BT-203, Basic Mechanical Engineering - RGPVONLINE.COM

BT-203, Basic Mechanical Engineering 2020

Dr. Alok Agrawal Page 1

BT-203 (CBGS)

B. Tech. I & II Semester

Examination, November 2019

Choice Based Grading System (CBGS)

Basic Mechanical Engineering

Time: Three Hours

Maximum Marks: 70

Note: i) Attempt any five question

ii) All question carry equal marks

iii) In case of any doubt or dispute the English version question should be treated as final

Question 1 (a): Discuss basic classification of engineering materials.

Solution:

Materials can be classified into following categories:

A. Metallic Materials:

a) Ferrous Metals.

b) Non-Ferrous Metals.

B. Non-Metallic Materials

a) Organic Materials.

b) Non-Organic Materials.

A. Metallic Materials:Metals are usually strong, good conductor of heat and electricity, and

opaque to light. Example: Iron, Aluminum, Copper, Silver, Gold and metal alloys.

a) Ferrous Metals:Ferrous metals mostly contain Iron as major constituent. A small

amount of other metals/elements added into ferrous metals to give the desired

properties. Iron as the base metal i.e. more than 98%. High alloy steel contains

more than 50% Fe and less than 50% other alloying elements. Iron is produced

from iron ore that mainly occurs in nature as chemical compounds such as

sulphides or oxides.The two most important oxides being Hematite and

Magnetite.Iron ore is reduced to pig iron using a blast furnace.

Examples: Mild Steel, Carbon Steel, Stainless Steel, Cast Iron and Wrought Iron.

Properties of Ferrous Metals

They are good conductor of heat and electricity.

They tend to react chemically with environment with the exception of noble metals.

They have good tensile strength and durability.



They are magnetic in nature.

They have little resistance to corrosion.

b) Non-Ferrous Metals:The non-ferrous metals are generally inferior in strength but

superior in corrosion resistance when compared to ferrous metals. Non-ferrous

metals are more costly.

Examples: Copper, Aluminum, Zinc, Lead, Alloys like Brass, Bronze, Duralumin etc.

B. Non-Metallic Materials:Non-metallic materials are generally bad conductor of heat and

electricity. Examples: Leather, Rubber, Asbestos, Plastics, glass etc.

a) Organic Materials:Materials which contains animal or vegetable cells or carbon

compounds are known as organic materials. They dissolve in organic liquids such

as alcohol, but they still not dissolve in water.

Examples: Leather, wood, plastics etc.

b) Non-Organic Materials:Materials which do not contains animal or vegetable cells

or carbon compounds are known as non-organic materials. They are generally

resulting from non-living sources, such as rocks or minerals. They tend to

dissolve in water.

Examples: Stone, Ceramics, Clay, Mud bricks, Glass etc.

Question 1 (b): What is Hardness? How do you measure it?

Solution:

Hardness is defined as the resistance of material to indentation, scratching, abrasion or cutting.

Hardness testing plays a vital role in material testing, quality control and acceptance or rejection

of components. Hardness testing of material before use is very important to access resistance to

plastic deformation, to determining resistance to penetration, to indicate how easily a material

can be machined. Hardness can be measure by various methods. Among them the most widely

used is as follows:

BrinellHardness Testing

The Brinell test used a hardened steel ball (5 or 10 mm diameter) indenter. The indenter is

applied to the test material below a load of 3000 kg. The load can be reduced to 1500 kg or 500

kg for testing soft materials to avoid excessive indentation. Force is applied for a specified time

usually between 10 to 30 seconds. The load time period has to ensure the plastic deformation of

the metal ceased.

Procedure:

In this test, a steel ball indenter is pressed into the specimen for a specific period of time

and with accurately controlled force.



When ball indenter removed, the material has a round indent.

Brinell test applies only a single test force.

The surface area of the indentation is the measure of hardness HB of the material.

The round impression is measured using a low-power microscope in millimeters.

This round indent produced is measured to calculate material hardness according to the

formula.

HB = Applied load (kg)/Surface area of the impression (mm2)

2

1

2

2DDDD

FH B

It is generally referred to as Brinell hardness number.

Question 2 (a): Explain the principal of working of micrometer.

