The McGraw-Hill Companies © 2012. Chapter Outline Shigley’s Mechanical Engineering Design.

The McGraw-Hill Companies © 2012

Chapter Outline

Shigley’s Mechanical Engineering Design

Design

To formulate a plan for the satisfaction of a specified needProcess requires innovation, iteration, and decision-makingCommunication-intensiveProducts should be◦ Functional◦ Safe◦Reliable◦Competitive◦Usable◦Manufacturable◦Marketable


Mechanical Engineering Design

Mechanical engineering design involves all the disciplines of mechanical engineering.

Example◦ Journal bearing: fluid flow, heat transfer, friction, energy

transport, material selection, thermomechanical treatments, statistical descriptions, etc.


The Design Process

Iterative in natureRequires initial estimation,

followed by continued refinement


Design Considerations

Some characteristics that influence the design


Computational Tools

Computer-Aided Engineering (CAE)◦Any use of the computer and software to aid in the

engineering process◦ Includes

Computer-Aided Design (CAD) Drafting, 3-D solid modeling, etc.

Computer-Aided Manufacturing (CAM) CNC toolpath, rapid prototyping, etc.

Engineering analysis and simulation Finite element, fluid flow, dynamic analysis, motion, etc.

Math solvers Spreadsheet, procedural programming language, equation solver,

etc.


Acquiring Technical Information

Libraries◦ Engineering handbooks, textbooks, journals, patents, etc.

Government sources◦Government agencies, U.S. Patent and Trademark, National

Institute for Standards and Technology, etc.Professional Societies (conferences, publications, etc.)◦American Society of Mechanical Engineers, Society of

Manufacturing Engineers, Society of Automotive Engineers, etc.

Commercial vendors◦Catalogs, technical literature, test data, etc.

Internet

Access to much of the above information


A Few Useful Internet Sites

www.globalspec.comwww.engnetglobal.comwww.efunda.comwww.thomasnet.comwww.uspto.gov


The Design Engineer’s Professional Responsibilities

Satisfy the needs of the customer in a competent, responsible, ethical, and professional manner.

Some key advise for a professional engineer◦Be competent◦Keep current in field of practice◦Keep good documentation◦ Ensure good and timely communication◦Act professionally and ethically


Ethical Guidelines for Professional Practice

National Society of Professional Engineers (NSPE) publishes a Code of Ethics for Engineers and an Engineers’ Creed.

www.nspe.org/ethics Six Fundamental CanonsEngineers, in the fulfillment of their professional duties, shall:◦Hold paramount the safety, health, and welfare of the public.◦ Perform services only in areas of their competence.◦ Issue public statements only in an objective and truthful

manner.◦Act for each employer or client as faithful agents or trustees.◦Avoid deceptive acts.◦Conduct themselves honorably, responsibly, ethically, and

lawfully so as to enhance the honor, reputation, and usefulness of the profession.


Standards and Codes

Standard◦A set of specifications for parts, materials, or processes◦ Intended to achieve uniformity, efficiency, and a specified

quality◦ Limits the multitude of variations

Code◦A set of specifications for the analysis, design, manufacture,

and construction of something◦ To achieve a specified degree of safety, efficiency, and

performance or quality◦Does not imply absolute safety

Various organizations establish and publish standards and codes for common and/or critical industries


Standards and Codes

Some organizations that establish standards and codes of particular interest to mechanical engineers:


Economics

Cost is almost always an important factor in engineering design.

Use of standard sizes is a first principle of cost reduction.

Table A-17 lists some typical preferred sizes.Certain common components may be less expensive

in stocked sizes.


Tolerances

Close tolerances generally increase cost◦Require additional

processing steps◦Require additional

inspection◦Require machines with

lower production rates


Breakeven Points

A cost comparison between two possible production methodsOften there is a breakeven point on quantity of production


Automatic screw machine25 parts/hr3 hr setup$20/hr labor cost

Hand screw machine10 parts/hrMinimal setup$20/hr labor cost

Breakeven at 50 units

EXAMPLE

Safety and Product Liability

Strict Liability concept generally prevails in U.S.Manufacturer is liable for damage or harm that results because

of a defect.Negligence need not be proved.Calls for good engineering in analysis and design, quality

control, and comprehensive testing.


Stress and Strength

Strength◦An inherent property of a material or of a mechanical element◦Depends on treatment and processing◦May or may not be uniform throughout the part◦ Examples: Ultimate strength, yield strength

Stress◦A state property at a specific point within a body◦ Primarily a function of load and geometry◦ Sometimes also a function of temperature and processing


Uncertainty

Common sources of uncertainty in stress or strength


Uncertainty

Stochastic method◦Based on statistical nature of the design parameters◦ Focus on the probability of survival of the design’s function

(reliability)◦Often limited by availability of statistical data


Uncertainty

Deterministic method

◦ Establishes a design factor, nd

◦Based on absolute uncertainties of a loss-of-function parameter and a maximum allowable parameter


◦ If, for example, the parameter is load, then

Example 1-1


Solution

Answer

Answer

Design Factor Method

Often used when statistical data is not availableSince stress may not vary linearly with load, it is more common

to express the design factor in terms of strength and stress.

All loss-of-function modes must be analyzed, and the mode with the smallest design factor governs.

Stress and strength terms must be of the same type and units.Stress and strength must apply to the same critical location in

the part.The factor of safety is the realized design factor of the final

design, including rounding up to standard size or available components.


