Handbook to SSC JE Mechanical-1
Short Description
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Description
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(crb)max "l.. => chances of failure "l.. For a given beam, (crb) ex y This fibre is subjected to compression
(crb)ll ·. (crb) is independent of 'x' . If beam is subjected to transverse shear load, the bending moment varies. ·. (crb) varies. To make beam a beam of uniform strength:(i) depth is varied.
Ix ~L
d = d x
(ii)
depth should be varied parabolically. width 'b' is varied
b = b[~] x
width should be varied linearly. Consider a log, out of which a rectangle is to be cut such that it is strongest in bending. band d ~ arbitrary dimensions of rectangle
NeutralAxis is neither in tension nor in compression
d
This fibre is subjected to tension (crb) =
(crb)max Y y max
.
D ~ diameter of log (given) ·. final dimensions of strongest rectangular cross-section are
A beam offering higher moment of resistance is stronger. I-section beams are strongest as they have high section modulus. Fora giveneross-sectionalareaandmaterialsquarecross-section Shear Stresses in Beams beam is strongerthan circular cross-sectionbeam as Zgquare > Zcircle PAy 't=---
INA' b
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A-12
P A y INA:
shear force on plane of cross-section. area. distance of hatched portion from neutral axis. moment of inertia of entire cross-section about neutral
b
width.
Engineering Mechanics and Strength of Materials ~ I section:
axis.
Consider a Beam of Rectangular Cross-Section Shear stress Distribution
~
T section:
~
Square with DiagonalsVertical:
b 9
By using the above formulae, we get
r max
I----~
= "8 t avg
1'=
.. r a: y2 (parabolic variation) Badboys2 As r f(y2) o:
:. As 's' t t ~ at extreme fibres l' = 0 DEFLECTION OF BEAMS P where, 1'avg = A·
Expression for Maximum Various Cross-Sections
Shear
Stress
Across
Deflection of beams plays an important role in design of beams for rigidity criterion. The expressions of deflections are further used for determination of natural frequencies of shaft under transverse vibrations. For a cantilever beam under any loading condition deflection is maximum at free end. In simply supported beam, deflection is maximum at mid-span (when beam is subjected to symmetric loading only).
Relationship between R, q and Y A = a2
A= bd
3 2
K= 2
K= K=-
For circle,
3
d2y -dx2
4 3
in a circular cross-section
"3
4
3
"2
1'max --=1'NA
9 8
1'avg' 1'NA = =
1.125.
"3
-
1'avg·
Mxx
--
sr.,
R
e=
4
1'max =
---)- For square, circle, rectangle, 1'NA is the maximum shear stress. But in triangular cross-section, it isn't so. In triangular cross-section, 1'max =
e : slope Y : deflection R : radius of curvature
dy dx
JMxx+ C, = EI(:) --> slope equation is obtianed If + C1x + C2 = EI(y) ---)-deflection eq" Mxx
1'avg
Sign Convention ---)- Deflection upwards (+ve)
---)-e ~
+ve
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Engineering Mechanics and Strength of Materials Deflection downwards (-ve) Also,
9 (-ve
J?
Wxx
ML EI'
9max = 9B =
'W' at its mid-span.
Alr(---L-/2--~!_B------~OC
d4y __,.4 times integration to = EI dx4----r obtain deflection 'y'
load intensity
Expression for Deflection in Cantilever Beams Case I: Cantilever beam subjected to point load W at free end X
WL2 9B = 9c = 9max = 8EI'
5 WL3 Ymax = Y c = 48 EI
Case VI: Cantilever beam subjected to uniformly distributed load over half its length from fixed end.
W
Aro~o~WN/m
L
7
Yc = Ymax = 384
WL4
ill'
DC
WL3 9max = 9B = 9c = 48EI
Expressions for Deflections in Simply Supported Beam Case I: Simply supported beam subjected to pure bending.
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G4$
AMr-----:
_WL3
..
ML2 Ymax = YB = 2EI
Case V: Cantilever beam of length 'L' subjected to point load
EI d3Y ~ 3 times integration to JC Fxx = dx3 obtain deflection 'y' shear force /
A-13
Ymax=~.
Case II: Cantilever beam subjected to uniformly distributed load
-------'ltF-----C Ll2
1
W N/m
~
:G_) 1
'"
ML 2EI;
9max=9BA= ,
A
Ll2
>1
IE
-------::I~
ML2 Ymax=YC=--· 8ill
Case II: Simply supported beam subjected to concentrated point load 'W' at mid-span. W
I
1
WL4 8EI Case III: Cantilever beam subjected to uniformly varying load
Arl
Ymax=YB=
4$
~
Ll2 L
WN/m
A
L
IB
WL4 Ymax = YB = 30EI Case IV: Cantilever beam subjected to concentrated moment 'M' at free end.
J.I-----"
1
~*~C------------~1B
-L
-DB)
~~.~------------------------~>M
WL
Mmax = 9
max
=9
4; A, B
=-
WL3 Ymax = Yc = 48EI WL2
16EI
Case III: Simply supported beam subjected to uniformly distributed load
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Engineering Mechanics and Strength of Materials
A-14
5 WL4 Ymax = Yc = 384 ill
;9max = 9B = 9A =
WI}
---~~::~_~_~~~
24EI
Case IV: Simply supported beam subjected to a concentrated point load acting not at mid-span
~~----~L------~
W
1
Cm~
P
b
Shear Stress Distribution
e: angle of twist shear angle L : distance of cross-section from fixed end :
s/
o
= c
/Y
3EIL
doesn't give max. slope ~
Torsion Equation
2
Wb (a - ab)
c
doesn't give max. deflection
Stiffness of beam =
Load Max. deflection
Higher flexural rigidity is an indicative of higher stiffness of beam but lower deflection and slope. ~
L J 9 : maximum angle of twist. T = (G1 J1 + G2 J2) L Pe =
..
91 = 92 = 9
CD G1 J1 = G2 J2 Net TM = T (anti-clock) Rxn = T (clock)
1'.
2
E Imin 2
Pe : Euler's buckling load. Imin : min [Ixx and Iyy]. L, : effective length of column. L : actual length of column. L =aL e
1.
1t
Le
Shafts with Both Ends Fixed
Short Columns (fail due to crushing)
As the length of structure, chances of it failing by buckling are more.
Shafts in Parallel T
Medium Columns (fail due to buckling as well as crushing)
f= ~
4 length fixity coefficient
1 a2 (end fixity coefficient)
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Engineering Mechanics and Strength of Materials
A-16
oe : buckling stress Both Ends Both Ends Fixed and Fixed and Hinged Fixed Free Hinged (BF) (F &H) (FF) (BH)
~
1
1
a
1
-
2
J2
1 11=-
1
4
2
0.2
cr
e
n2E =-s2
S t => Pe .,l.. => buckling tendency is increased (S)sc < (SMC)< (SLC) SC : Short Column MC : Medium Column LC : Long Column For steels, if S ~ 30 => short column S > 100 => long column 30 < S ~ 100 => medium column
2 1
-
4
If remaining all other parameters are same, (Pe)BF > (Pe)FH > (Pe)BH > (Pe)FF Which of the following column is stronger?
STRAIN ENERGY METHODS
(1)
Badboys2a4 12'
1
(Pe)l (Pe)2
(2)
nr4 na4 I = -=--=-
I =-
4
2
a4 4n
n2(4)
= 4n 072 = 4n(0.72) 12·
12
_
I
I
1
RR
PE
Pc
P
-0.513
. . (2) is stronger. Rankines formula: It is a combination of Euler and crushing load. It is also known as Rankine Gordon formula.