Solution:

For greater accuracy which is essential in the measurement of small sizes, we use micrometer.

The micrometer has five main parts: Frame, anvil, spindle, sleeve and thimble. The frame is U

shaped and made of steel. The gap of the frame permits the maximum dimension that can be

measured. On the left end of the frame, the anvil is fixed and at right end, there is a screwed

spindle. The diameter on anvil and spindle should be equal. A lock nut is provided to retain the

reading. Sleeve has reference line and main scale on it. A tubular cover fixed with spindle is

called as thimble. Thimble moves with spindle on main scale. Thimble has 50 equal parts on its

beveled edge. This is called the circular scale. Ratchet screw is provided to tighten the thimble.

When correct reading is achieved, ratchet slips automatically.

The basic operating principles of a micrometer are as follows:

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1. The amount of rotation of an accurately made screw can be directly and precisely

correlated to a certain amount of axial movement (and vice versa), through the constant

known as the screw's lead. A screw's lead is the distance it moves forward axially with

one complete turn (360°).

2. With an appropriate lead and major diameter of the screw, a given amount of axial

movement will be amplified in the resulting circumferential movement.

For example, if the lead of a screw is 1 mm, but the major diameter (here, outer diameter) is

10 mm, then the circumference of the screw is 10π, or about 31.4 mm. Therefore, an axial

movement of 1 mm is amplified (magnified) to a circumferential movement of 31.4 mm. This

amplification allows a small difference in the sizes of two similar measured objects to correlate

to a larger difference in the position of a micrometer's thimble.

Question 2 (b): Explain the process of velocity measurement of fluid.

Solution:

Velocity is an important parameter of fluid flow and therefore its measurement is quite essential.

Velocity of flowing fluid can be measured by Pitot tube. It is a device used to measure fluid flow

by determining the static pressure and stagnation pressure. It is a device used for measuring the

velocity of flow at any point in a pipe or channel. If the velocity of flow at any point decreases,

the pressure at that point increases due to the conservation of the kinetic energy into pressure

energy. It consists of a glass tube which bent at right angle.

Applying Bernoulli’s equation between stagnation point (S) and point (P) in the undisturbed flow

at the same horizontal plane, we get,

sss

ooo Z

g

V

w

pZ

g

V

w

p

22

22

,




But zo = zs and Vs = 0, po is static pressure and ps is stagnation pressure.

w

p

g

V

w

p so 2

2

w

po Pressure head at P = ho and w

ps pressure head at S = hs

so hg

Vh

2

2

,

hghhgV os 22

Question 3: What are the main parts of Lathe? Describe four operations which can be

performed in lathe.

Solution:

The lathe machine consists of the following six basic parts:

1. Bed: The bed of a lathe acts as the base on which the different fixed and operating parts

of the lathe are mounted. The top of the bed has two guideways to provide the support

and the sliding surfaces for the carriage and the tailstock. The head stock is permanently

fixed to the bed, the tailstock is adjustable for position to accommodate workpiece of the

different lengths. The lathe bed being the main guiding member for accurate machining

work, it should be sufficiently rigid to prevent deflection under cutting forces. Bed is

made up of cast iron.

2. Headstock: The headstock is permanently fastened to the left hand end of the lathe bed.

The main function of the headstock is to support the spindle and to house the main drive.

Spindle is the hollow rotating shaft used for holding the workpiece. The work holding

device such as chuck is mounted on the spindle. Main drive function is to drive the




spindle and to change the spindle speed with the help of gear box mounted. The main

drive is powered by an electric motor. Head stock is also called livestock as it turns with

work. The feed shaft is used for most turning operations and lead screw is used for

cutting threads.

3. Tailstock: The tailstock is located at the right hand end of the lathe bed. It can be moved

along the guideways on the lathe bed and can be clamped in any position on the lathe

bed. Its main function is to hold the dead center which can support the long workpiece

during machining and to hold the tools like drill, reamer for carrying out operation like

drilling, reaming or taping.

4. Carriage: The carriage is located between the headstock and tailstock of the lathe bed. It

can slides along the guideways on the lathe bed. The main function of the carriage is to

hold the cutting tool and to give longitudinal and/or cross feed to the cutting tool.

