DFM 2 Final 1

Chapter 2: Design Process

Objectives: Upon completion of this chapter the reader will be able to:1. Establish ground rules for any design process.2. Comprehend and implement the principles of product

design & development.3. Comprehend different stages of design activities.4. Implement the activities.

2.1 INTRODUCTION TO DESIGN PROCESS

In our present day life, whole environments are man- made. (Right form a needle to the most sophisticated aircraft or spacecraft systems). So we can say that, manufactured products are dominating very great part of our life. Design, construction, usage and salvation of these manufactured products rise from the need of living things.

(e.g.) Individual human beings, group of individuals, or a nation.

The need may arise because of the desire, want, interest, motive, drive or necessity of these living things. Generally speaking every industrial product whether it is a simple device like a tool or complex systems like a flexible manufacturing system, it is conceived, constructed, exploited and salvaged to satisfy the needs of the living things. To analyze this generalization let us consider the following products and systems.

(e.g.) A plough, a house, a bicycle or car, a restaurant.

Let us ask ourselves the following question.

“Why the above products or systems came into being?”

The answer to the above question obviously will be “to satisfy certain needs”. These needs may be classified as the needs of the manufacturer/seller/owner and the needs of the user. For example the plough is satisfying the need of the user which is ploughing his/her plot of agricultural land. On the other hand, the plough is also satisfying the economic need (financial income) of its manufacturer and seller. The same way a house or a building

Prepared by Prof.R.Panneer, Assistant Professor.

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22DESIGN PROCESS

Design for Manufacture, Design Process and DFX

which was constructed by a contractor satisfy the economic needs of its constructor, and the economic needs of the seller of the building materials, on the other hand the house or the building also satisfy the need of its owner namely sheltering. Similarly the car or the bicycle, which is used for transporting the people or the restaurant, which is providing drinks and food to the people, satisfies the economic need of the manufacturer of the car or bicycle and the owner or proprietor of the restaurant.

To conclude, we can state that “It is the need of the human beings (even other living things) for better living that leads to the conception, creation, conservation, exploitation and salvation of countless industrial and domestic products”

So, the life cycle of a man-made product or system (industrial/Domestic) can be shown as follows.


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Fig: 2.1 Life cycle of a Product/System

Statement of the need

System/Technical/Functional Analysis and

Design of the Product/system

Manufacturing and construction

Usage/exploitation

Salvation

Design of the geometric configuration shape

description/ form design /factors

Design & analysis ofDimensions/Size

Description/Theory of Dimensioning

Design & analysis of mechanisms for strength,

forces, moments & movements

Conceptualization and Creation of the Product/system

Need analysis



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Need for Product

Development from different

sources

Need Analysis

System Analysis

Functional Analysis

EvolveProduct Concept

Total structure design

Individual form design

First breakthrough!!Shape developed!

Analysis of Mechanisms

Analysis of forces

Analysis of strength,

vibrations, & other aspects

Analysis of Dimensions

Detailed Design

General Assembly

Component/parts

Pre- production prototype

Physical Performance

Testing

Technical breakthrough working prototype

developed!!!

Manufacturing systems design (Tooling design,

production planning etc.)

Production prototype

Concept options

Shape design options

Contd..

Contd..Need

coming from

different sources

Initial design analysi

s

Shape design

Evolving concept

Detailed design

Manufacturing systems design

Proto-type development

Fig 2.1 (contd.) Design activities at different stages of product development

Legend: All dashed lines indicates the iteration and revision process based on options and feedbacks


As can be seen from the above life cycle (fig 2.1)the creation of a new product or systems takes in many levels of activity and many skills as shown in the block diagram (fig 2.1 contd.)above.

The main aim of this text book is to analyze closely some of these design aspects like creation of a product, form design, form factors and appearance of the product. Apart form the above topic, this text book will also discuss topics such as Need Analysis, System Analysis, Technical Analysis, and Functional Analysis. Hence, the contents of this text book should be seen only as apart of design technique.

2.2 NEED ANALYSIS AND SPECIFYING THE NEEDS:

2.2.1 DEFINITION OF TERMS

For clarity and comprehension reasons, a brief explanation of the following terms has been outlined below.

2.2.1.1 Design

The word” design” has many meanings including the following. To plan, conceive, invent and to designate so as to transmit the plan to others. Design means creation of the purest sense. It is the initial process in the construction, manufacturing etc. of products.

2.2.1.2 Construction

It is the process of materialization of the design layout. Construction could be experimental (prototypes and models) for testing or manufacturing purposes, or final as in the case building and road constructions.

2.2.1.3 Manufacturing

It is the process of industrialization of a product for consumer applications. Manufacturing could be unit/job/batch/mass production using machine tools.

2.2.1.4 Mechanical design and construction

Unlike what is commonly called building construction (construction of buildings, roads, bridges etc.) mechanical design and construction deals with the creation and construction of mechanically or electro- mechanically operated mechanisms such as machine tools, devices, equipments or systems.


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Fig 2.2 Relationship between Design, Construction and Manufacturing

2.2.1.5 Mechanisms

In the context of book, a mechanism will be defined and interpreted as” every mechanical or technical product” (e.g. a tool, a machine, an equipment, a device, or a systems) which has been conceived, constructed and/or manufactured and exploited in view of satisfying a certain need of living things.

2.2.1.6 Technical object.

A Technical object is every piece, every tool, every equipment, every machine or in other words every mechanism that functions with an input or a set of inputs to produce an output. By analogy, with the science of life, which considers human being like a functional biological unit, we say that every technical object is a functional unit. The global function of the technical object is to bring an added value to the input, which is the output.

Fig 2.3 Technical object

However everything depends on the manner how we consider the system or technical object. That is for an user, an electric bulb is a functional unit and thus it is a technical object But for the manufacturer of the electric bulb filament is a functional unit and thus it becomes a technical object.


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Design

Construction

Model or Proto Type Product Assembly or Systems

Manufacturing

Product, Assembly or Systems

Mechanism OutputInput


2.2.1.7 Need.

Need may be simply defined as the desire or necessity for a given industrial product, expressed by a person termed as us user. A given need may be accurately specified through need analysis. The need may be satisfied through design, construction and exploitation of the desired product.

