Chapter 1
Introduction to
Mechanical Engineering
Design
Faculty of Engineering
Mechanical Dept.
Design
To formulate a plan for the satisfaction of a specified need
 Process requires innovation, iteration, and decision-making
 Communication-intensive
 Products 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.

Shigley’s Mechanical Engineering Design
The Design Process
Shigley’s Mechanical Engineering Design
Functions?
Function 1
Functions 1
2
Functions 1
2
3
Functions 1
2
3
4
5
Design Considerations

Some characteristics that influence the design
Shigley’s Mechanical Engineering Design
The Key Elements of Product Design

Function: All products and services have one or more.
It is the reason you are building the product.
It is the purpose of your design.

Form: The final form of a product, or a structure, is the result of
many factors:
-The way you achieve the function, the “solution”,
-The available material, their use , their properties, and their
influence on aesthetics, cost, environment etc.
 Materials: The availability and properties have direct effect on
the form of the product: strength, weight, flexibility, weathering,
environmental impact, aesthetics, temperature, cost, etc.
7
The Key Elements of Product Design
.
Function
Cost
Material
Form
8
Question 1
The final form of a structure is the result of the following
factor(s):
a.
b.
c.
d.
e.
The use of the material
The properties of the material
The way the function is achieved.
The cost and availability of the material
All of the above
Question 2

The three most important elements of a design are?
A.
Form, Shape, Looks
Form, Shape, Size
Purpose, Scope, Cost
Function, Form, Materials
Responsibility, Applicability, Compatibility.
B.
C.
D.
E.
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.
Shigley’s Mechanical Engineering Design
The Need
Research
Test
Generate
Solutions
Implement
Evaluate Solutions &
Select
The Engineering Design Process
Need

Define
Problem
◦ Who defines it? Society - Government
Market: Customer – Consumer
Research
Need for: Transportation, Communication, Health,
Education
Food-processing, preservation, delivery,
Defense, Entertainment, Comfort, Business , …
Generate
many
Solutions
Select best
one
Need for the product or the service
There has to be a need for the product or the service.

Customer Requirements
What customer ordered: product or services.
(You may have to help your customer define his need)
Test
The Engineering Design Process (cont.)
Need
Define
Problem
Research
Generate
many
Solutions

Define The Need: Once the need has been
established, the first step of the engineering process
is to define the problem on hand.

BUT, before you define a problem you need to
understand it.

Evaluate the project constraints.

Translate customer requirements into a workable/
engineering concept.

What are the requirements/specifications of the
turbine project???????
Select best
one
Test
Need
Define
Problem
Research
Generate
many
Solutions
Select best
one
Test
The Engineering Design Process (cont.)
Gather Information (Research) for all “How to”
elements.
 10 – 30% of an engineers time can be spend on this step.
 It is continuous rather than a one step activity
(“Learning Curve” applies)
◦ Applicable engineering principles
◦ Materials, properties, procurement .
◦ Tools and equipment, how to operate.
◦ Etc.

The Engineering Design Process (cont.)
Need

Brainstorming: An activity designed to generate a lot of
ideas/solutions.

Quantity is of importance - design has many solutions

Build on the ideas of others

Focus on Function
Define
Problem
Research
Generate
many
solutions
Select best
one/
Implement
- No criticism
- All ideas are welcome
- No limits or boundaries
- No hesitation
Sketching : a graphical presentation of an idea
Test
Need
The Engineering Design Process (cont.)
Define
Problem

Research



Generate
many
solutions
Select best
one/
Implement
Test




Modeling
Functional Performance
Data collection
Data Analysis
Decision
Analyze failures
Redesign
Utilize alternate solutions
17
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

Shigley’s Mechanical Engineering Design
Standards and Codes

Some organizations that establish standards and codes of
particular interest to mechanical engineers:
Shigley’s Mechanical Engineering Design
Economics
Cost is almost always an important factor in
engineering design.
 Use of standard sizes is a first principle of cost
reduction.
 Certain common components may be less expensive in
stocked sizes.

Shigley’s Mechanical Engineering Design
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 methods
Often there is a breakeven point on quantity of production
EXAMPLE
 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
Shigley’s Mechanical Engineering Design
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
Shigley’s Mechanical Engineering Design
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
Shigley’s Mechanical Engineering Design
Design Factor Method


Often 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.
 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.

Shigley’s Mechanical Engineering Design
Example 1-2
A solid circular rod of diameter d undergoes a bending moment M = 100N.m
inducing a stress σ = 16 M/πd3.
Using a material strength of 170 MPa and a design factor of 2.5, determine the
minimum diameter of the rod. Using Table A-17, select a preferred fractional
diameter and determine the resulting factor of safety.
Shigley’s Mechanical Engineering Design
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 %
Shigley’s Mechanical Engineering Design
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.

n
R   Ri
(1-5)
i 1

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%
Shigley’s Mechanical Engineering Design
Dimensions and Tolerances
The following terms are used generally in dimensioning :
 Nominal size. The size we use in speaking of an element. For
example, we may specify a 40-mm pipe a 12-mm bolt. Either the
theoretical size or the actual measured size may be quite different.
The theoretical size of a 40-mm pipe is 47.5 mm for the outside
diameter. And the diameter 12-mm bolt, say, may actually measure
11.8 mm.
 Limits. The stated maximum and minimum dimensions.
 Tolerance. The difference between the two limits.
 Bilateral tolerance. The variation in both directions from the basic
dimension. That is, the basic size is between the two limits, for
example, 25 ± 0.05 mm. The tow parts of the tolerance need not be
equal.
Shigley’s Mechanical Engineering Design
Dimensions and Tolerances
Shigley’s Mechanical Engineering Design
EXAMPLE 1–3
A shouldered screw contains three hollow right circular cylindrical parts on the
screw before a nut is tightened against the shoulder. To sustain the function, the
gap w must equal or exceed 0.08 mm. The parts in the assembly depicted in
Figure have dimensions and tolerances as follows:
a = 44.50 ±0.08 mm
b = 19.05 ±0.02 mm
c = 3.05 ±0.13 mm
d = 22.23 ±0.02 mm
Figure: An assembly of three
cylindrical sleeves of lengths b, c,
and d on a shoulder bolt shank of
length a. The gap w is of interest.
All parts except the part with the dimension d are supplied by vendors.
The part containing the dimension d is made in-house.
(a) Estimate the mean and tolerance on the gap w.
(b) What basic value of d will assure that w ≥ 0.08 mm?
Shigley’s Mechanical Engineering Design