INTRODUCTION TO
MECHANICAL
ENGINEERING
DESIGN
CHAPTER 1
Friday, February 06, 2015
Mohammad Suliman Abuhaiba, Ph.D., P.E. 1
CHAPTER OUTLINE
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1. Design
2. Mechanical Engineering Design
3. Phases and Interactions of the Design Process
4. Design Tools and Resources
5. The Design Engineer’s Professional Responsibilities
6. Standards and Codes
7. Economics
8. Safety and Product Liability
9. Stress and Strength
10.Uncertainty
11.Design Factor and Factor of Safety
12.Reliability
13.Dimensions and Tolerances
14.Calculations and Significant Figures
15.Power Transmission Case Study Specifications
DESIGN
Formulate a plan for the satisfaction of a
specified need
Process requires innovation, iteration, and
decision-making
Products should be
Functional
Safe
Reliable
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Competitive
Usable
Manufacturable
Marketable
MECHANICAL ENGINEERING
DESIGN (MED)
MED involves all disciplines of mechanical
engineering
Example - Journal bearing
fluid flow
heat transfer
Friction
energy transport
material selection
thermo-mechanical treatments
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THE DESIGN PROCESS
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DESIGN CONSIDERATIONS
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Functionality Cost Thermal properties
Strength/stress Friction Surface
Distortion/deflection/stiffness Weight Lubrication
Wear Life Marketability
Corrosion Noise Maintenance
Safety Styling Volume
Reliability Shape Liability
Manufacturability Size Remanufacturing/resource
recovery Utility Control
COMPUTATIONAL TOOLS
Computer-Aided Engineering (CAE): Any use of
computer & software to aid in the engineering
process
Computer-Aided Design (CAD)
Drafting, 3-D solid modeling, etc.
Computer-Aided Manufacturing (CAM)
CNC, rapid prototyping, etc.
Engineering analysis and simulation
FEA, fluid flow, dynamic analysis, motion, etc.
Math solvers: Spreadsheet, procedural
programming language, equation solver, etc.
MatLab, Mathematica, Maple, 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 Mohammad Suliman Abuhaiba, Ph.D., P.E. Friday, February 06, 2015 8
USEFUL INTERNET SITES
www.globalspec.com
www.engnetglobal.com
www.efunda.com
www.thomasnet.com
www.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
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ETHICAL GUIDELINES FOR
PROFESSIONAL PRACTICE
Six Fundamental Canons Engineers, in the fulfillment of their professional duties, shall:
1. Hold paramount safety, health, and welfare of the public
2. Perform services only in areas of their competence
3. Issue public statements only in an objective & truthful manner
4. Act for each employer or client as faithful agents or trustees
5. Avoid deceptive acts
6. Conduct themselves honorably, responsibly, ethically, and lawfully so as to enhance the honor, reputation, and usefulness of the profession.
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2
NSPE ENGINEERS’ CREED
As a Professional Engineer I dedicate my professional
knowledge and skill to the advancement and betterment
of human welfare.
I pledge:
• To give the utmost of performance
• To participate in none but honest enterprise
• To live and work according to the laws of man and the
highest standards of professional conduct
• To place service before profit, the honor and standing of
the profession before personal advantage, and the public
welfare above all other considerations.
In humility and with need for Divine Guidance, I make this
pledge. Mohammad Suliman Abuhaiba, Ph.D., P.E. Friday, February 06, 2015 1
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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 Mohammad Suliman Abuhaiba, Ph.D., P.E.
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STANDARDS AND CODES
Some organizations that establish standards and codes:
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Aluminum Association (AA)
American Bearing Manufacturers Association
(ABMA)
American Gear Manufacturers Association
(AGMA)
American Institute of Steel Construction
(AISC)
American Iron and Steel Institute (AISI)
American National Standards Institute (ANSI)
American Society of Heating, Refrigerating
and Air-Conditioning Engineers
(ASHRAE)
American Society of Mechanical Engineers
(ASME)
American Society of Testing and Materials
(ASTM)
American Welding Society (AWS)
ASM International
British Standards Institution (BSI)
Industrial Fasteners Institute (IFI)
Institute of Transportation
Engineers (ITE)
Institution of Mechanical
Engineers (IMechE)
International Bureau of Weights
and Measures (BIPM)
International Federation of
Robotics (IFR)
International Standards
Organization (ISO)
National Association of Power
Engineers (NAPE)
National Institute for Standards
and Technology (NIST)
Society of Automotive Engineers
(SAE)
ECONOMICS
Cost is an important factor in engineering
design
Use of standard sizes is a first principle of
cost reduction.
