selecting the best compromise, in the opinion of the design team, between the priorities of the system and the availability of hardware. In the initial stages some pruning of potential alternatives is called for. As far as the factors involving numerical data are concerned, some yield a go/no-go situation which will eliminate those which do not fit within the boundaries set. Other requirements of a more subjective nature should be compared on the basis of the elements' ability to meet the criteria as laid down in the Product Design Specification. The evaluation technique used here is similar to that used elsewhere in the design activity, particularly for initial concept selection. References for further reading are included at the end of each subsequent chapter and many elaborate on the details of a variety of techniques. Further advice on the detail of installation or specifying and ordering will be required from manufacturers' information. Normally this would be available in a catalogue but often it is necessary to communicate directly with a representative of the company. As an example of a more detailed component selection procedure consider the decisions which influence the type of spring most suitable for a particular application. The flow chart in Fig. 7.5 illustrates this. The selection of a type of spring depends mainly on the space available and the magnitude and direction of the loading. One further important Detail design GAS IS ZERO SPRING CONSTANT SPRING RATE REQUIRED? FORCE SPRING IS LOADING AXIAL? f- HELICAL TORSION SPRING yes IS LOAD COMPRESSIVE? IS SAFETY yes ^ 'CONSIDER A ' CRITICAL? ^ GAS SPRING iftAQ CDDIKin I yes REDESIGN FOR IS SPRING RATE yes COMPRESSION > 80 N/mm? IS THE LENGTH RESTRICTED? Z HELICAL EXTENSION SPRING yesi IS THE SPRING HELICAL COMPRESSION RATE > 650 N/mm? SPRING yes DIE SPRING DISC SPRING Figure 7.5 Selection chart for spring type consideration which influences the selection of a particular type of spring is that for some applications safety codes require that a compression spring is used. This is because a failed compression spring can continue to provide a stop and hold components apart, in effect providing a fail safe design. The flow chart in Fig. 7.5 illustrates the level of thought which must be applied to the smallest detail in any design. It only takes the failure of a relatively insignificant component to render the whole design worthless. 7.4 Robust design Design for reliability The reason for the steady increase in reliability engineering stems from the increasing awareness that the cost of ownership of a product or system comprises two components. The first is the capital cost and the second is the cost of operating, administering, maintaining and replacing the product or system. The second outlay, the running cost, can often exceed the capital cost and is a function of reliability. Indeed, because of the disastrous financial consequences of equipment failure most customers specify tightly reliability conditions. One hundred per cent reliability testing is unthinkable since this implies that there would be no products for sale. The time required for reliability testing depends on the failure rate of the items under test. In general reliability adds cost to a product and although unreliability carries with it a cost penalty, the optimum level of reliability is always a Engineering Design Principles Manufacturer's costs Manufacturing cost Cost after delivery Reliability Figure 7.6 Cost of reliability compromise between the two. Figure 7.6 shows the general relationship between reliability and cost. Reliability is concerned with the causes, distribution and prediction of failure. Failure is defined as the termination of the ability of a component or system to perform its required function. The parameter, ^failure rate' is given the symbol X(t). Another method of describ- ing the occurrence of failures is to state the mean time between successive failures. The two terms used, the mean time between failures (MTBF) and the mean time to fail (MTTF) are explained diagrammatically in Fig. 7.7. In many, but not all, cases MTTF and MTBF are the same. MTTF is the mean operating time between successive failures and the difference between the two terms is repair time. Hence, MTTF + mean time to repair = MTBF Components or systems which are not repaired do not extend beyond the point marked A in which case MTTF and MTBF are the same. The failure rate is not necessarily constant. If a reliability test were undertaken with a large sample and each product were tested until it failed and not replaced, the typical MTBF MTBF MTBF MTTF MTTF MTTF Operating Under repair Failure' Figure 7.7 Difference between MTBF and MTFF Detail design I 111 I Observed -^.- Figure 7.13 Determining high stress regions homogeneous with the original metal. As illustrated in the final diagram the pipe flow analogy shows two potential areas of high stress. These are caused by the lack of penetra- tion of the weld causing a blockage in the centre of the flow and the comers of the weld being sharp at the point of expansion and contraction of the flow. It is already established that a sudden notch or change in section will cause local stresses to be significantly increased. Extending the flow analogy a little further, it is at first sight surprising that removal of material to give a smooth profile can also extend fatigue life. Figure 7.14 shows a few examples of poor weld shapes which could be filled out by extra weld or could be dealt with more effectively by grinding away the sudden change in section. Many manufacturing processes leave residual or built in stresses in the component or structure. Most common of these are tensile stresses due to differential cooling following