CHAPTER 1: INTRODUCTION TO

ENGINEERING DESIGN

MECHANICAL ENGINEERING DEPARTMENT

POLITEKNIK SULTAN AZLAN SHAH

JJ513: ENGINEERING DESIGN

Engineering design is the process of devising a system, component, or process to meet desired needs. It is a decision making process in which basic sciences, mathematics, and engineering sciences are applied to convert resources optimally to meet stated objective

Definition

Phases in design process model

Identification of need generally starts the design process. Recognition of the need and phrasing the need often constitute a highly creative act, because the need may be only a vague discontent, a feeling of uneasiness, or a sensing that something is not right. The need is often not evident at all; recognition is usually triggered by a particular adverse circumstance or a set of random circumstances that arises almost simultaneously

Identification of need

The definition of problem is more specific and must include all the specifications for the object that is to be designed. The specifications are the input and output quantities, the characteristics and dimensions of the space the object must occupy, and all the limitations on these quantities. We can regard the object to be designed as something in a black box. In this case we must specify the inputs and outputs of the box, together with their characteristics and limitations.

The definition of problem

The synthesis of a scheme connecting possible system elements is sometimes called the invention of the concept or concept design. This is the first and most important step in the synthesis task. Various schemes must be proposed, investigated, and quantified in terms of established metrics.

Synthesis

As the fleshing out of the scheme progresses, analyses must be performed to assess whether the system performance is satisfactory or better, and, if satisfactory, just how well it will perform.

System schemes that do not survive analysis are revised, improved, or discarded. Those with potential are optimized to determine the best performance of which the scheme is capable. Competing schemes are compared so that the path leading to the most competitive product can be chosen.

Analysis and optimisation

Evaluation is a significant phase of the total design process. Evaluation is the final proof of a successful design and usually involves the testing of a prototype in the laboratory. Here we wish to discover if the design really satisfies the needs.

Evaluation

Communicating the design to others is the final, vital presentation step in the design process. Undoubtedly, many great designs, inventions, and creative works have been lost t0 posterity simply because the originators were unable or unwilling to explain their accomplishments to others.

Communicating the design

characteristic that influences the design of the element or, perhaps, the entire system.

1 Functionality 14 Noise

2 Strength/stress 15 Styling

3 Distortion/deflection/stiffness 16 Shape

4 Wear 17 Size

5 Corrosion 18 Control

6 Safety 19 Thermal properties

7 Reliability 20 Surface

8 Manufacturability 21 Lubrication

9 Utility 22 Marketability

10 Cost 23 Maintenance

11 Friction 24 Volume

12 Weight 25 Liability

13 Life

Design considerations

The consideration of cost plays such an important role in the design decision process that we could easily spend as much time in studying the cost factor as in the study of the entire subject of design.

The cost of manufacturing a single product will vary from city to city and from one plant to another because of overhead, labour, taxes, and freight differentials and the inevitable slight manufacturing variations.

Economics

A code is a set of specifications for the analysis, design, manufacture, and construction of something. The purpose of a code is to achieve a specified degree of safety, efficiency, and performance or quality. It is important to observe that safety codes do not imply absolute safety

Code and standard

A standard is a set of specifications for parts, materials, or processes intended to achieve uniformity, efficiency, and a specified quality. One of the important purposes of a standard is to place a limit on the number of items in the specifications so as to provide a reasonable inventory of tooling, sizes, shapes, and varieties.

Code and standard

The organizations of interest to mechanical engineers are:1. Aluminium Association (AA)2. American Gear Manufacturers Association (AGMA)3. American Institute of Steel Construction (AISC)4. American Iron and Steel Institute (AISI)5. American National Standards Institute (ANSI)5

6. ASM International67. American Society of Mechanical Engineers (ASME)8. American Society of Testing and Materials (ASTM)9. American Welding Society (AWS)10. American Bearing Manufacturers Association (ABMA)711. British Standards Institution (BSI)12. Industrial Fasteners Institute (IFI)13. Institution of Mechanical Engineers (I. Mech. E.)14. International Bureau of Weights and Measures (BIPM)15. International Standards Organization (ISO)16. National Institute for Standards and Technology (NIST)817. Society of Automotive Engineers (SAE)

Code and standard

The strict liability concept of product liability generally prevails in the United States. This concept states that the manufacturer of an article is liable for any damage or harm that results because of a defect. And it doesn’t matter whether the manufacturer knew about the defect, or even could have known about it.