2.2.2 NEED ANALYSIS

It is a method which tries to analyze systematically the kinds and qualities of forces at work and the manner of their interaction in satisfying the need. It consists of listing the user needs for the design in brief succinct phrases. Each user need should be identified with the basic need it represents. Needs are identified at many points in a business or agency. Most organizations have research and development components whose job is to create ideas that are relevant to the needs of the organization. Needs may come from the inputs of operating or service personal or from customers through sales or marketing representatives. Other needs are generated by outside consultants, purchasing agents, government agencies, or trade associations or by the attitudes and decisions of the general public.

Needs also arise from dissatisfaction with the existing system, or situation. They may be to reduce cost, increase reliability, or performance, or just change because the user has become bored with the product. The first, foremost and the most critical step in the need analysis is the definition of need or statement of the problem. The definition of a problem should include writing down formal problem statement which should express as specifically as possible what the design is intended to accomplish. The true problem is not always what is seems to be at first glance. Because this step requires such a small part of the total time to crate the final design, its importance is often overlooked. Fig 2.4, 2.5 and 2.6 illustrates how the final design can differ greatly depending upon how the problem is defined.

It is advantageous to define the problem as broadly as possible. If the definition is broad you will be less likely to overlook unusual or unconventional solutions. But in most cases, the extent to which you are able to follow a broad problem formulation will depend on the importance of the problem, the limits on time and money that been placed on the problem, and your own position in the organization.


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Fig 2.6 Note how the design is finally installed in relation to what is expected


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As proposed by the project sponsor

As specified in the project request

As designed by the senior designer

As produced by manufacturing

As installed at the user's site What the user wanted

Fig 2.4 Note how the design depends on Project Request

Fig 2.5 Note how the design depends on Designer and Manufacturer


The minimum definition of problem should include writing down a formal problem statement, which should include goals and objectives, definitions of any special technical terms; the constraints placed on the design and the criteria that will be used to evaluate the design. Perhaps the best way to proceed is develop a problem statement at the initial problem definition step and then in the second iteration after much information has been gathered, develop a much more detailed problem statement that is usually called the Need Analysis. This need analysis should be used to derive at the final problem analysis and it should be stated in the project document. Project documents are contractual documents established between the inventor or designer of the technical object and it’s future user termed as client.

The above process of initial statement of problem, gathering information preparing final statement of problem or problem analysis and preparation of project document for Need Analysis will be illustrated here with an example.

2.2.2.1 A Design example Need Analysis Step: 1 Type of Need or formal need statement

It can be easily deduced that the technical object with the above description is a ladle hook. Therefore we can proceed as follows.

Need Analysis Step: 2 First iteration - Preliminary information gathering


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A lifting object to be used at the end of the crane hook, which is capable of lifting transfer ladles along with molten metal from one place to another place to be used in a steel- melting furnace.

To transport the molten metal in the

ladle from one place or another placeWhy it is necessary?

Ladle and Crane

Hook

On what type of inputs does the mechanism or technical object depends?

Which is the technical object?

What is expected to accomplish?

Fig: 2.7 Need Analysis Step 2

Lift the ladle in empty condition and with molten metal


The above problem statement is too fragmentary. Many questions must be answered before we have sufficient information to complete the problem statement. In order of find out what questions or details/ information may be asked for? We may have to go in for a System Analysis.

2.3 SYSTEM ANALYSIS AND SPECIFYING THE SYSTEM STUDY:

2.3.1 Definition of Terms

2.3.1.1 System

Interaction between the human be being and his product with a certain environment.

2.3.1.2 Sub- systems

The individual parts that make –up the whole structure of system.

2.3.1.3 Open system

The one that interacts with the Environment

2.3.1.4 Closed system

The one that does not interact with the environment. Actually any system interacts with its Environment and the task is to determine the degree of openness.

2.3.1.5 Boundaries.

They are the points at which the product is in interaction with the external environment. A boundary must be well defined.

2.3.1.6 Flow

It is the movement of the material and the human energy through a system.

2.3.1.7 Input

It is the first phase of any system in which data, labor and other energy, materials, equipment and money is received from other system.

2.3.1.8 Process

It is phase of the system that changes or transforms input into a desired form is called as process (occasionally this phase of the system is called as processor)


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2.3.1.9 Feedback

It is the regulating force that compares the systems’ output ( what was produced) with the Standards of performance set for the system (what should have been done or produced?)

2.3.1.10. Control

It is the systems phase that dictates what can and can not be done in each of other phases.

2.3.2 INTRODUCTION TO SYSTEMS AND ENVIRONMENT

In our previous discussion, we said that an industrial product is conceived, constructed and used to satisfy a need. It follows that the human being is the master in all the above activities.

He/she manifests his/her wish or will. He/she plans the design of the product. He/she manufactures and uses his/he product. He/she discards it when it is not needed.

So, we can say that the human being is in continuous interaction with his creation. (i.e. product) We can also say that this interaction takes place within a certain surrounding environment is called a system.

Fig 2.8 System and Environment

A system in this case is a group of things or parts and the human being working together for the purpose of satisfying a need.

2.3.3 SYSTEM APPROACH TOWARDS NEEDS SPECIFICATION AND SATISFACTION.

The system approach towards need specification and satisfaction


External Environment

Internal Environment

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attempts to view the interaction of the human being and his product within their surrounding environment as a single integrated system of sub- systems.

It focuses on what role each part plays in the whole organization of the system. To study a need, crate and construct a product to its satisfaction, therefore will require the construction of a system mode shown in Fig 2.9.

Fig 2.9 Systems Approach

First of all a designer should establish whether he has got the required resources (shown in Fig 2.9) to construct a designed product. Without the possible availability of the minimum required resources to manufacture a product there is no point in making a design.

Hence using the systems approach towards need specification and satisfaction the designer should make sure that all resources are available at the disposal of the organization/Agency for which the design is made and then he can proceed with the design process.

Before proceeding further on the system analysis first let us discuss the various levels of systems (viz.,) manual systems, mechanical systems, automated systems etc.

