Chapter1 [Autosaved].ppt

D. M. Gadhave,
Lect. In Mechanical Engg.
Govt. Polytechnic, Bramhapuri
BASIC DESIGN
CONSIDERATION

Machine Design Philosophy
• A machine design is creation of new
and better machine and improving the
existing one. The process of design is
long and time consuming one. As per
customer, the machines are invented or
modified by creating an idea in mind.
The idea is then transformed in terms
of dimension and suitable drawing. The
drawing is then prepared with respect to
availability of men, machine and material.

General Procedure in Machine
Design
• In designing a machine component,
there is no rigid rule. The problems may
be arranged in several ways. The flow-
chart for general procedure in machine
design is shown in Fig. 1.

Analysis of forces
Material Selection
Design of elements (Size and stresses)
Modification
Detailed drawing
Production
Synthesis (Mechanism)
Need or Aim

• 1. Recognition of need :- First of all,
make a complete statement of the
problem, indicating the need, aim or
purpose for which the machine is to be
designed.
• 2. Synthesis or Mechanism :- Select the
possible mechanism or group of
mechanism which will give the desired
motion.
General Procedure in Machine Design contd…..

• 3. Analysis of forces :- Find the forces
acting on each member of the machine
and the energy transmitted by each
member.
• 4. Material Selection:- Select the
material best suited for each member of
the machine.

• 5. Design of element (Size and
stresses):- Find the size of each
member of the machine by considering
the forces acting on the member and
the permissible stresses for the material
used. It should be kept in mind that
each member should not deflect or
deform than permissible limit.

• 6. Modification:- Modify the size of
member to agree with the past
experience and judgment to facilitate
manufacture. The modification may also
be necessary by consideration of
manufacturing to reduce overall cost of.
• 7. Detailed Drawing:- Draw the detailed
drawing of each component and the
assembly of machine with complete
specification for manufacturing process.

• 8. Production :- The component, as per
drawing, is manufactured in the
workshop.
• Note:- When there are number of components and in
the market having the same quality, efficiency and
cost, then the customer will naturally attract towards
the most appealing product. The aesthetic and
ergonomics are very important features which gives
grace and value to product and dominates the
market.

General Consideration in
Machine Design.
• Following are the general
considerations in designing a machine
component.
• 1. Type of load and stresses caused by
the load :- The various types of load
such as steady load, variable load,
shock load and impact load acts on
machine components due to which
internal stresses are set up.

• 2. Motion of parts or kinematics of the
machine:- The successful operation of
any machine largely depends upon
simplest arrangement of parts which
will give the desired motion. The motion
of the parts may be:
• Rectilinear motion which include unidirectional or raciprocating
motions.
• Curvilinear motion which include rotary, oscillatory and simple
harmonic.
• Constant Velocity
• Constant or variable acceleration.
General Consideration in Machine Design contd…….

• 3. Selection of material:- It is essential
that the designer should have a through
knowledge of the properties of material
and their behavior under working
conditions i.e. strength, durability,
flexibility, weight, resistance to heat
and corrosion, ability to cast and weld,
machinability etc.

• 4. Form and size of part :- The form and
size are based on judgement. The smallest
practicable cross-section may be used, but
it may be checked that the stresses
induced in the designed cross-section are
reasonably safe. In order to design any
machine part for form and size, it is
necessary know the forces which the part
must sustain. It is also important to
anticipate suddenly applied or impact load
which may cause failure.

• 5. Frictional resistance and lubrication :-
There is always a loss of power due to
frictional resistance and it should be
noted that starting friction is higher
than running friction. It is therefore
essential that careful attention must be
given to the matter of lubrication of all
surfaces which move in contact with
others whether in rotating or sliding
condition.

• 6. Convenient and economical feature:-
In designing the operating features of
machines must be carefully studied.
The starting, controlling and stopping
level must be located on the basis of
convenient handling. Replacement or
change of part due to wear or breakage
should be easily accessible and
necessity for removing other parts
should be avoided.

• 7. Use of standard parts :- The use of
standard parts is closely related to cost
because the cost of standard part is very
much less than parts that are made by
order.
• 8. Safety of operation :- Some parts of
machines are dangerous to operator
especially which are operated at very high
speed and may cause injury. So it is essential
for designer to provide a safety device for
operator.

• 9. Workshop facility:- A design engineer
should know the limitation of employer
workshop in order to avoid the
necessity of having work done in some
other workshop.
• 10. Cost of construction :- It is most
important factor in design. If the
machine invented has its commercial
value, then it is possible to justify the
expenditure of money in design.