welding. These will relax with time but there are various techniques for speeding the process. Shot blasting, sand blasting or any similar process which compresses the surface reduces significantly any built in tensile stresses. In stress relieving the structure is heated and then allowed to relax. Imagine a small crack in a component of width 4 jim. The fatigue damage from each cycle will be proportional to crack movement, so a tensile load which opens the crack to 8 pm before allowing the component to relax does 4 pm worth of damage. In the same way, a compressive load closing the crack from 4 pm will do 4 pm worth of damage. A tension followed by a compression would do 8 pm worth of damage. Heating the structure to around 650°C followed by a slow cooling allows local high stresses to even out and cracks to reduce in width as a result. Following stress relieving, tensile loads still have the same Detail design 0rmd tofelandundercut G f W to blend out poof stait/slop puMe.sL.>AA>:i^>>;h-iv>;4>s^ik»>^";^^^ Grmd to improve shape of fillet Figure 7.14 Removing material improves strength damaging effect. However, since the crack is now closed up the damage caused by compression is reduced by approximately 75%. There is a great deal of very advanced science in the most refined areas of fatigue life prediction, but a good deal of common sense and the application of these basic principles should ensure that many potential failures are avoided. Materials which exhibit a greater resistance to fatigue should be selected for critical areas of a design. For example, ceramic materials are not thought to be susceptible to fatigue at all. Although general awareness of the phenomenon of fatigue began with the Comet aircraft disasters some years ago, the underlying principles were appreciated more than a century ago by the British engineer Sir William Fairbaim, who carried out classic experiments on wrought- iron girders. He found that a girder which was statically loaded would support 12 tonf for an indefinite period but would fail if a load of only 3 tonf were raised and lowered on it more than a million times. The quantification of the number of cycles that a product will last for, fatigue life, is based on testing data. There is a great wealth of this data which it is beyond the scope of this text to reproduce. However, the factors which must be taken into account in design are: mean stress alternating stress material ultimate tensile strength the type of loading: bending, axial or torsion the surface finish (the rougher the surface the lower the number of cycles the component will complete) the effect of any stress raiser the size of the component (since only relatively small components are normally tested to destruction). Engineering Design Principles 7.5 Principles Detail design principles Optimization The search is for the best compromise between conflicting criteria. Simplification Where possible rely on the expertise and knowledge of specialist manufacturers by using proprietary components. Analysis Ensure that all components have appropriate factors of safety and are not over designed. Robustness The designer should aim for a product which is fit for the purpose intended for the lifetime intended. Synthesis A solution is often arrived at by a combination of techniques and elements. Iteration Progress towards the production stage is made iteratively as knowledge of the important factors grows. 7.6 Exercises 1. Bolts installed on a production line are tightened with automatic wrenches. They are to be tightened sufficiently to yield the full cross-section in order to produce the highest possible initial tension. The limiting condition is twisting off the bolt head during assembly. The bolts have a mean twisting off torque of 20 Nm with a standard deviation of 1 Nm. The automatic wrenches have a standard deviation of 1.5Nm. What mean value of torque wrench setting would result in only 1 in 500 twisting off during assembly? 2. A shaft tolerance has a standard deviation of 0.01 mm. The hole tolerance has a standard deviation of 0.016 mm. The difference between the means is 0.045 mm. If the entire production is accepted for assembly, determine the proportion of assemblies with clearance less than the allowance of 0.01 mm and the proportion of assemblies expected to interfere. 3. A unit has a constant failure rate of 0.3% per 1000 hours. What is its MTBF? What are the probabilities of the unit successfully completing missions of 10000, 100000 and 1000000 hours? Design management Two different levels of design project management are presented, those most appropriate for student project work and those more advanced methods employed by companies. The relatively new International Standards covering design for quality are explained and the engineering design management control process outlined. Techniques covered include project planning and control by means of bar charts and Programme Evaluation and Review Technique (PERT) network analysis. An introduction to Quality Function Deployment (QFD) is presented, the requirements for formal design reviews are explained and the Value Analysis technique discussed. 