Advanced Method in Mechanical Design - ingmecc.uniroma1.it · Francesca Campana "Advanced method In...

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DRAFT VERSION - Francesca Campana "Advanced method In mechanical Design" - lecture notes vers0 Advanced Method in Mechanical Design lecture notes class of the II year of the Master Program in Mechanical Engineering Design prof. Francesca Campana room 24 @ Dip. di Ingegneria Meccanica e Aerospaziale [email protected] official webpage

Transcript of Advanced Method in Mechanical Design - ingmecc.uniroma1.it · Francesca Campana "Advanced method In...

DRAFT VERSION - Francesca Campana "Advanced method In mechanical Design" -

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Advanced Method in Mechanical Design

lecture notes

class of the II year of the Master Program

in

Mechanical Engineering Design

prof. Francesca Campana

room 24 @ Dip. di Ingegneria Meccanica e Aerospaziale

[email protected]

official webpage

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Introduction

The aim of the class is understanding the design workflow, its methods and tools useful

to achieve products that are well defined according to the client-company-community

requirements. Exercises will be carried out through computational and CAD-CAE

software. At the end of the course students will be able to set up a design workflow

plan, choosing the most relevant requirements and design approaches, for any product

of the industrial sector. In addition basics on the practical use of some CAD-CAE

software will be also given.

It will be organized in theory lessons and practical exercises that may ask for laptop.

The final exam will consist with technical reports that must be provided at least 14 days

before the attendance and a one-hour written exam.

Necessary prerequisites are: solid and applied mechanics, machine design,

manufacturing and technical drawings (2D and 3D).

Advisable knowledge: Matlab, Finite Element Analysis, CAD systems.

Office hours for explanations and suggestions: by appointment.

Syllabus

Part I - Design workflow

Design meaning, activities and related workflow according to product lifecycle

management and concurrent engineering. Overview of general principles and specific

tools for conceptual and executive design. Exercises

Part II - CAD and Reverse Engineering

CAD systems. Product Data Management, Product Lifecycle management. Solid and

surface modelling. New and advanced CAD approaches. Shape design and system

arrangement: general criteria and methods. File formats for data exchange. Exercises.

Reverse Engineering: acquisition methods, post-processing, examples and exercises.

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Part III - Virtual prototyping

Introduction to CAE systems. Multiboby simulation, Finite Element Analysis (FEA),

Topology optimization, Multiphysics, Ergonomic analysis. The role of simulations in

Design for "x" strategy. Examples for Design for manufacturing and assembly. Exercises

Part IV - Optimization methods for design

Probabilistic approach: source of randomness. Elements of Design for Reliability. Design

of Experiments. Robust Design. Response Surface. DOE versus DACE. Numerical

optimization through FEA. Topological Optimization. Examples for Additive

Manufacturing. Exercises

Part I - Design workflow

1.1 Design objectives in industrial environment

1.2 Definition of the requirement list

1.3 Design activities

1.4 From functional analysis to preliminary lay-out

1.5 From the concept design to the executive lay-out: design criteria

1.6 Tools and methods to organize the design activities.

1.7 Exercises

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Part I - Design workflow

Design meaning, activities and related workflow according to product lifecycle

management and concurrent engineering. Overview of general principles and specific

tools for conceptual and executive design.