• In engineering practice, the machine
parts are subjected to various forces
which may be due to energy
transmitted, weight of machine,
frictional resistance, change of
temperature or lack of balance of
moving parts. The different forces
acting on a machine parts produces
various types of stresses.
Type of Loads

Load
• Load is defined as any external force
acting upon a body.
• Loads are classified into four types.
Load
Steady or
Dead load
Fluctuating or
variable or Live
Load
Suddenly
applied or
shock load
Impact Load

Type of load contd…
• 1. Dead or steady Load :- A load is said
to be dead or steady load, if it does not
change its magnitude and direction.
• eg. Load on columns.
• 2. Variable or fluctuating or Live Load:-
A load is said to be variable load, if it
changes its magnitude or direction or
point of application with respect to
time. It is also called as fatigue load which may be
1) fluctuating 2) repeated 3) completely reversed.

• The load which vary from a minimum value to a
maximum value of the same nature (i.e. tensile or
compressive) is called fluctuating load.
• The load which vary from zero to a certain maximum
value of the same nature (i.e. tensile or compressive)
is called repeated load.
• The load which vary from a minimum value to a
maximum value of opposite nature (i.e. certain
minimum compressive to certain maximum tensile or
minimum tensile to maximum compressive) is called
alternating load.
• E.g. Force in IC Engine connecting rod, bending
moment of rotating shaft.

• 3. Suddenly applied or shock load :-
A load is said to Suddenly applied or
shock load, when it is suddenly applied
or removed.
• E.g. Leaf spring of Automobile or load
on shock absorber.
• 4. Impact Load :- A load is said to be
impact load, when it is applied with
some initial velocity.
• E.g. Blow of hammer.

• Stress:- When an external force applied
on body, the internal forces (equal and
opposite) are set up at various sections
of the body, which resists the external
force. This internal force per unit area
at any cross section of body is known
as stress.
• It is denoted by Greek letter sigma (σ).
• Mathematically σ = F / A
Concept of Stress and Strain :-

• where F= Force or load acting on body.
A = Cross-section area of body.
• S. I. unit of stress is N/mm2
• In SI unit stress is usually expressed in
Pascal (Pa) such that 1 Pa = 1 N/m2. In
actual practice, longer unit of stress i.e.
megapascal (MPa) and gigapascal (GPa)
• 1MPa = 1 x 106 N/m2= 1 N/mm2
• 1GPa = 1 x 109 N/m2= 1 x 103 N/mm2

Strain
• Strain :- When a external force acts on
a body, it undergoes some deformation.
This deformation per unit length is
called a strain.
• It is denoted by Greek letter epsilon (ε).
• Strain, ε = δl / l
• Where δl=Change in length of the body
l = Original length of the body

Hooke’s Law
• It states that when a material is laoded
within elastic limit, the stress is directly
proportional to strain, i.e.
σ α ε or σ =E. Ε
where E is constant of proportionality
known as Young’s modulus or modulus of
elasticity.

Stress-strain diagram for ductile
material (Mild Steel)
• In designing the various machine parts, it is
necessary to know how the material will
function in service. For this, certain
characteristics or properties of the materials
should be known. The mechanical properties
mostly used in mechanical engineering
practice are commonly determined from a
standard tensile test. This test consists of
gradually loading a standard specimen of a
material and noting the corresponding
values of load and elongation until the

specimen fractures. The load is applied and
measured by a testing machine . The stress is
determined by dividing the load values by the
original cross-sectional area of the specimen.
The elongation is measured by determining
the amount that two reference points on the
specimen are moved apart by the action of
the machine. The original distance between
the two reference point is known as gauge
length. The strain is determined by dividing
the elongation values by the gauge length.

• The values of stress and corresponding
strains are used to draw the stress-strain
diagram of the material tested. A stress-strain
diagram for a mild steel (ductile material) is
shown in fig. The various properties of the
material are:-

• 1. Proportional Limit (OA) :- From the
graph i.e. from point O to A is straight line
which represent that stress is directly
proportional to strain. Beyond point A, the
curve slightly deviates from straight line.
Thus the Hooke’s law hold good up to
point A and is known as proportional limit.
Thus proportional limit is defined as stress
at which the stress-strain curve begins to
deviates from straight line.

• 2. Elastic Limit (AB) :- If the load is
increased point A to point B, the material
is regain its shape and size when the load
is removed. This means that the material
has elastic properties up to point B. This
point is known as elastic limit. It is defined
as stress developed in the material without
permanent deformation. Since two limits
are very close to each other, for all
practical purpose, it is taken to be equal.

