The Concept of Engineering Design Factor of Safety

Mechanical Eng. Design is a complex field requiring many different skills.
The object of engineering is to provide society with the requirements of the modern civilization.

1. MMB 322 Machine Component Design
Lecture 1
The Concept of Engineering Design
Factor of Safety

2. The concept of Engineering Design
1. Introduction
Mechanical Eng. Design is a complex
field requiring many different skills.
The object of engineering is to provide
society with the requirements of the
modern civilization.
The complexity of the subject requires a
sequence in which ideas are introduced
and iterated.

3. As an applied science engineering uses
scientific knowledge to achieve specific
objectives.
By definition the Design is the process
whereby a requirement is converted to a
meaningful and functional plan.
Therefore the Design is the formulation
and creation of plans for machine
components, products, structures, or
processes to perform a desired function
and satisfy specific human deed.
Introduction

4. Introduction
For Instance:

•Building Design
•Drill bit & Rim Design
•Shaft, Gear, or Brake Design
•Manufacturing Process Design
•Farm Machinery Design, etc.
The Eng. Design is classified according to a
particular product or professional field.

5. Introduction
In contrast to any mathematical or scientific
problems, the design problems have no unique
answers.
For most design activities the Designer utilizes:
Mathematics, Statics, Strength of Materials,
Dynamics, Mechanics of Machines, Solid
Mechanics, Engineering Drawing, Computer
Aided Drawing, Material Science, and many
other subjects.
It is for this reason the Engineering Design is
classified as interdisciplinary subject.

6. Recognition of need
Fig. 1-1 The phases of design with loops, after Shigley
2. Phases of Design

7. 2.1 Recognition of need
Sometimes the design begins when an engineer
recognizes a need being triggered by a particular
adverse or random circumstance such as:
–Customer report on product quality (failure/breakdown)
–Development of patents onto an engineered product.
The important things are:
–To recognize that the need exists
–To use all of his senses and background experience to
focus on the need
–To justify its gratification (usefulness)
–To gather as much information as possible about it

8. Definition of problem
Fig. 1-1 The phases of design with loops, after Shigley
2. Phases of Design

9. 2.2 Definition of the problem
This must include all the specifications for the
things that is to be designed, which may include:
–The input and output data,
–The characteristics and dimensions of the
space the things must occupy,
–All the limitations on these quantities, etc.
Obvious items in the specifications are:
–The running speed of the parts,
–Temperature limitations,
–Dimensional and weight limitations, etc.

10. Other designer’s limitations could be:
•The manufacturing processes available
together with the facilities at the plant
•The quality of the labor skills
•The materials and sizes available
•The competitive situation at the market, etc.
All the above limitations constitute the implied
specifications.
Therefore everything which limits the designer’s
freedom of choice is a specification.

11. Synthesis
Analysis & Optimization
Fig. 1-1 The phases of design with loops, after Shigley
2. Phases of Design

12. 2.3 Synthesis, Analysis & Optimization
The next step in the design process is the
synthesis of the optimum solution.
This can not take place without both analysis and
optimization, because the system must comply
with the design specifications (limitations).
The analysis and optimization require that we
construct abstract models of the system in the form
of mathematical models.

In creating these models it is our hope that we can
find one, which will very well represent the system.
If the design fails the procedure must begin again.

13. Evaluation
Fig. 1-1 The phases of design with loops, after Shigley
2. Phases of Design

14. 2.4 Evaluation
The evaluation is a final prove of a successful design
and usually involves:
–Prototype building
–Testing of the prototype in the laboratory
At this stage we wish to discover if the design really
satisfies the needs, thus to get positive answers to:
–Is the design reliable?
–Will it compete successfully with similar product?
–Is it economical to manufacture and to use?
–Is it easily maintained?
–Can a profit be made from its sale and use?

