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 systemsadopted for a new application. Conveyor belts, control system of machinesand mechanisms or haulage systems are some of the examples whereexisting design systems are adapted for a particular use.

Developmental design

Here we start with an existing design but finally a modified design isobtained. A new model of a car is a typical example of a developmentaldesign .

New design

This type of design is an entirely new one but based on existing scientificprinciples. No scientific invention is involved but requires creative thinkingto solve a problem. Examples of this type of design may include designing asmall vehicle for transportation of men and material on board a ship or in adesert. 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 Elements

ModificationDetailed 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.



STEPS IN DESIGN PROCESSDesign 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 PROPERTIESThe 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 Compression Test



Stress-Strain Diagram

Strain ( ) (DL/Lo)

41

2

3

5

Elastic

Region

Plastic

Region

Strain

Hardening Fracture

ultimatetensile strength

Elastic region

slope =Young’s (elastic) modulus

yield strength

Plastic region

ultimate tensile strength

strain hardening

fracture

necking

yieldstrength

UTS

y

εEσ

ε

σE

12

y

ε ε

σE



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 itsoriginal shape when the load is removed is called elasticity. This isdesired in shock absorber.

PLASTICTY :

The ability of a material to deform under load and retain its newshape when the load is removed is called plasticity. This is desired inforging 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

LLEL%

o

of

• Another ductility measure: 100xA

AARA%

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

dDSmaller indents mean larger hardness.

increasing hardness

most plastics

brasses Al alloys

easy to machine steels file hard

cutting tools

nitridedsteels 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

Fracture Toughness



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

FormabilityIt 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 PairThe two links or elements of a machine, when in contact with each other, are said to forma pair. If the relative motion between them is completely or successfully constrained (i.e. ina definite direction), the pair is known as kinematic pair.

Types of Constrained MotionsFollowing 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 directionof 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 motionof the piston is limited to a definite direction (i.e. it will only reciprocate) relative to thecylinder 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 shaftwith collars at each end in a circular hole, as shown in Fig. 3, are also examples ofcompletely 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 theconstrained 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 mayrotate in a bearing or it may move upwards. This is a case of incompletelyconstrained motion. But if the load is placed on the shaft to prevent axialupward movement of the shaft, then the motion of the pair is said to besuccessfully constrained motion.

The motion of an I.C. engine valve and the piston reciprocating inside anengine 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 makesa partial rotation or oscillates is known as lever or rocker orfollower and the link CD (link 3) which connects the crankand lever is called connecting rod or coupler. The fixed linkAB (link 1) is known as frame of the mechanism.When the crank (link 4) is the driver, the mechanism istransforming 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

Engineering

Transcript of Design Fundamentals