General considerations of MAchine Design

GENERAL CONSIDERATIONS OF MACHINE DESIGN

S K Chand (Assistant Professor)
Government Engineering College, Raipur
INTRODUCTION
➢ Design is essentially a decision making process.
➢ For every problem, we need to design a solution.
• Design is to formulate a plan to satisfy a particular
need and to create something with a physical reality.

Definition: It is defined as an iterative decision making
activity to produce a drawing or a plan to convert resources
optimally into a product or device or system to satisfy the
human needs.
The ultimate aim of design is to select appropriate
shape, material, size and manufacturing process details in
such a way that the resulting machine component should
perform its given function satisfactorily (i.e. without any
failure).

Design of a chair
Factors need to be considered
• The purpose for which the chair is to be designed.
• Whether the chair is to be designed for a grown up person or a
child
• Material for the chair: Strength and cost need to be determined
• Aesthetics of the designed chair

Definition of Machine
Machine is defined as a combination of resisting
bodies with successfully constrained relative
motions which is used to transform other form of
energy into mechanical energy or transmit and
modify available energy to do some useful work.
Machines can receive mechanical energy and
modify it so that a specific task is carried out.

Modification/ Transformation of
energy
Machine Elements
Machine design involves primarily designing machine
elements so that they transmit the forces and perform their
task successfully.

TYPES OF
DESIGN
ADAPTIVE DESIGN DEVELOPMENTAL
DESIGN
NEW DESIGN

Adaptive Design: Based on existing design,
standard products or systems are adopted for new
application.
e.g.- Conveyor belt, control system of machines etc.
Developmental Design: Starting with an existing
design, finally a modified design is obtained.
e.g.- A new model of a car.

New Design
• This type of design is an entirely new one but
based on existing scientific principles.
• No scientific invention is involved but requires
thinking to solve a problem.
• Some research activity may be necessary.
e.g.- ATV

TYPES OF DESIGN
BASED ON METHODS
RATIONAL DESIGN EMPIRICAL DESIGN
INDUSTRIAL
DESIGN

Rational Design: Based on the determining the
stresses and strains of components and thereby
deciding their dimensions.
Industrial Design: Based on the industrial
considerations and norms viz. market survey,
external look, production facilities, low cost, use
of existing standard products.

Empirical Design: Based on empirical formulae
which in turn is based on experience and
experiments.
e.g.- When we tighten a nut on a bolt the force exerted or the stresses
induced can’t be determined exactly but experience shows that the
tightening force may be given by ‘P = 284d’ where d is the bolt
diameter in mm and P is the applied force in kg.
There is no mathematical backing of this equation but is
based on observations and experience.

ENGINEERING MATERIALS
Choice of materials for a machine element
depends on
➢Properties
➢Cost
➢Availability

Common
Engineering
Materials
Ferrous
Materials
Non-ferrous
Materials
Non Metals

Important ferrous metal
➢Cast iron
➢Wrought iron
➢Steel

Cast iron
➢ Alloy of iron, carbon and silicon
➢ Hard and brittle
➢ Carbon content within 1.7% to 3%
➢ Carbon presence: Free carbon/ iron carbide Fe3C.
Types of cast iron:
• Grey cast iron
• White cast iron
• Malleable cast iron
• Spheroidal or nodular cast iron
• Austenitic cast iron
• Abrasion resistant cast iron

Grey cast iron
➢Carbon here is mainly in the form of graphite.
➢Inexpensive.
➢High Compressive Strength.
➢Graphite is an excellent solid lubricant which
makes it easily machinable but brittle.
Designated as FG20, FG35 or FG35Si15
The numbers indicate ultimate tensile strength in
MPa and 15 indicates 0.15% silicon.

White cast iron
➢Carbon present in the form of iron carbide
Fe3C which is hard and brittle.
➢The presence of iron carbide increases
hardness and makes it difficult to machine.
➢Very good abrasion resistant property

Malleable cast iron
➢ These are white cast iron rendered malleable by annealing.
➢ These are tougher than GCI and they can be twisted or bent without
fracture
➢ Excellent machining property
➢ Inexpensive
➢ Used for making parts where forging is expensive
e.g. Hubs for wagon wheels, brake supports
Designated based on the method of processing
Black heart BM32, BM30
White heart WM42, WM35

Spheroidal or Nodular graphite cast iron
➢Graphite is present in the form of spheres or
nodules
➢High tensile strength
➢Good elongation property
Designated as: SG50/7, SG80/2
First number gives the tensile strength in MPa
Second number indicates percentage elongation.

Austenitic cast iron
Depending on the form of graphite present these
cast iron is classified broadly under two headings
➢Austenitic flake graphite iron, AFGNi16Cu7Cr2
➢Spheroidal/nodular graphite iron, ASGNi20Cr2
Used for making automobile parts: cylinders,
pistons, piston rings, brake drums etc.

Abrasion resistant cast iron
These are alloy cast iron and the alloying
elements provide abrasion resistance.
Typical designation: ABR33Ni4Cr2
Indicates a tensile strength in MPa with 4%
nickel and 2% chromium.

Wrought iron
• This is very pure iron where the iron content is of the
order of 99.5%.
• It is produced by re-melting pig iron and addition of
very small amount of silicon, sulphur and phosphorus.
• It is tough, malleable and ductile and can easily be
forged or welded.
• It is poor in shock/impact loading.
• Chains, crane hooks, railway couplings and such other
components may be made of this iron.