• 3. Yield Point (C & D) :- If the material is
stressed beyond point B, the plastic stage
will reach i.e. on the removal of load, the
material will not regain its original shape.
A little consideration will show that beyond
point B, the strain increases at a faster
rate with any increase in stress until the
point C is reached. At this point, the
material yield before the load and there is
appreciable strain without increase in
stress.

• In case of ductile material, it will be
seen that a small load drops to D,
immediately after yielding commences.
Hence there are two yield points C and
D. The points C and D are called upper
and lower yield points respectively. The
stresses corresponding to yield points
are called as yield point stresses.

• 4. Ultimate Point (E) :- At D, if the load is
increased, there is increase in stress up to
point E. At E, the material becomes hot
and strain attains a maximum value which
is known as ultimate stress.
Simultaneously strain increases which is
followed by decreased in cross sectional
area. Ultimate stress is defined as largest
value of load to the original cross sectional
area of test piece.

• 5. Breaking Point (F) :-After the
specimen reaches ultimate stress, neck
formation begains which decreases the
cross sectional area of workpiece. At
this point stress required to break away
the specimen is less than maximum
stress. The stress corresponding to
point F is known as breaking stress.

• 6. Percentage reduction in area:- It is the
difference between original cross-sectional
area and cross sectional area at neck (i.e.
where the fracture takes place). This
difference is expressed as percentage of the
original cross-sectional area.
• Let A = Original cross sectional area, and
a = Cross-sectional area at the neck.
then reduction in area = A - a
and % reduction in area = x
A
A - a 100

• 7. Percentage elongation:- It is the
percentage increased in standard gauge
length (i.e. original length) obtained by
measuring the fractured specimen after
bringing the broken parts together.
• Let l =Original (Gauge) length, and
L= Length of specimen after fracture (Final length)
Elongation = L – l
and percentage elongation = x
l
L - l 100

Stress-strain diagram for brittle
material (Cast Iron)

• Brittle material shows lack of plastic
deformation and will fail with less than
5% elongation. When the load is
applied to specimen, there is
continuous elongation from O to P
where stress is proportional to strain.
The point P is called proportional limit.
For hard and brittle material the elastic
limit is above proportional limit. The
point Y is elastic limit.

• After Y i.e. elastic limit, the curve shows
a permanent and non recoverable
plastic deformation. The point B is
breaking point at which the material
fails. B is also Ultimate Point.
• For ductile material design is based on
yield point stress on the other hand for
brittle material design is based on
ultimate stress.

Types of Stresses
• 1. Tensile Stress:- When a body is
subjected to two equal and opposite axial
pulls F (also called tensile load) as shown in
fig (a), then stresses induced at any section
of body is known as tensile stress as shown
in fig. (b). A little consideration will show that
due to tensile load, there will be decrease in
cross-sectional area and an increase in
length. It is denoted by σt.
• Let F = Axial tensile force
A = cross-section area of body

Tensile stress contd..
• Then tensile stress σt =
• For example:- Stresses in chain of Automobile or
connecting rod.
F
A

Compressive Stress
• 2. Compressive Stress:- When a body is
subjected to two equal and opposite axial
pushes F (also called compressive load) as
shown in fig (a), then stresses induced at any
section of body is known as compressive stress
as shown in fig. (b). A little consideration will
show that due to compressive load, there will
be an increase in cross-sectional area and
decrease in length. It is denoted by σc.
• Let F = Axial compressive force
A = cross-section area of body

Compressive Stress contdd
• Then compressive stress σc =
• For example:- stresses induced due to hydraulic press or beam.
• In case of tension or compression, the area
involved is perpendicular to the force applied.
F
A

Transverse shear stress
• 3. Transverse shear stress :- When a
body is subjected to two equal and
opposite forces acting tangentially
across sections such that one body
resists over the other, then the stresses
induced is called transverse shear
stress. It is expressed by Greek letter
(tau) ζ.
• Mathematically ζ =
Tangential Force
Resisting Area

Transverse shear stress contdd…
• There are two types of transverse shear
stress.
• 1. Single shear stress
• 2. Double shear stress.
• 1. Single Shear Stress :-

• Consider a body consisting of two
plates connected by rivet as shown in
fig. (a). In this case, the tangential
force is resisted by one cross section of
the rivet (or when shearing takes place
at one cross section of the rivet), then
the rivets are said to be in single shear.
• In this case,
• Area resisting shear A= x d2

