Design Fundamentals

Presented By,
A. J. Bhosale
Asst. Prof.
Mechanical Engineering
AISSMS COE, Pune
Unit 2
Design fundamentals

102013 Basic Mechanical Engineering A J Bhosale
AISSMS College of Engineering, Pune
Syllabus
 Design: Steps in design process, Mechanical Properties
(Strength, Toughness, Hardness, Ductility, Malleability,
Brittleness, Elasticity, Plasticity, Resilience, Fatigue, Creep)
and selection of Engineering materials, Applications of
following materials in engineering- Aluminium, Plastic,
Steel, Brass, Cast Iron, Copper, Rubber.
 Mechanism: Definition and comparison of Mechanism and
Machine, Four Bar Mechanism, Slider Crank Mechanism.

Design
 Designing is the process of making many decisions that converts
an abstract concept into a hardware reality.
Concept Product

Design process
 Design: Design is essentially a decision-making process. If we have a
problem, we need to design a solution. In other words, to design is to
formulate a plan to satisfy a particular need and to create something
with a physical reality.
 Consider for an example, design of a chair.A number of factors need
be considered first:
→The purpose for which the chair is to be designed such as whether it is
to be used as an easy chair, an office chair or to accompany a dining
table.
→Whether the chair is to be designed for a grown up person or a child.
→ Material for the chair, its strength and cost need to be determined.
→Finally, the aesthetics (Look) of the designed chair.

 Types of Design:-
 Adaptive design
 This is based on existing design, for example, standard products or systems
adopted for a new application. Conveyor belts, control system of machines
and mechanisms or haulage systems are some of the examples where
existing design systems are adapted for a particular use.
 Developmental design
 Here we start with an existing design but finally a modified design is
obtained. A new model of a car is a typical example of a developmental
design .
 New design
 This type of design is an entirely new one but based on existing scientific
principles. No scientific invention is involved but requires creative thinking
to solve a problem. Examples of this type of design may include designing a
small vehicle for transportation of men and material on board a ship or in a
desert. Some research activity may be necessary.
DESIGN PROCESS

PURPOSE OF DESIGN
To create new products and gadgets for use.
To improve existing commodities to make them more
user friendly and comfortable.
To satisfy changes of human needs of enjoyment and
beauty.
To introduce automation.
To improve efficiency.
To face competition in market.

Design is needed:-
To solve existing problem
To improve the performance
To increase safety
To improve economy
To reduce cost
To increase human comfort.
To develop new products

STEPS IN DESIGN PROCESS
Need or
Aim
Synthesis
(Mechanisms)
Analysis of
Forces
Selection of
Materials
Design of
ElementsModification
Detailed
DrawingProduction

Need or Aim:
 Define the problem or make a complete statement of the
problem, indicating the need or purpose for which the component
is made.
 For example: Suppose we have to design a mouse for the
computer which is used for operating the computer.
Synthesis (Mechanisms):
 Synthesis means selecting the possible mechanism or group of
mechanisms which will give the desired motion or output.
 For example : While designing the mouse select a mechanism to
click and scroll.

Analysis of Forces :
 Find out the forces acting on each member of the
component(machine) and the motion transmitted by each member.
 For example: In the design of a mouse, the main force is the
weight of hand of the operator and the normal reaction from the
surface.
Selection of Material:
 Select the best suitable material for each member of the
component ( machine).
 For example : The best material for mouse is plastic (outer
body), scrolling part is made of rubber and sensing element is
made of glass.

Design of Elements :
Find the shape and size of each member of the component by
considering forces acting on the member and permissible stresses
for the used material.
 For example: In the design of a mouse, the scrolling part is round
in shape, base is flat and the upper part is of curved shape.
Modification:
 Modify the shape and size of the component as per past
experience of the designer.
The modification may also reduce the overall cost of
manufacturing.
 For example: While designing a mouse, modify the shape of the
base, make the hand resting part as more comfortable to the
operator, increase the diameter of scrolling part, etc.