2.3.4 SYSTEM LEVELS

Systems may be classified in terms of processing power and the degree of automation involved. They are

Manual systems: in a manual system the earliest and still most prevalent type of system is a system where the human being is the data and the process processor. Remark: Animals can also be integrated as process processors in this system.

Mechanical systems: Also called electro- mechanical system where most of the effort needed for the process is furnished by mechanical or electro- mechanical sub- systems such as wind, water electricity, chemicals etc.


Resources:Man PowerMaterialMachineInformationMoney

InputProcessTransformation

OutputProducts

and Services

SYSTEM

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Automated systems: Unlike the other systems, the involvement of the human being in this system is minimal or non- existent. In this system the process is controlled by programmed information

2.3.5 SYSTEM STUDY.

Generally the process of creating or improving a system is called system study. In order to create a system first we have to determine the inputs, commanding part and other interacting environments, which are shown schematically below.

To illustrate how the above are determined, let us consider the lifting hook again as an example for a technical object and continue our discussion further as below.

Fig 2.10 Elements that influence the use of a technical object

The internal, integral and related elements to the technical object

2.3.5.1 Inputs

The technical object operates on one or several inputs. The use of these inputs must be specified with precision on the project document. We can say that every input is an element that influences the use of the technical object. The principal input of the Lifting Hook is the ladle with the molten metal or the empty ladle. The output being lifting the ladle with molten metal or in empty condition. The project document therefore must specify the characteristics of the ladle and the molten metal like.

The volume & weight of molten metal that ladle should carry

The empty weight of ladle. The dimensions of the ladle


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Input

Command

Energy Environment

TechnicalObject

TechnicalEnvironment

PhysicalEnvironment

HumanEnvironment


The above details should be written down in the project document as a specification.

2.3.5.2. Commanding part.

Every technical object will have a commanding part or otherwise it will be controlled by some commanding part. So, We can say that, the commanding part is an element, which influence the use of a technical object.

It is the user (Human- being) and the Crane who is the commanding part of the lifting hook. The user lifts the hook, load the hook on the ladle, and remove the hook from the ladle. The crane lifts the ladle and moves the ladle using the hook. So the hook must be adaptable to suit the crane attachment, the crane’s carrying capacity, it’ s speed of movement and the user’s hand.

2.3.5.3 Energy Environment

Any technical object is operated by a particular type of energy or it operates under particular energy environment. So, the energy environment is also an element which influences the use of the technical object.

The project document should specify how energy is used by the technical object or otherwise it has to state the relationship between the energy environment and the technical object.

It is evident that the crane and the user again who/which furnishes the required energy for manipulating, loading, removing and moving the hook. The shape of the hook which receives the user’s hand and the muscular energy have to be adaptable to the shape of the hand, to the intensity (magnitude) of the body, and to the muscular energy, which a person normally exerts in various positions.

Note: The study of the shape and movements of the human organs will include study of ergonomics which is further discussed later in the text.

The hook should be also adaptable to the carrying capacity of the crane.

The external elements to the object

Finally the output must be obtained within a certain environment or external environment. We will be able to distinguish three different elements, which influence the use of ht object.


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2.3.5.4 Technical environment

It is the industrial and technical environment of the object, which influences the use of the object. In reality the technical object could be integrated into a vast structure in which it enters in relation with other technical objects. Hence, the project document should specify the nature of the technical environment and its’ relations with the technical object.

In case of the lifting hook these relationship can be different depending on:

its use, that is whether it is to be used with a particular type of ladle or with different types of ladles

whether it is to be used with a particular type of crane and crane attachment

whether it is to be used with different types of crane and crane attachments.

In the above example the technical environment is highly reduced or simply it associated mainly with the type of ladle and crane with which it is to be used. But in case of complex technical objects like large systems the technical environments and the relationship will be also very complex.

2.3.5.5. Physical environment.

Generally speaking, it is the physical universe or certain physical phenomena that influence the use of the technical object or vice- versa. For example the presence of humidity in the air brings about corrosion to metals or it may spoil the wood etc. The project document should therefor specify the specific or proper physical environment that is required for the proper function of the technical object over the stated period of time.

The physical environment of the lifting hook may be.

the climate existing inside the steel- mill ( humidity, temperature etc.)

the temperature of the molten-metal. The temperature of ladle etc.

2.3.5.6 The human environment.

The human environment consists of the human being and the living things at large. The use of certain technical objects can cause accidents including deaths. In a general manner the project document must include the influence of technical object towards


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the human environment in view of assuring its’ safe working conditions.

The effect of pollution or the pollution that will be crated by a technical object might raise a question to its necessity (e.g. Atomic power plants)

Action and reaction of its manipulation errors (e.g. Accident occurred at Chernobyl Atomic power plant because of manipulation error.)

Generally it is possible to represent schematically the influencing elements/ environments on the use of the lifting hook as follows.

Fig 2.11 Elements that influence the use of a lifting hook

2.4 FUNCTIONAL ANALYSIS AND CREATION OF STRUCTURE & FORM:

2.4.1 Properties of the product

Any object (product, machine, or systems) possesses characteristic properties. Some of these properties may be desired, but others may be more or less unwanted. The most important property of all is the primary function of the product, because it is this that helps the user in his need. The other desirable properties may be: pleasing appearance, ease of handling, safety, durability and reliability, maintainability, ease of manufacturing etc. Before the product is designed the designer should list the required


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INPUT(Ladle with molten

metal)

ENERGY ENVIRONMENT

User and Crane(Hand And Muscular

Energy, Lifting Energy)

1

2

3

COMMANDINGPART

User and Crane(Fixing, removing

moving etc)

1

2

3

TECHNICALOBJECT

(Lifting Hook)

TECHNICALENVIRONMENT(Types of ladle and

crane)

PHYSICALENVIRONMENT

(Temperature, climate etc)

HUMANENVIRONMENT

(Possible accidents to human being due to

failure)


properties, perhaps in collaboration with the user. During the design period when the product is created, it is these properties that determine the decisions and choices that are made. Unfortunately one cannot design a product in such a way that the desired properties are determined one after the other, for they are not independent variables. We find, however, that five properties can be distinguished form all others, in that together they completely define the product. They are:

For the product as a whole: Structure (i.e. the elements of the Product and their relationship) For each element: Form, Material, Dimension, surface

The aim in designing is that the qualities present in the finished product should correspond to the properties required. As this aim, however, is not always achieved, we must distinguish between the desired properties and the realized ones. Thus we can arrive at a model as shown in Fig 2.5. This shows the step by- step process from the analysis of the problem to the finished product. In the initial analysis stage, the problem is examined from all sides. This results on the one hand in concrete formulation of the desired function, and on the other hand, in a list of the desired properties, which constitute the criteria, that must make up the background for the selection of solutions.

