ENT345 Mechanical
Component Desing Sem 1-
2015/2016
DR. HAFTIRMAN
SCHOOL OF MECHATRONIC UniMAP
1
LECTURE NOTES
ENT345
MECHANICAL COMPONENTS DESIGN
Lecture 2
18/9/2015
DESIGN
Dr. HAFTIRMAN
SCHOOL OF MECHATRONIC ENGINEERING
MECHANICAL ENGINEEERING PROGRAM
UniMAP
COPYRIGHT©RESERVED 2015
Outline
Design Philosophy
Phase and interactions of the design
process
Design factor
Design analysis methods
General design procedure
ENT345 Mechanical
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Design
CO1:
ABILITY TO DESIGN OF MEHANICAL
COMPONENTS IN MECHANICAL SYSTEMS
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Design Philosophy
Design is either to formulate a plan for the satisfaction of a
specified need or to solve a problem. If the plan results in
the creation of something having a physical reality, then
the product must be functional, safe, reliable, competitive,
usable, manufacturable, and marketable.
Design is an innovative and highly iterative process. It is
also a decision-making process. The engineering designer
has to be personally comfortable with a decision-making,
problem-solving role.
Design is a communication-intensive activity in which
both words and pictures are used, and written and oral
forms are employed.
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Design Philosophy
Engineers have to communicate effectively and work with
people of many disciplines. These are important skills, and an
engineer’s success depends on them.
A designer’s personal resources of creativeness, communicative
ability, and problem-solving skill are intertwined with
knowledge of technology and first principles.
Engineering tools such as mathematics, statistics, computers,
graphics, and languages are combined to produce a plane 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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Design Philosophy
Some general considerations to ensure that a machine part is
safe for operation under reasonably foreseeable conditions.
Application.
Environment
Load
Type of Stresses
Material
Confidence
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Design Philosophy
Some general considerations to ensure that a machine part is safe for
operation under reasonably foreseeable conditions.
Application.
Is the component to be produced in large or small quantities?
What manufacturing techniques will be used to make the
components?
What are the consequences of failure in terms of danger to people
and economic cost?
How cost sensitive to design?
Are small physical size or low weight important?
Will the component be inspected and service periodically?
ENT345 Mechanical Component Desing
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ENT345 Mechanical
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Design Philosophy
Environment
To what temperature range will the component be
exposed?
What is potential for corrosion?
Is low noise important?
What is the vibration environment?
Design Philosophy
Loads
Consider all modes of operation, including startup,
shutdown, normal operation, and foreseeable
overloads.
The loads should be characterized as static, repeated,
and reversed, fluctuating, shock, or impact.
Magnitudes od loads are the maximum, minimum, and
mean.
Will high mean loads be applied for extended periods
of time, particularly at high temperature, and creep.
ENT345 Mechanical
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Design Philosophy
Type of stresses
What kinds of stresses will be created: direct
tension, direct compression, direct shear,
bending, or torsional shear?
Will two or more kinds of stresses be applied
simultaneously?
Are stresses developed in one direction
(uniaxially), two directions (biaxially), or three
directions ( triaxially)?
Is buckling likely to occur?
ENT345 Mechanical
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Design Philosophy
Material
Consider the required material properties of yield strength,
ultimate tensile strength, ultimate compressive strength,
endurance strength, stiffness, ductility, toughness, creep
resistance, corrosion resistance, and others in relation to the
application, loads, stresses, and the environment.
Will the component be made from a ferrous or non ferrous metal?
Is the material brittle or ductile ? Ductile materials are highly
preferred for components subjected to fatigue, shock, or impact
loads?
Is the application suitable for a composite material?
ENT345 Mechanical
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Design Philosophy
Confidence
How reliable are the data for loads, material
properties, and stress calculations?
Are controls for manufacturing processes
adequate to ensure that the component will be
produced as designed with regard to
dimensional accuracy, surface finish, and final
as made material properties?
ENT345 Mechanical
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Phases and Interactions of the
Design Process
Identification of need
Definition of Problem
Synthesis
Analysis and optimization
Evaluation
Presentation
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Identification of need generally starts the design process.
