Chapter 1: Introduction
WHAT IS A MACHINE
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MACHINE : A device for transforming or
transfering energy
An apparatus consisting of interrelated units
(machine elements)
A device that modifies force and motion
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A machine receives energy in some available
form and uses it to do some particular kind of
work
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A petrol engine is a machine, which may use the
heat energy derived from the combustion of the
fuel to propel a vehicle along the road
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A lathe is a machine which receives mechanical
energy from the line shaft through the belt or
gears and uses that energy to remove metal from
a bar or other piece of work
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LINK OR ELEMENT : Each part of a
machine which has motion relative to some
other part
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STRUCTURES : Made up of series of members
of regular shape that have a particular function
for load carrying
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SYNTHESIS : Concerned with the problem of
selecting the size of the mechanism to perform
a given function
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STRESS : Internal reacting force per unit area
due to the effects of external applied forces
DESIGN
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Formulate a plan for the satisfaction of a human
need
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The need for the problem has to be identified
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Design problem have no unique answer
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A good answer today may well turn out to be a
poor answer tomorrow, if there is a growth of
knowledge during the period
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A design is always subject to certain problemsolving constraints
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A design problem is not a hypothetical problem
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Design has an authentic purpose
the creation of an end result by taking
definite action, or
the creation of something having physical
reality
ENGINEERING DESIGN

The process in which scientific principles and
the tools of engineering mathematics,
computers, graphics and English are used to
produce a plan which, when carried out, will
satisfy a human need
MECHANICAL ENGINEERING
DESIGN

Design of things and systems of mechanical
nature, machines, products, structures, devices,
and instruments

For the most part, mechanical design utilizes
mathematics, the materials sciences, and the
engineering mechanics sciences

The ultimate goal in machine design is to
size and shape the parts
choose appropriate material and
choose manufacturing process
So that resulting machine can be expected to
perform its intended function without failure

An engineer should be able to calculate and
predict the mode and conditions of failure for
each element and then design it to prevent that
failure
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This requires stress and deflection analysis for
each part

Stresses are functions of applied and inertial
loads
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An analysis of the forces, moments, torques and
dynamics of system must be done before
stresses and deflections can be completely
calculated
Design

A design must be:
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Functional- fill a need or customer expectation
Safe- not hazardous to users or bystanders
Reliable- conditional probability that product will perform its
intended function without failure to a certain age.
Competitive- contender in the market
Usable- accommodates human size and strength
Manufacturable- minimal number of parts and suitable for
production
Marketable- product can be sold and serviced
Design Process Actions
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Conceive alternative solutions
Analyze, test, simulate, or predict performance
of alternatives
Choose the “best” solution
Implement design
Design is…
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An innovative and iterative process
A communication intensive activity
Subject to constraints
Steps to Design
Design Considerations
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Strength
Stiffness
Wear
Corrosion
Safety
Reliability
Friction
Usability
Utility
Cost
Processing
Weight
Life
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
Noise
Styling
Shape
Size
Control
Thermal Properties
Surface
Lubrication
Marketability
Maintenance
Volume
Liability
Recovery
Codes and Standards

Code- a set of specifications for the analysis,
design, manufacture, and construction of
something

Standard- a set of specifications for parts,
materials, or processes intended to achieve
uniformity, efficiency, and a specified quality
Organizations
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Aluminum Association (AA)
American Gear Manufacturers
Association (AGMA)
American Institute of Steel
Construction (AISC)
American Iron and Steel Institute
(AISI)
American National Standards
Institute (ANSI)
American Society for Metals (ASM)
American Society of Mechanical
Engineers (ASME)
American Society of Testing
Materials (ASTM)
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American Welding Society (AWS)
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American Bearing Manufacturers
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)
American Society of Agricultural
and Biological Engineers (ASABE)
Economics
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Cost plays an important role in design decision
process
 No matter how great the idea may be, if it’s not
profitable it may never be seen
The use of standard sizes and large manufacturing
tolerances reduce costs
Evaluating design alternatives with regard to cost
 Breakeven Points
 Cost Estimates
Product Liability

“Strict liability” concept prevails in the U.S.
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Manufacturers are liable for any damage or harm
that results from a defect.
Uncertainty
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Roman Method- repeat designs that are proven
Factor of Safety Method of Philon- separate the loss-offunction load and the impressed load using a ratio
Loss of Function
nd 
Impressed Load
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Permissible Stress- fraction of significant material
property (i.e., strength)
Uncertainty
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Design Factor Method- factor of safety is increased with
rounding error to achieve nominal size (5.3 mm designed
bolt size is increased to 6.0 mm)
Stochastic Design Factor Method- uncertainty in stress
and strength is quantified for linearly proportional loads
s
Average Strength
nd  
Average Stress

Measures of Strength
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S – Strength
Ss – Shear Strength
Sy – Yield Strength
Su – Ultimate Strength
S - Mean Strength
Measures of Stress
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t – Shear Stress
 – Normal Stress
1 – Principal Stress
y – Stress in y-direction
r – Radial Stress
t – Tangential Stress
Stress Allowable
(AISC)
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Tension: 0.45 Sy ≤ all ≤ 0.60 Sy
Shear:
tall = 0.40 Sy
Bending: 0.60 Sy ≤ all ≤ 0.75 Sy
Bearing: all = 0.90 Sy
Loads Used to Obtain Stresses
F   Wd   Wl   kFl  Fw   Fmisc
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Where:
Wd- dead loads
Wl- live loads
k- service factor
Fw- wind load
Fmisc- locality effects (earthquakes)
Service Factors
Applications
Elevators
Traveling Crane Supports
Light Machinery Supports
Reciprocating Machinery
Supports
Floor and Balcony
Supports
k
2
1.25
1.20
1.50
1.33
Factor of Safety

Design factors (nd) are defined as:
and
strength
nd 
stress
nd  nsn z
where
ns-accounts for uncertainty of strength
nd-accounts for uncertainty of loads
Realized Factor of Safety
nr 
nr 
S

Ss
t
Reliability
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Probability that a mechanical element will not
fail in use
0≤R≤1
Reliability approach to design: judicious
selection of material, processes, and geometry to
achieve reliability goal
Factor of Safety Method- time proven, widely
accepted
Reliability Approach- new, requires data