Next follows the stage of synthesis, i.e. the stage in which the product is created. This is done by roughly determining step by step on the basic properties of structure, form, material, dimension, and surface. When the basic properties are decided and the design of the product is finished, then it can be manufactured. After manufacture, the product exists, and possesses some ‘realized properties’ which hopefully are close to the ‘desired properties’ that were formulated during the initial analysis.

2.4.2 Functional Analysis

The model shown in Fig 2.12 is a greatly simplified one that serves only to give a general view of the design process. It cannot be used as a recipe for designing a product. It can, however, be elaborated to try to achieve this. As we are primarily concerned at the first stage with the quality of ‘form’, we will only make the model more detailed in the stages where the basic properties are laid down. We can call the detailed model as the product synthesis, as it shows the individual steps through which the product is created.


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Refer Fig 2.13. The black arrow shows the time sequence. The product synthesis takes as its starting point the two outputs from the problem analysis, namely on the one hand the formulation of the desire function- the main function (possibly several sub-ordinate main functions), on the other hand the list of desire properties, which can also be described as criteria for an optimum product.

Figure 2.12 The basic properties

Fig 2.13 Functional analysis and creation of structure


Problem Analysis

Main Functions

Sub- Functions and Means

Basic Structure

Quantified Structure

Total Form Form of Elements

MaterialDimensionSurface

CRITERIA

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DesiredFunctions

Basic Properties:Structure

FormMaterial

DimensionSurface

RealizedFunctions


In the product synthesis the very important stage of creating the structure of the product is divided into a series of steps, beginning with a division of the desired function into sub-function. Then follows an examination of possible means of realizing the sub functions, a combination of these into a basic structure and finally an adaptation into a quantified structure, where critical parameters are optimized and where the relative arrangement of the element is determined.

Form is treated in two parallel branches, since the total form and the form of the constituent elements are determined simultaneously. The detailed form of the elements includes a specification of materials, dimensions and surfaces. We see from the product synthesis, Fig 2.13 that the criteria for an optimum product are used through the whole design process as a guideline and control for each step where a decision is taken.

The following paragraphs outline the individual stages in the product synthesis and functional analysis and typical examples are given.

2.4.3 Global functions

The Global function of a product is the way in which output is determined by input. If we conceive the product as a compound system we can discuss functions at all levels from the function of the total systems (global function, or possibly several parallel global functions) to the functions of sub- systems and of elements (principal and elementary functions). The idea of function is a very important tool for analyzing a problem into a series of clearly formulated components that express what the product must be able to do.

Example: The global function of a car is to transport people

and their property. The global function of a lathe machines is to shape

work materials to the desired form and size. The global function of a Ladle hook is to lift hot-

metal ladle with the aid of a crane.

Note: Every set of organs or pieces which is associated with a global function can be called as a mechanism.


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2.4.4 Principal, Elementary Functions and Means

Every relationship between technical object and one of the surrounding environments (such as for example the physical or human environment which influences the proper use of the mechanism) is called a principal function.

In order to obtain a global function, it may be necessary first, to obtain a set of principal functions. The global function of a technical object/mechanism thus can be considered as an organized set of principal functions.

When a principal function is complex enough for analysis it will be necessary to break it down into a number of simple functions called as elementary functions. This breakdown will then permit a better analysis of the principal function.

By means, we understand a solution, i.e. a method, a sub- systems or an element, with which a given function can be realized. The division of the main function into principal functions and further into elementary functions takes place alternately with the search for means to realize these.

One possible procedure of obtaining the global, principal, elementary functions and means consists of arranging a so-called function/ means tree shown in Fig 2.14

Example:

The lifting hook of a hot- metal ladle functions with a set of inputs such as the ladle with hot-metal and the lifting pin. The functions from the point of view of the relationship between the hook and these inputs are considered as principal functions. For example the hot-metal has to be poured down from the ladle after it was lifted and transported to a particular place. Hence the distance between the lifting pin and the trunnion must be high enough to accommodate one half of the diameter of the ladle if it is tipped to its’ maximum extent. So, this principal function determines the height of the hook.

Figure 2.14 show how the first stage in the function/ means tree for an automatic tea maker may look. Theoretically the function/ means tree can be detailed until the means become machine elements, or parts of machine elements. We stop when we find means to the most important principal functions.


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Figure 2.14 The function/means tree of an automatic tea maker

2.4.5 Basic Structure

A solution is achieved by connecting one process for each principal function, which we call the basic structure. The basic structure can be expressed in block diagrams, working (or basic) drawings (machine symbols, hydraulic, pneumatic, electric symbols, etc) or otherwise simplified drawings. No decisions are made at this stage as to ‘quantities’ such as dimensions, relative arrangement etc. Fig 2.15 shows different basic structures of the tea maker.

2.4.6 Quantified Structure

The quantified structure is one where the important parameters of the individual elements are optimized and specified, together with the relative arrangement of the elements. However, nothing is yet decided concerning the form design of the elements. Different quantified structures are shown in Fig 2.16.


Automatic Tea

Making

Tea Process with tea extract

Normal Tea Process

Tea Process with perfusion

Heating Water

Combine water and tea leaves

Control brewing

time

Separate tea from

tea leaves

Pass thro’ heating surface

Water Tea Tea Water Remove tea Remove leaves

Water Tea

Measure Time Measure Tea Concentration

Measure time dependent statusBring

energy to element

Bring water to element

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2.4.7 Total form

The total form of the product is determined alternately with the form of the elements. The requirements of the total design depend on the product we are dealing with. If aesthetic criteria are important (i.e. in cars, boats, cameras, etc) the design of the elements must be adapted to the total design. If technical and economic criteria are what matters most (i.e. carburetors, gearboxes, satellites, etc) the design of the elements must take precedence over the total design. Suggestions for total form of a tea maker is shown in Fig 2.17.

