Shigley’s Mechanical Engineering Design
9th Edition
Richard G. Budynas and J. Keith Nisbett
Chapter 1
INTRODUCTION TO
MECHANICAL ENGINEERING
DESIGN
Prepared by
Dr. Saleh S. Al-Hayek
Assistant Professor of Mechanical Engineering
University of Tabuk
1 Introduction to Mechanical Engineering Design
Chapter
Outline
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 Engineer’s Professional Responsibilities
1-6
Standards and Codes
1-7
Economics
1-8
Safety and Product Liability
1-9
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
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 with
an excess of partially contradictory information.
• Engineers have to communicate effectively and work with
people 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 ,safe,
reliable, competitive, usable, manufacturable, and marketable,
regardless of who builds it or who uses it.
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Mechanical Engineering Design
Mechanical engineering design involves all disciplines of
mechanical engineering.
• A simple journal bearing involves fluid flow, heat transfer,
friction, energy transport, material selection, thermomechanical
treatments, statistical descriptions, and so on.
• A building is environmentally controlled. The heating,
ventilation, and air-conditioning considerations are sufficiently
specialized that some speak of heating, ventilating, and airconditioning design as if it is separate and distinct from
mechanical engineering design.
• Similarly, internal-combustion engine design, turbomachinery
design, and jet-engine design are sometimes considered discrete
entities.
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Standard Design Process
• The complete design process from
start to finish, is often outlined as in
the figure.
• Begins with an identification of need
and a decision to do something
about it.
• After many iterations, the process
ends with the presentation of the
plans for satisfying the need.
• Several design phases may be
repeated 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 sense
that something is not right.
• The definition of problem is more specific and must include all the
specifications for the object that is to be designed.
• The synthesis of a scheme connecting possible system elements is
sometimes called the invention of the concept or concept design.
This is the first and most important step in the synthesis task.
• Analyses must be performed to assess whether the system
performance is satisfactory.
• Synthesis, analysis and optimization are intimately and iteratively
related.
• Evaluation is the final proof of a successful design and usually
involves the testing of a prototype in the laboratory.
• Presentation is a selling job.
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Design Considerations
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Functionality
Strength/stress
Distortion/deflection/stiffness
Wear
Corrosion
Safety
Reliability
Manufacturability
Utility
Cost
Friction
Weight
Life
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Noise
Styling
Shape
Size
Control
Thermal properties
Surface
Lubrication
Marketability
Maintenance
Volume
Liability
Remanufacturing/resource
recovery
The Design Engineer’s Responsibilities
• In general, design engineering is required to satisfy the needs of
customers ( management, clients, consumers, etc. ) and is expected
to do so in a competent, responsible, ethical, and professional
manner.
• Careful attention to the following action steps will help you to
organize your solution processing technique.
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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 engineer’s professional obligations include conducting
activities in an ethical manner.
8
Standards and Codes
1.
A standard is a set of specifications for parts, materials, or processes
intended to achieve uniformity, efficiency, and a specified quality.
2.
A code is a set of specifications for the analysis, design, manufacture, and
construction of something.
3.
All of the organizations and societies listed below have established
specifications for standards and safety or design codes.
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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)
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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)
Economics
• The consideration of cost plays an
important role in the design decision
process.
• The use of standard or stock sizes
is a first principle of cost reduction.
• Among the effects of design
specifications on costs, tolerances
are perhaps most significant.
• When two or more design
approaches are compared for cost,
there occurs a point corresponding
to equal cost, which is called the
breakeven point.
10
Stress and Strength
• The survival of many products depends on how the designer
adjusts the maximum stresses in a component to be less than
the component’s strength at specific locations of interest.
• Strength is a property of a material or of a mechanical element.
The strength of an element depends on the choice, the
processing of the material.
• Stress is a state property at a specific point within a body,
which is a function of load, geometry, temperature, and
manufacturing processing.
• We shall use the capital letter S to denote strength, the Greek
letters σ (sigma) and τ (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 n d is defined as
12
Reliability
• The reliability method of design is one in which we obtain the
distribution of stresses and the distribution of strengths and
then relate these two in order to achieve an acceptable
success rate.
• The reliability R can be expressed by a number having the
range
0  R 1
• In the reliability method of design, the designer’s task is to
make a judicious selection of materials, processes, and
geometry (size) so as to achieve a specific reliability goal.
• It is important to note that good statistical data and estimates
are essential to perform an acceptable reliability analysis.
13
Dimensions and Tolerances
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Normal size
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Limits. The stated maximum and minimum dimensions.
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Tolerance. The difference between the two limits.
Bilateral tolerance. The variation in both directions from the basic
dimension, i.e. 2 5  0 .0 5 m m
Unilateral tolerance. The basic dimension is taken as one of the limits, and
variation is permitted in only one direction,
25
 0 .0 5
 0 .0 0
mm
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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 in
which the internal member is larger than the external member.
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Allowance. The minimum stated clearance or the maximum stated
interference for mating parts.
14
Units
• In the symbolic units equation for Newton’s second law, F=ma.
Units chosen for any three of these quantities are called base
units.
• The International System of Units (SI) is an absolute system.
The base units are the meter, the kilogram (for mass), and the
second.
15
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, and
0.00219 are assumed to be numbers with three significant figures.
• To display 706 to four significant figures, insert a trailing zero and
display either 706.0, 7.06×102, or 0.7060×103.
• Computers and calculators display calculations to many significant
figures. However, you should 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.
• For example, determine the circumference of a solid shaft with a
diameter of d=11mm. The circumference is given by C   d .Since
d is given with two significant figures, C should be reported with only
two significant figures.
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