Detailed Drawing :
Draw the detailed drawing of each member and assembly of the
component with complete specification for the suggested
manufacturing process.
 For example: In the design of a mouse, draw the detailed
drawing of left and right click, scrolling part, hand resting part,
etc.
Production:
As per the drawn detailed drawing, the component is
manufactured and assembled in the workshop.
For example: The most of the parts of a mouse are generally
made on injection moulding machine and assembled by press fit.

General Consideration in Design
1) Type of Load
2) Selection of Materials
3) Shape and Size
4) Friction and lubrication
5) Operational Safety
6) Machine availability
7) Use of Standard Part
8) Motion of Element
9) Production quantity
10)Maintenance of element
11)Life of element
12)Capacity of element
13)Weight of element
14)Cost of Element

MECHANICAL PROPERTIES
The characteristics of material that describe the behavior under the action of
external loads are referred as its mechanical properties. The common mechanical
properties are as follows
STRENGTH :
 It is defined as the ability of a material to resist loads without failure.
 It is usually expressed or measured in terms of maximum load per unit
area(i.e maximum stress or ultimate strength) that a material can withstand
failure and it varies according to the type of loading . Further the strength is
divided into three types they are
 Tensile Strength:
 The tensile strength or tenacity is defined as the ability of material to resist
a stretching (tensile) load without fracture.
 Ex- Steel,Aluminum, Iron, have high tensile test.
Tensile strength

Strength
 Tension and CompressionTest

Stress-Strain Diagram
Strain ( ) (DL/Lo)
4
1
2
3
5
Elastic
Region
Plastic
Region
Strain
Hardening Fracture
ultimate
tensile
strength
Elastic region
slope =Young’s (elastic) modulus
yield strength
Plastic region
ultimate tensile strength
strain hardening
fracture
necking
yield
strength
UTS
y
εEσ 
ε
σ
E 

12
y
εε
σ
E



Compressive strength :
The ability of a material to resist squeezing (compressive) load without fracture is called
compressive strength. Ex- Cast iron, Concrete have high compressive strength.
Shear strength :
The ability of a material to resist transverse loads i.e. loads tending to separate (or cut)
the material is called shear strength. Ex- Diamond,Tungsten, Carbides etc.
STIFFNESS :
It is the ability of material to resist deformation or deflection under load. Within the
elastic limit, stiffness is measured by the modulus of elasticity. This property is desired in
spring, tires, shock absorbers etc.
Shear strength Compressive strength

ELASTICITY :
The ability of a material to deform under load and return to its
original shape when the load is removed is called elasticity. This is
desired in shock absorber.
PLASTICTY :
The ability of a material to deform under load and retain its new
shape when the load is removed is called plasticity. This is desired in
forging operation.

DUCTILITY :
It is the ability of a material to be deformed plastically without
rupture under tensile load. Due to this property material can
drawn out into fine wire without fracture. Ex- Wire Drawing,
Tube Drawing.
Ductility
x 100
L
LL
EL%
o
of


• Another ductility measure:
100x
A
AA
RA%
o
fo
-
=

MALLEABILTY :
It is the ability of a material to be deformed plastically without rupture
under compressive load. Due to this property metals are hammered and
rolled into thin sheets. Ex- Sheet metal working (Plate formation)
BRITTLENESS :
It is the property of sudden fracture without any visible permanent
deformation. Ex- Concrete, Cast iron, Glass.
Malleability

TOUGHNESS :
 It is defined as the ability of the material to absorb energy up to
fracture during the plastic deformation. Toughness of a metal
offers the resistance to breaking when force is applied.
 It is measured by using ImpactTesting Machine.
 Ex- Desired in Bumpers,
hammers, Gun barrels.
ImpactTest
Machine

Toughness
very small toughness
(unreinforced polymers)
Engineering tensile strain, 
E ngineering
tensile
stress, 
small toughness (ceramics)
large toughness (metals)
Brittle fracture: elastic energy
Ductile fracture: elastic + plastic energy