1.1 Design objectives in industrial environment

For a mechanical engineer designing means to define a component or an assembly able to work under assigned conditions with the specified performances. It is related to the definition of its physical behaviours (kinematic, dynamic, structural, chemical, electromagnetic, ....), its shape and then its manufacturability. Defining "assigned conditions", "specified performances" and evaluating shapes and physical behaviours that must be derived from them cannot be simple due to many interacting aspects and constraints, first of all cost reduction. To give a general and methodological approach to the set-up of the design requirements, it is necessary to understand the scenario where the engineers usually works. Design is a step of the industrial process named product design and development. A general component or assembly from the enterprise point of view is a product. Product design and development starts according to a customer need that can be satisfied by industrial earnings. Developing new products in an industrial environment is a complex task that involves many efforts from different departments (management, marketing, technical depts, manufacturing, ...) and a clear understanding of the needs of the costumers. It is true for all kind of products, it does not matter if they concern with niche market or large mass production. The enterprise starts the development according to its business strategy after a marketing survey and a technological study of feasibility. Product-Process design gathers all these information to define and produce the component/system. Figure 1 summarizes this concept adding another actor: the community (or society) where the product is going to be used. The community may constrain different aspects of the product by means of culture, trends and common sense (e.g. type of use, shape, market spreading); the society, in the meaning of state, may constrain specific technical aspects by laws and standards. According to the type of product (that means the type of enterprise) marketing survey, technology feasibility and community constraints have different relevance. In some cases a specific dept. for technology feasibility is present (e.g. research and development dept. in chemical industries), in other cases standards and laws strictly define technical requirements (e.g. nuclear sector or aeronautical).

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The design process accomplishes these needs through the organization of the work that must start from the definition of the requirement list. The requirement list defines all the needs and the technical aspects that are necessary for design. To give a general procedure to define the requirement list, needs and technical aspects are set in two classes: internal and external properties.

Figure 1 - Actors who constrain Product-Process Design

Internal properties are technical requirements of the product (power, efficiency, maximum speed, acceleration, strength, reliability, ...). They are mainly defined as performance values of the product and they represent part of the costumer's needs besides part of the technical constraints. For example let analyse the performance values of a racing car: maximum speed or acceleration time are costumer's requests, type of engine, weight or type of transmission may be seen as technical constraints, that are not necessarily known by costumers . External properties summarize at glance the voice of customer, community and standards, the requests of production, distribution, sale and service. They define macro-categories of technical properties. Each of them satisfies a specific costumer or enterprise’s request that may rise during the lifecycle of the product (Figure 2).

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Figure 2 - Product Life Cycle Scheme

Pursuant to this reasoning the external properties are assigned to each step of the lifecycle as reported in table 1. Table 1 - Introduction to External Properties

when what who cares about custumer enterprise community Use functionalities and performance x x

ergonomics x reliability x maintenance x x safety x x aesthetics x fitness to standards and laws x x x environmental sustainability x x x

Distribution and sale

fitness to transportation x x fitness to storing x

Production manufacturability x fitness to assembly/disassembly x

Disposal and End of life

recycling x x

cost x x

More in details their definitions and basic annotations are the following reported in table 2. Table 2 - External Properties: Definition Property Definition Annotation

functionalities The set of functions that the component/assembly must do and have to work properly.

Some of them are requested by costumer other of them must be identify by the engineer to make the system work (e.g. engine cooling, frameworks, ...)

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Functionality is trictly related to performance, thus it can be seen as part of performance

performance Target values and admissible domains for the numerical evaluation of the functionalities

Some of them are requested by costumer other of them must be identify by the engineer to make the system work with quality

ergonomics "Ergonomics (or human factors) is the scientific discipline concerned with the understanding of interactions among humans and other elements of a system, and the profession that applies theory, principles, data and methods to design in order to optimize human well-being and overall system performance" (http://www.iea.cc/whats/index.html)

Property related to all the aspects (functions and performances) that are involved in the correct interaction between man and system during its use. It can be distinguished in physical ergonomics, cognitive ergonomics and organizational (http://erg.sagepub.com/content/17/4/7.full.pdf+html)

reliability It is the probability of a system of work properly (without failures) in the assigned conditions with the defined duration

It is a quantitative requirement, it is strictly related to the system arrangement, component and process quality, maintenance policy. Loss of reliability may cause loss of safety not vice-versa

maintenance It defines the policy of intervention to guarantee a proper reliability

safety It concerns with functions and performances related to guarantee safety of users and inner/out environment where the system acts.

aesthetics It defines the shape, finishing and general perceived aspects of the system in relationship with users and environment

fitness to standards and laws

It obliges to analyse and respect all the standards related to the lifecycle of the system and under the enterprise responsability

environmental sustainability

It concerns with the reduction of pollution and bad use of the natural goods.