Example; the need to do something about a food-packaging
machine may be indicated by noise level, by a variation in
package weight, by variations in the quality of the packaging
or wrap.
The definition of problem is more specific and must include all
the specifications for the object that is to be designed. The
specifications define the cost, the number to be manufactured,
the expected life, the range, the operating temperature, and the
reliability. Specified characteristics can include the speeds,
feeds, temperature limitations, maximum range, expected
variations in the variables, dimensional and weight limitations.
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The synthesis of a scheme connecting possible system elements
is sometimes called the invention of the concept design. For
example, the design of a system to transmit power requires
attention to the design and selection of individual components
e.g., gear, bearings, shaft. In order to design the shaft for stress
and deflection, it is necessary to know the applied forces. If the
forces are transmitted through gears, it is necessary to know the
gear specifications in order to determine the forces that will be
transmitted to the shafts.
Rough estimates will need to be made in order to proceed
through the process, refining and iterating until a final design is
obtained that is satisfactory for each individual components as
well as for the overall design specifications.
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Both analysis and optimization require that we construct or
devise abstract models of the system that will admit some
form of mathematical analysis. In creating them it is our hope
that we can fins one that will simulate the real physical system
very well.
Evaluation is the final proof of a successful design and
usually involves the testing of a prototype in the laboratory.
Here we wish to discover if the design really satisfies the
needs.
Presentation is selling job. The engineer when presenting a
new solution to administrative, management, or supervisory
persons, is attempting to sell or to prove to them that their
solution is a better one.
Design Considerations
The strength required to an element in a system is an important factor in the
determination of the geometry and the dimensions of the element. The expression
design consideration is referring to some characteristic that influences the design
of the element or the entire system. Usually quite a number of such characteristics
must be considered and prioritized in a given design situation.
Functionality Manufacturability Styling
Strength/Stress Utility Shape
Distortion/deflection/stiffness Cost Size
Wear Friction Control
Corrosion Weight Thermal
Safety Life Surface
Reliability Noise Lubrication
Marketable Maintenance Volume
Liability Remanufacturing/Resource recovery
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Design Considerations
When several parts are assembled, the gap (interference) depends on the
dimensions and tolerances of the individual parts. The following terms are
used generally in dimensioning:
Nominal size
Limits
Tolerance
Bilateral tolerance
Unilateral tolerance
Clearance
Interference
Allowance
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DESIGN FACTOR AND FACTOR OF
SAFETY
“nd “ is called the design factor
The factor of safety has the same definition as the design factor. The factor safety is called “n”
The term design factor (nd) is a measure of the relative safety of a load-carrying component.
DESIGN FACTOR AND FACTOR OF
SAFETY Factor of safety is a term describing the structural
capacity of a system beyond the applied loads or
actual loads.
There are two distinct uses of the Factor of Safety:
one as a calculated ration of strength (structural
capacity) to actual applied load. This is a measure
of the reliability of a particular design
The other use of Factor of safety is a constant
value imposed by law, standard, specification,
contract or custom.
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DESIGN FACTOR AND FACTOR OF
SAFETY
The design factor is what the part is
required to be able to withstand. The design
factor is for an application.
The safety factor is how much the designed
part actually will be able to withstand. The
safety factor is for actual part that was
designed.
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DESIGN FACTOR AND FACTOR OF
SAFETY
Say a beam in a structure is required to
have a design factor of 3. The engineer
chose a beam that will be able to withstand
10 times the load. The design factor is still
3, because it is the requirement that must be
met, the beam just happens to exceed the
requirement and its safety factor is 10.
ENT345 Mechanical
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DESIGN FACTOR AND FACTOR OF
SAFETY
The safety factor should always meet or exceed
the required design factor or the design is not
adequate. Meeting the required design factor
implies that the design meets but does not exceed
the minimum allowable requirements. A high
safety factor well over the required design factor
sometimes implies "over engineering" which
results in excessive weight and/or cost. In
colloquial use the term, "required safety factor" is
functionally equivalent to the design factor.
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DESIGN FACTOR AND FACTOR OF
SAFETY Appropriate factors of safety are based on several
considerations. Prime considerations are the accuracy of
load, strength, wear estimates and the environment to
which the product will be exposed in service; the
consequences of engineering failure, and the cost of over-
engineering the component to achieve that factor of safety.