HARDNESS :
+ It is defined as the ability of a material to resist scratching or indentation by another
hard body. Hardness is directly related to strength. Ex- Diamond, Tungsten, Carbides,
Ceramics etc have high hardness.
+ Large hardness means:
+++ resistance to plastic deformation or cracking in compression and better wear
properties
+ Measured in BHN(Brinell Hardness Number), VPN( Vickers Pyramid No.), RC
(Rockwell C scale) etc.
e.g.,
10 mm sphere
apply known force measure size
of indent after
removing load
dD
Smaller indents
mean larger
hardness.
increasing hardness
most
plastics
brasses
Al alloys
easy to machine
steels file hard
cutting
tools
nitrided
steels diamond

CREEP :
+ The slow and progressive deformation of a material with
time at constant stress is called creep.
+ It is more severe in materials that are subjected to heat
for long periods. It is seen into turbine blades, nuclear
power plants, jet engines, heat exchangers etc.
FATIGUE :
+ Failure of material under repeated or reversal stresses is
called fatigue. Machine parts are frequently subjected to
varying stresses and it is important to know the strength of
materials in such conditions.
+ For ex- Shafts, Gears, rotating parts of machine etc.

RESILIENCE :
It is a property of material to absorb energy and to resist shock and impact
loads. It is measured by the amount of energy absorbed per unit volume within
the elastic limit.
MACHINABILITY:
+ The ease with which a given material may be worked or shaped with a cutting
tool is called machinability. Machinability depends on chemical composition,
structure and mechanical properties.
+ Ex- Cast iron,Aluminum, Magnesium.
Resilience
Machinability

CASTABILITY :
 Castability of metal refer to the ease with which it can be cast into different
shapes and is concerned with the behavior of metal in its molten state.
 Ex- Cast iron,Aluminum have high castability.
STRAIN HARDENING :
The strengthening effect produced in metals by plastic deformation (cold
working ) is called strain hardening or work hardening. Strain hardening
reduces ductility and corrosion resistance but, raises the hardness and electrical
resistance.
Rolling Operation

WELDABILITY:
It is the ability of material to be joined by welding. Weldability depends
on chemical composition, physical properties and heat treatment to which
they are subjected.

SELECTION OF ENGINEERING MATERIALS
THE ROLL OF MATERIAL SELECTION IN DESIGN
FractureToughness

The material should be already available in the market in the
abundant quantity.
The cost of the material selected for a particular job from
several alternatives should be minimum.
The properties of the materials selected should meet the
functional requirements and the service conditions.
Availability
Cost
Material Properties
It has been the most important factor while selecting the material
for a particular job.
The materials should be selected for particular part based on the
process by which it is going to be manufactured.
Manufacturing
Considerations
SELECTION OF ENGINEERING MATERIALS

The effect of environmental conditions [Like temperature,
humidity, etc.] should be given more attention during
selection of material.
Environmental
Considerations
Machinability is the case with which a given metal can be
machined. Machinability of the material depends upon hardness,
strength and chemical Composition of materials.
Machinability
Formability
It is an indication of suitability of the metal for a machine part
that requires forming. Forming depends upon ductility and tensile
Strength.

Engineering Materials
Metallic (Metal &
Alloys)
Ferrous Alloys
Cast Iron
Alloy Cast
Iron
Plain
Carbon
Steel
Alloy
Steel
High Alloy Steel
(Stainless Steel)
Non- Ferrous
Alloys
Aluminum
Nickel
Zinc
Titanium
Non- Metallic (Non-Metals)
Organic
Plastic
Wood
Rubber
Paper
Leather
Inorganic
Sand
Brick
Concrete
Cement
Plaster
Glass
Graphite
Composites
Particulate
Composites
Reinforced
Composite

Application of materials in engineering:-
 Aluminum:-
 IC engine cylinder blocks, cylinder heads, piston
 Gear box casing
 Crank case, chain conveyors, Pulley, Fan blades
 Parts in air craft parts, ship building
 Window and door frames, etc..

 Plastic:-
 Electrical insulation
 Machine guards
 Fuel Containers
 Pipes
 Refrigerator Parts
 Toys, Covers, Typewriter, Keys etc..