Recently introduced it confirms that external properties are a results of the technical and community development

fitness to transportation

It is related to the internal properties that allow component s transportation without damages

fitness to storing

It is related to the internal properties that allow component

For example think about the internal properties asked to

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storages without damages design methane or hydrogen tanks, or nuclear disposal

Manufacturability

It concerns with the fitness to be produced as designed according to the proper efficiency in terms of cost and quality

It is the base of the so-called integrated product-process design

fitness to assembly/disassembly

It concerns with the fitness to be assembled as designed according to the proper efficiency in terms of cost and quality

recycling It concerns with the optimization of the material selection according to recycling, also in relationship with the fitness of manufacturing and disassembly

cost It concerns with all the costs involved during the product lifecycles

It pertains all the choices related to the other external properties

Looking at the external properties and their definitions is easy to understand that they are correlated each other through their link with the internal properties. Putting aside Functionalities and Performances that are obviously related to target values, Ergonomics, Reliability and Aesthetics pertain to functionalities, shape, system design, surface finish and materials. Environmental Sustainability involves efficiency but also Recycling and Manufacturing process. Manufacturability involves shape and material properties but also Functionalities for what concerns with tolerances. Recycling pertains to materials and thus to Manufacturing and strength (that means Performances). Obviously Cost is related with all the properties and in many cases it constrains their improvement. It includes all the costs necessary to define the product, from the company work during the product development to the raw material to the manufacturing and distribution. Among them the cost related to the product development are also known as cost of the time to market. Time to market is the duration which is necessary to develop new products until the completion of the product process planning, or in other words: it is the amount of time it takes to design and manufacture a product before it is available to buy. During this period no incomings are earned, so it is extremely important its optimization. During the development, cost increases exponentially as soon as the design process becomes executive and the process planning is defined. Changes are easier if they are made in the first part of the design development. This consideration represents one the basis for the time to market optimization. To reduce time to market it is necessary to avoid design changes at the end of the product development, thus design must always start and evolve keeping in mind clear objectives that are able to satisfy all the required needs. This moves the attention towards a new scheme of workflow that welcomes parallel steps and recursive optimization, avoiding sequential steps, with final optimizations with partial redesigns. The final aim is a harmonious growth of the project "well-posed from the beginning", that means adopting a design for quality approach.

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The analysis of the product lifecycle and external properties are the two key issue for approaching the design for quality. Enterprise competitiveness and thus the answer to the voice of management passes through the development of products with the highest quality/cost ratio. Increasing quality means to make possible costumer's needs, expected or not, during the whole lifecycle. Reducing cost means to cut wastefulness and time to market by adopting proper methodologies and best practice. In product design and development Concurrent or Simultaneous Engineering defines the best workflow to achieve an optimal quality/cost ratio. It aims to join design problems of different sectors as soon as possible. It obliges many parallel works and concurrent decisions so that interface problems and their strategic choices (e.g. type of manufacturing) must be faced as soon it is possible. This is necessary to reduce time to market and to prevent a bad assessment of the lifecycle properties. In medium-large companies it means that the product design and development is organized by different depts. that are asked to work together to assess requirements and design solution in many steps of the flow. In small enterprises it can mean that the engineer must be multidisciplinary and able to prevent questions and problems of the next-step collegue!

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1.2 Definition of a requirement list Design starts from an economic and technical study of feasibility. The technical study highlights the technological principle adopted to solve the required need. The technological principle is the physical law that accomplishes the product definition. It fixes many of the internal properties necessary to design and, by the knowledge of the state of the art, it may gives not only a preliminary idea of basic lay-outs but, above all, the knowledge of a possible lifecycle. By them and by the voice of costumers the list of requirements is built1. The list of requirements must show the set of requirements that must be satisfied in the product, therefore implemented in the design process. From the external properties (that means the voice of costumers, company, society, ….) the related internal proprieties must be exploited and organized, detailing also the numerical values for the related performances (if any). In some cases these values can be ranges. Often they are called “target values”. In Fig. 3 an overview about the steps necessary to define a list of requirements is described. Each step can be a seen as a loop that must converge as soon as possible.