-=-+-
Strain energy is defined as energy absorbtion capacity of the component during its functionality. Resilience is energy absorbtion capacity of the component within elastic region. Energy absorbtion capacity of a component just before fracture is known as toughness.
SE of bar = work done by load P 1 P2L cr2 crEAL Po =-- =- x AL =--. 2 2AE 2E 2 Strain energy of solid circular shaft subjected to torsion
Strain energy of bar ~
=-
where, PR = Rankine's Load PE = Crippling load by Euler's formula Pc = crushing load h PR were
cry.A = _--=__
l+a(~r where K = radius of gyrotion (minimum) a = Rankini's constant A = Area of cross - section of column Slenderness Ratio ~ Used to compare buckling loads of various columns having same material and same cross-section.
t=-
T Zp '
where T : twisting moment. Zp : polar section modulus for circular
~ =
C~)d3 2
2
SE = _!_ T9 =_!_ T L =2._(AL). 2 2 GJ 4G
x
section.
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Engineering Mechanics and Strength of Materials ~
Strain energy of hollow circular d : Inner diameter. D : Outer diameter.
x
A-17 Mxx : moment at section x-x X
section shaft.
I I
A
d K= -
r-
B~~
~;
D
~1(------7)
X
K = 0 for solid K32 (d) S~32
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Engineering Mechanics and Strength of Materials 101. IfS = slenderness ratio, then the value of's' for long column should be in range of: (a) S> 120 (b) S< 120 (c) S= 120 (d) S= 60 102. Euler's buckling formula is associated with: (a) Short column (b) long column (c) medium column (d) None of these 103. If'D' is the diameter of a circular column, then radius of gyration (K) will be given by:
104.
105.
106.
107. Badboys2
108.
109.
110.
111.
112.
D
D
(b)2 4 (c) 2D (d) 4D A beam column is described as a column which carries: (a) axialloads only (b) transverse loads (c) axial and transverse loads (d) None of these When two forces are in equilibrium, then which of the following conditions is true. (a) Magnitudes are equal (b) opposite directions (c) collinear in action (d) All of the above In case of co-planar non-concurrent forces, when EH = 0, EV = 0, then the resultant may be: (a) moment (b) couple (c) force (d) None of these When a sphere is placed on a smooth surface, then the reaction will act: (a) inclined to contact plane (b) perpendicular to contact plane (c) horizontal to contact plane (d) All of the above For aquiring equilibrium condition, How many are minimum number of coplaner and non - collinear forces required? (a) 1 (b) 5 (c) 3 (d) 4 If three co-planar and concurrent forces are acting on a rigid body at different points then the body will be in : (a) equilibrium (b) not in equilibrium (c) mayor may not be in equilibrium (d) None of these A body having a weight of 50 N is resting on a rough horizontal floor, then the force of friction acting on the body will be equal to: (a) 50N (b) lOON (c) zero (d) None of these Angle offrictiion is defined as the angle between (a) normal reaction and the resultant of frictional force and normal reaction (b) Normal reaction and frictional force (c) Force on the body and normal reaction (d) None of these The maximum inclination of the plane at which the body just starts to move is termed as : (a) Cone of friction (b) Angle of repose (c) friction angle (d) None of these (a)
A-25 113. The value of frictional force depends on (a) weight of the body (b) area of contact (c) Normal reaction (d) roughness of surface 114. The value of maximum force of friction when the body begins to slide over another body/contacting surface is known to be: (a) limiting friction (b) rolling friction (c) sliding friction (d) None of these 115. IfF = limiting friction, R = normal reaction, then coefficient offriction (u) is given as: (a)
Jl=-
R F
(b) Jl=RxF
F (d) Jl=F2R R 116. If = angle of friction, Jl = coefficient of friction, then which of the following relation is true? (a) Jl = cot (b) Jl = sin (c) Jl = tan (d) Jl = cos 117. If = angle of friction, a = angle of repose, then which of the following relation is true? (c)
Jl=-
(a)
=a
(b)
¢=-
1
a (c) = a2 (d) a= 2 118. If = angle of friction, u= coefficient offriction, then which ofthe following relation is true? (a) =COC1(Jl) (b) =tan-1(Jl) (c) =sin-1(Jl) (d) =cos-1(Jl) 119. When a block of weight w is resting on a rough inclined plane with angle of inclination being 'a', the force offriction will be equal to : (a) wsin (b) w cos (c) wtan (d) w cot 120. If Jls = coefficient of static friction, Jlk= coefficient ofkinefic friction, R = Normal reaction, then frictional force of a moving body with constant velocity will be equal to :
e e
e e
&_
(a)
Jlk
(b)
(c)
Jl~
(d) Jl~
R
R
121. During calculation of shear force, the upward forced to the left the of the section are taken as : (a) Negative (b) Positive (c) Zero (d) None of these 122. In a shear force and bending moment diagrams, Area of load diagrams provides: (a) shear force change (b) bending moment (c) shear force (d) Point of contra flexure 123. There is a cantilever beam whose length is L and it carried a point load F at its true end. Shear force at the center of the beam will be equal to: (a)
(c)
FL
2 F
(b) FL F
(d) 2
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Engineering Mechanics and Strength of Materials
A-26
124. There is a cantilver beam whose length is L and it carries a point load at its free end. Then the bending moment at the centre of the beam will be equal to: 9 (a) --FL (b) - 2 FL 5 FL FL (c) 2 (d) 8 125. A simply supported beam oflength 'I' is carrying a 4dl of intensity w/unit length. Then the bending moment at the centers ofthe beam will be equal to : (a) w z2
(b)
w/2 4
(d)
(c)
wz2 8
w/2 16
126. A simply supported beam oflength 'I' is carrying a Udl of intensity w/unit length. Then the maximum bending moment will be equal to : (a)
wz2
(b)
8
(d)
wz2 2
wz2
135. The unit of moment is: (a) N-m (c)
(a)
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(a)
T
-
J
T
r R
=-
M
(b)
-
o
-
J
141.
142.
T
Ymax
=-
143.
144.
0
(c)
J3F
F (d) 4
132. Factor of safety is the ratio of (a) breaking stress to working stress (b) ultimate stress to working stress (c) elastic limit to working stress (d) breaking stress to ultimate stress 133. Effect of a force on the body will depend upon : (a) Direction (b) Magnitude (c) Line of action (d) All of the above 134. The law of parallelogram offorces gives the resultant of: (a) Parallel forces (b) Like parallel forces (c) two coplanar concurrent forces (d) Non coplaner concurrent forces
(d) N/m4
10 3
(b) 5
2 3 (d) 5 10 Tangent of angle offriction is equal to: (a) kinetic friction (b) Limiting friction (c) Frictional force (d) coefficient of friction The coefficient of rolling resistance for a wheel of200 mm diameter which rolls on a horizontal steel roll, is 0.3 mm. The steel wheel carries a load of 600 N. The force required to roll the wheel will be equal to: (a) 90N (b) 180N (c) 45N (d) 270N The ratio oflinear stress to linear strain is given by : (a) Modulus of elasticity (b) Modulus of rigidity (c) Bulk modulus (d) Poisson's ratio the value of Poisson's ratio is always: (a) less than one (b) greater than one (c) equal to one (d) None of these Young's modulus of elasticity for a perfectly rigid bodies (c)
= --
G9 r G9 (d) - = L R L 130. Which of the following is a vector quantity (a) Energy (b) Mass (c) Angle (d) Force 131. Twoforcesof equal magnitude 'F' act an angle of 120 with each other. Then their resultant will be equal to: (a) 2F (b) F (c)
(b) N/m
136. The quantity,which is equal to rate of change ofmomentum is known to be: (a) Force (b) Acceleration (c) Impulse (d) displacement 137. If Dynamic friction = td, static friction = ts' then their relationship will be: (a) tdts (c) td = ts (d) None of these 138. When the applied force is less than the limiting frictional force, the body will : (a) start moving (b) remain at rest (c) slide backward (d) None of these 139. In comparisonto rolling friction,the value of sliding friction will be: (a) less (b) more (c) equal (d) double 140. A bodyof weight 30N rest on a horizontal floor.A gradually increasing horizontal force is applied to the body which just starts moving when the force is 9N. The coefficient of friction between the body and floor will be :
16
127. The greatest value of the Poisson's ratio is equal to: (a) 2 (b) 1 (c) 0.5 (d) 0.25 128. In S.1.system of units, the unit for strain is: (a) Pa (b) KPa (c) GPa (d) None of these 129. Which ofthe followingequation is associated for designing of shaft base on strength is given by:
N/m2
145.