For example, components whose failure could result in
substantial financial loss, serious injury or death usually
can use a safety factor of four or higher (often ten). Non-
critical components generally might have a design factor of
two.
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DESIGN FACTOR AND FACTOR OF
SAFETY
Example
A rod with a cross-sectional area of A and loaded in tension with an axial force of P = 9 kN undergoes a stress of σ = P/A. Using a material strength of 168 N/mm2 and a design factor of 3.0.
Determine the minimum diameter of d solid circular rod. Using Table A-17, select a preferred fractional diameter and determine the rod’s factor of safety.
ENT345 Mechanical
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DESIGN FACTOR AND FACTOR OF
SAFETY
Solution
From Table A-17, the next higher preferred size is 16 mm
Thus, according to the same Eq developed earlier, the factor of safety n is
The rounding the diameter has increased the actual design factor.
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DESIGN FACTOR AND FACTOR OF
SAFETY Ductile Materials
1. n =1.25 to 2.0. Design of structures under static loads for which there is a high level of confidence in all design.
2. n =2.0 to 2.5. Design of machine elements under dynamic loading with average confidence in all design data.
3. n =2.5 to 4.0 Design of static structures or machine elements under dynamic loading with uncertainty about loads, materials properties, stress analysis, or the environment.
4. n =4.0 or higher. Design of static structures or machine elements under dynamic loading with uncertainty about some combination of loads, material properties, stress analysis, or the environment. The desire to provide extra safety to critical components may also justify these values.
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DESIGN FACTOR AND FACTOR OF
SAFETY
Brittle materials
1. n=3.0 to 4.0. Design of structures under static
loads for which there is a high level of
confidence in all design data.
2. n=4.0 to 8.0. Design of static structures or
machine elements under dynamic loading with
uncertainty loads, material properties, stress
analysis, or the environment.
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REALIBILITY
Reliability of element is the statistical measure of the probability that a mechanical element will not fail in use.
The reliability R can be expressed by a number having the range 0 ≤ R ≤1
A reliability of R=0.90 means that there is a 90 percent chance that the part will perform its proper function without failure.
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.
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REALIBILITY
The failure of 6 parts out of every 1000 manufactured might be considered an acceptable failure rate for a certain class of products. This represents a reliability of
𝑅 = 1 − 𝑝𝑓
pf is the probability of failure.
𝑅 = 1 −6
1000= 0.994 𝑜𝑟 99.4%
If the reliability of component i is Ri in a series system of n components, the reliability of the system is given by 𝑅 = 𝑅𝑖
𝑛𝑖=1
For example consider a shaft with two bearings having reliabilities of 95 percent and 98 percent. The overall reliability of the shaft system is 𝑅 = 𝑅1𝑅2 = (0.95)(0.98)=0.93 or 93 percent
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DIMENSIONS AND TOLERANCES
The terms are used in dimensioning;
Nominal size=>The size we use in speaking of an element.
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. The basic size is between two limits.
Unilateral tolerance=>The basic dimension is taken as one of the limits, and variations is permitted in only one direction.
Clearance=> A general term that refers to the mating of cylindrical parts such as a bolt and a hole. The word clearance is used only when internal member is smaller than the external member. The diametral clearance is the measured difference in the two diameters.
Allowance=>The minimum stated clearance or the maximum stated interference for mating parts.
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Hardness
Hardness is the resistance of a material to
penetration by a pointed tool.
ASTM standard hardness method E-18
Rockwell hardness scale are designated as A, B,
C.
The indenters are describes as a diamond, a 1.6
mm diameter ball, and a diamond for scales A, B,
and C respectively, where the load applied is
either 60, 100, or 150 kg.
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Hardness
HB = the Brinell hardness.
For steels
Su = 0.5𝐻𝐵 𝑘𝑝𝑠𝑖3.4𝐻𝐵 𝑀𝑃
For cast Iron
Su = 0.23𝐻𝐵 − 12.5 𝑘𝑝𝑠𝑖1.58𝐻𝐵 − 86 𝑀𝑃𝑎
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Hardness
Example 2-2
It is necessary to ensure that a certain part supplied by a foundry always
meets or exceeds ASTM No. 20 specifications for cast iron (see Table A-
24).
