Machine Design- Chapter 1

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Chapter 1INTRODUCTION TO

MECHANICAL ENGINEERINGDESIGN

Copyright 2011 by The McGraw-Hill Companies, Inc. ermission re!"ire# $or repro#"ction or #isplay.

%higley&s Mechanical 'ngineering (esign

)th'#ition in %I "nits*ichar# G. +"#ynas an# . eith isbett

Prepared by

"ei-/"an Chanssociate ro$essor o$ Mechanical 'ngineering

ational Cheng "ng niersity


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1 Intro#"ction to Mechanical 'ngineering (esign

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. Itis also a decision-making process.

Decisions sometimes have to be made with limitedinformation, occasionally with just the right amount ofinformation, or with an excess of partially contradictoryinformation.

ngineers have to communicate e!ectively and workwith people of many disciplines.

ngineering tools "such as mathematics, statics,computers, graphics, and languages# are combined toproduce a plan that, when carried out, produces aproduct that is functional ,safe, reliable,cometiti!e, usable, manufacturable, an"mar#etable, regardless of who builds it or who uses it.


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%echanical ngineering

%echanical engineering design involves all disciplines ofmechanical engineering.

& simple journal bearing involves 'uid 'ow, heat transfer,friction, energy transport, material selection,thermomechanical treatments, statistical descriptions,and so on.

& building is environmentally controlled. (he heating,ventilation, and air-conditioning considerations aresu)ciently speciali*ed that some speak of heating,ventilating, and air-conditioning design as if it is separateand distinct from mechanical engineering design.

+imilarly, internal-combustion engine design,turbomachinery design, and jet-engine design aresometimes considered discrete entities.


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+tandard Design rocess

(he complete design process

from start to nish, is oftenoutlined as in the gure.

/egins with an identication ofneed and a decision to dosomething about it.

&fter many iterations, theprocess ends with thepresentation of the plans forsatisfying the need.

+everal design phases may be

repeated throughout the life ofthe product.


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hase of the Design rocess

I"enti$cation of nee" generally starts the design process.(he need may only be a vague discontent, a feeling ofuneasiness, or a sense that something is not right.

T%e "e$nition of roblem is more specic and must includeall the specications for the object that is to be designed.

T%e s&nt%esis of a sc%eme connecting possible systemelements is sometimes called the invention of the concept orconcept design. (his is the rst and most important step inthe synthesis task.

Anal&sesmust be performed to assess whether the systemperformance is satisfactory.

S&nt%esis, anal&sisand otimi'ationare intimately anditeratively related.

E!aluationis the nal proof of a successful design andusually involves the testing of a prototype in the laboratory.

(resentationis a selling job.


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Design 2onsiderations


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(he Design ngineer4s 5esponsibilities

In general, design engineering is re6uired to satisfy theneeds of customers " management, clients, consumers, etc.# and is expected to do so in a competent, responsible,ethical, and professional manner.

2areful attention to the following action steps will help youto organi*e your solution processing techni6ue.

7nderstand the problem.

Identify the known.

Identify the unknown and formulate the solution strategy.

+tate all assumption and decision.

&naly*e the problem.

valuate your solution.

(he design engineer4s professional obligations includeconducting activities in an ethical manner.


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+tandards and 2odes

9. & stan"ar"is a set of specications for parts, materials, or processes

intended to achieve uniformity, e)ciency, and a specied 6uality.:. & co"eis a set of specications for the analysis, design, manufacture,

and construction of something.

3. &ll of the organi*ations and societies listed below have establishedspecications for standards and safety or design codes.


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conomics

(he consideration of cost plays

an important role in the designdecision process.

(he use of standard or stock si*esis a rst principle of costreduction.

&mong the e!ects of designspecications on costs,tolerances are perhaps mostsignicant.


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+tress and +trength

(he survival of many products depends on how thedesigner adjusts the maximum stresses in acomponent to be less than the component4s strengthat specic locations of interest.

Stren*t%is a property of a material or of a mechanicalelement. (he strength of an element depends on the

choice, the processing of the material.

Stressis a state property at a specic point within abody, which is a function of load, geometry,temperature, and manufacturing processing.

"sigma# and ? "tau# to designate normaland shear stresses, respectively.


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5eliability

(he reliability method of design is one in which we

obtain the distribution of stresses and the distribution ofstrengths and then relate these two in order to achievean acceptable success rate.

(he reliability Rcan be expressed by a number havingthe range

In the reliabilit& met%o" of "esi*n, the designer4stask is to make a judicious selection of materials,processes, and geometry "si*e# so as to achieve aspecic reliability goal.

It is important to note that good statistical data andestimates are essential to perform an acceptablereliability analysis.

! "R


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Dimensions and (olerances

Normal si'e

Limits. (he stated maximum and minimum dimensions.

Tolerance. (he di!erence between the two limits.

Bilateral tolerance. #he variation in both directions from the basic dimension,i.e.

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

Clearance. & general term that refers to the mating of

cylindrical parts such as a bolt and a hole. Inference)(he opposite of clearance, for mating cylindrical

parts in which the internal member is larger than the externalmember.

Allo+ance.(he minimum stated clearance or the maximum

stated interference for mating parts.

$% !.!%mm

+

!.!%$%

!.!!mm


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7nits

In the symbolic units e6uation for @ewton4s secondlaw, ABma. 7nits chosen for any three of these6uantities are called baseunits.

(he International +ystem of 7nits "+I# is an absolutesystem. (he base units are the meter, the kilogram "formass#, and the second.


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+ignicant Aigures

(he number of signicant gures is usually inferred by the

number of gures given "except for leading *eros#. Aorexample, 1;0, 3.9$, and ;.;;:98 are assumed to benumbers with three signicant gures.

(o display 1;0 to four signicant gures, insert a trailing*ero and display either 1;0.;, 1.;0C9;:, or ;.1;0;C9;3.

2omputers and calculators display calculations to manysignicant gures. owever, you should never report anumber of signicant gures of a calculation any greaterthan the smallest number of signicant gures of thenumbers used for the calculation.

Aor example, determine the circumference of a solid shaftwith a diameter of dB99mm. (he circumference is given by

.+ince d is given with two signicant gures, 2should be reported with only two signicant gures.

=C d

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Machine Design- Chapter 1