 Steel:-
 Plain Carbon Steel:-
 Automobile body
 Spindle, Levers, Light duty Gears.
 Axle, Nut & Bolt, Connecting rods,
 Coil Spring, Leaf Spring.
 High Alloy Steel:-
 High temperature chemical handling equipment- Boiler,
Shells, Food Processing equipment, Springs etc.

 Cast Iron:-
 Machine tool beds, Columns, Guide-ways
 Bearing Housing, Plummer bock
 IC Engines cylinder block, cylinder heads
 Hydraulic cylinders
 Gears , Pulleys, Flywheel, Couplings
 Brake drum
 Clutch Plate, etc..

 Copper :-
 Brass (Alloy of Copper + Zinc)
 Sliding Contact bearing
 Wires, Tubes
 Plates, Locks, etc..
 Bronze (Alloy of Copper + Tin (Silicon Al,
Beryllium ))
 Journal Bearing
 Centrifugal Pumps parts
 Fitting for high pressure chemical plants.

 Rubber:-
 Natural Rubber
 Vehicle tyres
 vehicle tubes
 Heavy duty conveyors belt
 Brushes, etc..
 Synthetic Rubber-
 Conveyors
 V- Belts
 Gaskets
 Washers
 Tank lining, etc..

Kinematic Link or Element
 Each part of a machine, which moves relative to some other part, is known
as a kinematic link (or simply link) or element.
 A link may consist of several parts,which are rigidly fastened together, so that they
do not move relative to one another.
 For example, in a reciprocating steam engine, as shown in Fig. 1, piston,
piston rod and crosshead constitute one link ; connecting rod with big and
small end bearings constitute a second link ; crank, crank shaft and flywheel
a third link and the cylinder, engine frame and main bearings a fourth link.

Types of Links
In order to transmit motion, the driver and the follower may be connected by the
following three types of links :
1.Rigid link.
A rigid link is one which does not undergo any deformation while transmitting motion. Strictly
speaking, rigid links do not exist. However, as the deformation of a connecting rod,
crank etc. of a reciprocating steam engine is not appreciable, they can be considered as
rigid links.
2. Flexible link.
A flexible link is one which is partly deformed in a manner not to affect the transmission of
motion. For example, belts, ropes, chains and wires are flexible links and transmit tensile
forces only.
3. Fluid link.
A fluid link is one which is formed by having a fluid in a receptacle and the motion is transmitted
through the fluid by pressure or compression only, as in the case of hydraulic presses, jacks
and brakes.

Link or element:
It is the name given to any body which has motion relative to another.All
materials have some elasticity.A rigid link is one, whose deformations are so
small that they can be neglected in determining the motion parameters of the
link.
 Binary link: Link which is connected to other links at two points. (Fig.a)
 Ternary link: Link which is connected to other links at three points. (Fig.b)
 Quaternary link: Link which is connected to other links at four points. (Fig.
c)

Kinematic Pair
The two links or elements of a machine, when in contact with each other, are said to form
a pair. If the relative motion between them is completely or successfully constrained (i.e.in
a definite direction), the pair is known as kinematic pair.
Types of Constrained Motions
Following are the three types of constrained motions :
1. Completely constrained motion.
When the motion between a pair is limited to a definite direction irrespective of the direction
of force applied, then the motion is said to be a completely constrained motion.
For example, the piston and cylinder (in a steam engine) form a pair and the motion
of the piston is limited to a definite direction (i.e. it will only reciprocate) relative to the
cylinder irrespective of the direction of motion of the crank.
The motion of a square bar in a square hole, as shown in Fig. 2, and the motion of a shaft
with collars at each end in a circular hole, as shown in Fig. 3, are also examples of
completely constrained motion.

2. Incompletely constrained motion.
When the motion between a pair can take place in more than one direction, then
the motion is called an incompletely constrained motion.
The change in the direction of impressed force may alter the direction of
relative motion between the pair.
A circular bar or shaft in a circular hole, as shown in Fig. 4, is an example of
an incompletely constrained motion as it may either rotate or slide in a
hole.These both motions have no relationship with the other.