What hardness should be specified?
From Eq (2-22), with (Su)min= 138 MPa.
HB=𝑆𝑢+86
1.58=
138+86
1.58
HB=142=> If the foundry can control the hardness within 20 points,
routinely, then specify 145 <HB<165. This imposes no hardship on the
foundry and assures the designer that ASTM grade 20 will always be
supplied at a predictable cost (ASTM grade 20 hardness 156).
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DESIGN ANALYSIS METHODS
The recommended methods for design
analysis based on the type of material
(brittle or ductile), the nature of the loading
(static or cyclical), and the type of stress
(uniaxial or biaxial)
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GENERAL DESIGN PROCEDURE
The procedure is set up assuming that the
following factors are known or can be specified or
estimated:
General design requirements: Objectives and
limitations on size, shape, weight, and desired
precision.
Nature of the loads to be carried.
Types of stresses produced by the loads.
Type of material from which the element is to be
made. ENT345 Mechanical
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General description of the manufacturing process
to be used, particularly with regard to the surface
finish that will be produced.
Desired reliability.
GENERAL DESIGN PROCEDURE
Specify the objectives and limitations, if any, of the design, including
desired life, size, shape, and appearance.
Determine the environment in which the element will be placed,
considering such factors as corrosion potential and temperature.
Determine the nature and characteristics of the loads to be carried by
the element.
Determine the magnitudes for the loads and the operating conditions=>
maximum expected load, minimum expected load, and expected
number of cycles of loading.
Analyze how loads are to be applied to determine the type of stresses
produced=> direct normal stress, bending stress, and shear stress.
Proposes the basic geometry for the element, paying particular
attention to its ability to carry the applied loads safety.
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Proposes the method of manufacturing the element with
particular attention to the precision required for various
features and surface finish that is desired.
Specify the material from which the element is to be made,
along with its condition. For metals the specific alloy should
be specified, and the condition could include such
processing factors as hot rolling, cold drawing, and a
specific heat treatment.
Determine the expected properties of the selected material.
Example ultimate strength, yield strength, ductility as
represented by percent elongation, stiffness as presented by
modulus of elasticity, E or G.
Specify an appropriate design factor.
Determine which stress analysis method (See logic
diagram).
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Determine which stress analysis method (See logic diagram).
Compute the appropriate design stress for use in the stress analysis. If
fatigue loading is involved, the actual expected endurance strength of the
material should be computed.
Determine the nature of any stress concentrations that may exist the
design at places where geometry changes occur.
Complete required stress analyses at all points where the stress may be
high and at changes of cross section to determine the minimum
acceptable dimensions for critical areas.
Specify suitable, convenient dimensions for all features of the element.
After completing all necessary stress analyses and proposing the basic
sizes for all features, check all assumptions made earlier in the design to
ensure that the element is still safe and reasonably efficient.
Specify suitable tolerances for all dimensions, considering the
performance of the element, its fir with mating elements, the capability
of the manufacturing process, and cost.
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Check to determine whether some part of the
component may deflect excessively.
Document the final design with drawings and
specifications.
Maintain a careful record of the design analyses
for future reference. Keep in min that others may
have to consult these records whether or not you
are still involved in the project.
DESIGN EXAMPLE
A large electrical transformer will be suspended below a roof
truss inside a building. The total weight of the transformer is
142.33 kN. Design the means of support.
Solution
Objective :Design the means of supporting the
transformer.
Given : The total load is 142.33 kN. The transformer will be
suspended below a roof truss inside a building. The load can be
considered to be static. It is assumed that it will be protected from
the weather and that temperatures are not expected to be severely
cold or hot in the vicinity of the transformer.
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Basic Design Decisions : Two straight, cylindrical rods will be used to
support the transformer, connecting the top of its casing to the bottom
chord of the rod truss. The ends of the rod will be threaded to allow them
to be secured by nuts or by threading them into tapped holes. This design
example will be concerned only with the two rods. It is assumed that
appropriate attachment points are available to allow the two rods to share
the load equally during service. However, it is possible that only one rod
will carry the entire load at some point during installation. Therefore,
each rod will be designed to carry the full 142.33 kN. We will use steel
for the rods, and because neither weight nor physical size is critical in this
application, a plain, medium carbon steel will be used. We specify AISI
1040 cold drawn steel which has a yield strength of 489.54 MPa and
moderately high ductility as represented by its 12% elongation. The rods
should be protected from corrosion by appropriate coatings.