Figure 2.15 Alternate Basic Structures for an Automatic Tea Maker


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Figure 2.16 Quantified Structures for main elements of a Tea Maker


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Figure 2.17 Suggestions for total form of the tea maker


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2.4.8 Form Design based on functional surfaces:

How can one make a start on the form design of a specific element? We must ask ourselves, what it is that characterizes the element in question? The element is a part of both a basic structure and of a quantified structure. We can therefore say that the element has been defined only by its function and by its functional relationship to its surroundings. The starting point of the form design must consequently be to formulate the functions the element must perform. Thereafter one can sketch the most important surfaces- or functional surfaces- and from these the rest of the element may be designed.

In this book a functional surface is taken to mean a surface that has an active function during use-for example, the slot in the head of a screw; the area of impact on the head of a hammer; the surface of a chair seat; the cogs on a wheel; etc. We now examine the connection between the functional surfaces and the form. For example, let us select a simple element- a bottle opener shown in the following figure.

Figure 2.18 shows two types of opener which do not appear to have much in common; however, the functional surfaces are almost identical, in Figure 2.19, the bottle opener possesses three functional surfaces as shown, the difference between the two types illustrated consists in the different special arrangement of the material connecting the functional surfaces.

We can therefore identify two steps in the design of an element, on the one hand determining the functional surfaces and, on the other, deciding how these will be connected together. As


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Figure 2.19 The functional surface of the two bottle openers

Figure 2.18 Two different bottle Openers with apparently nothing in common


already mentioned, Figure 2.19 shows this last step, while Figure 2.20 illustrates how other arrangements of the functional surfaces give rise to other form design possibilities.

Functional surfaces are therefore the basis of the form design of any product. It is therefore appropriate to discuss in more detail what, in fact, functional surfaces are. In a product consisting of more than one element there are two types of functional surfaces external and internal. External surfaces have an active function in relation to the surroundings, such as a handle, a supporting surface, etc. the internal surfaces have an active function in relation to other elements of the product.

This can be illustrated by imagining a product as a system consisting of a number of elements with certain relationships to each other. The vice in Figure 2.21 may thus be described as a system shown in Figure 2.22 where the elements are represented by blocks and the relationship between them and the surroundings by lines.

Figure 2.20 Different choices of functional surfaces give rise to different design possibilities

Figure 2.21 A vice. The starting point for figures 2.22 to 2.24


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If we consider a particular element of the vice e.g. the sliding jaw we can see that that the relations correspond exactly to the above mentioned functional surfaces. The sliding jaw has an external surface, consisting of the surface, which presses on the subject as well as of the tip horizontal surface. The internal surfaces consist of the hole for the spindle and the two holes for the rods. The functional surfaces are illustrated in Figure 2.23.

As shown in figure 2.24, a specific arrangement of functional surfaces can be the basis for many form designs, and other arrangements can give other series of form designs. In the following paragraphs it will be apparent how a great deal of effort is needed to determine which functional surfaces are to be used in order that a firm and broad basis for the design work is achieved

Figure 2.22 A vice. Relationship of elements

Figure 2.23 A vice. The functional surfaces of the sliding jaw

2.4.8.1 The method of variation of the functional surfaces

A specification of the parameters that determine the functional surfaces of an element may form the basis of variation methods


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for generating ideas. By systematic variation of the parameters it becomes possible to list a number of arrangements of functional surfaces for a given element. The relevant parameters that can be varied are: numbers, arrangement, form geometry and dimension.

Figure 2.24 Suggested form designs for the sliding jaw, based on two different groups of the functional surfaces.

Figure 2.25 Examples of variation of internal functional surfaces based on the four variation parameters. The

examples shown are- a hinge, overhead projector, a socket for a camera lens and a socket for an electric light bulb


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Figures 2.25 and 2.26 show a number of examples of products, where the functional surfaces are emphasized. The products are presented in pairs in order that the four variation parameters may be observed, partly for the internal and partly for the external functional surfaces.

2.26 Examples of variation of external functional surfaces based on the four variation parameters. The examples are; a wheel for

a chair, an electric drill, a hotplate and an electric switch

2.4.8.2 Restrictions of form design

Let us imagine that we have a proposal for the form design of the functional surfaces of an element. How then do we move on from there? As has already been mentioned, the functional surfaces must be connected together. The problem is now to arrange the connections so that the element can function in use. The role of the element when in use must therefore be assessed and taken into consideration. The conditions that may have to be taken account of in the form design of an element can be formulated as follows:

2.4.8.3 Banned areas:

1. Areas in space which are structurally conditioned must not be obstructed, i.e. other elements must not be hampered (this applies to both stationary and movable elements).


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2. Areas in space which are functionally conditioned must not be obstructed, (e.g. the objects in the process, rays of light and jets of water).

3. Areas in space, which are operationally conditioned, must not be obstructed (e.g. room for a hand, room for an operator, etc).

On the basis of these banned areas one can now draw up a number of form design suggestions that roughly show where in space the connection must be put. The next step is to decide on the form geometry and the dimensions- first as rough sketches and, thereafter, in detail drawings, judged on for instance technological or aesthetic criteria.

It is important to note from the preceding comments that the form design of an element contains both a qualitative and a quantitative part. Any decision on dimensions is irrelevant until it has been decided how the material will be arranged, e.g. whether a functional surface will be supported at one point or at several. The number of elements and the relative arrangement of the connections belong to the qualitative part of the form design, while geometry and dimension belong to the quantitative one. The following section explains how the variation of parameters can be applied.

Taking a typical example-the frame in a hydraulic press- we now observe how the variation parameters can be used in designing an element. The frame of the press contains two functional surfaces, namely the fastening areas for respectively the hydraulic cylinder and the pressure plate.