15. Presentation
Fig. 1-1 The phases of design with loops, after Shigley
2. Phases of Design

16. 2.5 Presentation
The purpose of this section is to note the
importance of the presentation as a final step in
the design process.
The designer when presenting a new solution to
the management or supervisory person is
attempting to prove to them that his solution is a
better one as compared to the existing solutions.

To achieved this it is required to be technically
competent and versatile in the following:
Writing, Speaking, and Drawing abilities
These three forms of communication are skills,
and they can be acquired only by a long practice.

17. Phases of Eng. Design
Fig. 1-1 The design flow diagram
with feedback loops, after Didier

18. Phases of Eng. Design
Fig. 1-1 The phases of design for a machine, after Hall et. al.

19. 3. Design Considerations
These are the characteristics, which influence the
design process of elements or the entire system.
For example:
Sometime the strength of a mechanical element is
an important factor in determining the geometry and
dimensions of the element.
In such a situation we say that the strength is an
important design consideration.
The following list of factors may be considered:
1. Strength-Static or Fatigue, 2. Noise, 3. Friction
4. Deflection, 5. Lubrication, 6. Wear, 7. Cost etc

20. 4. Code and Standards
•A standard is a set of specifications for parts,
materials, or processes intended to achieve:
uniformity,
efficiency, and
specified quality.
One of the important purpose of the standard is to
place a limit on the number of items in the
specifications, so as to provide:
1. A reasonable inventory of tooling,
2. Dimensional Sizes, 3. Shapes, 4. Varieties.

21. A Code is a set of specifications for the analysis,
manufacture and construction of something. The
purpose of the code is to achieve a specified
degree of:
safety,
efficiency,
performance , and
quality
Code and Standards
It is important to observe that safety codes do
not imply absolute safety.
Absolute safety is impossible to obtain, because
unexpected events does happen.

22. All of the organizations and societies listed below
have established specifications for standards and
safety or design codes:
Code and Standards
Anti-friction Bearing Manufacturers (AFBMA)
British Standard Institution (BSI)
American National Standard Institution (ANSI)
Aluminum Association (AA)
American Iron and Steel Institute (AISI)
International Standards Organization (ISO)
National Bureau of Standards (NBS)
Industrial Fasteners Institute (IFI)

23. 5. Economics
Cost consideration plays an important role in the
design decision process. Herein a few general
approaches and simple rules are listed as follows:
i) Standard sizes
ii) Large Tolerances
iii) Cost Estimates
The use of standard or stock sizes is the first
principle of cost reduction.
Thus the designer must have an access to the
stock list of the preferred sizes, not to these listed
usually in the catalogs. The reason is, that some of
the materials are not readily available because are
rarely used and are not stocked.

24. There are many standard parts and devices such as
motors, pumps, bearings and fasteners (bolts,
nuts, etc.), which are specified by the designers.
In such a case the designer should specify parts
which are readily available, or to have access to
the list of preferred sizes.
See Table A-12 for the preferred millimeter sizes in
your collection of design tables.
5. Economics – i) The use of standard sizes

25. Tolerances cover dimensional variations and
surface roughness and the variation in
mechanical properties resulting from heat
treatment and other processing operations.
In terms of effect on the cost the tolerances have
significant contribution as compared to the other
specifications, since they influence the productivity
of the end product in many ways:
Small tolerances lead to a high cost or
impractical production,
Large tolerances lead to low cost and good
producibility.
5. Economics – ii) To use large tolerances

26. 5. Economics – iii) Cost Estimates
Breakeven point
(CNC machine)
(Lathe machine)
Fig. 1-2 The breakeven point,
after J. E. Shigley
Sometimes, when two design approaches are compared for
cost, then the choice depends on other factors such as
quantities of production, the speed of the assembly lines,
etc. There will be a point corresponding to an equal cost –
called Breakeven Point.