3. Successfully constrained motion
When the motion between the elements, forming a pair, is such that the
constrained motion is not completed by itself, but by some other means,
then the motion is said to be successfully constrained motion.
Consider a shaft in a foot-step bearing as shown in Fig. 5. The shaft may
rotate in a bearing or it may move upwards. This is a case of incompletely
constrained motion. But if the load is placed on the shaft to prevent axial
upward movement of the shaft, then the motion of the pair is said to be
successfully constrained motion.
The motion of an I.C. engine valve and the piston reciprocating inside an
engine cylinder are also the examples of successfully constrained motion.

Degrees of freedom (DOF):
 It is the number of independent coordinates required to describe the
position of a body in space.
 A free body in space (fig 1.5) can have six degrees of freedom. I.e., linear
positions along x, y and z axes and rotational/angular positions with
respect to x, y and z axes.
 In a kinematic pair, depending on the constraints imposed on the motion,
the links may loose some of the six degrees of freedom.

Classification of Kinematic Pairs
1.According to the type of relative motion between the elements.
(a) Sliding pair.
When the two elements of a pair are connected in such a way that one can
only slide relative to the other, the pair is known as a sliding pair.
The piston and cylinder, cross-head and guides of a reciprocating
steam engine, ram and its guides in shaper, tail stock on the lathe bed
etc. are the examples of a sliding pair. A little consideration will show,
that a sliding pair has a completely constrained motion.

(b)Turning pair.
When the two elements of a pair are connected in such
a way that one can only turn or revolve about a
fixed axis of another link, the pair is known as
turning pair.
(c) Spherical pair.
When the two elements of a pair are connected in such
a way that one element (with spherical shape)
turns or swivels about the other fixed element,
the pair formed is called a spherical pair. The
ball and socket joint, attachment of a car mirror,
pen stand etc., are the examples of a spherical
pair.

(d) Rolling pair.
When the two elements of a pair are connected in such a
way that one rolls over another fixed link, the pair
is known as rolling pair. Ball and roller bearings
are examples of rolling pair.
(e) Screw pair.
When the two elements of a pair are connected in such a
way that one element can turn about the other by
screw threads, the pair is known as screw pair.
The lead screw of a lathe with nut, and bolt with
a nut are examples of a screw pair.

2. According to the type of contact between the elements.
(a) Lower pair.
When the two elements of a pair have a surface contact when relative motion takes place and the
surface of one element slides over the surface of the other, the pair formed is known as
lower pair. It will be seen that sliding pairs, turning pairs and screw pairs form lower
pairs.
(b) Higher pair.
When the two elements of a pair have a line or point contact when relative motion takes place and
the motion between the two elements is partly turning and partly sliding, then the pair is
known as higher pair. A pair of friction discs, toothed gearing, belt and rope drives, ball and
roller bearings and cam and follower are the examples of higher pairs.

Kinematic Chain
 When the kinematic pairs are coupled in such a way that the last link is
joined to the first link to transmit definite motion (i.e. completely or
successfully constrained motion), it is called a kinematic chain.
 In other words, a kinematic chain may be defined as a combination of
kinematic pairs, joined in such a way that each link forms a part of two pairs
and the relative motion between the links or elements is completely or
successfully constrained.

Mechanism
 When one of the links of a kinematic chain is fixed, the chain is known as
mechanism.
 It may be used for transmitting or transforming motion
 A mechanism with four links is known as simple mechanism, and the
mechanism with more than four links is known as compound mechanism.
 When a mechanism is required to transmit power or to do some particular
type of work, it then becomes a machine.
Crank
Connecting
Rod
Turning Pairs
Piston
Sliding Pair

Inversions of mechanism:
• A mechanism is one in which one of the links of a kinematic
chain is fixed.
• Different mechanisms can be obtained by fixing different
links of the same kinematic chain.
• These are called as inversions of the mechanism.
• By changing the fixed link, the number of mechanisms
which can be obtained is equal to the number of links.
• Except the original mechanism, all other mechanisms will
be known as inversions of original mechanism.
• The inversion of a mechanism does not change the motion of
its links relative to each other.