Figure 2.27 Functional surfaces and banned areas in connection with the form design of frame for a hydraulic press


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Refer Figure 2.27, when designing the frame there are three banned areas:

1. There must be room for the piston in all its positions.

2. There must be room for an object of a closely defined maximum size

3. There must be room for the object to be put into and taken out of the press.

In other words the frame must be designed so that the two functional surfaces are connected in a way which takes account of the banned areas, and which allows it to fulfill its function- to transmit the necessary forces.

Figure 2.28 Form concepts for frame for a hydraulic press


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Figure 2.28 shows how the variation of number and arrangement of elements may be used to examine where the material and arrangement of elements may be used to examine where the material can lie. After that, the variation of form geometry and dimension make it possible to detail a number of rough design suggestions or form concepts. For comparison, figure 2.29 shows the design of a number of existing presses.

Figure 2.29 Hydraulic press.


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A more detailed use of the variation parameters is possible through a closer indication of the material areas between the functional surfaces. This is illustrated in the following example. Figure 2.30 shows the functional surfaces in a fork joint with a single bearing at one end and double bearings at the other. In varying the material area it is appropriate to use three sorts of symbols; a line for something that is approximately a rod (straight or curved), a hatched plane for something flat, and finally a hatched area for something solid, i.e. material in three dimensions. Variations of form geometry and dimension can result in a series of proposals as shown in figure 2.30. Note that it is useful to work at two levels of abstraction, namely, with a series of solutions where number and arrangement are varied and one where form geometry and dimension are varied (Figure 2.30& 2.31). Note also the considerable difference in illustration technique. The form proposals must now be further detailed, and it becomes necessary to take into consideration the form factors that actually exist. In the example of the fork joint, the manufacturing process becomes a decisive factor for the choice of design, which is discussed later.

Figure 2.30 Form concepts for a fork joint at the most abstract level, where the number and arrangement of the material

areas are examined


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Figure 2.31 Form concepts for a fork joint drawn up on the basis of figure 2.30 and variation of form geometry and

dimension.

2.4.8.4 The form division method

If the examples in the previous section are studied closely one more parameter can be identified. This, through conscious variation, can give rise to ideas for a number of suggestions for the design.

This choice of dividing into more elements or integrating into a few is a choice, which is always available. The division need not lead to more physically separate elements, as it may be caused by a visual division on a physically whole element.

A deliberate variation of the number of elements may be suitably called the form division method, bearing in mind that it may be


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question of a division into more elements as well as an integration into a few- possibly into a single whole one. In the examples on the following pages the physical and the visual divisions are considered. Incidentally it is not stated whether the elements are physically separable or not, as either type may be possible when a specific design is considered.

Figure 2.32 shows a pawl with four functional surfaces, the area of the breaking function, the area of the bearing, the area of finger pressure and the area of pressure for a mechanical systems, which must be moved simultaneously with the pawl being released. If it is assumed that the pawl must be form designed approximately as shown in the illustration, that is to say that the material areas are laid down the form division method may give rise to the form design proposals shown.

Note that the number of part elements goes form 1 (complete integration) to 5. It must be emphasized that the form division has no functional importance, but it may be very important for the manufacturing process and so for the economics. In Figure 2.32 pawl 4 will be the cheapest one if only one is to be made, whereas pawl 1 may be cheapest in mass production.

Figure 2.32 Form concepts for a pawl based on variation of the form division


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We will now demonstrate the application of the form division method on an element by one more example. The bearing in Figure 2.33 contains two areas of bearing and a supporting surface, and the approximate form design is shown. The rest of the figure contains a number of ideas to which the form division methods give rise. Here again, an essential factor in the choice of form design will be the manufacturing process.

Figure 2.33 Form concepts for a double bearing

In the introduction to this topic, it was mentioned that the methods in question may be used in designing either elements or complete products. This also applies to the form division method.

2.4.8.5 A coherent example: a pulley

It is usually appropriate to vary the five form variation parameters in the following order: number and arrangement; form geometry and dimension; form division. It is, however, not certain that in a given situation all five parameters can be used. For instance, the arrangement of material area may be ruled by so many conditions that there is only one place for it. Alternatively, form geometry may be decided in advance. An example of the general situation, where all five- variation parameters can be used, is shown below.


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The object to be examined is a pulley, e.g. for a conveyor belt. The pulley has two functional surfaces, the rolling surface and the bearings. Variation of functional surfaces has been carried out in Figure 2.34. where four variation parameter as will as maximum and minimum surfaces are illustrated.Two groupings of functional surfaces have been chosen for further examination, and in Figure 2.35 possible material areas are shown based on a variation of numbers and relative arrangement. A division of the material areas into the form of rods, planes or solids is useful. Figure 2.36 shows how, by varying the form geometry and the dimension, a number of more specific form design ideas can be given. Possible form divisions for a few of these ideas are shown in Figure 2.37

The final decisions on the form depends largely on the choice of material and manufacturing process and possibly also on an evaluation of the appearance. By using sketches, models and scale drawings one can decide on all the details, which are then documented in a set of working drawing with the accompanying assembly drawing. Naturally this plan, which corresponds to Figures 2.34 to 2.37, is very schematic. This is in order to underline the steps one must basically take in designing.

Figure 2.34 Variation of functional surfaces for a pulley on a conveyor belt


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Figure 2.35 Form concepts for a pulley at the most abstract level where the number and arrangement of the material

areas are examined

Figure 2.36 Form concepts for a pulley arrived at by varying the form geometry and dimension of selected solutions form

Figure 2.35


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Figure 2.37 Form concepts for a pulley. Two of the suggested solution in Figure 2.36 have been detailed to a certain level by

varying the form division. For the final detailed design the design engineer must first decide on the manufacturing

processes to be used.


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2.5 TECHNICAL ANALYSIS AND SPECIFYING DIMENSIONS AND MATERIALS:

Having established the relations between the technical object and the surrounding environment using the systems analysis, the next step in the design process will be analyzing these relations between the technical object and its’ surrounding environment in order to determine the end result viz., Nominal form, Nominal size, Material required etc.