IS:
(a) Zero (b) One (c) infinity (d) None of these 146. Which ofthe following is a diamensionless quantity: (a) stress (b) strain (c) Pressure (d) Modulus of elasticity 147. A cylindricalshellof diameter200 mm and wall thicknessof 5 mm is subjected to internal fluid pressure of lON/mm2. Maximum stress streas developed in the shell will be : (a) 50 Nzmm? (b) 100 Nzmm(c) 200 Nzmm? (d) 400 Nzmm148. The bulk moduless of elasticity: (a) does not increase with pressure (b) increases with pressure (c) independent of pressure (d) None of these
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Engineering Mechanics and Strength of Materials 149. To represent, stress - strain relations for a livearlyelastic homogeneous and isotropic material, minimum number of material constants required are: (a) 2 (b) 3 (c) 1 (d) 4 150. A tension member with a cross- sectionalarea of30 - mmresists a load of 60 KN. The normal stress induced on the plane of maximum shear stress will be : (a) 2 KN/mm2 (b) 4 KN/mm2 (c) 8 KN/mm2 (d) 3 KN/mm2 151. Ifelasticmodulus(E) = 12GPa, shearmodulus(G) = 50 GPa, then the value of Poisson's ration for the material will be eqaul to: (a) 0.1 (b) 0.4 (c) 0.2 (d) OJ 152. Poisson's ratio ofthe material is used in : (a) One dimensional bodies (b) two dimensional bodies (c) three dimensional bodies (d) both band c 153. Hook's Law holds good upto : (a) yield point (b) proportionalitylimit (c) breaking point (d) elastic limit 154. When a cast iron specimen is subjected to tensile test, then the percentage reduction in area will be equal to: (a) 0010 (b) 5% (c) 10% (d) 15% 155. Ifequal and opposite forces are applied to a body tending to elongate, if, then which of the following type of stress is developed? (a) twisting stress (b) compressive stress (c) tensile stress (d) shear stress 156. A 100 kg lamp is supported by a single cable of diameter 4 mm. The stress carried by the cable will be equal to : (a) 40 MPa (b) 78 MPa (c) 48 MPa (d) 88 MPa 157. The modulus of elasticity and rigidity of a material are 200 GPa and 80 GPa respectively.Then the Poisson's ratio will be equal to: (a) 1 (b) 0.55 (c) 0.75 (d) 0.25 158. If a composite bar of copper and aluminium is heated, then the stresses induced in copper and aluminium will be (a) compressive and tensile (b) bending and tensile (c) shear and bending (d) compressive and shear 159. Slowplastic deformation ofmetals under constant loadingl stress as a function of time is known as : (a) Fatigue (b) creep (c) Elastic deformation (d) Plastic deformation 160. The fatigue life ofa part can be improved by: (a) shot peening (b) coating (c) Polishing (d) carburizing 161. Flow stresses are associated with: (a) Breaking point (b) Plastic deformation (c) Fluid motion (d) Fracture stress
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A-27 162. The stress strain curve below represents for:
Strain
163.
164.
165.
166.
(a) Brittlematerial (b) Hard material (c) Softmaterial (d) ductilematerial Necking pheneomenon in stress-strain curve is observed for: (a) ductilematerial (b) brittlematerial (c) (a) and (b) both (d) None of them When a wire is stretched to double its length, then the longitudinal strain produced in it will be equal to: (a) 0.5 (b) 1 (c) 1.5 (d) 2.0 In a composite bar, the resultant strain produced will be equal to: (a) Sum of the strains produced by individual bars (b) Same as stress produced in each bar (c) Same as strain produced in each bar (d) difference of strains produced by the individual bars The total extension ofthe bar loaded as shown in figure is : lOT~
~
r
~
9T
IE 10mm "IE 10mm "IE lOmm "I
(a)
26xlO ---mm
16x 10 (b) --mm
(c)
6xl0 --mm
32xlO (d) ---mm
AE
AE
AE
AE
167. A composite bar is made up of steel and Aluminium strips each having area of cross = section on cm2 The composite bar is subjected to an axial load of 12000 N. If Esteel= 3 xE AI' then the stress in steel will be equal to: (a) 10N/mm2 (b) 20N/mm2 (c) 30N/mm2 (d) 40 N/mm2 168. If a beam is of a rectangular cross-section, then the distribution of shearing stress across a section is : (a) triangular (b) rectangular (c) Parabolic (d) Hyperbolic 169. The reaction at the prop in a propped cantilever beam subjected to U.D.L will be equal to : (a)
wI 4
(b)
3wl 8
wi wi (d) 8 6 170. Uniformalydistributedload 'WI is acting overper unit length ofa cantilever beam of Sm length. If the shear force at the midpointofbeam is 6 KN. then th value ofw willbe equalto : (a) 2 KN/m (b) 3 KN/m (c) 4KN/m (d) 6KN/m (c)
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A-28
171. The ratio of the compressive critical load for a long column fixed at both ends and a column with one end fixed and the other end being free is : (a) 2:1 (b) 4:1 (c) 8 : 1 (d) 16: 1 172. A simple supported beam PQ oflength 9 m, carries a Udl of 10 KN/m for a distance of 6 m from end P.What will be the reactions forces at P and Q. (a) 40N,20N (b) 20N,20N (c) 30N,20N (d) 80N,40N 173. A simply supported beam of 1 m length is subjected to a Udl of0.4 N/m. The maximumbending moment occuringin the beam will be: (a) O.OSN-m (b) 1N-m (c) 2N-m (d) 4N-m 174. A hollow shaft has external and internal diameters of 10em and Scm respectively. Torsional sectional modulus of the shaft will be: (a) 184cm3 (b) 384cm3 (c) 284cm3 (d) 37Scm3 175. A solid shaft of diameter 20 mm can sustain a maximum shear stress of 400 kg! cm-. The the torque transmitted by the shaft will be equal to : (a) 0.628Kg-cm (b) 62.8Kg-cm (c) 628 Kg-cm (d) 324Kg-cm 176. For designing a connecting rod, which of the following formula is utilized? (a) Rankin's formula (b) Euler's formula (c) both (a) and (b) (d) None of these 177. When a connecting rod is subjected to an axial force, then the buckling of the connecting rod may be with (a) X - axis as neutral axis (b) X - axis or y-axis as neutral axis (c) Z-axis as neutral axis (d) None of these 178. A column which is failed under the application of direct stress is known as : (a) Shortcolumn (b) mediumcolumn (c) long column (d) None of these 179. If L, = buckling load, Lc= crushing load, then which ofthe following relationship is true for long columns? 00 ~>~ ~ ~>~ (c) ~ =Lc (d) None of these 180. In case of compression numbers, they tend to buckle in which ofthe following direction? (a) Maximum cross-section (b) Neutral axis (c) Horizontalaxis (d) Minimum radius of gyration 181. Two books of mass 1 kg each are kept on a table, one over the other. The coefficient of friction on every pair of contacting surfaces is 0.3, the lower book is pulled with a horizontal force F. The minimum value ofF for which slip occurs between the two books is (a) zero (b) 1.06N (c) S.74N (d) 8.83N 182. Ifa system is in equilibrium and the position ofthe system depends upon many independent variables, the principle of virtual work states that the partial derivatives of its total potential energy with respect to each of the independent variable must be
Engineering Mechanics and Strength of Materials (a) -1.0 (b) zero (c) 1.0 (d) infinite 183. A block of mass M is released from point P on a rough inclined plane with inclination angle 9, shown in the figure below. The coefficient of friction is J..!.IfJ..!< tan 9, then the time taken by the block to reach another point Q on the inclined plane, where PQ = s, is
(a)
(c)
2s g cos 9(tan 9 -1-1) 2s gsin9(tan9-1-1)