Figure 3 – Steps for defining a list of requirements

1 In design theory the most general approach says that the list of requirements anticipates the choice of the technological principle that must be chosen by the engineer [REF HUBKA and Pahl]. In our opinion it is partially true in the industrial environment: if we see together the conceptual design research and the product development from the Company point of view, the adoption of a specific technological principle is a natural consequence of the Company where the engineer works. In other words, if you decide to design a mechanical system to go from place A to place B and you work for Boeing, the technological principle is already decided and it is not the same of Citroen.

STEPS TO DEFINE A LIST OF REQUIREMENTS

1. COLLECT voice of costumers, state of the art, standard and technical constraints by enterprise

2. ANALYSE functionalities by costumer's point of view and by technical point of view

3. DEFINE functionalities and performance, and their relevant external and internal properties

4. HIGHLIGHT other external properties relevant for the customer and their correlations with performance and functionalities

5. DEFINE correlated internal properties for the external ones of point 4.

6. HIGHLIGHT product-process constraints to define manufacturability and assembly requirements

7. DEFINE correlated internal properties for the external ones of point 6.

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The final aspect of a requirement list is a sort of table with rows collected according to the external properties and associated to columns able to quantify the detailed definition of each property by its internal properties. To better explain how defining a list of requirements let us start from an example. Imagine you want to design a small table for a train seat. The voice of the costumer and the needs of the Company can be defined according to the following: - Design a train seat table anchored to the back. It should be opened/closed

manually and reduce the risk of downfall of things put there, it also has to be suitable as bookrack.

- Maximum in plane size 400 x 350 mm, with height from the floor of about 800 mm.

- The system must be agronomical, easy to be assembled and manufactured (medium/high volume), without maintenance, safe and reliable for intensive use.

Please notice that the train seat can be orientable: its back can rotate of max 30° from the vertical axis (see fig. 4). Consider that this solution is found to be preferred in the respect of a table anchored near the armrest of the seat (solution that would make the seats less comfortable). Starting from these considerations the 7 steps to the define the requirement list must be follow. The results will be a recursive change of the table that at the end will look similarly to that of table 3.

Figure 4 – train seat configuration according to the type of table that is object of the

design

In this case some external requirements have been added according to some technical consideration (for example, ISO standard or laws must be verified). Moreover where possible external properties have been better detailed into internal properties (see for example functionalities and reliability). Be careful that: - some internal properties are not defined yet (for example the red ones in the table). They must be defined at the end of the list of requirement set-up - some external properties could be without related internal properties - many internal properties are correlated with others (for example Roughness is relevant for perception and friction, thus aesthetics and safety).

max 30

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Table 3 – List of Requirements for a train seat table anchored to the back (red values means that the table is not complete)

External Properties Detailed Description Values

Performances and functionalities

foldable table anchored to a train seat

in plane size 400x350 mm

height from the floor about 800 mm consider 100 mm from the

armrest of the seats

allowed weight on the table

less than 20 kg

Maximum load for design 200 N cantilever beam totally loaded at

the edge 100 N

maximum deflection at the edge 5 mm

completely foldable consider that train seat can

change orientation 5°-30° from the vertical axis

capability of using as bookrack

regulation of rotation max 120°

Ergonomics opening/closing procedure easy to be understood

easiness for manual usage required force minimum

ergonomics related to positioning during usage

regulation possible (how many?)

space for objects to prevent downfall

for glass/bottle for book

extruded rim at the edge

easiness to be clean avoid small pockets Width less than ??

use suitable materials

Aesthetics Good perception Materials

Roughness

Color according to the train equipment

Safety Avoiding unexpected opening

Avoiding unappropriete usage

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Avoiding sharp edges

Avoiding cutting elements from the mechanism

Avoiding object sliding friction coefficient > 0,19

Reliability and Maintenance Reliable to be opened/closed

number of clycles per year 80000 cycle/year(?)