Knowledge of these relations will reveal the nature of deformations and the intensity of the applied forces on the technical object which will enable the designer to,

Select the material for the technical object Calculate the sections of the organs of the assembly of

the technical object Selection of arrangement of pieces within the assembly

etc.

2.5.1 Technical Analysis Step: 1

Other questions might also be suggested, but the above will be sufficient for staring the process. Certainly the list does point up the need for information. The student can gather information about the above questions from an experienced steel-mill engineer, by making phone calls to ladle manufactures and suppliers, and referring some published information about steel- mill equipment.


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1. What is a ladle hook and how does it function? 2. How much molten steel does an average ladle hold?3. What is the empty weight of ladle?4. What are the dimensions of the trunnions where the ladle

hooks meet with the ladle? 5. What are the service conditions? What are the maximum

and minimum temperatures? 6. What about corrosion and wear? 7. What is the expected life of a ladle hook? 8. What are the limitations on cost? 9. Are there any special precautions with regard to safety? 10. Is here any Design code? 11. Should the hook be so designed as to be adaptable to use

in handing scrap boxes or charging the furnace with Ferro alloy additions?


2.5.2 Technical Analysis Step: 2

Gathered Information:

1. Fig 2.38 shows a teeming ladle that transfers molten steel from the melting furnace to the ingot mold. Note that ladle is supported form the crane by two ladle hooks. The hooks make contact with the ladle at the bearing surfaces called trunnions.

2. Steel mill cranes typically have a capacity of 100 to 200 tons. We will assume a total ladle weight of 150 tons, since the ladle is made of heavy steel plate lined with refractory brick, its capacity for holding molten metal is only about 60 percent of the total weight.

Fig 2.39 Details of Dimensions of Ladle


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300

900 36003000

Fig 2.38 Teeming Ladle


3. The specific dimensions of the ladle are given in Fig 2.39 taken form the design of hot metal ladles.

4. The conditions of service: the ladle hook will operate in a steel mill building. The radiation from the molten-steel in the uncovered ladle may result in temperatures as high as 6000C to 10000C in part of the hook surface. Impact loading and fatigue loading (very low frequency and low number of cycles will be present. But they are not likely to be the controlling factors.

5. Corrosion is likely to be a minor problem but wear could be factor where the hook pins contact the crane and where it supports

6. Steel- mill equipments are heavy & rugged and built to last long. An average life of 10-15 years would be a reasonable estimation for a ladle hook

7. We can not have any special guide-lines on cost of the hook other than that ladle is a fairly standard item. But we should bear in our mind that the failure of a ladle hook that dropped a ladle full of molten steel would be catastrophic

8. The design must have a high safety factor, reliable and yet be economical

With the above elaboration we can develop the complete problem statement and problem analysis as follows.

2.5.3 Technical Analysis: Step: 3

Problem Statement:

Design a hook for lifting hot metal ladles with a maximum weight of 150 tons . The hook should be compatible with the ladle ( details given in Fig 2.38 & 2.39). The hook eye should receive a 200-mm diameter pin for attaching to the crane.


We first need to determine the form or shape of the hook, which should be compatible with the ladle and crane attachment. The hook is a load- connecting member between the crane and the ladle. From this information, we can deduce that the hook will have general load configuration as shown in Fig 2.40 (But this may not be the case for all design problems. Generally a detailed analysis is required for arriving at an acceptable form of the product, which is discussed in detail earlier).


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Fig 2.41 Ladle Hook


C C

B B

A A

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Fig 2.40 Ladle

P=150 tons


The next step is to establish the stress regions in order to further refine and develop the form of the hook. (Refer Fig 2.41). There are three critical stress regions in the hook. Section A (at bight of the hook): Hook is subjected to 1) Direct tensile loading 2) Bending stress as a curved beam and 3) Lateral bending stress due to deflection of trunnions and concentration of load towards the outer surface of hook thickness. Section B (at shank section of hook): Hook is subjected to 1) Direct axial stress and 2) lateral bending stress. Section C (at eye of hook): Hook is subjected to 1) Direct stress with stress concentration factor land 2) Lateral bending.

Using some intuition, we can hypothesize that section A will be critical section and stresses acting at section ‘A’ will establish the thickness of the hook. Section ‘B’ will establish the shank width and Section ‘C’ will establish eye details.

2.5.5 Technical Analysis: Step: 5 Material selection and working stress It is almost automatic that a load- bearing member of 2.5m to 3m tall, in situation where deadweight is not critical will be made from steel. Moreover the need for high reliability in large section, coupled with modest cost, suggests that a structure built up form steel plates is preferable to a monolithic cast or forged steel body. Therefore we need to make a selection from the standard grades of carbon and low- alloy steel plates that are commercially available form the local market.

TABLE 2.1 Characteristics of Structural Steel ASTMAPEC.

DESCRIPTIONC%

MN

%OTHERS

YSN/Mn2

RELATIVE COST

A36 Carbon steel 0.29 1.00 -- 250 1.0A441 HSLA steel 0.22 1.25 0.02 345 1.15A442 HSLA steel 0.15 1.10 0.05 V0.3cu 345 1.25A514 Alloy steel 0.15 0.80 Ni-Cr-Mo 690 2.0

Based on ASTM Specification (Table 2.1) structural quality steel falls into three categories.

1. PLAIN HOT –ROLLED CARBON STEELS 2. HIGH STRENGTH LOW ALLOY (HSLA) STEELS 3. ALLOY STEELS


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Since welding may be used for fabricating this hook only weldable grades with less than 0.30 percent carbon will be considered. The A36 has impact and brittle fracture characteristics that make it unsuitable for this application. A 514 is ruled out because of high cost. The balance two HSLA steels defter chiefly in their resistance to atmospheric corrosion. Since corrosion resistance is not a crucial property for this application we can decide to use the cheaper A441 steel. The effect of high temperature or unrecognized stress concentration owing to fabrication or metallurgical changes must be taken into consideration in our analysis because safety is paramount. Therefore we reduce the yield strength (YS) by a factor of safety. Because weight is not an important consideration in our application we have the luxury of using conservatively a high factor of safety of 4

Working stress = 86 N/mm2

Although the method of manufacturing the designed hook will be considered after we have a better understanding of the Hooks’ size and the detailed dimensions, for present purposes we can assume that the ladle hook will be manufactured from built- up thickness of steel plates held together by rivets or welding.