• Shear stress on the rivet cross section,
• ζ =
• All lap joints and single cover butt joints
are in single shear.
F
x d2

• 2. Double shear stress :-
• Now let us consider two plates
connected by two cover plates as
shown in Fig. (a)

• In this case, the tangential force F tends
to shear off the rivet at two cross section
shown in Fig (b). Hence when the
tangential force is resisted by two cross-
sections of the rivets (when shearing takes
place at two cross-sections of the rivet),
then the rivets are said to be in double
shear.
• In this case,
• Area resisting shear A=2 x x d2

• Shear stress on the rivet cross section,
• ζ =
• But joints with double cover plates are
in double shear.
• In case of shear, area involved is
parallel to the external force applied.
F
x d2
2 x

Important note
• When the holes are to be punched or drilled
in the metal plates, then the tools used to
perform the operations must overcome the
ultimate shearing resistance of the material to
be cut. If a hole of diameter d is to be
punched in a metal plate of thickness t, then
area to sheared, A = ᴨ d x t
• And the maximum shearing resistance of the
tool or force required to punch a hole,
• F = A x ζu = ᴨ d x t x ζu
• ζu=Ultimate shear strength of material of plate

4. Crushing or Bearing Stress
• A localised compressive stress at the
surface of contact between two
member of machine parts, that are
relatively at rest is known as crushing
stress or bearing stress.
• E.g. Riveted joint, knuckle joint, cotter
joint etc.
• Let us consider a riveted joint subjected
to Load F as shown in fig.

• In such cases crushing stress is given as
• σcr =
• Where d is diameter of rivet, t is thickness of
plate and n is no. of rivet.
F
d x t x n

5. Bearing pressure stress
• A localised compressive stress at the surface
of contact between two member of machine
parts, that are in relative motion is known as
bearing pressure stress.
• E.g. Journal supported in bearing, pin for
lever, clutch lining.
• Let us consider a journal rotating in a fixed
bearing as shown in fig. The journal exert a
bearing pressure on the curved surface of
bearing below it. The distribution of bearing
pressure will not be uniform as shown in fig.

• Average bearing pressure is given as
• σbp =
• Where F is Radial Load on the journal, d is
diameter of journal, l is length of the journal
F
d x l

6. Bending Stress
• When a body or machine component is
subjected to bending moment M as
shown in fig, the stress induced in it is
known as Bending stress.
• E.g. Beam subjected to bending
moment.
• Consider a beam subjected to a
bending moment M as shown in Fig.

• A little consideration will show that when a
beam is subjected to the bending moment,
the fibers on the upper side of the beam will
be shortned due to compression and those on
lower side will be elongated due to tension.
Also there are some surfaces which are
neither shortened nor elongated. Such
surface is called neutral surface. The
intersection of the neutral surface with any
normal cross-section of the beam is known as
neutral axis. The stress distribution of a beam

is shown in fig. The bending equation is
given by,
= =
σb = M/I x y
σb = M/I/y
σb = M/Z
M
I y R
E
σb

• where M= Bending moment in Nmm
• σb =Bending stress N/mm2
• I = Moment of inertia of c/s about neutral axis mm4
• y = distance from neutral axis to extreme fiber
• E = Modulus of elasticity
• R = Radius of curvature
• Z = Section Modulus

7. Torsional shear stress
• When a machine member is subjected to
the action of two equal and opposite
couples (torque or twisting moment), then
the machine member is said to be
subjected to torsion. The stress set-up by
torsion is known as torsional shear stress.
It is zero at the centroidal axis and
maximum at the extreme fibers.
• Consider a shaft fixed at one end and
subjected to a Torque (T) at the other end

Principal Stresses
• In a design of machine component, the
uniaxial state of stress is rarely used
because most of the component are
subjected to axial load, bending
moment and twisting moment. The bi-
axial or tri-axial is most common
condition of the surface of component.
• Consider an element of plate subjected
to two dimensional stresses as shown in
fig.

• As we know that, in tension or
compression, the stress is in a plane
which is at right angle to the line of
• action. But it has been observed that at
any point in a strained material, there
are three planes, mutually
perpendicular to each other which carry
direct stresses only and no shear stress.

• Out of these three direct stresses, one
will be maximum and other will be
minimum. These perpendicular planes
which have only direct stresses and no
shear stresses are known as Principal
Plane and the direct stresses acting
along these planes are known as
Principal Stresses. The shear stresses
acts along plane inclined at 450 to
principal plane.