27. iii) Cost Estimates
Two or more designs for example: two engine
designs can be roughly compared by using some
cost estimates depending upon the application:
Output Torque, N-m
Useful power delivered, kW
Maximum operating speed, rpm
Total engine cylinder’s capacity, cm
3
Speed at which max. toque or power is achieved, rpm
Counting the No. of parts involved in the design, etc.
In general, the design having the smallest number
of parts is likely to cost less.

28. Factor of Safety
The strength is an inherent property of a part, that
is a property build into the part because of the use
of a particular material and a manufacturing
process.
Letter S is used to denote strength, and using
appropriate subscript to designate the kind of
strength: S
u, S
y, S
s, etc.
The term Factor of Safety (FS) is used to
evaluate the safeness of a machine member.

Let a mechanical element be subjected to a general
type of load, designated as F. This can be a force,
BM, Torque, deflection or some kind of distortion.

29. If F is increased, eventually it will become so large,
that any additional small increase would
permanently impair the ability of the member to
perform its proper function being designed for.
If we designate the limit of F as F
u, then the FS is
defined as
(1-1)
When F=F
u
, then n=1, and hence there is no safety
for the member at all.
 The term margin of safety is widely used in the
engineering practice and is defined as
(1-2)
F
F
n
u

1nm
Factor of Safety

30. However, since the terms Fu and F are statistically
varying quantities, it should be noted that a FS n>1
does not preclude failure.
The greatest use of FS occurs when we compare
the stress σ and the strength S in order to
estimate the amount of safety.
In engineering practice FS is used to account for
two separate and usually unrelated effects:
1. When many parts are to be manufactured
from various shipment of materials. There occurs
a variation in the strength for the following reasons:
•Material processing technology,
•Hot and cold working,
•Geometry of the cross section.
Factor of Safety

31. To address the above problems engineers are
using the FS approach to account for:
 The uncertainties that may occur in the strength
 The uncertainties that may occur with the load
 2. When a part is assembled into a machine,
and the machine is acquired by the ultimate user,
there will be a variation in the loading of the part
and hence the stresses induced by that loading.

Therefore, the manufacturer and designer have
no control on that situation.
Factor of Safety

32. There are three distinct procedures in which the
FS is used by the engineers. They depend upon
whether the FS is designated as a single quantity
or it is factored into components:
Case 1. The entire FS is applied to the strength
(1-3)
Where the stresses σ and τ are called the safe, or the
design stresses; And S and S
s are the normal and shear
strength of the material.
Factor of Safety
n
S

n
S
s

Since in Eqn. 1-3 a single FS is used, thus n must
include allowances for:
Uncertainties in strength and uncertainties in load

33. It should be noted that Eqn. 1-3 implies that the
stress is linearly related to load.
If there is any doubt about the above relation,
then Case 1 should not be used.
 In case that the part has been designed, and the
geometry, loading and strength of the material
known, the FS can be obtained from (1-3) → (1-4)
Factor of Safety
Case 2. The entire FS is applied to the load or
to the stress that results from this load.
(1-5)
Where F
p is called allowable or permissible load
And σ
p is allowable or permissible stress
FnF
p   n
p

34. Factor of Safety
The first relation of Eq. (1-5) should always be used
when the stress is not proportional to the load.
If the geometry of the part is selected then Eq. (1-5)
can be used for design purposes, such that the
permissible stress is never greater than the strength
Case 3. The total or overall FS is factored into
components, and separate factors are used for the
strength and for the loads, or for the stresses
produced by those loads.
(*)
S
p
21..nnnn
s

35. Where:
n
s
– is used to account for all the variations or
uncertainties concerned with the strength,
n
1 – accounts for all the uncertainties concerned
with load 1,
n
2 – accounts for all the uncertainties regarding
load 2.
When we apply FS – n
s
to the strength we
consider that the resulting strength will never
be any smaller. The smallest value of the
strength is (1-6)
Factor of Safety
s
min
n
S
S

36. End of Lecture 1