5. Drive: The center lathe has primarily two motions. The primary cutting motion and feed

motion. These motions are accomplished by means of corresponding drives, which are

system of mechanisms for transmitting power from its source (the electric motor) to the

operative units of lathe that is to the spindle for primary cutting motion (main drive) and

to the carriage for feed motion (feed drive)

6. Work holding device: The common work holding devices used for a center lathe are as

follows:

a) Centers: Long workpiece (shaft or axle) with L/D ratio > 4, are turned lengthwise

between centers. The workpiece in those ends center holes has been previously

drilled and on which a driving dog has been clamped on one end is mounted

between headstock and tailstock centers. The workpiece is rotated by driving the

lathe dog. The lathe dog is driven with the help of a drive plate.

b) Chucks: Workpiece with a length L<4d may be clamped in a chuck without the need

of additional support of the free end. It may be 3 jaw or four jaw chuck.

c) Mandrel: It can be described as solid shaft or spindle which is used for holding

bored parts for machining their outside surfaces on lathe. Mandrels are usually

employed for those jobs which have a finished hole which is concentric with the

outer surface that is to be machined.

Four basic operations performed on Lathe machine:

1. Turning: It is the process of removing the material from the cylindrical surface of the

workpiece to reduce its diameter. In turning operation, the tool motion is longitudinal i.e.

parallel to the axis of the lathe spindle. The tool used for the turning operation is called

turning tool.



2. Facing: It is the process of removing the material from the end surface or face of

workpiece. The facing operation produces a flat surface. The facing operation is used for

reducing the length of the workpiece. In facing operation, the tool motion is

perpendicular to the axis of the lathe spindle. The tool used for this operation is called

facing tool.

3. Chamfering: it is a process of beveling the sharp edge of a workpiece. It is provided for

avoiding the injuries to the persons handling the finished products. The tool used for this

operation is called chamfering tool.

4. Grooving: It is a process of providing a narrow groove on the cylindrical surface of the

workpiece. In this operation, the shape of the tool is reproduced on the workpiece.

Question 4 (a): Differentiate between Newtonian and Non-Newtonian fluids.

Solution:

S.No. Newtonian Fluids Non-Newtonian Fluids

1 Fluid which obeys Newton’s law of

viscosity is known as Newtonian fluid.

Fluid which does not obey Newton’s law

of viscosity is known as Newtonian fluid.

2 Gases and liquid which have simpler

molecular formula and low molecular

weight are generally come under this.

Material with complex structure and high

molecular weight are generally come

under this.

3 Relation between shear stress and shear

rate is linear.

Relation between shear stress and shear

rate is not linear.

4 The curve passes through the origin. The curve may or may not pass through

the origin.

5 Constant of proportionality is the familiar

dynamic viscosity.

Constant coefficient of viscosity can not

be defined.

6 It is time dependent. It may be time dependent or may not be

time dependent.

7 Examples: water, benzene, ethyl alcohol

etc.

Chocolate, ketchup, blood, saliva,

toothpaste, paint etc.

Question 4 (b): Describe the working of Pelton turbine.

Solution:

It is tangential flow impulse turbine. The energy available at the inlet of the turbine is only

kinetic energy. The pressure at the inlet and outlet of the turbine is atmospheric. This turbine is

used for high heads and low specific speed. The main parts of Pelton turbines are:

1) Nozzle and flow regulating arrangement: The amount of water striking the buckets of the

runner is controlled by providing a spear in the nozzle. When the spear is pushed forward



into the nozzle the amount of water striking the runner is reduced. And when the spear is

pushed back, the amount of water striking the runner increases.

2) Runner with buckets: It consists of a circular disc on the periphery of which a number of

buckets are fixed. The shape of the bucket is of a double hemispherical cup. Each bucket

is divided into two symmetrical parts by a dividing wall which is known as splitter. The

splitter divides the jet into two equal parts and the jet comes out at the outer edge of the

bucket. The buckets are shaped in a way that the jet gets deflected through 160o-170

o.

3) Casing: The function of the casing is to prevent the splashing of water and to discharge

water to tail race. It also acts as safeguard against accidents.

4) Breaking jet: When the nozzle is completely closed by moving the spear in the forward

direction, the amount of water striking the runner reduces to zero. But the runner due to

inertia goes on revolving for a long time. To stop the runner in a short time, a small

nozzle is provided which directs the jet of water on the back of the vanes. This jet of

water is called breaking jet.