• The maximum and minimum principal
stresses is given by
• And maximum shear stresses is given
by

Creep Strain & Creep Curve
• When a part is subjected to a constant
stress at high temperature for a long
period of time, it will undergo slow &
permanent deformation called as creep.
This property is considered in designing
I C Engine, Boilers, & Turbines. This
phenomenon is observed in metals, non-
metals, glass and polymer.
• To understand the phenomenon of
creep completely, the graph showing

creep strain vs time is shown.
A
B
E
F
D
O
C

• If the stress σ is applied below
proportional limit, the elastic strain will
occur immediately upon the application
of load regardless the duration of
application. The strain characteristic
under this condition will be represented
by the curve OAB.
• At the elevated temperature, the strain
OC occurs upon the application of the
same stress F, OC being greater than OA

The strain OC may be entirely elastic or
elastic plus plastic depending upon the
material, temperature and stress.
• At suitable stress-temp condition, the
strain increases as the time of load
application is extended, the strain
follows the curve CDEF. This continued
increase in strain with time while under
constant stress is called Creep-Curve.

• The creep curve is divided in three stages.
• I-stage :- It is shown by the curve CD. Here
creep rate decreases.
• II-Stage:- It is shown by DE. Here creep rate
is constant. It occupies a major portion of life
of component.
• III-stage:- It is shown by EF. Here point F is
fracture point. Here creep rate rapidly
increases due to necking and finally result in
fracture at point F.

• When the material is subjected to
repeated stresses i.e. the stresses
which vary from zero to certain
maximum value or alternating stresses
i.e. which vary from a minimum value
to maximum value of opposite nature, it
fails below its yield point stresses. Such
type of failure of a material is known as
fatigue.
Fatigue

• The fatigue failure is caused by means
of progressive crack formation and the
phenomenon of failure or fracture is
known as fatigue failure.
• Fatigue failure begins at irregularities
on the surface of material which acts as
stress raiser. Under static loading, the
stress concentration is uniformly
distributed. But under repeated stress,

• a progressive damage due to
fragmentation and crystal breakup
starts and some microscopic cracks are
formed. These cracks will enlarge and
join together and results in visible
cracks and the material will fails below
the yield point. The failure may occur
even without prior indication.

Endurance Limit & S-N Curve
• Endurance limit is defined as maximum
value of completely reversed bending
stress that the standard specimen can
withstand without failure, for infinite
number of cycles (usually 107 cycle).
• Consider a standard mirror polished
specimen rotating in a fatigue testing
machine as shown in Fig. a.

• As the specimen rotates, the bending
stress at the upper fiber vary from
maximum compression to maximum
tension while bending stress at lower
fiber vary from maximum tensile to
maximum compression. Hence the
specimen is subjected to completely
reversed bending stress as shown in fig
(b).

• A record is kept of the number of cycles
required to produce the failure at a
given stress and the results are plotted
in stress-cycle curve called as S-N curve
as shown in fig ©

• A little consideration will show that if
the stress is kept below a certain value
as shown by dotted line in fig (c), the
material will not fail whatever may be
the number of cycles. This stress, as
represented by dotted line, is known as
endurance limit or fatigue limit (σe). It
is defined as maximum vale of the
completely reversed bending stress
which a polished standard specimen can

• withstand without failure, for infinite
number of cycles (usually 107 cycle). It
may also be defined as safe maximum
stress which can be applied to the
machine part working under actual
condition.

1.2 Factors in Design
• Working Stress :- When designing a
machine parts, it is desirable to keep the
stress lower than the maximum or
ultimate stress at which failure of material
takes place. This stress is known as
working stress or design stress. It is also
known as safe or allowable stress.
• By failure it is not mean actual breaking of material.
Some machine parts are said to fail when they have
plastic deformation set in them, and they no more
perform their function satisfactory.

Factor of safety
• It is defined as the ratio of maximum
stress to the working stress.
Mathematically,
• Factor of safety =
• In case of ductile material, i.e. mild-
steel, where the yield-point is clearly
defined, the factor of safety is based
upon the yield-point stress. In such cases
Maximum Stress
Working or Design Stress

• In case of brittle material e.g. Cast-iron,
the yield point is not well defined.
Therefore the factor of safety is based
upon on Ultimate stress.
Yield point stress
Ultimate stress

Factors affecting the selection of
Factor of Safety
• The selection of a proper factor of
safety to be used in designing any
machine components depends upon
numbers of factors such as the
material, mode of manufacturing, type
of stress, general service conditions and
shape of parts. Before selecting a
proper factor of safety, a design
engineer should consider the following
points:-

• 1. The reliabilty of properties of the
material and change of these properties
during service.
• 2. The reliabilty of test results and
accuracy of application of these results
to actual machine parts.
• 3. The reliabilty of applied load.
• 4. The certainty of exact mode of
failure.