Detailed Stress Analysis

I) Stresses at section A-A:


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Fig 2.42 The ladle hook as curved beam.

A A’

P/2

380 C C

Centroid Axis

a c c

Y Y

v

R

t

Centroid Axis

Neutral Axis

SECTION AA’

r

R

Y


Three different stresses can act at the bight section of the hook. Of the three, the bending stress in the curved section of the hook that is due to the weight of the ladle is the most significant. Because of geometry, the hook at section A is classified as a curved beam. A characteristic of the curved beam is that the neutral axis does not coincide with the centroid axis (Fig 2.42).

Even if your previous course on Strength of Materials did not cover curved beams, you should have received enough background to read about the subject and apply new information to this problem. 1. The maximum tensile stress in section ‘A-A’. The bending stress in section A-A is given by

where r = v+y (refer Fig 2.15) A = 2ct

The bending moment is M =

Maximum Tensile Stress max =

Since minimum value of (r-y) is ‘a’ and the maximum value of y is (c - )

By definition;


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if we let 2c = 600mmthen

and max =

2. The direct (axial) stress in section ‘A-A’ is,

d =

3. The lateral (sidewise) bending stress in section ‘A-A’ is,

The lateral (sidewise) bending stress can be determined by assuming that the force on the hook is applied at 25 mm from the outer surface of the plate.

Fig 2.43. Lateral Bending Stress.

These three stresses act simultaneously and at the critically stressed location of A:


lt

2c

25 mm

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=

=

Since we have already decided to limit the working stress to 86 N/ mm2

By assuming the hook thickness as 125 mm, 150mm, 175mm etc, we determine the total working stress as

HOOK THICKNESS SUM OF STRESSES (in mm) at point ‘A’ N/mm2

125 114.34 150 96.95 175 84.12

Thus if the dimension of the hook at A-A are 2c = 600mm and t =175mm the stress level is kept to a conservative value of 86 N/mm

II) Stresses in the shank, Section B-B:

1. Direct stress: d =

Where ‘w’ is the width of the shank at BB.

2. Lateral Bending: σlb

=

These two stresses act simultaneously at the location of B:


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III) Stresses in Section ‘CC’ through eye of the hook:

Fig 2.44 Section ,CC’ of the Hook

1. Direct stress:

2. Lateral Bending: σlb

=

=

j =181. 5mm 182 mm

2.5.7 Final Dimensions:


200 175j j

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Fig 2.45 Final Dimensions

We have determined the dimensions of the critical elements of the hook on the basis of the stresses that are expected to be present. These dimensions have been determined by keeping the nominal stresses at a level below 86N/mm2. At this point the drawing of the ladle hook should be made to a suitable scale to check the validity of the calculated dimensions and establish the remaining dimensions. Some of the remaining dimensions will be determined by functional requirements of the hook and its surrounding environments. Other dimensions will be determined by engineering common sense. Hence in order to the remaining dimensions functional analysis of the technical object is essential.

2.6 NEED LIMITATION:

First let us discuss the different types design strategies in order to explain the need limitation with respect to a particular type of


C

B

A

182182 200

264

600380

3000

175

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strategy. Within the realm of mechanical and electrical there are four design strategies as listed below.

1. One- of – a kind design 2. Design for mass production 3. Large expensive systems 4. Design to code.

The first type is a design for a new product which will be produced only once or produced few in numbers (like a lifting hook for a ladle). This design would likely be characterized by minimum of analysis and optimization. In design for mass production, emphasis is on cost and quality. Design would proceed through the various phases with extensive analysis, prototype testing and optimization.

With a large expensive system like a 100 HW Steam turbine, testing with prototypes is not affordable. But we must carry out the design process as much as possible from analysis and from field experience. In some situations public health or safety issues are so paramount that available deigns have been circumscribed by codes or standards. The design of a steam boiler is such an example, where the code specifies the limiting stress and methods for calculating it.

Even though the first type of the design will not consume much time, design for mass production and design or large systems will extend over a certain period of time. It is worth to mention here that the conclusions derived over this period of time will have absolute significance only within that time period of investigation. After certain period of time the design evolved may not be adaptable or the developed systems or product might loose its demand. Even in the case of design for code type the codes may be changing over the period of time. Hence the product designed may not be according to the new code. We must consider these limiting factors when we specify our need.

2.7 DESIGN PHILOSOPHY:

The above discussion on the various aspects of Design Process gives the student an overall view on the step- by step procedure of determining the nominal form, size and the material of a technical object which will be used for satisfying a need or solving a problem in the field of Mechanical Engineering. But the student shall consider this procedure only as apart of the design technique.


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The procedure explained in this chapter is not a universal method which is applicable to each and every object or mechanism.

The danger of formulating a systematic/universal method for designing a product is that we will be led into a thinking that a systematic approach necessarily gives the right design. This is just not so. The most effective design is achieved by the right balance of systematic and intuition. Therefore the students are advised to use the suggested systematic approach as the foundation for the appropriate attitude to innovation. This approach is an understanding of the fact that one can, through conscious effort look objectively and systematically at all the design criteria and arrives at the optimum solution.

To create something new without any previous design to guide, a designer will fail miserably unless he has in-depth knowledge and higher level of understanding. The designers cannot acquire this by attending lectures and reading books. Understanding coupled with powers of logical deduction and judgment is not a capability that can be acquired from outside. It is something purely personal and inward, acquired only by diligent thinking and working with the knowledge already possessed.

A set of wrong and right examples or a list of design rules cannot by themselves cultivate the beginner, the habit of methodical planned thinking. The advantage of working to a methodical plan lies mainly in avoidance of superfluous repetitions. Only by working to a methodical plan a designer can escape straying into a blind alley and must start again. By adopting the right method of working and thinking carefully about the design, a designer can save time, avoid wasteful metal efforts, and thereby increase the effectiveness of his work.

Working to a method is a welcome prospect. But it does not provide the creativity and imagination.


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DFM 2 Final 1

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