Maintenance absent

Manufacturing & Assembly Use of standard components

Reduced number of components

Easiness of assembly

Standard and laws To be checked To be checked

Cost Manufacturing cost less than 40 euros

Exercise 01: As exercise please define a correlation/interaction matrix among external and internal properties of the train-seat table.

1.3 Design activities

From the requirement list the design starts according these main steps:

1. concept design that leads to the preliminary lay-out

2. executive design that leads to the final lay-out of the assembly, sub-assemblies and components

3. final testing and prototyping

4. process-design

According to the type of product these activities may involve one of more teams and design depts. Each activity is structured in sub-phases that may ask for interactions according to the principle of simultaneous engineering. Specific technical issues, tools and methodologies, described in the next, must be applied in each of them, so that the product withstands to continuous definition of major details, that must be optimized concordantly to the requirement list and the interaction matrix. Final outputs of each activity is the design documentation, that means technical drawings (blueprints, CAD models, exploited views, preliminary sketches) and technical reports. Each design step has its own elective type of drawing. At the very beginning of the conceptual design step concepts or schematically designed sketches are made. They must highlight functionalities, their way of acting and a general overview of the lay-out needs (how many functional districts are necessary? How/Where do they interact?). These kind of drawings summarise the functional analysis that represents the first part of the conceptual design.

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After this sub-assemblies definition and design must be provided. It means that components of each sub-assembly is defined by its shape and size according to technical constraints due to the internal arrangement of the lay-out (physical constraints, joint types, materials, strength,…) and to the list of requirements. In this step CAD models are made and subsequently detailed together with the assembly/component optimizations. Finally, when tolerances and roughness can be defined according to functional and manufacturing constraints, technical drawings are made together with exploded views of the assembly. Prototyping obviously represents the necessary test phase of a mechanical design. Generally speaking we can distinguish different levels of prototyping: virtual prototyping, made by numerical simulations of one or more physical behaviours (structural, electromagnetic, fluid dynamics, ergonomics, kinematic, multiphysics,...); functional mock-up by means of rapid prototype; final prototyping for product-process set-up. Although virtual prototyping represents one of the most well-known approach to reduce time to market, final tests on real components are still necessary to release a complex product. In some areas (e.g. aerospace) experimental tests are obliged training-steps to guarantee functionality.

1.4 From functional analysis to preliminary lay-out

Functional analysis is the first design step after the list of requirements. It is devoted to focus the basic actions the product must do and to help the choice of the components/sub-assemblies (also called "actuators"2) that are able to do them. Functional analysis must obviously take care not only of the functionalities but also of the functions related to the other external properties (e.g. ergonomics of command interface can be made by specific visors, safety by specific lockers, ...). It must highlights the time sequence of their actions but also the correlations. For this reason two schemes are usually made:

- sequential functional analysis and

- functional tree analysis.

They are complementary to accomplish both time sequence and correlation among requirements. Sequential Functional Analysis (SFA) analyses the use of the product looking for the temporal link among the required functionalities, including also the set-up before and after the use (if they are necessary). Doing so the product work can be assigned to main functions and auxiliary ones. For example if you are studying a washing machine, electrical wires and plugs are ancillary to the main functions (containing clothes, adding water/soap/…, rotating, heating, ….), as well as support everything by a structure, …

2 The term “actuator” must be preferred to "component", since it highlights a specific function, not a technical solution, that may represent the way the actuator works. For instance I can joint two metal sheets by welding, glue or bolt = different solution define different constraints and internal properties that I must evaluate according to the lay-out of my product.

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This type of information is relevant to think a proper lay-out of the sub-assemblies of the product (set of functions related in terms of time, input and outputs can be managed as sub-assemblies). Tree analysis of the functions looks for the functions related to the external properties of the list of requirements. It is devoted to look for what functions are able to implement an external property not to look for time/input-output relationship among functions. It starts from the top, declaring the product, then the main leaves are the external properties and at each of them secondary leaves are defined through specific functions able/necessary to implement that property.