2s g cos 9(tan 9 + 1-1)
(b)
2s gsin9(tan9+ 1-1)
(d)
184. A 1 kg block is resting on a surface with coefficient of friction 1-1 = 0.1. A force of 0.8 N is applied to the block as shown in figure. The frictional force is
08±L
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(a) zero (b) 0.8N (c) 0.89N (d) 1.2N 185. A block R of mass 100 kg is placed on a block S of mass ISO kg as shown in the figure. Block R is tied to the wall by massless and inextensible string PQ. If the coefficient of static friction for all surface is 0.4, the minimum force F (in kN) needed to move the block S is
00
Q~
~
Q~
(c) 0.98 (d) 1.37 186. A ball weighing 0.01 kg. hits a hard surfaceverticallywith a speed ofS mls and rebounds with the same speed. The ball remains in contact with the surface for 0.01 second. The average force exerted by the surface on the ball is (a) 0.1N (b) 1N (c) 8N (d) ION 187. A pin-ended column of length L, modulus of elasticity E and second moment of the cross-sectional area I is loaded centrically by a compressive load P, the critical buckling load (Pcr) is given by (a)
E1 Pcr=~ nL
rrEI
(c) Pcr=7
(b) p
c
n2E1 3L2
=--
n2E1 (d) pc =--L2
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Engineering Mechanics and Strength of Materials
A-29
188. For a circular shaft of diameter d subjected to torque T, the
194. A solid circular shaft of diameter d is subjected to a
maximum value of the shear stress is
combined bending moment M and torque T. The material property to be used for designing the shaft using the relation
(a)
.!.i_.J M2 + T2
(b) 16T
tid'
(c)
(d)
189. A 200
x 100 x 50 mm steel block is subjected to a hydrostatic pressure of 15 MPa. The Young's modulus and Poisson's ratio of the material are 200 GPa and 0.3 respectively. The change in the volume ofthe block in mm ' is (a) 85 (b) (c) 100 (d) 110 190. A uniformly loaded propped cantilever beam and its free body diagrams are shown below. The reactions are
195.
so
iillllllllll]l M(illllllilit q
R,
IJIII
5qL
(a)
196.
q
L
1II1II
s, = -8-' R2=
is
nd3
8T nd'
197.
R2
3qL qL2 -8-' M= 8
198.
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3qL 5qL qL2 (b) s, = -8-' R2= -8-' M= 8 5qL 3qL (c) R, = -8-' R2= -8- , M=O 3qL 5qL (d) Rl = -8-' R2= -8- ,M=O
199.
(a) ultimate tensile strength (Su) (b) tensile yield strength (Sy) (c) torsional yield strength (Ssy) (d) endurance strength (Se) Ifthe principal stress in a plane stress problem are crl = 100 MPa, crl = 40 MPa, the magnitude ofthe maximum shear stress (in MPa) will be (a) 60 (b) 50 (c) 30 (d) 20 The state of plane-stress at a point is given by crx= -200 MPa, cry = 100 MPa and txy = 100 MPa. The maximum shear stress in MPa is (a) 111.8 (b) 150.1 (c) 180.3 (d) 223.6 A column has a rectangular cross-section of 10 mm x 20 mm and a length of! m. The slenderness ratio ofthe column is closed to (a) 200 (b) 346 (c) 477 (d) 1000 A thin cylinder of inner radius 500 mm and thickness 10 mm is subjected to an internal pressure of 5 MPa. The average circumferential (hoop) stress in MPa (a) 100 (b) 250 (c) 500 (d) 1000 A solid steel cube constrained on all six faces is heated so that the temperature rises uniformly by LlT. Ifthe thermal coefficient ofthe material is a, Young's modulus is E and the Poisson's ratio is v, the thermal stress developed in the cube due to heating is
a(LlT)E
191. The strain energy stored in the beam with flexural rigidity EI
(a)
(l-2v)
2a(LlT)E (b)
(l-2v)
and loaded as shown in the figure is P
1+-[;
L
t
2L
.;
(c)
3EI 4p2J3
--
(c)
L--+I
~
p2J3 (a)
(b)
a(LlT)E
3a(LlT)E
P
2p2L3
--
3EI 8p2JJ
(d)
3EI 3EI 192. A rod of length L and diameter D is subjected to a tensile load P. Which ofthe following is sufficient to calculate the resulting change in diameter? (a) Young's modulus (b) Shear modulus (c) Poisson's ratio (d) Both Young's modulus and shear modulus 193. The transverse shear stress acting in a beam ofrectangular cross-section, subjected to a transverse shear load, is (a) variable with maximum at the bottom of the beam (b) variable with maximum at the top of the beam (c) uniform (d) variable with maximum of the neutral axis
(1- 2v)
(d)
3(l-2v)
200. For a long slender column of uniform cross-section, the ratio of critical buckling load for the case with both ends clamped to the case with both ends hinged is (a) 1 (b) 2 (c) 4 (d) 8 201. A cantilever beam oflength L is subjected to a moment M at the free end. The moment of the inertia ofthe beam crosssection about the neutral axis is I and the Young's modulus is E. The magnitude ofthe maximum deflection is (a) (c)
ML2 2EI
(b)
ML2 EI
(d)
4ML2 EI
202. The maximum allowable compressive stress corresponding to lateral buckling in a discretely laterally supported symmetrical I-beam, does not depend upon (a) modulus of elasticity (b) radius of gyration about the minor axis (c) span/length of the beam (d) ratio of overall depth to thickness of the flange
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Engineering Mechanics and Strength of Materials
A-30 203. The number of strain readings (using strain gauges) needed on a plane surface to determine th principal strains and their directions is (a) 1 (b) 2 (c) 3 (d) 4 204. The buckling load in a steel column is (a) related to the length (b) directly proportional to the slenderness ratio (c) inversely proportional to the slenderness ratio (d) non-linearly to the slenderness ratio 205. Ifmoment M is applied at the free end of centilever then the moment produced at the fixed end will be (a) M (b) Ml2 (c) 2M (d) zero 206. A thin walled cylindrical pressure vessel having a radius of 0.5 m and wall thickness 25 mm is subjected to an internal pressure of700 kPa. The hoop stress developed is (a) 14 MPa (b) 1.4 MPa (c) 0.14MPa (c) 0.014MPa 207. If J.l = Poisson's ratio G = Modulus of rigidity, K = bulk modulus then (a)
Badboys2(c)
3K-G J.l= 2G+6K
(b)
K-G J.l= 2G+6K
(d)
K-G u= G+3K
3K-2G
u= 2G + 6K
208. For the shear force diagram shown in figure, the loaded beam will be .------.14t
9t
~4m""""I----
4t
209. When a column is fixed at both ends, corresponding Euler's critical load is 2n2EI
(a)
n2EI L2
(b)
(c)
3n2EI L2
(d)
L2 4n2EI L2
210. Consider the beamAB shown in figure below. PatAC ofthe beam is rigid. While part CB has the flexural rigidity EI. Identify the current combination of deflection at end Band bending moment at end A respectively
I:
(
c L
(a)
pC 2PL
(c)
8PC 2PL
B
L ----.j
.j4
(b)
3EI'
3EI'
(d)
pC PL 3EI'
8Pr! PL 3EI'
211. In a stressed body, an elementary cube of material is taken at a point with its faces perpendicular to X and Y reference axes. Tensile stresses equal to 15 kN/cm2 and 9 kN/cm2 are observed on theses respective faces. They are also accompanied by shear equal to 4 kN/cm2. The magnitude of the principal stresses at the point are (a) 12 kN/cm2 tensile and 3 kN/cm2 tensile (b) 17 kN/cm2 tensile and 7 kN/cm2 tensile (c) 9.5 kN/cm2 compressive and 6.5 kN/cm2 compressive (d) 12 kN/cm2tensile and 13 kN/cm2 tensile 212. Under torsion, brittle materials generally fail (a) along a plane perpendicular to its longitudinal axis (b) in the direction of minimum tension (c) along surfaces forming a 45° angle with the longitudinal
axis (d) not in any specific manner 213. A simply supported beam ofspan L and flexural rigidity EI, carries a unit point load at its centre. The strain energy in the beam due to bending is (a)
r!