The best approaches to the prevention of product liability are good engineering in analysis and design, quality control, and comprehensive testing procedures.

Safety and product liability

“The word ergonomics implies the study of man at work while the word human factors implies the study of ma n in relation to equipment and environment.”

Ergonomic factors

ergonomic factors

Design and performance parameter that can contribute to achieving customer requirements

Design Requirements

Design specificationsRefer to detail description of the completed

design, including all dimensions, material properties and fabrication instructions

Power to be delivered: 20 hp Input speed: 1750 rev/minOutput speed: 85 rev/minTargeted for uniformly loaded applications, such as

conveyor belts, blowers,and generatorsOutput shaft and input shaft in-lineBase mounted with 4 boltsContinuous operation6-year life, with 8 hours/day, 5 days/wkLow maintenanceCompetitive costNominal operating conditions of industrialized locations Input and output shafts standard size for typical couplings

Design Requirement example

Power to be delivered: 20 hp Power efficiency: >95% Steady state input speed: 1750 rev/min Maximum input speed: 2400 rev/min Steady-state output speed: 82–88 rev/min Usually low shock levels, occasional moderate shock Input and output shaft diameter tolerance: ±0.001 in Output shaft and input shaft in-line: concentricity ±0.005 in, alignment ±0.001 rad Maximum allowable loads on input shaft: axial, 50 lbf; transverse, 100 lbf Maximum allowable loads on output shaft: axial, 50 lbf; transverse, 500 lbf Base mounted with 4 bolts Mounting orientation only with base on bottom 100% duty cycle Maintenance schedule: lubrication check every 2000 hours; change of lubrication every 8000 hours of operation; gears and bearing life >12,000 hours; infinite shaft life; gears, bearings, and shafts replaceable Access to check, drain, and refill lubrication without disassembly or opening of gasketed joints.

Design specification example

Materials SelectionThe designer of any product, other than

software must get involved with material selection.

Only occasionally will the exact grade of material be specified by the customer.

Even then the designer must understand the material to be able to design the product.

Materials Selection forEngineering Design

So many materials, so much information.How do we decide?How do we begin to choose?

First we need to look at the function of the product – product analysis

Materials Selection

Product AnalysisJust what it says – analyse the product!What does it do?How does it do it?Where does it do it?Who uses it?What should it cost?

Materials Selection

What is the function of a bike – obvious?How does the function depend on the typeof bike?

RacingTouringMountain bikeCommuterChilds

Case Study – a bike

How is it made to be easily maintained?What should it look like (colours etc.)?What should it cost?How has it been made comfortable to ride?How do the mechanical parts work and

interact?

TextbooksManufacturer’s literatureInternet Sites

Where do I find the data?

Materials Selection Charts

Modulus spans 5 orders of magnitude 0.01 GPa for foams to 1000 GPa for diamond

The charts therefore use logarithmic scales, where twice the distance means ten times.

This makes it possible to show the full range on one chart

Materials Selection Charts

Allow easy visualisation of propertiesShow lots of different materialsCan be ‘drilled down’ to specificsShow balances of properties e.g. strength v

costIdeal for a first ‘rough cut’ selection

Materials Selection Charts

1. Think about the design from ergonomic and functional viewpoint.

2. Decide on the materials to be used.3. Choose a suitable process that is also

economic4. Is the product performance driven or cost

driven?This makes a huge difference when choosing materials

Although we usually choose materials first sometimes it is the shape and process which is the limiting factor

Summary

Drink ContainerWhat are the requirements

Case Study

Provide leak free environment for storing liquid.

Comply with food standards & protect liquid from health hazards.

For fizzy drinks, withstand pressure.Brand image & identityEasy to openEasy to store & transportCheap for high volumes

Specifications

SteelAluminiumGlassPlastic

Possible Materials