Figure 5 - Sequential functional analysis of the train-seat table

The functional analysis derives from the requirement list, when many correlations among properties are present, attention must be paid to simplify the problem and to uncouple the functions that are correlated. It means that in this step the engineer can decide if a specific function involved in more than one requirement is really important for only one of them. This will lead to optimize in the next that function only according to that requirement. Final aim of the functional analysis is to define one actuator per function and how they must work together. Time sequence and function interactions can help to define a preliminary actuator scheme, so that a simplified functional analysis is of the utmost importance for a correct preliminary design.

Functional analysis describes the system in words, each function can be technically made by different kind of actuators. Their difference can be related to the principle of action, cost or other external properties (e.g. analyse the function "helping to support a rotating axis reducing friction". It can be done by crankshaft bearing, ball bearing, fluid dynamic bearings, electromagnetic bearings, oil or other kind of materials with friction coefficient lower than that of the shaft). To avoid wrong selections each function can be associated to many possible actuators in a matrix called morphological table.

Rows of the table are related to the functions, columns to their specific actuators. Each row has its own specific number of actuators, that should be grouped by technological principles if different. The actuators must be defined not only by its name but also by a sketch. This will help the engineer to understand the design trends and advancement.

open the table easily, when you are sit down

put objects on (bottle or glass, book, mobile, …) firmly

make regolations of the slope when you read

close the table easily

allow stability on the

back of the seat

guarantee load support

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Figure 6 - Example of Functional Tree Analysis (test-case of the train-seat table)

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Figure 7 - Functional Tree Analysis with the highlight of connected functions (test-case of the train-seat table)

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DRAFT VERSION - Francesca Campana "Advanced method In mechanical Design" -

lecture notes vers0

Fig. 08 shows an example of morphological table. It gives at glance an overview of the solutions related to each sub-function. Fast sketches are mandatory to give the idea of the solutions in an intuitive way.

Figure 8 - General scheme for a morphological matrix (taken from The Industrial Design Engineering Wiki @ TU Delft http://www.wikid.eu/index.php/Morphological_chart

(last access 26/03/2017)

Through this matrix (or chart) a preliminary concept may be done selecting the most suitable actuator for each function (concept aggregation). Obviously this is a result of reasoning based on:

1. technical knowledge behind the actuators and the product you are designing,

2. the requirements defined in the list of requirements.

It is going to be possible that some actuators are more suitable to fulfil specific properties than others, or may work better if associated with other particular actuators, thus concept aggregation is a results of many considerations that must accomplish quality, design and manufacturing constrains, besides cost. In many cases this leads to more than one concept and to help an optimized aggregation each actuators can be evaluated on a qualitative scale (good, sufficient, insufficient), according to the most relevant properties of the function or product.

Figure 9 shows an example of this practice and its role for the concept design. Since actuators can be ranked according to their link with the external properties two aggregation strategies can be found:

1. choice the highest ranked actuator when it is relevant for a specific property, this can help to balance overall quality and cost;

2. choice the actuators according to the ranking of one specific property, this can help to maximize a specific property.

DRAFT VERSION - Francesca Campana "Advanced method In mechanical Design" -

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Figure 9 - Evaluation of the solutions according to the requirements (colors represent solutions accepted for 3 concepts)

The aggregation of the concepts is made declaring the selected actuators and drawing a rough scheme of their position and interaction (Fig. 10 and 11). After their drawing, the concepts must be evaluated to select the preliminary lay-out of the concept that will be developed. Although the evaluation is based on the priorities of the requirement list, a different result from the evaluation of the single actuator is being expected. In fact the concept evaluation is carried out on the overall conceptual scheme, not looking single actuators, thus it judges the interaction and harmonization of functions and properties. To not loose time during the concept evaluation, it is usually suggested the definition of not more than three concepts (if more than 3 are chosen, it is better they change only minor details).