C
A
f
(a)
48EI
(b)
192EI
(d)
16EI
C (c)
C
96EI
214. The design ofa eccentrically loaded column needs revision when 1.5 tim
(c)
Ai.__
~i~10_t
1.5 tim
(d)
(a)
fe' + f; RH.S. then it is a locked chain L.HS. = RH.S. then it is a kinematic chain L.HS. < RH.S. then it is an unconstrained chain
Types: (i) (ii) (iii)
Beam engine Locomotive coupling rod Watts indicator ©©©©mechanism
(b) Single slider crank chain
GRUBLER'S CRITERION In a mechanism total no. of degrees of freedom is given by F = 3(n-l)-2j where n is no. oflinks and j = no. ofjoints (simple hinges) most ofthe mechanism are constrained so F = 1 which produces 1 = 3(n-l)-2j => 2j - 3n + 4 = 0 this is called Grubler's criterion. If there are higher pairs also no. of degrees of freedom is given by F = 3(n-l)-2j-h where h = no. of higher pairs. Also known as Kutz Bach criterion to determine the number of degree of freedom. This statement says that if the higher pairs are present in the mechanism like as slider crank mechanism or a mechanism in which slipping is possible between the wheel and fixed links. Higher pair: When the two element of a pair have a line or point contact when relative motion takes place and the motion between two elements is partly turning and partly sliding. E.g. Cam and follower, bale and bearing, belt and rope drive etc. Number of degree of freedom (movability): The number of independent parameters that define its configuration. The number of input parameters which must be independently controlled in order to bring the mechanism into useful engineering purpose.
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GRASHOF'S CRITERIA Grashof's criteria is applied to pinned four bar linkages and states that the sum ofthe shortest and longest link of a planar four-bar linkage cannot be greater than the sum of remaining two links if there is to be continuous relative motion between the links.
Types : (i) (ii) (iii) (iv) (v)
Pendulum pump Oscillating cylinder engine Rotary internal combustion engine Crank and slotted liver quick return mechanism Whitworth quick return mechanism
(c) Double Slider crank chain Types:
(i) (ii) (iii)
Elliptical trammel Scotch-yoke mechanism Oldham's coupling
Klein's Construction: It is defined as a graphical method to achieve the magnitudes of velocity and acceleration oflinks as well as required points on the links. Klein's construction is drawn on the configuration diagram. And It does not need to be drawn two or three different diagrams. Limitation: It is applicable to slider crank mechanism only.
INSTANTANEOUS CENTRE A point located in the plane (of motion of a body) which has zero velocity. The plane motion of all the particles ofthe body may be considered as pure rotation about the point. Such a point is called the instantaneous centre ofthe body. Ifthere are three rigid bodies in relative planar motion and share three instantaneous centre, all lie on the straight line, called Kennedy's theorem. Instantaneous axis of rotation: The axis passing through the instaneous centre of the body at right angles to the plane of motion is called instantaneous axis of rotation. Axode: The instantaneous centre changes every moment, its locus is called centrods, and the surface generated by the instantaneous axis is called the axode.
Methods to Locate Instantaneous Centre Locating the instantaneous centre of a body depends on the situation given. Following are some examples: (i)
Fig. Linkage shown in Fig. 1 is Grashoftype if
The instantaneous
Va along the (J)
perpendicular to the direction of velocity Va at point A on a rigid body shown in Fig.
s+l
)
>
?> :>
p
Single V-Butt joint The tensile strength of the single V-Butt joint is given by P t x I x at where throat thickness or thickness of thinner plate at allowable tensile stress for weldment in N/mm2 replace at by ac in case it is designed for compression I = Length of weld Double V-Butt joint: Tensile strength for doub V-butt joint shown in Fig. is given by P t x I x at where t = tl + t2 P = (tl + t2) x I x at => where I = length of weld = width of plates t 1 = throat thickness at the top
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Theory ofMachines and Machine Design
A-56
t2 = throat thickness at the bottom crt = allowable tensile stress for weldment inN/mm2
pi -
j~ -
t
---Inn -t -:t p
(ii) Rectangular sunk key: A rectangular sunk key is shown in Fig. The width of the key is equal to ~ and 4 thickness is equal to ~ . ,._~ 12
d
w=T
t=_!'
12 1 = 1.5 d
Double V-Butt joint Metric Thread There are various forms of screw threads, metric thread is an Indian Standard (I.S.0) thread having an included angle of 60°, these are two types, coarse threads and fine threads. For a particular value of diameter, coarse threads have large pitch and lead as compared to fine threads. Coarse threads are more in strength and chances of thread shearing and crushing is very less. They are preferred for vibration free applications as they offer less resistance to unscrewing. Fine threads give better adjustment in fitment and are used where high vibrations take place as they offer high resistance to unscrewing. Fine threads are designated as Md x P for example M50 x 5 which indicates an isometric fine thread which has nominal diameter of 50 mm and pitch 5. While in case of coarse threads only Md is mentioned for example M50. Todesignate tolerance grade we use the values of each tolerances like 7 for fine grade, 8 for normal and 9 for coarse grade. For example a bolt thread of 6 mm size of coarse pitch and with allowance on threads and normal tolerance grade is designated as M6-8d.
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KEYS Toprevent the relative motion of the shaft and the machinery part connected to it we use a piece of mild steel called key. Keys are temporary fastenings and are subjectedto considerable crushing and shearing stresses. Different types of keys are listed below. (a) Sunk keys: These keys are designed in such a way that they are half way in the key way of the hub of pulley and half in the key way of the shaft. There are basically five types of sunk keys listed as following: (i) Square sunk key: A square sunkkeyis shownin Fig. If d is the diameter of the shaft width of the square sunk key is equal to d/4 and the thickness is same as width. -f--t
1+-'fJ""+I I
_t.
----
J_~~ _L~2 Shaft cross-section d
w=t=T
length of the key ( = 1.5 d
I
Square sunk key
Rectangular sunk key (iii) Gib-head key: Cross-sectional view of a Gib-head sunk key is shown in Fig.
w=_Q_
4
t=2w=_Q_ 3 6 I =1.5d where d = diameter of shaft
Gib head sunk key (iv) Parallel sunk key: It is a taperless key and may be rectangular or square in cross-section. It is used where the pulley, gear or other mating piece is required to slide along the shaft. (v) Feather key: A special type of parallel key which transmits a turning moment and also permits axial movement. (b) Tangent keys: These keys are fitted in pair at right angles, each key is to withstand torsion in one direction only. Tangent keys are used for heavy duty applications. A crosssectional view of a tangent key is shown in Fig.