Working: Water is transferred from a high head source through a penstock which is fitted with a

nozzle. Through the nozzle, the water flows out as a high speed jet. A needle spear moving

inside the nozzle controls the flow of water through the nozzle and at the same time provides a

smooth flow with a negligible energy loss. The available potential energy of water is thus

converted into kinetic energy before the jet strikes the buckets. The pressure of wheel is

atmospheric and constant so that energy transfer occurs due to purely impulse action. Thus the

wheel rotates in the direction of jet producing mechanical work. Speed of the turbine is kept

constant by a governing mechanism that automatically regulates the quantity of water flowing

through the runner in accordance with any variation of load.

Question 5 (a): State Zeroth law of thermodynamics.

Solution:



When a body A is in thermal equilibrium with body B and also separately with body C, then B

and C also will be in thermal equilibrium with each other.

This law helps us to measure thermodynamic property called temperature. A reference system

i.e. thermometer is brought in contact separately with two systems and if the thermometer shows

the same reading in both the cases, the two systems are at same temperature and are in

thermodynamic equilibrium with each other.

Question 5 (b): Describe the work done in compressing 1 kg of air from a volume of 0.15 m3

at a pressure of 1 bar to a volume of 0.05 m3 when the compression is (i) Isothermal, (ii)

Adiabatic

Solution:

Given,

Mass of air = 1 kg,

Initial volume of air, V1 = 0.15 m3, Initial pressure of air, P1 = 1 bar = 10

5 N/m

2,

Final volume of air, V2 = 0.05 m3

1. Isothermal work done:

We know isothermal work done is given as

kJJW

V

VVpW

473.1618.1647315.0

05.0ln15.010

ln

5

21

1

21121



Negative work means work is done on the system.

1. Adiabatic work done:

1

221121

VpVpW

2211 VpVp 5

4.1

4.15

2

112 1065.4

05.0

15.010

V

Vpp

kJJW 5.2020500

4.0

232.015.010

14.1

05.01065.415.010 555

21

Negative work means work is done on the system.

Question 6 (a): Differentiate between natural and artificial draught.

Solution:

S.No. Natural Draught Artificial Draught

1 Initial cost is high Initial cost is not so high

2 Natural draught cannot be controlled Artificial draught is better in control

3 In natural draught rate of combustion is

low

In artificial draught rate of combustion is

very high

4 In the natural draught, low-grade fuel

cannot be burnt properly

In the artificial draught low-grade fuel can

be used

5 Natural draught is considerably affected by

the atmospheric temperature

Artificial draught is not affected by the

atmospheric temperature

6 Natural draught has lower efficiency Artificial draught has higher efficiency.

7 In natural draught, fuel consumption is

more

In artificial draught fuel consumption is

less

Question 6 (b): What are the requirements of good boiler?

Solution

A good boiler should have the following characteristics:

1. The boiler should have maximum steam generation rate with

minimum fuel consumption.

2. It can be started or stopped quickly.

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3. Its initial cost, running and maintenance cost should not be high.

4. Its erection time should be less, and its parts should be easily dismantable.

5. The boiler should have positive controls and safety apparatus.

6. It should have a high rate of heat transfer and better combustion efficiency.

7. It should be able to accommodate the load variation.

8. It should occupy less floor space.

9. It should be trouble free and require less attention and less maintenance.

10. It should be free from manufacturing defects.

11. Mud should not get deposited on the heating surface. Soot or scale should not be

deposited on the tubes.

12. All parts of the boiler should be accessible for cleaning and inspection.

13. It should conform to Indian Boilers Regulations Acts.

Question 7: Describe the expression for thermal efficiency of Otto cycle. Calculate the value

for a compression ratio of 8.

Solution:

Thermal efficiency

Since the processes 1-2 and 3-4 are adiabatic, no heat transfer takes place during these processes.

The heat transfer is limited to addition of heat during the constant volume process and rejection

of heat during constant volume process 4-1.