The objective of the design analysis that follows is to determine the size of
the rod.
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Analysis: The rod are to be subjected to direct normal tensile stress.
Assuming that the threads at the ends rods are cut or rolled into the
nominal diameter of the rods, the critical place for stress analysis is in the
threaded portion.
Use the direct tensile stress formula σ = F/A
We will first compute the design stress and then compute the required
cross-sectional area to maintain the stress in service that value. Finally, a
standard thread will be specified from the data. The design stress is
σd = Sy /N
We specify a design factor of N=3, because it is typical for general machine
design and because there is some uncertainty about the actual installation
procedures that may be used. Then
σd = Sy /N = (489.54 MPa)/3= 163.18 MPa.
Results: In the basic tensile stress equation σ = F/A, we know F , and we
will let σ= σd . Then the required cross-sectional area is
A =F/ σd = (142330N)/(163.18MPa) = 872 mm2
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A standard size thread will now be specified from
data on fasteners. You should be familiar with such
data from Table American Standard threads.
A 11/2 -6 UNC thread (11/2 – in- diameter rod with 6
threads per in) has a tensile stress area of 906.4 mm2
which should be satisfactory for this application.
Comments
The final design specifies a 11/2 – in-diameter rod
made from AISI 1040 cold-drawn steel with 11/2 -6
UNC threads machined on each end to allow the
attachment of the rods to the transformer and the
truss.
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DESIGN EXAMPLE
Figure shows the two gears A and B in a gear box mechanism.
Compute the relative deflection between them in the plane of
the paper that is due to the forces. These separating forces or
normal forces that it is customary to consider the loads at the
gears and the reactions at the bearing to be concentrated. The
shafts carrying the gears are steel and have uniform diameters as
listed in the figure.
Solution
Objective :Compute the relative deflection between gears A and
B in Figure.
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Given : The layout and loading pattern are shown in Figure.
The separating force between gears A and B is 1067.52 N.
Gear A pushes downward on gear B, and the reaction force of
gear B pushes upward on gear A.
Shaft 1 has a diameter of 19.05 mm and a moment of inertia
of 0.064511x105 mm4 . Shaft 2 has a diameter of 25.4 mm and a
moment of inertia of 0.2043x105 mm4 . Both of shaft are steel.
Use E = 116.85 Gpa.
Analysis : Use the deflection formulas to compute the
upward deflection of shaft 1 at and downward of shaft 2 at
gear B. The sum of the two deflections is the total deflection
of gear A with respect to gear B. Case(a) from Table applies
to shaft 1 because there is a single concentrated force acting
at the midpoint of the shaft between the supporting bearings.
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Analysis : We will call that deflection yA. Shat 2 is a simply
supported beam carrying two nonsymmetrical loads. No
single formula matches that loading patter from Appendix.
But we can use superposition to compute the deflection of the
shaft at gear B by considering the two forces separately as
shown in Part (d) of Figure. Case (b) from Table is used for
each load. We first compute the deflection at B due only to
the 1067.52 N force, calling it yB1. Then we compute the
deflection at B due to the 1423.36 N force, calling it yB2. The
total deflection at B is yB = yB1 + yB2
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2015/2016
DR. HAFTIRMAN
SCHOOL OF MECHATRONIC UniMAP
54
Results :
ENT345 Mechanical
Component Desing Sem 1-
2015/2016
DR. HAFTIRMAN
SCHOOL OF MECHATRONIC UniMAP
55
Equations for
Deflected beam shape
ENT345 Mechanical
Component Desing Sem 1-
2015/2016
DR. HAFTIRMAN
SCHOOL OF MECHATRONIC UniMAP
56
ENT345 Mechanical
Component Desing Sem 1-
2015/2016
DR. HAFTIRMAN
SCHOOL OF MECHATRONIC UniMAP
57
THANK YOU