8.1 Introduction As Chapter 1 illustrates we have been designing products for many centuries. Despite this the standards of many modem products in terms of design leave much to be desired. Most people have experienced products which are ugly, difficult to use, unreliable, difficult to maintain, too costly or not intrinsically safe. It is also true that even today many products are very difficult to dispose of at the end of their useful life even though design for recycling is acknowledged as of paramount importance. A recent survey conducted by the UK Design Council concluded that an average product could be redesigned to reduce manufacturing costs by 24% and to improve market demand by 29%. The message is that design must be managed more effectively. The assertion that design is a creative activity and cannot be managed is not true. Reference to the design process shown in Fig. 1.5 illustrates that creativity is just one section of the design process as a whole and that much of what is called design can and must be controlled. Much more complex tasks, such as planning, opening and running a new factory or developing a new market, are successfully managed. It is essential that companies manage the design process and it is equally essential that students manage project work with the same rigour. It is also important to manage the design function since, as illustrated in Fig. 3.1, the cost of manufacturing a product is mainly dictated by design. Many enlightened companies recognize this fact and are placing much greater emphasis on the early stages of the design process. Upstream design changes do not cost much. As we proceed downstream we find that the cost per phase increases exponentially. The prime, and some would say only, reason for a manufacturing company to exist is to make profits for shareholders. If a company does not achieve this then it will go out of business. The design function has a major role to play in this profit making and competi- tive and therefore successful products are essential. It is possible to make enormous profits from single innovative ideas for a short time, without a professional and comprehensive design approach. However, if a company wishes to stay competitive over a period of time then a well managed, resourced and modem approach to design is essential. 126 I Engineering Design Principles Board of directors H Legal Marketing | 1 Sales | 1 Technical ] 1 Finance | 1 Manufacture] Quality manager Development Chief Research manager engineer manager Design Drawing office manager manager Design Section engineers leaders I Draughtsmen | Figure 8.1 Typical technical management structure A necessary precursor to successful management of design is that the company wide management structure be clearly defined. The company wide interfaces with design were discussed in Chapter 1 and illustrated in Fig. 1.6. It is clear that the design department must communicate with many other departments within a company on a regular basis. It is important also to consider the typical management structure as illustrated in Fig. 8.1. The major divisions are generally sales, manufacturing and technical. The chief engineer is normally responsible to the technical director and is responsible for research, development and design. Design engineers generally work in teams and the major output is data, in the form of scheme drawings and supporting documentation, which is fed via the drawing office manager to draughtsmen. 8.2 Management of design for quality In recent years there has been a recognition that improvements in the quality control of the design process itself were necessary. Figure 8.2 illustrates the major stages of the design process with the necessary management control elements. These elements are project planning, Quality Function Deployment (QFD), design reviews and value analysis/ engineering. QFD is a series of management control matrices which helps keep customer requirements firmly to the fore during all stages of a particular project. Each of these management control elements is covered in more detail later in the chapter. The management of design for quality is governed by standards with which an accredited company must comply. Most countries have adopted the international standard whose constituent parts have collectively become known as ISO 9000 and include ISO 9000-9004. ISO 9000 is a guide to the selection and use of the appropriate part of the ISO 9000 series. Design management [ 127] Design brief Project planning PDS QFD Stage 1 - Product characteristics Concept generation Concept selection Preliminary design review Embodiment QFD Stage 2 - Parts characterisitics Value analysis/engineering Intermediate design review Detail design I Testing Final design review Production Figure 8.2 Engineering design management process ISO 9001 relates to quality specifications for design, development, production, installa- tion, and servicing when the requirements of goods or services are specified by the customer. ISO 9002 sets out requirements where a firm is manufacturing goods or offering a service to a published specification or to the customer's specification. ISO 9003 specifies the quality system to be used in final inspection and test procedures. ISO 9004 is a guide to overall quality management and the quality system elements within the ISO 9000 series. Collectively these standards set out how a company can establish, document and maintain an effective quality management system which demonstrates to customers a commitment to quaUty. The standards cover the whole range of company activities and the management of design is (in the main) covered by ISO 9000 and 9001. The needs are to 128 I Engineering Design Principles establish and control the functions of design planning, assigning activities to qualified staff with adequate resources, controlling interfaces between different disciplines, documenting design input requirements and design output in terms of requirements and calculations. Design output must be assessed to verify that it meets design input requirements and documented procedures must be used to control all design changes and modifications. Careful, planned and documented control at each stage ensures a smooth passage from concept to end product. There are many stages and methods recommended by quality standards for the effective control of the design