Post on 30-Dec-2015
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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
Shigley’s Mechanical Engineering Design
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.
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The Design Process
Iterative in natureRequires initial estimation,
followed by continued refinement
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Design Considerations
Some characteristics that influence the design
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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.
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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
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A Few Useful Internet Sites
www.globalspec.comwww.engnetglobal.comwww.efunda.comwww.thomasnet.comwww.uspto.gov
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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
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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.
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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
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Standards and Codes
Some organizations that establish standards and codes of particular interest to mechanical engineers:
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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.
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Tolerances
Close tolerances generally increase cost◦Require additional
processing steps◦Require additional
inspection◦Require machines with
lower production rates
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Breakeven Points
A cost comparison between two possible production methodsOften there is a breakeven point on quantity of production
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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.
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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
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Uncertainty
Common sources of uncertainty in stress or strength
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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
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Uncertainty
Deterministic method
◦ Establishes a design factor, nd
◦Based on absolute uncertainties of a loss-of-function parameter and a maximum allowable parameter
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◦ If, for example, the parameter is load, then
Example 1-1
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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.
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Example 1-2
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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
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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%
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1
(1-5)n
ii
R R
Power Transmission Case Study Specifications
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Power Transmission Case Study Specifications
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Power Transmission Case Study Specifications
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Chapter Outline
Shigley’s Mechanical Engineering Design
Example 1-2
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Solution
Answer
Answer
Standard Tensile Test
Used to obtain material characteristics and strengthsLoaded in tension with slowly increasing PLoad and deflection are recorded
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Fig. 2–1
Stress and Strain
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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
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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
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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.
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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 .
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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.
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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
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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
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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
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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
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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
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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)
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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
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Strength and Hardness
For many materials, relationship between ultimate strength and Brinell hardness number is roughly linear
For steels
For cast iron
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Example 2-2
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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
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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
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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
Shigley’s Mechanical Engineering Design