DRAFT VERSION - Francesca Campana "Advanced method In mechanical Design" -

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Figure 10 - Example of concept definition (test-case of the train-seat table)

Figure 11 - Sketch design for concept definition (test-case device for making hot chocolate. For exercise try to derive functional groups of these concepts)

The evaluation can be made according to a comparison with a datum (that represents the ideal solution, that with maximum scores). Again we divide the evaluation in terms of quality properties and cost properties. Cost properties will take into account expected number of parts, materials, difficulties of manufacturing and assembly. Quality in terms of relevant requirements. Also in

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this case a qualitative evaluation is made (on three or five scores). Defining an ideal concept as that with the maximum score in every property, each concept can be ranked by normalizing its average score with ideal one. An overview of the quality/cost ratio among the concepts is then made comparing their scores in a 2D-diagram of quality versus 1/cost (fig. 12). In terms of maximum quality/cost ratio the best concept will be that nearest to 1 along the 45° line. Different solutions can be found assuming different weight to quality or 1/cost. The best concept will be developed by a preliminary lay-out design that will define the preliminary draft of the sub-assemblies and components of the product.

Figure 12 - Evaluation of concepts via datum definition

The procedure that has been described is based on the Hubka’s design methodology [Hubka, Eder, "Theory of Technical Systems: A Total Concept Theory for Engineering Design, Springer Verlag, DOI 10.1007/978-3-642-52121-8]. This methodology has been defined as a general approach for designing. It is consistent with other design strategies analysed by other authors and standards. According to Design for Quality principles, if the voice of costumers in the list of requirements is well defined, with minor modifications it is also proposed by other more recent authors and specific methods like Axiomatic Design and Quality Function Deployment.

1.5 From the concept design to the executive lay-out: design criteria for shapes and sizes.

Concept schemes, also called system architecture, are made of symbolic sketches that give a general idea of the actuators and their interactions. The mechanical design of the optimal concept must be carried out according to the specific design criteria of each sub-assembly of the product. To develop an assembly from conceptual to its executive design, two classes of technical expertise are required:

DRAFT VERSION - Francesca Campana "Advanced method In mechanical Design" -

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1. knowledge about drawing optimal topologies3, shapes and functional interfaces;

2. knowledge about design criteria for section sizes (longitudinal lengths are usually defined by requirement constraints or interface design).

From the first one the preliminary lay-out is drawn. It defines 3D shapes, physical constraints and functional couplings. It defines how power is transmitted and then the application points of the forces, thus the kinematic and the stress-strain conditions of the structures. From the second one, the preliminary design is developed to find its lengths. In other words, preliminary design is the design step in which shapes and lengths of the actuators are developed.

1.5.1 Defining shapes: basic rules

Basic classes on solid mechanics and mechanical engineering explain the reason why these solutions must be preferred [R.g. Budynas, J. K. Nisbett, “Shigley’s

Mechanical Engineering Design – 9th Edition” – MacGraw Hill]. Some practical rules, and related common mistakes, are shown in table 4.

Table 4 – Practical rules for shapes and loads DO DO NOT

Avoid load outside the plane of the structure. Adopt wireframe applied on the vertexes.

Avoid deviation of the stress flow, especially in joints

Avoid stress-concentration

Interface of moving parts must share the same nominal surface to achieve uniform loads, thus minimum wear. Be carefull movement is related to gap tolerance

Obviously other specific solutions concern with particular issues (e.g. bearings, kinematic chains, ...) are related to other different technical recommendations and are strictly determined by the personal knowledge. Complexity of the topology is manly related to the technological aspects of manufacturing, since it constraints the freedom of obtaining complex components. In terms of simultaneous engineering the manufacturing system must be defined as soon as possible, so that optimization of its related aspects can be made.

1.5.2 Defining sizes: the concept of safety margin

3 Topology is a mathematical property that defines the degree of connectivity of the space domain of the component.

More information will be given in a following chapter.

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According to [R.g. Budynas, J. K. Nisbett, “Shigley’s Mechanical Engineering Design – 9th Edition” – MacGraw Hill] different design criteria may be adopted to derive lengths. It depends on the type of loads (static, fatigue, compression stability, crash, temperature, fracture, wear, ...) and related hypotheses of equivalent stress computation according to the structural part we are analysing (beam, shell, plate, ...). Nevertheless the general description of a design criterion for length definition, can be written in the form of:

Where stands for the maximum equivalent condition in the component, e.g.

the equivalent stress in the point of maximum stress, stands for the limit condition related to the adopted material and X represent a Safety Factor (or margin of safety). When the design criterion is expressed in terms of strain or energy stresses in the above formula should be replaced with strain or energy computed according to the component section/point and the limit of the material.