Tangent key (c) Saddle keys: These are taper keys fitted in key way and designed such that it is flat on the shaft. (d) Wood ruff keys: This key is made of a piece from a cylindrical disc of segmental cross-section. (e) Round keys: These keys are circular in cross-section and are fitted partly into the shaft and partly into the hub. (f) Splines: When splines are integrated with the shaft which finally fits into the keyways of the hub. These are stronger than a single keyway. Design of Keys A key may fail due to shearing and crushing, it is equally strong in shearing and crushing if following condition satisfies.
Badboys2
Theory of Machines and Machine Design
(iii) Combined loading: When a shaft is subjected to combined
w=~ where
w= t= O"c = r=
A-57
2~ width of the key thickness of the key permissible crushing stress permissible shearing stress
twisting moment and bending moment, then the shaft is designed on the basis of maximum normal stress theory and maximum shear stress theroy and larger size is adopted. According to maximum shear stress theory (Guest's theory) the maximum value of shear stress in the shaft is given by 1 I 2 2 \J (o.,) + 4~ 2
SHAFTS
~max = -
Shafts are used to transmit power from one place to another, these are normally of circular cross-section. Mild steels are hot rolled and then finished to actual size by turning, grinding or cold drawing to manufacture shafts. Alloy steels with composition of nickel, chromium and vanadium is also used to impart high strength. The cold rolled shafts are stronger than hot rolled shafts, but with higher residual stresses.
=
r
max
~
Types of Shafts There can be two types of shafts (a) Transmission shaft such as counter shafts, line shafts, over head shafts, etc. (b) Machine shaft such as crank shaft Design of Shaft Shafts are designed on the basis of (a) Strength: On the basis of strength of the shaft material we design a shaft considering three types of stresses induced in the shafts. (i) Torsionalload (ii) Bending load (iii) Combined torsional and being loads (i) Torsional load: If the shaft is subjected to pure torsional load then torsional shear stress is given by
16Txdo n do - dj
( 4
4) ,
N/m2
=>
n T -- -x~xdo 16
where
do = outer diameter of shaft in m
3
d, = inner diameter of the shaft in m (ii) Bending load: When the shaft is subjected to a bending moment only, then the value of stress induced is given by O"b =
32~ for solid shaft nd where O"b = bending stress and for a hollow shaft 32M
J2
'M2+T2
\j
nd3 M2+T2 = __ ~ 16 max
r, =
nd3 16~max
where T, =equivalent twisting moment = ~ M2 + T2
Now, According to maximum normal stress theory (Rankine's theory the maximum normal stress in the shaft is given by O"b(max) =
-1 O"b + -1 ~ (O"b) 2 + 4~ 2 2 2
,,~_)~ ::3 [~(M M +
_!_ (M + ~M2 + T2 ) = nd3
2
r = 16; N/m2 for solid shaft
~=
= ~ nd3
=>
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nd where d = shaft diameter in m T = torsional moment in N-m For a hollow shaft
32M + 4 x (16 T nd3 nd3
_!_ 2
32
Me=
where
+
T2)]
O"b(max)
nd3 --xO"b(max)
32 Me = equivalent bending moment = ~ (M + ~M2 + T2 )
(b) Design of Shafts on the basis of rigidity and stiffness A shaft of small diameter and long length the maximum deflection is expressed as (5max S; 0.75 mm/length in meters also (5max S; 0.06 Lin mm where L = distance between load and bearings in m. These deflections are minimised by using support bearings. If gear is mounted on the shaft then 3 (5max S;
f
where f = gear face with mm If shaft crosses these limits then deflections are minimized by using self aligning bearings. SPUR GEARS When two parallel and coplanar shafts are connected by gears having teeth parallel to the axis of the shaft, its arrangement is called spur gearing, and gear used is spur gear. While designing spur gear it is assumed that gear teeth should have sufficient
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Theory of Machines and Machine Design
A-58 strength so that they do not fail under static as well as dynamic loading.
Lewis Equation Lewis equation is used to determine the beam strength of a gear tooth. Each tooth is considered as a cantilever beam which is fixed at the base. The normal force acting on the tip of the gear is resolved into radial and tangential component as shown in Fig. The radial component induces a direct compress stress of small value, so it is ignored. Tangential component FT is duces a bending stress that can break the tooth.
-__ r Tangent to the
=
0.124 _ 0.684 T .
Y for 20° full depth mvolute system
T
0.841 Y for 20° stub system = 0.175 - -T The permissible working stress (O"w)in the Lewi's equation depends upon the material for which, allowable static stress (0"0) may be determined. Allowable static stress is the stress at the elastic limit of the material also known as basic stress. Barth Formula: According to Barth formula, the permissible working stress is given by O"w= 0"0 X C, Cv = velocity factor where
Cv = --- 4.5
- - - base circle
0.912
= 0.154 - --
4.5+v
. at for care full y cut gears operatmg velocities upto 12.5 mls
-- 3
.
.
Maximum value ofbendmg stress
My
= O"w= -
... (1)
I where M is maximum bending moment (i.e. at BC) M= Ftxh
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M
from (1)
=
h
O"wI Y
. Now for y for beam of height t
c. lor
. at or dimary cut gears operatmg
velocities upto 12.5 mls
Static Tooth Load ... (2)
M
Ft= -
3+v
Beam strength or static tooth load is given by Fs = O"eb P, Y = O"eb 1t my where O"e= Flexural endurance limit For safety against breakage Fs > FD where FD is the dynamic tooth load which takes place due to inaccurate tooth spacing, irregularities in profiles and tooth deflection under the effect of load.
BEARINGS
t
= -
A machine element which permits a relative motion between the contact surfaces of the members while carrying the load. It supports journal. The bearings are mainly classified as (a) Sliding contact bearings (b) Rolling contact bearings
2
Sliding Contact Bearings
= O"wX bt3 = O"wbt2
F
12 x ~ 6h 2 Now if circular pitch is P, then we can represent t, and h in t
terms of Peas
O"wbK~ Pc
6K2
= O"wbPc K~ 6K2
K2
Let
y=_l_
where
F t = o"wb Pc Y Lewis Equation y = form factor called Lewis form factor b = width of gear face
6K2
Y for 14!.: composite and full depth involute system
2
In these bearings, the sliding takes place along the surfaces of contact between the moving element and the fixed element. These are also known as plain bearings. According to the thickness oflayer ofthe lubricant between the bearing and the journal, sliding contact bearings can be classified as (a) Thick film bearings: Bearings in which the working surfaces are completely separated from each other by the lubricant. These are also called a hydrodynamic lubricated bearings. (b) Thin film bearings: In these bearings although lubricant is present, the working surfaces partially contact each other atleast part of the time. Such type of bearings are also called boundary lubricated bearings. (c) Zero film bearings: Bearings which operate without any lubricant are known as zero film bearings. (d) Hydrostatic bearings: Bearings which can support steady loads without any relative motion between the journal and the bearings because there is externally pressurized lubricant between the members.
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Theory of Machines and Machine Design
A-59
Hydrodynamic Journal Bearing Terminology Cross-sectionalview of a hydrodynamicjournal bearing is shown in Fig.
where K is a factor for end leakages for
0.75 <
d (ix) Short and long bearings: Short and long bearings are decided on the basis of the ratio lid. [
If
- < 1 then bearing is said to be short d [ -
Journal I I I I
d
C2=R-r=--=2
2
clearance ratio: Ratio between diametral clearnace to journal diameter .