Consider 1 kg of air

Heat added during process 2-3 = 23 TTcv

Heat rejected during process 4-1 = 14 TTcv



Therefore, Work done = Heat added - Heat rejected = 1423 TTcTTc vv

Thermal efficiency, η = Wok done/Heat supplied,

23

1423

TTc

TTcTTc

v

vv

=

23

141TT

TT

Now compression ratio V1/V2 = expansion ratio V4/V3 = r

Also, r = (Swept volume + clearance volume)/clearance volume

For ideal gas pv = RT and pvγ = constant

Considering isentropic processes 1-2 and 3-4, we have 1

2

1

1

2

V

V

T

Tand 1

1

3

4

4

3

rV

V

T

T, So 1

12

rTT and 1

43

rTT

Hence, substituting

11

14

14

1

1

1

4

14 1111

rrTT

TT

rTrT

TT

Note that the thermal efficiency of Otto cycle is a function of compression ratio and ratio of

specific heat. As specific heat is assumed to be a constant for any working fluid, the efficiency is

increased by increasing the compression ratio. Further, the efficiency is independent of heat

supplied and pressure ratio. The use of gases with higher specific heat value would increase

efficiency of Otto cycle.

Value of efficiency for compression ratio of 8 can be calculated by substituted the value of

compression ratio in efficiency formula

%46.565646.0435.01297.2

11

8

11

11

14.11

r

Question 8: Write short notes on any two:

a) Stress strain diagram of ductile material

b) Use of Vernier Calliper

c) Boiler mountings

d) Working of 2 stroke engine

Solution:

a) Stress strain diagram of ductile material

Suppose that a metal specimen be placed in tension-compression-testing machine. As the axial

load is gradually increased in increments, the total elongation over the gauge length is measured

at each increment of the load and this is continued until failure of the specimen takes place.

Knowing the original cross-sectional area and length of the specimen, the normal stress σ and the

strain ε can be obtained. The graph of these quantities with the stress σ along the y-axis and the



strain ε along the x-axis is called the stress-strain diagram. The diagram shown below is that for

a medium-carbon structural steel (Mild steel).

The various important points achieved in this curve are discussed below:

Point A:The elastic limit is the limit beyond which the material will no longer go back to its

original shape when the load is removed, or it is the maximum stress that may be developed such

that there is no permanent or residual deformation when the load is entirely removed.

Point B: It represents upper yield point of the material. It is the point where material starts

yielding or elongation. After this point the curve is no longer a straight line. After this point, the

material undergoes more rapid deformation. This point gives the yields strength of the material.

Yield stress is defined as the stress after which material extension takes place more quickly with

no or little increase in load.

Point C: It represents the lower yield point of the material. It is point after which material try to

regain its strength.

Point D: It represents the ultimate strength of the material. It is the maximum stress value that

material can withstand. It is the point of interest for design engineers. This ultimate strength is

referred as the tensile strength of material.

Point E: It represents breaking point. It is the point occurred after maximum deformation. The

stress associates with this point known as breaking strength or rupture strength.

b) Use of Vernier Calliper

It uses two scales one fixed and the other movable, and then the difference between these two

scales can be utilized, to enhance the accuracy of measurement. It consists of two scales, main



scale and vernier scale. Main scale is engraved on solid L-shaped frame and vernier scale slides

on the main scale. Vernier scale, which slides on the main scale, has a movable jaw, which slides

on the frame. The movable jaw can be locked at any desired position with the help of locking

screw. The work to be measured is kept between the jaws for measuring the outside dimensions.

Inside dimensions can be measured using inside jaws.

Least count = Smallest division on main scale/Total number of divisions on Vernier scale

= 1/10 = 0.1mm.

Following are the important parts of Vernier calipers:

1. Outside jaw: Used to take external measures of objects.

2. Inside jaws: Used to internal measures of objects.

3. Depth probe: Used to measure the depth of objects.

4. Main scale.

5. Vernier scale

Application of Vernier caliper:

It is used for both internal and external measurements. It is generally used by closing the jaws on

to the work and taking the reading from the main as well as the vernier scale. To obtain the

reading, the number of divisions on the main scale is read off. The vernier scale is then examined

to determine which of its division coincide or most coincident with a division on the main scale.

c) Boiler mountings

There are different fittings and devices which are necessary for the operation and safety of boiler.

