DESIGN OF MACHINE

ELEMENTS - 1

18ME52
Review of Session 1:

1. What is Design and its Phases?

2. What is a Machine?

3. What is Machine Design?

4. What are the Types and Methods of Design?

5. Factors involved in Machine Design?

1. Type of load and stresses caused by the load.
2. Motion of the parts or kinematics of the machine.
3. Selection of materials.
4. Form and size of the parts.
5. Frictional resistance and lubrication.
6. Convenient and economical features.
7. Use of standard parts.
8. Safety of operation.
9. Workshop facilities.
10.Number of machines to be manufactured.
11.Cost of construction.
Engineering Material Selection
• Availability of the materials
• Suitability of the materials for the working conditions in service
• The cost of the materials.
• Strength aspect.
• Ease of manufacturing.
• Weight of the materials.
 MECHANICAL PROPERTIES
• Strength
• Stiffness
• Elasticity
• Plasticity
• Ductility
• Brittleness
• Malleability
• Toughness
• Machinability
• Resilience
• Creep
• Fatigue
• Hardness
 Engineering Materials

Metals Non-Metals

Ferrous Non - Ferrous 1. Timber

2. Leather
1) Cast Iron 1) Light metal group such as aluminium and its 3. Rubber
2) Wrought Iron alloys, magnesium and manganese alloys.
4. Plastics
2) Copper based alloys such as brass (Cu-Zn),
3) Steel 1. Thermosetting
bronze (Cu-Sn).
2. Thermoplastics
3) White metal group such as nickel, silver,
white bearing metals eg. SnSb7Cu3,
Sn60Sb11Pb, zinc etc.
Ferrous Materials – Cast Iron, Wrought Iron, & Steels
Cast Iron:
• It is an alloy of iron, carbon and silicon and it is hard and brittle.

• Carbon content may be within 1.7% to 3%.

• In general the types of cast iron are

1. Grey cast iron
2. White cast iron

3. Malleable cast iron

4. Spheroidal or nodular cast iron

5. Austenitic cast iron

6. Abrasion resistant cast iron.

1. Grey Cast Iron:
• Carbon here is mainly in the form of graphite.

• This type of cast iron is inexpensive and has high compressive strength.

• Graphite is an excellent solid lubricant and this makes it easily machinable but brittle.
• Some examples of this type of cast iron are FG20, FG35 or FG35Si15.

• The numbers indicate ultimate tensile strength in MPa and 15 indicates 0.15% silicon.

2. White Cast Iron:

• In these cast irons carbon is 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. Consequently these
cast irons are abrasion resistant.
3. Malleable Cast Iron:
• These are white cast irons rendered malleable by annealing.
• These are tougher than grey cast iron and they can be twisted or bent without fracture.
• They have excellent machining properties and are inexpensive.
• Malleable cast iron are used for making parts where forging is expensive such as hubs for wagon wheels,
brake supports.
• Depending on the method of processing they may be designated as black heart BM32, BM30 or white
heart WM42, WM35 etc.

4. Spheroidal or nodular graphite cast iron

• In these cast irons graphite is present in the form of spheres or nodules.
• They have high tensile strength and good elongation properties.
• They are designated as, for example, SG50/7, SG80/2 etc., where the first number gives the tensile
strength in MPa and the second number indicates percentage elongation.
3. Austenitic cast iron:
• Depending on the form of graphite present these cast iron can be classified broadly under two headings:

• Austenitic flake graphite iron designated, for example, AFGNi16Cu7Cr2

• Austenitic spheroidal or nodular graphite iron designated, for example, ASGNi20Cr2.

• These are alloy cast irons and they contain small percentages of silicon, manganese, sulphur, phosphorus
etc.

• They may be produced by adding alloying elements viz. nickel, chromium, molybdenum, copper and
manganese in sufficient quantities.

• These elements give more strength and improved properties.

• They are used for making automobile parts such as cylinders, pistons, piston rings, brake drums etc.
4. Abrasion resistant cast iron
• These are alloy cast iron and the alloying elements render abrasion resistance.

• A typical designation is ABR33 Ni4 Cr2 which indicates a tensile strength in kg/mm2 with 4% nickel
and 2% chromium.

Wrought iron
• This is a very pure iron where the iron content is of the order of 99.5%.

• It is produced by re-melting pig iron and some small amount of silicon, Sulphur, or phosphorus may
be present.

• It is tough, malleable and ductile and can easily be forged or welded.