process. These are simplified and represented in a form which could be used in student project work. (1) A design programme which breaks down the design process into separate elements and is presented as a chart of the design activities against time. The chart should show major events such as design reviews so that progress can be measured. (2) Design management control procedures should be documented to show the relation- ship with various departments and with contractors. It is essential that the design team is not isolated from staff of other disciplines. Design responsibilities should be defined at a level appropriate to the design task. Design documentation should consist of numbered drawings and specifications, log books, calculations with assumptions, sketches, the design concept, analysis and test results. (3) If innovation is proposed, then the desirability of each innovation should be assessed by analysis and/or testing in order to justify objectively its adoption in preference to established alternatives. For the introduction of a new material, tests must be conducted and results compared with the stated performance specification. (4) Procedures should be prescribed for the identification and revision status of design documents, records of changes made and their distribution, control, recall and for the approval of all document changes by the person responsible for the design. (5) Design control should ensure that tolerances are adequate to provide the required quality. They should be no tighter than necessary. (6) National and international legal requirements, including health and safety standards, placing constraints on designs should be identified. (7) Reliability requirements should be included in the specification. The specification should contain details of the proposed reliability testing programme and how the test results are to be analysed. (8) Value engineering tasks should be undertaken and documented. (9) Systematic procedures should be established to ensure the use of data gained from previous designs and user experience. (10) Regular design reviews, which are documented and systematic critical appraisals of the design, must take place. The objectives are to ensure that the design satisfies the specification, that other viable solutions have been considered and that the design can be produced, inspected, installed, operated and maintained in a satisfactory manner. The design reviews must also ensure that there is adequate supporting documentation defining the design. It is obviously impossible outside a company environment and particularly during a student project for all of these factors to be taken into account. However, some form of control of the design process is always desirable. The steps recommended for the manage- ment of a complete engineering design project are listed in Fig. 8.2. Design management 1129] 8.3 Project planning and control The two primary tasks involved in project management are planning and control. During the planning phase project timescales, costs and resources must be identified. During the project the emphasis is on the use of simple methods which will readily provide key information for the control of the project without creating too many demands on time and resources. Gantt chart A simple bar chart, as illustrated in Fig. 8.3, is probably the most widely used technique for monitoring of projects. Bar charts can be developed for a whole project, as illustrated, or can be used to identify the effort required by particular personnel at particular times. The chart in Fig. 8.3 is unusual since milestones, in the shape of design reviews, are indicated. Also, in the industrial environment it is more usual to employ months as the units of time. However, student-based project work is of necessity generally of relatively short duration and weeks are more appropriate. The horizontal bars on the chart are all blank at the start of a project and are filled in as work progresses. In Fig. 8.3 the project has reached the end of week 6. The chart indicates that the embodiment work is ahead of schedule but that the cost estimate for the selected concept is not complete. This could be critical since the careful management of costs must run in parallel with the management of time and decisions made during the preliminary design review are based to some extent on cost information. In short the chart indicates where efforts need to be increased if the original targets are to be met. Weeks Activities 1 1 2 3^ ±^ 5 J 2_ 8 ^ 10 V[ ^2 13 14 "Tsl PDS rj -.^... 1 Project planning 1 QFD1 1 Concept generation Concept selection 1 Cost estimation cz] cm czj Embodiment QFD2 1 Value analysis 1 Detail design 1 Bought out parts Make prototypes 1 1 I Testing | 1 1 Design reviews \ ^ jk. j^ Figure 8.3 Bar chart for design project 130 I Engineering Design Principles It is essential that for all project work, particularly student design project work, that as detailed a bar chart as possible is developed at the outset. It then follows that the chart should be regularly updated and the bars filled in so that progress can be monitored. It is worth noting that one of the main aims in project planning is to reduce lead times to the minimum possible so that a product may be on sale and earning profits at the earliest opportunity. This is why there should be a significant amount of overlap of the activities on any bar chart. One disadvantage of the bar chart is that the complex interdependence of activities and the dependence of one activity on the completion of another is not clearly indicated. PERT networks There are many network techniques and the Critical Path Method (CPM) and Programme Review and Evaluation Technique (

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