These devices are mounted over boiler shell. The various boiler mountings are as follows:

1. Water level indicator: Its function is to indicate the level of water in the boiler constantly.

It is also called water gauge. Normally two water level indicators are fitted at the front



end of every boiler. When water is being heated in the boiler transforms into steam the

level of water in the boiler shell goes on decreasing. For proper working of boiler. Water

must be kept at certain level, if the water level falls below the certain level, boiler goes on

producing steam without the addition of feed water, then great damage like crack and

leak can occur to the parts of the boiler which are uncovered from water. This can result

in the stoppage of steam generation and boiler operation.

2. Pressure gauge: Its function is to measure the pressure exerted inside the vessel. The

gauge is mounted on the front top of the shell or the drum. It is usually contracted to

indicate up to double the maximum pressure. They are of two types i.e. Bourdon tube

pressure gauge and diaphragm type pressure gauge.

3. Safety valve: The function of the safety valve is to release the excess steam when the

pressure of steam inside the boiler exceeds the rated pressure. As soon as the pressure of

steam inside the boiler exceeds the rated pressure the safety valve automatically open and

excess steam rushes out into the atmosphere till the pressure drops down to the normal

value. It is fitted on the top of the shell. As per boiler regulation every boiler must be

fitted with at least two safety valves.

4. Fusible plug: Its function is to protect the boiler against damage due to overheating for

low water level. It is fitted on the fire box crown plate or over the combustion chamber. It

consists of a hollow gun metal body screwed into the fire box crown. The body has a

hexagonal flange to tighten it into the shell. A gun metal plug separated from the metal

plug by an annulus of fusible metal. The fusible metal is protected from the fire by flange

on the hollow gun metal plug.

5. Blow-off cock: It performs two functions: (a) It may discharge a portion of water when

the boiler is in operation to blow out mud, scale or sediment periodically. (b) It may

empty the boiler when necessary for cleaning, inspection and repair.

It is fitted on the boiler shell directly or to a short branch pipe at the lowest part of the

water space. When more than one boiler is working and they drain in the same waste pipe

line, an isolating valve is necessary to prevent the discharge of one boiler, from entering

into the other.

6. Feed check valve: Its function is to control the supply of water to the boiler and to

prevent the escaping of water from the boiler when the pump pressure is less or pump is

stopped. The feed check valve is fitted in the water space of the boiler slightly below the

normal level of water.

7. Junction or stop valve: It is valve which is placed directly over a boiler and connected to

a steam pipe which carries steam to the engine. It is usually termed as stop valve.

Junction valve and stop valve are essentially the same, the larger size are called junction

valve and the smaller size are called stop valve. Its function is to regulate the flow of

steam from one steam pipe to the other or from the boiler to the steam pipe.



d) Working of 2 stroke engine

In two strokes engine the engine the cycle is completed in one revolution of the crankshaft. The

main difference between two stroke and four stroke engine is in the method of filling the fresh

charge and removing the burnt gases from the cylinder. In the four strokes engine operations are

performed by the engine piston during the suction and exhaust stroke respectively. In two stroke

engine, the filling process is accomplished by the charge compressed in crankcase or by a

blower. The induction of the compressed charge moves out the product of combustion through

exhaust port. Therefore, no piston strokes are required for these two operations. Two strokes are

sufficient to complete the cycle, one for compressing the fresh charge and other for expansion or

power stroke.

The air or charge is inducted into the crankcase through the spring loaded inlet valve when

pressure in the crankcase is reduced due to upward motion of the piston during compression

stroke. After compression and ignition, expansion takes place in a usual way.

During the expansion stroke the charge in the crankcase is compressed. Near the end of the

expansion stroke, the piston uncovers the exhaust port and the cylinder pressure drops to the

atmospheric pressure as the combustion products leave the cylinder. Further movement of the

piston uncovers the transfer port, permitting the slightly compressed charge in the crankcase to

enter the engine cylinder. The top of the piston has usually a projection to deflect the fresh

charge towards the top of the cylinder before flowing to the exhaust ports. This serves the double

purpose of scavenging the upper part of the cylinder of the combustion products and preventing

the fresh charge from flowing directly to the exhaust ports.

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BT-203, Basic Mechanical Engineering - RGPVONLINE.COM