Types of Kinematic Chains
• The most important kinematic chains are those which consist of
four lower pairs, each pair being a sliding pair or a turning pair.
• The following three types of kinematic chains with four lower
pairs are important from the subject point of view :
1. Four bar chain or quadric cyclic chain,
2. Single slider crank chain, and
3. Double slider crank chain.

Four Bar Chain or Quadric Cycle Chain
 The simplest and the basic kinematic chain is a four bar chain or quadric cycle chain, as
shown in Fig.
 It consists of four links, each of them forms a turning pair at A,B,C and D.The four links may
be of different lengths.
 A very important consideration in designing a mechanism is to ensure that the input crank
makes a complete revolution relative to the other links. The mechanism in which no link
makes a complete revolution will not be useful.
 In a four bar chain, one of the links, in particular the shortest link, will make a complete
revolution relative to the other three links. Such a link is known as crank or driver.
In Fig. AD (link 4 ) is a crank. The link BC (link 2) which makes
a partial rotation or oscillates is known as lever or rocker or
follower and the link CD (link 3) which connects the crank
and lever is called connecting rod or coupler. The fixed link
AB (link 1) is known as frame of the mechanism.
When the crank (link 4) is the driver, the mechanism is
transforming rotary motion into oscillating motion.

FOUR BAR MECHANISMS(BOX LIFTING)

Grashof’s Law
 For Planner Four bar linkage, sum of shortest and longest link
lengths can not be grater than sum of remaining two link lengths,
if there is to be continuous relative motion between two
members.
 If l + s < p + q, Crank- Rocker Mechanism is Possible.
 If l + s = p + q, Double Crank Mechanism is Possible.
 If l + s > p + q, Double Rocker Mechanism is Possible.

Single Slider Crank Chain
 A single slider crank chain is a modification of the basic four bar chain.
 It consist of one sliding pair and three turning pairs. It is, usually, found in reciprocating steam engine
mechanism.
 This type of mechanism converts rotary motion into reciprocating motion and vice versa.
 In a single slider crank chain, as shown in Fig., the links 1 and 2, links 2 and 3, and links 3 and 4 form
three turning pairs while the links 4 and 1 form a sliding pair.
 The link 1 corresponds to the frame of the engine, which is fixed. The link 2 corresponds to the crank
; link 3 corresponds to the connecting rod and link 4 corresponds to cross-head.
 As the crank rotates, the cross-head reciprocates in the guides and thus the piston reciprocates in the
cylinder.

Example of Mechanism
Can crusher
Simple press
Rear-window wiper

Example of Mechanisms
Moves packages from an assembly bench
to a conveyor
Lift platform
Microwave carrier to assist
people on wheelchair

Lift platform
Front loader
Device to close the top
flap of boxes

Conceptual design for an
exercise machine
Rowing type exercise machine

Machine:
Machine is a combination of resistant bodies, with definite
constrained motion, which is used for transmitting or
transforming available energy so as to do some particular kind
of work.

Sr.
No.
Mechanism Machine
1 If one of the links or elements of a
kinematic chain is fixed, the transmitting or
transforming the motion. It is then termed
as a mechanism.
When a mechanism is required to transmit
power or to do some particular kind of work,
the various links or elements have to be
designed so as to withstand the forces to which
they are subjected. The arrangement is then
known as a machine.
2 The primary function of mechanism is to
transmit or transform the motion.
The primary function of machine is to transmit
or transform the energy.
3 Every mechanism is not necessarily a
machine.
Every machine is either a mechanism or a
combination of more than one mechanisms.
4 Examples of mechanism are:
Clock, type-writer,, P.V. diagram indicator
of lockengine, etc.
Examples of machine are:
I.C. engine, shaping machine, hand pump, etc.

Design Fundamentals

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