Example 1-2


Solution

Answer

Answer

Reliability

Reliability, R – The statistical measure of the probability that a mechanical element will not fail in use

Probability of Failure, pf – the number of instances of failures per total number of possible instances

Example: If 1000 parts are manufactured, with 6 of the parts failing, the reliability is


or 99.4 %

Reliability

Series System – a system that is deemed to have failed if any component within the system fails

The overall reliability of a series system is the product of the reliabilities of the individual components.

Example: A shaft with two bearings having reliabilities of 95% and 98% has an overall reliability of

R = R1 R2 = 0.95 (0.98) = 0.93 or 93%


1

(1-5)n

ii

R R

Power Transmission Case Study Specifications






Chapter Outline


Example 1-2


Solution

Answer

Answer

Standard Tensile Test

Used to obtain material characteristics and strengthsLoaded in tension with slowly increasing PLoad and deflection are recorded


Fig. 2–1

Stress and Strain


The stress is calculated from

where is the original cross-sectional area.

The normal strain is calculated from

where l0 is the original gauge length and l is the current length corresponding to the current P.

Stress-Strain Diagram

Plot stress vs. normal strainTypically linear relation until

the proportional limit, plNo permanent deformation

until the elastic limit, elYield strength, Sy , defined at

point where significant plastic deformation begins, or where permanent set reaches a fixed amount, usually 0.2% of the original gauge length

Ultimate strength, Su , defined as the maximum stress on the diagram


Ductile material

Brittle material

Fig. 2–2

Elastic Relationship of Stress and Strain

Slope of linear section is Young’s Modulus, or modulus of elasticity, E

Hooke’s law

E is relatively constant for a given type of material (e.g. steel, copper, aluminum)

See Table A-5 for typical values

Usually independent of heat treatment, carbon content, or alloying


Fig. 2–2 (a)

True Stress-Strain DiagramEngineering stress-strain diagrams

(commonly used) are based on original area.

Area typically reduces under load, particularly during “necking” after point u.

True stress is based on actual area corresponding to current P.

True strain is the sum of the incremental elongations divided by the current gauge length at load P.

Note that true stress continually increases all the way to fracture.


True Stress-strain

Engineeringstress-strain

(2-4)

Compression Strength

Compression tests are used to obtain compressive strengths.Buckling and bulging can be problematic.For ductile materials, compressive strengths are usually

about the same as tensile strengths, Suc = Sut .

For brittle materials, compressive strengths, Suc , are often greater than tensile strengths, Sut .


Torsional Strengths

Torsional strengths are found by twisting solid circular bars. Results are plotted as a torque-twist diagram. Shear stresses in the specimen are linear with respect to the radial

location – zero at the center and maximum at the outer radius. Maximum shear stress is related to the angle of twist by

◦ q is the angle of twist (in radians)◦ r is the radius of the bar◦ l0 is the gauge length◦ G is the material stiffness property called the shear modulus or

modulus of rigidity.


Torsional Strengths

Maximum shear stress is related to the applied torque by

◦ J is the polar second moment of area of the cross section◦ For round cross section,

Torsional yield strength, Ssy corresponds to the maximum shear stress at the point where the torque-twist diagram becomes significantly non-linear

Modulus of rupture, Ssu corresponds to the torque Tu at the maximum point on the torque-twist diagram


Resilience

Resilience – Capacity of a material to absorb energy within its elastic range

Modulus of resilience, uR

◦ Energy absorbed per unit volume without permanent deformation

◦ Equals the area under the stress-strain curve up to the elastic limit

◦ Elastic limit often approximated by yield point


Resilience

Area under curve to yield point gives approximation

If elastic region is linear,

For two materials with the same yield strength, the less stiff material (lower E) has greater resilience


Toughness

Toughness – capacity of a material to absorb energy without fracture

Modulus of toughness, uT

◦ Energy absorbed per unit volume without fracture

◦ Equals area under the stress-strain curve up to the fracture point


Toughness

Area under curve up to fracture point

Often estimated graphically from stress-strain dataApproximated by using the average of yield and ultimate

strengths and the strain at fracture


Resilience and Toughness

Measures of energy absorbing characteristics of a materialUnits are energy per unit volume◦ lbf·in/in3 or J/m3

Assumes low strain ratesFor higher strain rates, use impact methods (See Sec. 2-5)


Hardness

Hardness – The resistance of a material to penetration by a pointed tool

Two most common hardness-measuring systems◦Rockwell

A, B, and C scales Specified indenters and loads for each scale Hardness numbers are relative

◦Brinell Hardness number HB is the applied load divided by the

spherical surface area of the indentation


Strength and Hardness

For many materials, relationship between ultimate strength and Brinell hardness number is roughly linear

For steels

For cast iron


Example 2-2


Material Numbering Systems

Common numbering systems◦ Society of Automotive Engineers (SAE)◦American Iron and Steel Institute (AISI)◦Unified Numbering System (UNS)◦American Society for Testing and Materials (ASTM) for cast

irons


UNS Numbering System

UNS system established by SAE in 1975Letter prefix followed by 5 digit numberLetter prefix designates material class◦G – carbon and alloy steel◦A – Aluminum alloy◦C – Copper-based alloy◦ S – Stainless or corrosion-resistant steel


UNS for SteelsFor steel, letter prefix is GFirst two numbers indicate composition, excluding carbon content

Second pair of numbers indicates carbon content in hundredths of a percent by weight

Fifth number is used for special situationsExample: G52986 is chromium alloy with 0.98% carbon


The McGraw-Hill Companies © 2012. Chapter Outline Shigley’s Mechanical Engineering Design.

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