• 5. The extent of simplifying
assumptions.
• 6. The extent of localised stresses.
• 7. The extent of localised stresses set-
up during manufacturing.
• 8. The extent of loss of life if failure
occurs.
• 9. The extents of loss of property if
failure occurs.

• Each of the above factors must be carefully
considered and evaluated. The high factor of safety
result in unnecessary risk of failure. The value of
factor of safety for different material and type of load
are in the following table.
Material Steady Load Live Load Shock Load
Cast-Iron 5 to 6 8 to 12 16 to 20
Wrought Iron 4 7 10 to 15
Steel 4 8 12 to 16
Soft material and alloys 6 9 15
Leather 9 12 15
Timber 7 10 to 15 20

Stress Concentration
• Whenever the machine components
changes the shape of its cross-section,
the simple stress distribution no longer
holds good. This irregularities in stress
distribution caused by the abrupt
changes of form is called stress
concentration. It occurs for all kinds of
stresses in the presence of fillets,
notches, holes, keyways, splines,
surface roughness or scratches etc.

• To understand the concept of stress
concentration, consider a part with
different cross section under tensile
load as shown in fig.

• A little consideration will show that the
nominal stress distribution is uniform in
both sides i.e. right and left hand side,
but in the region where cross-section is
changing, a redistribution of force
within the parts takes place. The
material near the edges is stressed
considerably higher than average value.
This concentration of high stress due to
discontinuities or abrupt change is called as
stress concentration.

• The various of causes of stress
concentration are :-
• 1. Abrupt changes in cross-section like
fillets, notches, holes, keyways, steps,
grooves etc.
• 2. Poor surface finish :- The surface
irregularities also acts as the source of
•
Causes of Stress Concentration

• 3. Localized loading :- Concentrated
loads applied at minimum areas of
machine parts such as contact between
gear teeth, cam and follower etc causes
• 4. Variation in Mechanical properties
such as internal cracks, cavities or air
pockets are also acts as source of stress
concentration.
Causes of Stress Concentration

• The presence of stress concentration
can not be totally eliminated but can be
reduced to the some extent. The
following methods are use to reduce
stress concentration. A concept of
reducing the stress concentration
means that the stress flow lines shall
maintain their spacing as far as
possible.
Remedies to reduce the stress concentration.

• 1. Providing additional fillets, notches in
the member subjected to tension as
shown in fig (a). As we see that stress
lines tend to bunch up and cut very
close to the sharp re-entrant corners. In
order to improve the situation, fillets
and notches may be provided as shown
in fig (b) and © to give more equally
spaced flow lines.

• 2. Methods of reducing stress
concentration in cylindrical members
with holes by drilling additional holes.

• 3. Providing fillets, notch and undercut
for cylindrical member with shoulder
subjected to bending.

• 4. Methods of reducing stress
concentration in cylindrical members
with thread.

Theoretical Stress Concentration
Factor
• The theoretical stress concentration
factor is defined as the ratio of the
maximum stress in a member (at notch
or fillet) to the nominal stress at the
same section based upon based upon
net area. It is denoted by Kt.
Mathematically,
• Kt =
Maximum Stress
Nominal Stress

• In cyclic loading, stress concentration in
ductile material is always serious
because ductility is not effective in
relieving concentration of stress caused
by cracks. If the stress at any point is
above endurance limit of material, the
crack may develop under the action of
repeated load and causes failure.

• The value of Kt depends upon the
material and geometry of part.
• In static loading, stress concentration in
ductile material is not so serious as in
brittle materials, because in ductile
material yielding takes which reduces
stress concentration. In brittle material,
cracks may appear at these local
concentration which will increase the
stress over rest of section.

Fatigue Stress Concentration
Factor

• 1. Service Factor :-
• This factor is considered in design of
ball bearing. In ball bearing, the basic
dynamic radial load is calculated. It is
then multiplied by service factor (Ks) to
get design Load.
• Design Load = Ks x Actual Load
Converting actual Load and Torque into design
Load/Torque using design factors

• 2. Velocity factor :-
• This factor is considered in design of
gears. It depends on velocity of
operating gears. It is represented by Cv.
• Design Load = Cv x Actual Load
• 3. Factor of Safety:-
• It is always necessary to keep the load
less than maximum load for safety of
the element.