Table A-17: some typical preferred sizes
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EC
ON
OM
IC
S
Table A-17
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TOLERANCES
Close tolerances
generally increase
cost & require:
• additional
processing steps
• additional
inspection
• machines with
lower production
rates
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BREAKEVEN POINTS
A cost comparison between two possible production methods
There is a breakeven point on quantity of production
Mohammad Suliman Abuhaiba, Ph.D., P.E.
Automatic screw
machine
25 parts/hr
3 hr setup
$20/hr labor cost
Hand screw machine
10 parts/hr
Minimal setup
$20/hr labor cost
Breakeven at 50 units
EXAMPLE
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SAFETY & PRODUCT LIABILITY
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 Mohammad Suliman Abuhaiba, Ph.D., P.E. Friday, February 06, 2015 2
1
UNCERTAINTY
Common sources of uncertainty in stress or strength
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 thermo-mechanical treatment on properties.
Intensity and distribution of loading.
Validity of mathematical models used to represent reality.
Intensity 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.
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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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DESIGN FACTOR METHOD
Used when statistical data is not available
Since 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.
Factor of safety is the realized design factor of final
design, including rounding up to standard size or
available components.
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Mohammad Suliman Abuhaiba, Ph.D., P.E.
EXAMPLE 1-2
Solution
RELIABILITY
Reliability, R: statistical measure of the
probability that a mechanical element will not
fail in use
Probability of Failure, pf : 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
Mohammad Suliman Abuhaiba, Ph.D., P.E.
or 99.4 %
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RELIABILITY
Overall reliability of a series system = 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%
Mohammad Suliman Abuhaiba, Ph.D., P.E.
1
(1-5)n
i
i
R R
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DIMENSIONS AND TOLERANCES
Nominal size: size we use in speaking of an
element.
Is not required to match the actual dimension
Limits: stated max & min dimensions
Tolerance: difference between the two limits
Bilateral tolerance: variation in both
directions from the basic dimension, e.g.
1.005 ± 0.002 in.
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DIMENSIONS AND TOLERANCES
Unilateral tolerance: basic dimension is taken
as one of the limits, and variation is permitted
in only one direction, e.g.
Clearance: difference in sizes of two mating
cylindrical parts such as a bolt and a hole.
Diametral clearance: difference in the two
diameters
Radial clearance: difference in the two radii
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DIMENSIONS AND TOLERANCES
Interference: opposite of clearance, where
internal member is larger than the external
member
Allowance: minimum stated clearance or
maximum stated interference of mating parts
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Mohammad Suliman Abuhaiba, Ph.D., P.E.
EXAMPLE 1-3
Figure 1-4
Mohammad Suliman Abuhaiba, Ph.D., P.E.
EXAMPLE 1-3 (CONTINUED)
Solution
CALCULATIONS AND
SIGNIFICANT FIGURES
The accuracy of a real number depends on number
of significant figures describing the number.
Unless otherwise stated, no less than three
significant figures should be used in your
calculations.
The number of significant figures is usually inferred
by the number of figures given.
706, 3.14, and 0.00219 are assumed to be
numbers with three significant figures
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CALCULATIONS AND
SIGNIFICANT FIGURES
Consider a number such as 91600.
For three significant figures express the
number as 91.6 × 103. For four significant
figures express it as 91.60 × 103
Never report a number of significant figures
of a calculation any greater than the smallest
number of significant figures of the numbers
used for the calculation.
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LINKED END-OF-CHAPTER PROBLEMS
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Friday, February 06, 2015
35
Mohammad Suliman Abuhaiba, Ph.D., P.E.
POWER TRANSMISSION CASE STUDY
SPECIFICATIONS
Design Requirements
Power to be delivered = 20 hp
Input speed = 1750 rpm
Output speed = 85 rpm
Targeted for uniformly loaded applications, such as
conveyor belts, blowers, and generators
Output shaft and input shaft in-line
Base mounted with 4 bolts
Continuous operation
6-year life, with 8 hours/day, 5 days/week
Low maintenance
Competitive cost
Nominal operating conditions of industrialized locations
Input and output shafts standard size for typical
couplings
Mohammad Suliman Abuhaiba, Ph.D., P.E.
POWER TRANSMISSION CASE STUDY
SPECIFICATIONS
Mohammad Suliman Abuhaiba, Ph.D., P.E.
POWER TRANSMISSION CASE STUDY
SPECIFICATIONS