The margin of safety is greater than one and it is necessary to give assurance of resistance in case of possible variations of the the equivalent condition and the material limit. In the actual practice, reason of variations can be related to 8 reasons (Ti i=1,8):

1) limit about the analytic formula used to compute the equivalent condition (Tesca, Mohr, Von Mises, Simplified impact energy, ...);

2) limit about the evaluation of the actual loads (are they fixed or probabilistic? Are they completely known?) and their evaluation as local stress conditions (e.g. shear load in beam sections, or stress evaluation in beam with large curvature, or nearby the supports within De Saint Venant theory, ...);

3) limit about the knowledge of the probabilistic behaviour of the material strength or their acceptance tolerance (are you sure that a yielding stress is always the same for a given batch of material?);

4) knowledge of the operative conditions, that means understanding if environmental conditions (weather, umidity, temperature, ...) may impact the severity of the stress conditions or the material strength;

5) maintenance requirements and checks, since proper maintenance planning may reduce risks of failures;

6) severity of failures in the respect of safety;

7) standards/law requests;

8) weight reduction.

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Generally speaking, increasing the uncertainty, reasons from 1) up to 7) may increase the C value. In the case of 8) increasing its necessity makes decreasing C (it is in opposite condition to the other ones).

During design these reasons must be considerate to avoid underestimation of loads, material limits and safety consequences. Nevertheless a balance with weight reduction should be found.

They can be used as a checklist or can be applied to systematically find a value for C. According to [J. A. Collins, “Mechanical Design of Machine Elements and machines”, Wiley and Son], C can be computed as a function of the 8 issues by means of: C= 1 + ((10+

)^2)/100 if

≥ -6

otherwise: C=1.15 Ti can be derived in a qualitative way from a 9 degree scale that goes from -4 (extreme necessity of reducing C according to the ith reason) up to +4 (extreme necessity of increasing C according to the ith reason).

0 value stands for no necessity of change.

±1 slight necessity

±2 moderate necessity

±3 relevant necessity

±4 extreme necessity

If adding all the Ti the sum is below -6 the C should be set =1.15 that represents the minimum possible value.

1.6 Tools and methods to organize the design activities.

Design activities need some tools and methods able to aid them.

In the very first part of the design activity two methods are particularly useful: Gantt's Diagram and Design Review.

By your own look for examples about them. Concerning Design Review, please check its use in the ISO standards. Where is it applied?

1.7 Exercises

1. Give the definition of product. Can you distinguish its definition from the Enterprise point of view and from the mechanical design point of view?

2. What is the main goal of the Design product and development?

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3. What is the meaning of external property? Can you describe them according to their correlation among them?

4. Can you define a list of importance for the external properties according to different type of products (e.g. a golf caddy, electrical oven, goods lift, limb prosthesis, ...)

5. What is the meaning of Simultaneous Engineering? and Concurrent Engineering?

6. Why does Concurrent engineering reduce time to market?

7. Moving from A to B can be made by different technological principles: by flight with or without engine, by car, bicycle/skateboard, segway, by train, by boat with or without engine. Can you tell what technological principles are related to each of them?

8. Adopting the rules of uniform stress and minimum weight, please define the shape of the two components in following assembly, taking into account the details for interfacing the two parts by interference

F= 600 N

L1=200 mm

L2=300 mm

hmax=bmax=30 mm

List of Basic References

Hubka, Eder, "Theory of Technical Systems: A Total Concept Theory for Engineering Design, Springer Verlag, DOI 10.1007/978-3-642-52121-8

J. A. Collins, “Mechanical Design of Machine Elements and machines”, Wiley and Son

R.g. Budynas, J. K. Nisbett, “Shigley’s Mechanical Engineering Design – 9th Edition” – MacGraw Hill