Steel
• This is by far the most important engineering material and there is
an enormous variety of steel to meet the wide variety of engineering
requirements.
• Steel is an alloy of iron and carbon in which the carbon content can
be less than 1.7% and carbon is present in the form of iron carbide
(𝐹𝑒3𝐶) to impart hardness and strength.
Two main categories of steel
➢ Plain carbon steel
➢ Alloy steel

Plain carbon steel
• The properties of plain carbon steel depend mainly on the carbon
percentage. Other alloying elements usually not present in more than
0.5% to 1%.
• Designated as C01, C14, C45, C70, where the numbers indicate the
carbon percentage.
Categorization of plain carbon steel
➢ Dead mild steel- up to 0.15% C
➢ Low carbon steel or mild steel- 0.15% to 0.46% C
➢ Medium carbon steel- 0.45% to 0.8% C
➢ High carbon steel- 0.8% to 1.5% C
In general higher carbon percentage indicates higher strength.

Alloy steel
• These are steels in which elements other than carbon are added in sufficient
quantities to impart desired properties, such as wear strength, corrosion resistance,
electric and magnetic properties.
Chief alloying elements
Nickel: Strength and toughness
Chromium: Hardness and strength
Tungsten: Hardness at elevated temperature
Vanadium: Tensile strength
Manganese: High strength at hot rolled/ heat treated condition
Silicon: High elastic limit
Cobalt: Hardness
Molybdenum: Extra tensile strength
e.g. 35Ni1Cr60, 30Ni4Cr1, stainless steel(18/8 steel): 18% Cr and 8% Ni

Non-Ferrous Metals
Metals containing elements other than iron as their chief
constituents are usually referred to as non-ferrous metals.
Aluminium
• This is a white metal produced from alumina.
• In its pure state it is weak & soft but addition of small amounts
of copper, manganese, silicon and magnesium makes it hard
and strong.
• It is also corrosion resistant, low weight and non-toxic.

Duralium
This is an alloy of 4% Cu, 0.5% Mn, 0.5% Mg and aluminium. It
is widely used in automobile and aircraft components.
Y-alloy
This is an alloy of 4% Cu, 1.5% Mn, 2% Ni, 6% Si, Mg, Fe and
rest is Al. it gives large strength at high temperature. It is used for
aircraft engine parts such as cylinder heads, piston etc.

Magnalium
This is an aluminium alloy with 2 to 10% magnesium. It also
contains 1.75% Cu. Due to its light weight and good strength it is
used for aircraft and automobile components.
Copper-alloys
Copper is one of the most widely used non-ferrous metal in
industry.it is soft, malleable and ductile and is a good conductor
of heat and electricity.

Brass (Cu-Zn alloy)
• It is fundamentally a binary alloy with Zn up to 50%.
• Up to ~37%, as Zn percentage increases, ductility increases.
Beyond that ductility falls.
• Small amount of other elements viz. lead or tin imparts other
properties to the brass. Lead gives good machining quality and
tin imparts strength.
• Brass is highly corrosion resistant, easily machinable and
therefore a good bearing material.

Bronze (Cu-Sn alloy)
This is mainly a copper-tin alloy where tin percentage may vary
between 5 to 25. It provides hardness but tin content also oxidizes
resulting in brittleness. Deoxidizers such as Zn may be added.
Gun Metal
It is an alloy where 2% Zn is added as deoxidizing element and
typical compositions are 88% Cu, 10% Sn and 2% Zn. This is
suitable for working in cold state. It was originally made for
casting guns but now used for boiler fittings, bushes etc.

Non-metals are used in engineering practice mainly due to their
low cost, flexibility and resistance to heat and electricity.
Timber
• This is a relatively low cost material.
• Bad conductor of heat and electricity.
• Good elastic and frictional property.
• Widely used: foundry pattern, water lubricated bearing.
Non-Metals

Leather
This is widely used in engineering for its flexibility and wear
resistance.
Uses: belt drives, washers etc.
Rubber
It has high bulk modulus and is used for drive elements, sealing,
vibration isolation and similar applications.

Plastics
These are synthetic materials which can be moulded into desired
shapes under pressure with or without application of heat. These
are now extensively used in various industrial applications for
their corrosion resistance, dimensional stability and relatively low
cost.
There are two main types of plastics-
➢ Thermosetting plastic
➢ Thermoplastics

Thermosetting Plastics
• Thermosetting plastics are formed under heat and pressure.
• It initially softens and with increasing heat and pressure,
polymerisation takes place. This results in hardening of the
material.
• These plastics can not be deformed or remoulded again under
heat and pressure.
• Some examples of thermosetting plastics: Phenol formaldehyde
(Bakelite), Phenol-furfural (Durite), Epoxy resins, Phenolic
resins.

Thermoplastics
• Thermoplastics do not become hard with the application of heat
and pressure and no chemical change takes place.
• They remain soft at elevated temperatures until they are
hardened by cooling.
• They can be re-melted and remoulded by application of heat and
pressure.
• Some examples of thermoplastics are: Cellulose nitrate
(Celluloid), Polythene, Polyvinyl acetate, Polyvinyl Chloride
(PVC).

General considerations of MAchine Design

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