.
Diametral clearance ratio
C1
= -
d (iv) Eccentricity: It is the radial distance between the centre (0) of the bearing and the displaced centre (0') of the bearing under load. Eccentricity is denoted bye. (v) Eccentricity ratio (Attitude): Ratio of eccentricity to radial clearance is called eccentricity ratio. e E=
-
C2 (vi) Sommerfield number: A dimensionless number used in design of bearings. It's value is given by Sommerfield number
=
1 bearing is called square bearing
[
I
Hydrodynamic journal bearing Diameter of the bearing = D = 2R Diameter of the journal = d = 2r Length of the bearing = l Terminologies associated with a hydrodynamic journal bearing are defined as following. (i) Diametral clearance: Difference between the diameter of bearing and journal is called diametral clearance C1 = D-d (ii) Radial clearance: It is the difference between the radii of bearing and journal D-d C1
Badboys2 (iii) Diametral
!:_ < 2.8, K = 0.002
=
(z;)(~J
- > 1 then bearing is said to be long d (x) Heat generation and rejection in bearing: Due to fluid friction and solid friction heat is generated in the bearing which can be expressed as Qgen= J,.tWV N-m/s where W = load on the bearing V = rubbing velocity in mls Heat rejection is given by Qrejection = Kh A (t, - ta) JIS where Kh = heat dissipation coefficient in W/m2/C A = prejected area of the bearing tb = bearing surface temperature ta = ambient temerature In case of pressure fed bearings ift, is the inlet temperature of oil and to is outlet temperature of the oil then heat rejection is given by Qrejection = pCoil(to- ti) where p = density of oil Coil= specific heat of oil Bearing Characteristic Number The factor ZN is known as bearing characteristic number and P it is a dimensionless number. where Z = Absolute viscosity of the lubricant in kg/m-s N = Speed ofjournal in r.p.m. P = Bearing pressure on the projected bearing area in Nzmm-'
W
P
where N = Journal speed in r.p.m., Z = lubricant viscosity, P = bearing pressure normally we take its value as 14.3 x 106 (vii)Critical pressure in journal bearing: The pressure at which the oil film breaks and metal to metal contact takes place is known as critical pressure. It's value is given by
= -,
.
W = Load on the Journal
[·d The variation of coefficient offriction with respect to the bearing characteristic number is shown in Fig.
t
r-:------,_ Thin film or boundary lubrication 1
(unstable)
3 c
c;J2 ( [+[ d J NI mm
PZN ( d - 4.75 x 106 where
2
N = Journal speed in r.p.m. Z = Absolute viscosity of the lubricant (viii)Coefficient of friction: Coefficient of friction can be expressed as
o ""B
E (5
c
Partial lubrication
"~-+---~--
~
o K ZN P
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Theory of Machines and Machine Design
A-60
Variation of coefficientof friction with the bearing characteristic number (
z: J
Rolling Contact Bearings Bearing which operate on the basis of principle of rolling, i.e. the contact between the bearing surfaces is rolling are known as rolling contact bearings. These are also called anti friction bearings as they offer low friction. Mainly there are two types of rolling contact bearings. (i) Ball bearing (ii) Roller bearing Average life (Median life) of a bearing: It is the number of revolutions or number of hours at a constant speed that 50% of a batch of ball bearing will complete or may be exceed and 50% fail before the rated life is achieved. It is denoted by L5o. Life a
1 (Load)? Dynamic load rating: Value of radial load which bearing can suffer for I million revolutions of inner ring with only 10% failure is known as dynamic load rating or basic dynamic capacity or specific dynamic capacity. Rating Life L ~ (~
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Disc clutch Frictional torque acting on an element dr is given by T, = 2n /-! pr2 dr where p = axial pressure intensity /-! = coefficient of friction For uniform pressure the intensity of pressure is given by P=
P = load C = dynamic basic load rating
J
1/3
IfN is r.p.m. the Life in hours is given by CJ3 106 L= ( x-- hours P 60N P = Cx
dr
J'
P= C (-I L
or
surfaces in contact. Due to friction heat is generated which should be dissipated rapidly. Friction clutches are further classified into (a) Disc or plate clutch (b) Cone clutch (c) Centrifugal clutch (a) Disc clutch: Cross-sectional view of a disc clutch is shown in Fig.
6 ]1/3
W
n(r?-d)
where
rl = external radius of the surface r2 = internal radius of the surface W = axial value of thrust which holds the frictional surfaces together. Total torque transmitted is given by
[rfrf-d- d ]
T = ~ /-!W cosec a 3 (b) Cone clutch: Total torque transmitted in the cone clutch is given by
_!.Q_
[ 60NL
CLUTCHES Clutch is a connection between the driving and driven shafts with the provision to disconnect the driven shaft instantaneously without stopping the driving shaft. Main functions of cluthces are to stop and start the driven member without stopping the driving member, to maintain torque, power and speed, and to eradicate the effects of shocks while transmitting power. Clutches are classified into two types: (1) Positive clutches: These are used where there IS requirement ofpositive drive for examplejaw or claw clutch. (2) Friction clutches: Friction clutch transmits the power by friction without shock. It is used where sudden and complete disconnection of two rotating shafts are necessary, and the shafts are in axial alignment. The power transmission takes place due to two or more concentric rotating frictional
Tcme= ~ /-!W [r? - r~] cosec a 3 r2 - r2 1 2 where
a = semi-angle of frictional surfaces with the
clutch axis. (c) Centrifugal clutch: Total torque transmitted in case of centrifugal clutch is given by T = /-! (C - S) r, x n where C = Spring force acting on shoe = mrwm = mass of shoe r = distance of centre of gravity of shoe from centre w = angular velocity of rotating pulley in rad/s ri = inside radius of pulley rim S = Inward force due to spring-m (W12)r 3 Wl= -w 4 n = number of shoes C - S = mr w- -
_2._ mrw- = }_ mr w16
16
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Theory of Machines and Machine Design
I···.. 1.
EXERCISE
A rotating disc of 1 m diameter has two eccentric masses of 0.5 kg each at radii of 50 mm and 60 mm at angular positions of 00 and 1500, respectively. A balancing mass of 0.1 kg is to be used to balance the rotor. What is the radial position of the balancing mass?
2.
(a) 50 mm (b) 120 mm (c) 150mm (d) 280mm The number of degrees of freedom of a planar linkage with 8 links and 9 simple revolute joints is (a) 1 (b) 2
3.
Match the items in Column I and Column II.
(c)
3
...,
A-61
(c) (d)
Geneva mechanism is an intermittent motion device Grubler's criterion assumes mobility of a planar mechanism to be one 10. Mobility of a statically indeterminate structure is (a) ::;;-1 (b) zero
(c) 1 11. A double-parallelogram
(d) ?:2
mechanism is shown in the figure. Note that PQ is a single link. The mobility ofthe mechanism is P Q
(d) 4
Column I
ColumnII
Higher kinematic pair 1. crubler's equation Q. Lower kinematic pair 2. Line contact R Quick return mechanism 3. Euler's equation S. Mobility of a linkage 4. Planar 5. Shaper 6. Surface contact (a) P-2, Q-6,R-4, S-3 (b) P-6, Q-2, R-4, S-l (c) P-6, Q-2, R-5, S-3 (d) P-2, Q-6, R-5, S-l Match the items in Column I and Column II. P.
12.
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Column I P.
Q.
5.
R S. (a) (c) The
(a) (c) 6.
7.
8.
9.