• It cannot however take sudden shock.

• 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 basically 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 to impart hardness and
strength.

• Two main categories of steel are

• Plain carbon steel

• Alloy steel.
1. Plain carbon steel
• The properties of plain carbon steel depend mainly on the carbon percentages and other alloying elements
are not usually present in more than 0.5 to 1% such as 0.5% Si or 1% Mn etc.
• There is a large variety of plane carbon steel and they are designated as C01, C14, C45, C70 and so on where
the number indicates the carbon percentage.
• Following categorization of these steels is sometimes made for convenience:
1. Dead mild steel - up to 0.15% C
2. Low carbon steel or mild steel - 0.15 to 0.46% C
3. Medium carbon steel - 0.45 to 0.8% C.
4. High carbon steel - 0.8 to 1.5% C

• Detailed properties of these steels may be found in any standard handbook but in general higher carbon
percentage indicates higher strength.
2. Alloy steel
• These are steels in which elements other than carbon are added in sufficient quantities to
impart desired properties, such as wear resistance, corrosion resistance, electric or magnetic
properties.
• Chief alloying elements added are usually:
• Nickel for strength and toughness
• Chromium for hardness and strength
• Tungsten for hardness at elevated temperature

• Vanadium for tensile strength

• Manganese for high strength in hot rolled and heat treated condition
• Silicon for high elastic limit
• Cobalt for hardness

• Molybdenum for extra tensile strength.

• Some examples of alloy steels are 35Ni1Cr60, 30Ni4Cr1, 40Cr1Mo28, 37Mn2.

• Stainless steel is one such alloy steel that gives good corrosion resistance.

• One important type of stainless steel is often described as 18/8 steel where
chromium and nickel percentages are 18 and 8 respectively.

• A typical designation of a stainless steel is 15Si2Mn2Cr18Ni8 where carbon

percentage is 0.15.
Non Ferrous Materials
• Metals containing elements other than iron as their chief constituents are usually referred
to as non-ferrous metals.
• There is a wide variety of non-metals in practice.
1. Aluminium- This is the white metal produced from Alumina. In its pure state it is weak and soft but
addition of small amounts of Cu, Mn, Si and Magnesium makes it hard and strong. It is also corrosion
resistant, low weight and non-toxic.

2. Duralumin- This is an alloy of 4% Cu, 0.5% Mn, 0.5% Mg and aluminium. It is widely used in automobile
and aircraft components.

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

5. Copper alloys: Copper is one of the most widely used non-ferrous metals in industry. It is soft,
malleable and ductile and is a good conductor of heat and electricity. The following two important
copper alloys are widely used in practice:

• Brass (Cu-Zn alloy): It is fundamentally a binary alloy with Zn up to 50% . As Zn percentage increases,
ductility increases up to ~37% of Zn beyond which the ductility falls.

• 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 is one such alloy where 2% Zn is added as deoxidizing agent and typical
compositions are 88% Cu, 10% Sn, 2% Zn. This is suitable for working in cold state. It was originally
made for casting guns but used now for boiler fittings, bushes, glands and other such uses.
Non-metals
• Non-metallic materials are also used in engineering practice due to principally their low
cost, flexibility and resistance to heat and electricity.
• Though there are many suitable non-metals, the following are important few from design
point of view:
1. Timber- This is a relatively low cost material and a bad conductor of heat and electricity. It has also
good elastic and frictional properties and is widely used in foundry patterns and as water lubricated
bearings.
2. Leather- This is widely used in engineering for its flexibility and wear resistance. It is widely used for
belt drives, washers and such other applications.
3. Rubber- It has high bulk modulus and is used for drive elements, sealing, vibration isolation and
similar applications.
4. 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 plastics: Thermosetting plastics are formed under heat and pressure. It initially softens and with
increasing heat and pressure, polymerization takes place. This results in hardening of the material. These
plastics cannot be deformed or remolded again under heat and pressure. Some examples of thermosetting
plastics are phenol formaldehyde (Bakelite), phenol-furfural (Durite), epoxy resins, phenolic resins etc.

➢ 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. These can be
re-melted and remolded by application of heat and pressure. Some examples of thermoplastics are cellulose
nitrate (celluloid), polythene, polyvinyl acetate, polyvinyl chloride ( PVC) etc.
 GREY CAST IRON
NYLON

BRASS
STEELS

COPPER

Bronze Duralumin
RUBBER