shigley 9e si chap01

8/11/2019 Shigley 9E SI Chap01

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Chapter 1

INTRODUCTION TO

MECHANICAL ENGINEERING

DESIGN

Copyright 2011 by The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Shigleys Mechanical Engineering Design

9thEdition in SI unitsRichard G. Budynas and J. Keith Nisbett

Prepared by

Kuei-Yuan ChanAssociate Professor of Mechanical Engineering

National Cheng Kung University


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1 Introduction to Mechanical Engineering Design

1-1 Design

1-2 Mechanical Engineering Design

1-3 Phases and Interactions of the Design Process

1-4 Design Tools and Resources

1-5 The Design Engineers Professional Responsibilities

1-6 Standards and Codes

1-7 Economics

1-8 Safety and Product Liability1-9 Stress and Strength

1-10 Uncertainty

1-11 Design Factor and Factor of Safety

1-12 Reliability

1-13 Dimensions and Tolerances

1-14 Units

1-15 Calculations and Significant Figures

1-16 Design Topic Interdependencies

1-17 Power Transmission Case Study Specifications

ChapterOutline


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Design

Design is an innovative and highly iterative process. It is also a

decision-making process.

Decisions sometimes have to be made with limited information,occasionally with just the right amount of information, or withan excess of partially contradictory information.

Engineers have to communicate effectively and work withpeople of many disciplines.

Engineering tools (such as mathematics, statics, computers,graphics, and languages) are combined to produce a plan that,when carried out, produces a product that is functional safereliable competitive usable manufacturable and marketable,

regardless of who builds it or who uses it.


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Mechanical Engineering

Mechanical engineering design involves all disciplines of

mechanical engineering.

A simple journal bearing involves fluid flow, heat transfer,friction, energy transport, material selection, thermomechanicaltreatments, statistical descriptions, and so on.

A building is environmentally controlled. The heating,ventilation, and air-conditioning considerations are sufficientlyspecialized that some speak of heating, ventilating, and air-conditioning design as if it is separate and distinct frommechanical engineering design.

Similarly, internal-combustion engine design, turbomachinery

design, and jet-engine design are sometimes considereddiscrete entities.


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Standard Design Process

The complete design process from

start to finish, is often outlined as inthe figure.

Begins with an identification of needand a decision to do somethingabout it.

After many iterations, the processends with the presentation of theplans for satisfying the need.

Several design phases may berepeated throughout the life of the

product.


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Phase of the Design Process

Identification of need generally starts the design process. The need

may only be a vague discontent, a feeling of uneasiness, or a sensethat something is not right.

The definition of problem is more specific and must include all thespecifications for the object that is to be designed.

The synthesis of a scheme connecting possible system elements issometimes called the invention of the concept or concept design.This is the first and most important step in the synthesis task.

Analysesmust be performed to assess whether the systemperformance is satisfactory.

Synthesis, analysisand optimizationare intimately and iterativelyrelated.

Evaluationis the final proof of a successful design and usuallyinvolves the testing of a prototype in the laboratory.

Presentationis a selling job.


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Design Considerations

Functionality

Strength/stress

Distortion/deflection/stiffness

Wear

Corrosion

Safety

Reliability

Manufacturability

Utility

Cost

Friction

Weight

Life

Noise

Styling

Shape

Size

Control

Thermal properties

Surface

Lubrication

Marketability

Maintenance

Volume

Liability

Remanufacturing/resourcerecovery


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The Design Engineers Responsibilities

In general, design engineering is required to satisfy the needs of

customers ( management, clients, consumers, etc. ) and is expectedto do so in a competent, responsible, ethical, and professionalmanner.

Careful attention to the following action steps will help you toorganize your solution processing technique.

Understand the problem. Identify the known.

Identify the unknown and formulate the solution strategy.

State all assumption and decision.

Analyze the problem.

Evaluate your solution.

The design engineers professional obligations include conductingactivities in an ethical manner.


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Standards and Codes

1. A standardis a set of specifications for parts, materials, or processes

intended to achieve uniformity, efficiency, and a specified quality.2. A codeis a set of specifications for the analysis, design, manufacture, and

construction of something.