• It depends on the type of load which
may be steady, impact or suddenly
applied. If the factor of safety is given
then
• Design Load = FOS x Actual Load

Properties of Engineering Material
• 1. Static Strength :- It is the ability of
material to resists stress without failure
when subjected to static loading.
• 2. Fatigue Strength :- It is the ability of
material to resists stress without failure
when subjected to fatigue loading.
• 3. Elasticity :- It is the ability of material
to regain its original shape & size after
the removal of external load.

• 4. Plasticity :- It is the ability of material
to retain the permanent deformation
after the external load is removed.
• 5. Stiffness:- It is the ability of material
to resists deformation under load.
• 6. Ductility :- It is the ability of material
to drawn into wires.
• 7. Brittleness:- It is the ability of
material to rupture without negligible
deformation.

• 8. Malleability:- It is the ability of
material to drawn into thin sheets.
• 9. Hardness:- It is the ability of material
to resists penetration, abrasion or
plastic deformation.
• 10. Resilience:- It is the ability of
material to absorbs energy when
deformed within elastic range.

Designation of Material as per IS
• 1. Cast Iron :-
• A. Grey Cast Iron :-
• It is designated by letter FG followed by
minimum ultimate tensile strength in
N/mm2.
• Example :- FG200 means Grey Cast
Iron having Ultimate tensile strength
200 N/mm2.

• B. Nodular or Spheroidal cast Iron:-
• It is designated by letter SG followed by
fig indicating
• 1. Minimum Ultimate tensile strength in
N/mm2.
• 2. Percentage elongation.
• Example :- SG 800/2 means Spheroidal Cast
Iron having Ultimate tensile strength 800
N/mm2.and minimum elongation 2%.

• 2. Plain Carbon Steel:-
• It is designated by two ways.
• A) By specifying tensile strength
• It is again designated by two ways
• i) It is designated by letter Fe followed
by minimum Ultimate tensile strength in
N/mm2.
• Example :- Fe500 means Plain Carbon Steel
having Ultimate tensile strength 500 N/mm2.

• ii) It is designated by letter FeE
followed by minimum Yield strength in
N/mm2.
• Example :- FeE400 means Plain Carbon Steel
having Ultimate yield strength 400 N/mm2

• B) On the basis of Chemical
Composition.
• It is designated by xxCx.
• 1) The first two digit indicates 100 times the
average % of carbon.
• 2) Letter C for Carbon
• 3) Last digit indicate 10 times average
Manganese content.
• Example :- 30C8 means Plain Carbon Steel
having 0.3% avg. carbon content & 0.8% Mg.

• 3. Alloy Steel :-
• They are designated by fig. indicating
100 times the average percentage
carbon followed by chemical symbol for
alloying elements each followed by the
figure for its average % content
multiplied by a factor as given below.
Element Multiplying Factor
Cr, Co, Ni, Mn, Si and W 1/4
Al, Be, V, Pb, Cu, Nb, Ti, Ta, Zr and Mo 1/10
P, S and N 1/100

• Example :- 35Mn2Mo28 means Alloy
Steel having 0.35% carbon, 0.5%
Manganese and 2.8% of Molybdenum.
• 5. High Ally Steel
• It is designated by letter X followed by
figure indicating 100 times % of carbon
& chemical symbol of alloying element
followed by fig. for its average %
content rounded off to nearest fig.

• Example :- X20Cr18Ni2 means High Alloy
Steel having 0.20% carbon, 18%
Chromium and 2% of Nickel.
• 6. Cast Steel :- They are designated by
letter CS followed by no. showing
ultimate tensile strength in in N/mm2.
• Example :- CS840 means Cast Steel
having Ultimate tensile strength 840
N/mm2

Standardization
• A standard is a set of specification,
defined by a certain body or an
organisation to which various
characteristics of a component should
confirm. The characteristics include
dimension, shape, tolerance, surface
finish, material, method of testing etc.
A standardization is defined as the
process of establishing standards so as
to minimize varieties in the characterstic

• Standard used in design are,
• 1. Standards for material, their
mechanical and chemical properties.
• 2. Standards for dimensions
• 3. Standards for fits, tolerances and
surface finish.
• 4. Standards for engineering drawing of
components.

Benefits of standardisation
• 1. Interchangeability of machine
component is possible.
• 2. Better product quality, reliability &
longer service life.
• 3. Mass production of components at
low overall cost.
• 4. Easy and quick replacement of worn
out parts.
• 5. Low overall cost and ensure safety.

Use of Design data book
• When a designer wants to design and
develop a product, he require a lot of
information such as material specification,
physical and mechanical properties of
material, standards, different manufacturing
processes, process specification, details of
standards machine parts such as motors,
bearings and the standards dimension of
nuts, bolts, shafts etc. are available in design
data book. Use of design data book makes
easy for the designer to collect above data.

Preferred Number Series
• When a machine is to be made in
several sizes with different powers or
capacities, it is necessary to decide
what capacities will cover a certain
range with number of sizes. It has been
shown by experience that a certain
range can be covered efficiently when it
follows a geometrical progression with a
constant ratio. The preferred numbers
are the conventionally rounded off

• These four series are called basic series.
For establishing series, first number is
taken. By multiplying it by a series
factor, second number is obtained. By
multiplying second number by a series
factor, third number is obtained and
continued.
• Eg. For R5, 1, 1.6,2.5,4,6.30,10.0 etc

Theories of Elastic Failure
• Strength of component is based upon
mechanical properties of material used.
Since these properties are determined
from simple tension or compression test
in uniaxial system, therefore predicting
failure in members subjected to uniaxial
system is simple and straight forward.
But when a component is subjected to
bi-axial or tri-axial system, predicting
failure is more complicated.

• The various theories of failure subjected
to bi-axial stress are:
• 1. Maximum Principal or Normal Stress
Theory (Rankine Theory)
• 2. Maximum Shear Stress Theory
(Guest Theory)
• 3. Maximum Distortion Energy Theory
(Henky Theory)

Maximum Principal or Normal
Stress Theory
• According to this, the failure or yielding
occurs at a point in a member when
maximum principal or normal stress
reaches the limiting strength of material
in a simple tension test.
• Since the limiting strength for ductile
material is yield point stress and for
brittle material is ultimate stress,
therefore, according to this theory,
taking Factor of Safety into consideration,

• The maximum principal or normal stress
in bi-axial system is given by,
• σt1 = (for Ductile material)
• σt1 = (for Brittle Material)
• Where σyp= Yield point stress in simple tension test &
• σult= Ultimate stress
σyp
FOS
σult
FOS

• Since this theory considered failure only
in tension or compression and ignores
the possibility of failure in shear, hence
it is not used for ductile material which
are strong in tension or compression
but weak in shear. However, for brittle
materials which are relatively strong in
shear but weak in tension or
compression, this theory is generally
used.

2. Maximum Shear Stress
Theory
• According to this, the failure or yielding
occurs at a point in a member when
maximum shear stress in bi-axial
system reaches a value equal to the
shear stress at yield point in a simple
tension test.
• The maximum shear stress in bi-axial
system ζmax is given by,

• ζmax =
• Where ζyp = Shear stress at yield point as
determined from simple tension test.
• Since shear stress at yield point in a
simple tension test is equal to one-half
the yield stress in tension, therefore
ζ yp
FOS

• ζmax =
• Where σyp = Yield point stress in
tension
• This theory is used for ductile material
because they are strong in tension or
compression and weak in shear.
σ yp
2 x FOS

3. Maximum Distortion Energy Theory
• According to this theory, the failure or
yielding occur at a point in a member
when the distortion strain energy (also
called shear strain energy) per unit
volume in a bi-axial stress system
reaches the limiting distortion energy
per unit volume as determined from
simple tension test.
• Mathematically, the maximum distortion
strain energy for yielding is given as

• (σt1)2+ (σt2)2 – 2.σt1. σt2 =
• This theory is used for ductile material
only.
σyp
FOS
2

Factors to be considered under
Modern Design
• It is essential that modern designer
should consider following factors.
• 1. Design for Safety
• 2. Ecological consideration
• 3. Societal Consideration
• 4. Creativity in Design

Design for Safety
• The designer of product is responsible
for ensuring that products are safe to
use and there is no danger during
normal operation or in case of failure.
He must consider the Total life cycle,
off-design performance, make safety
integral, fail-safe design through
safeguard, provision for hazard
warnings and labels etc.

Ecological & Societal
Consideration
• Ecological design is defined as any form
of design that minimizes
environmentally destructive impact by
integrating with living processes. The
life cycle of product is usually divided
into procurement, manufacture, use
and disposal.
• Consider all aspects of basic design for
soundness, modular design for
replacement, Recycling factors in select-

• -ing materials, consider pollution
problem, energy consumption and
packaging.

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