3. All of the organizations and societies listed below have establishedspecifications for standards and safety or design codes.

Aluminum Association (AA) American Gear Manufacturers

Association (AGM)

American Institute of Steel Construction

(AISC)

American Iron and Steel Institute (AISI)

American National Standards Institute

(ANSI)

ASM International

American Society of Mechanical

Engineers (ASME)

American Society of Testing and

Material (ASTM)

American Welding Society (AWS) American Bearing Manufactures

Association (ABMA)

British Standards Institute (BSI)

Industrial Fasteners Institute (IFI)

Institution of Mechanical Engineers

(I.Mech.E)

International Bureau of Weights and

Measures (BIPM)

International Standards Organization

(ISO)

National Institute for Standards and

Technology (NIST)

Society of Automotive Engineers (SAE)


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Economics

The consideration of cost plays an

important role in the design decisionprocess.

The use of standard or stock sizesis a first principle of cost reduction.

Among the effects of designspecifications on costs, tolerancesare perhaps most significant.

When two or more designapproaches are compared for cost,there occurs a point corresponding

to equal cost, which is called thebreakeven point.


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Stress and Strength

The survival of many products depends on how the designer

adjusts the maximum stresses in a component to be less thanthe components strength at specific locations of interest.

Strengthis a property of a material or of a mechanical element.The strength of an element depends on the choice, theprocessing of the material.

Stressis a state property at a specific point within a body,which is a function of load, geometry, temperature, andmanufacturing processing.

We shall use the capital letter S to denote strength, the Greekletters (sigma) and (tau) to designate normal and shear

stresses, respectively.


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Uncertainty

1. Examples of uncertainties concerning stress and strength include

Composition of material and the effect of variation on properties.Variations in properties from place to place within a bar of stock.

Effect of processing locally, or nearby, on properties.

Effect of nearby assemblies such as weldments and shrink fits on stress conditions.

Effect of thermomechanical treatment on properties.

Intensity and distribution of loading.

Validity of stress concentrations.

Influence of time on strength and geometry.

Effect of corrosion.

Effect of wear.

Uncertainty as to the length of any list of uncertainties.

2. Engineers must accommodate uncertainty.

3. The design factor is defined as

dn


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Reliability

The reliability method of design is one in which we obtain the

distribution of stresses and the distribution of strengths andthen relate these two in order to achieve an acceptablesuccess rate.

The reliability Rcan be expressed by a number having therange

In the reliability method of design, the designers task is tomake a judicious selection of materials, processes, andgeometry (size) so as to achieve a specific reliability goal.

It is important to note that good statistical data and estimatesare essential to perform an acceptable reliability analysis.

0 1R


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Dimensions and Tolerances

Normal size

Limits. The stated maximum and minimum dimensions.

Tolerance. The difference between the two limits.

Bilateral tolerance. The variation in both directions from the basicdimension, i.e.

Unilateral tolerance. The basic dimension is taken as one of the limits, andvariation is permitted in only one direction,

Clearance.A general term that refers to the mating of cylindrical parts

such as a bolt and a hole.

Inference. The opposite of clearance, for mating cylindrical parts inwhich the internal member is larger than the external member.

Allowance

. The minimum stated clearance or the maximum statedinterference for mating parts.

25 0.05mm

0.0525

0.00mm


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Units

In the symbolic units equation for Newtons second law, F=ma.

Units chosen for any three of these quantities are called baseunits.

The International System of Units (SI) is an absolute system.The base units are the meter, the kilogram (for mass), and thesecond.


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Significant Figures

The number of significant figures is usually inferred by the number of

figures given (except for leading zeros). For example, 706, 3.14, and0.00219 are assumed to be numbers with three significant figures.

To display 706 to four significant figures, insert a trailing zero anddisplay either 706.0, 7.06102, or 0.7060103.

Computers and calculators display calculations to many significant

figures. However, you should never report a number of significantfigures of a calculation any greater than the smallest number ofsignificant figures of the numbers used for the calculation.

For example, determine the circumference of a solid shaft with adiameter of d=11mm. The circumference is given by .Since

d is given with two significant figures, C should be reported with onlytwo significant figures.

C d

shigley 9e si chap01

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