Addendum Instantaneous centre of velocity Section modulus Prime circle P-4, Q-2, R-3, S-l P-3, Q-2, R-1, S-4 number of inversions for 6
4
ColumnII 1. Cam 2.
Beam
3.
Linkage
Gear (b) P-4, Q-3, R-2, S-l
4.
13.
(d) P-3, Q-4, R-1, S-2 a slider crank mechanism is (b) 5
(d) 3
For a four-bar linkage in toggle-position, the value of mechanical advantage is (a) zero (b) 0.5 (c) 1.0 (d) infinite The speed of an engine varies from 210 rad/s to 190 rad/s. During a cycle, the change in kinetic energy is found to be 400 N-m. The inertia ofthe flywheel in kg-m2 is (a) 0.10 (b) 0.20 (c) 0.30 (d) 0.40 The rotor shaft of a large electric motor supported between short bearings at both deflection of 1.8 mm in the middle of the rotor. Assuming the rotor to be perfectly balanced and supported at knife edges at both the ends, the likely critical speed (in rpm) of the shaft is (a) 350 (b) 705 (c) 2810 (d) 4430 Which of the following statements is incorrect? (a) Gashoffs rule states that for a planar crank-rocker four bar mechanism, the sum of the shortest and longest link lengths cannot be less than the sum of remaining two link lengths (b) Inversions of a mechanism are created by fixing different links one at a time
14.
15.
16.
(a) -1 (b) zero (c) 1 (d) 2 A circular object of radius r rolls without slipping on a horizontal level floor with the centre having velocity V. The velocity at the point of contact between the object and the floor is (a) zero (b) Vin the direction of motion (c) Vopposite to the direction of motion (d) Vverticallyupward from the floor For the given statements: I. Mating spur gear teeth is an example of higher pair. Il. A revolute joint is an example oflower pair. Indicate the correct answer. (a) Both I and IIare false (b) I is true and II is false (c) I is false and II is true (d) Both I and II are true In a mechanism, the fixed instantaneous centres are those centres which (a) Remain in the same place for all configuration of mechanism (b) Large with configuration of mechanism (c) Moves as the mechanism moves, but joints are of permanent nature (d) None of the above Maximum fluctuation of energy is the (a) Ratio of maximum and minimum energies (b) sum of maximum and minimum energies (c) Difference of maximum and minimum energies (d) Difference of maximum and minimum energies from mean value In full depth 114 degree involute system, the smallest number of teeth in a pinion which meshes with rack without interference is (a) 12 (b) 16 (c) 25 (d) 32
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Theory of Machines and Machine Design
A-62
17. The two-link system, shown in the figure, is constrained
26.
to move with planer motion. It possesses y
) o
18.
19.
20.
22.
23.
24.
25.
x
(a) 2 degrees of freedom (b) 3 degrees of freedom (c) 4 degrees of freedom (d) 6 degrees of freedom Ifthe ratio of the length of connecting rod to the crank radius increases, then (a) primary unbalanced forces will increase (b) primary unbalanced forces will decrease (c) secondary unbalanced forces will increase (d) secondary unbalanced forces will decrease In a cam mechanism with reciprocating roller follower, the follower has a constant acceleration in the case of (a) cycloidal motion (b) simple harmonic motion (c) parabolic motion (d) 3 - 4 - 5 polynomial motion A flywheel fitted in a steam engine has a mass of 800 kg. Its radius of gyration is 360 mm. The starting torque of engine is 580 N-m. Find the kinetic energy of flywheel after 12 seconds? (a) 233.3 kJ (b) 349.8kJ (c) 487.5 kJ (d) None of these In a slider-crank mechanism, the maximum acceleration of slider is obtained when the crank is (a) at the inner dead centre position (b) at the outer dead centre position (c) exactly midway position between the two dead centres (d) none of these Ifthe rotating mass of a rim type flywheel is distributed on another rim type flywheel whose mean radius is half the mean radius ofthe former, then energy stored in the later at the same speed will be (a) four times the first one (b) same as the first one (c) one fourth of the first one (d) one and a halftimes the first one What will be the number of pair of teeth in contact ifarc of contact is 31.4 mm and module is equal to 5. (a) 3 pairs (b) 4 pairs (c) 2 pairs (d) 5 pairs The distance between the parallel shaft is 18 mm and they are conntected by an Oldham's couling. The driving shaft revalues at 160 rpm. What will be the maximum speed of sliding the tongue ofthe intermediate piece along its grow? (a) 0.302 m/s (b) 0.604 m/s (c) 0.906m/s (d) None of these Two spur gears have a velocity ratio of 113. The driven gear has 72 teeth of 8 mm module and rotates at 300 rpm. The pitch line velocity will be (a) 3.08m/s (b) 6.12 mls (c) 9.04 mls (d) 12.13 mls
Badboys2 21.
27.
Instantaneous centre of a body rolling with sliding on a stationary curved surface lies (a) at the point of contact (b) on the common normal at the point of contact (c) at the centre of curvature of the stationary surface (d) Both (b) and (c) If Cf is the coefficient of speed fluctuation of a flywheel then the ratio of O)~O)min will be (a)
1-2Cf 1+2Cf
(b)
2-Cf
(c)
l+Cf 1-Cf
(d)
2+Cf 2-Cf
28. A rotor supported at A and B, carries two masses as shown in the given figure. The rotor is
29.
(a) dynamically balanced (b) statically balanced (c) statically and dynamically balanced (d) not balanced A body of mass m and radius of gyration k is to be replaced by two masses m, and m2 located at distances h, and h2 from the CG of the original body. An equivalent dynamic system will result, if (a)
30.
h}+h2=k
(b)
hT+h;=k2
A cord is wrapped around a cylinder of radius 'r' and mass 'm' as shown in the given figure. If the cylinder is releasd from rest, velocity of the cylinder, after it has moved through a distance 'h' will be
31.
(a)
.J2 gh
(c)
l3
(b)
Jib {gh3h \/3
(d) gh There are six gears A, B, C, D, E, F, in a compound train. The number ofteeths in the gears are 20, 60, 30, 80,25 and 75 respectively. The ratio of the angular speeds of the driven (F) to the driver (A) ofthe drive is (a)
(c)
1
1 24 4 15
(b)
8
(d) 12
Badboys2 0
""'"
("I') ("I')
ill
Theory of Machines and Machine Design 32.
33.
34.
35.
In the four-bar mechanism shown in the given figure, links 2 and 4 have equal lengths. The point P on the coupler 3 will generate a/an (a) ellipse 2 3 4 (b) parabola (c) approximately straight line (d) circle p A system of masses rotating in different parallel planes is in dynamic balance if the resultant (a) force is equal to zero (b) couple is equal to zero (c) force and the resultant couple are both equal to zero (d) force is numerically equal to the resultant couple, but neither of them need necessarily be zero. A bicycle remains stable in running through a bend because of (a) Gyroscopic action (b) Corioliss' acceleration (c) Centrifugal action (d) Radius of curved path The maximum fluctuation of energy E[, during a cycle for a flywheel is (a) l( (1)2max - (1)2min) (b) 1/2 1(1)av ((1)2 max - (1)2min)
1
(C)
(a)
40.
41.
42.
43.
2
(d) (where I= Mass moment of inertia of the flywheel (1)av = Average rotational speed K; = coefficient of fluctuation of speed) The road roller shown in the given figure is being moved over an obstacle by a pull 'P'. The value of'P' required will be the minimum when it is
44.
45.
(a)
horizontal
(b) vertical
37.
(c) at45° to the horizontal (d) perpendicular to the line CO Two gear 20 and 40 teeth respectively are in mesh. Pressure
46.
angle is 20°, module is 12 and line of contact on each side of the pitch point is half the maximum length. What will be the height of addendum for the gear wheel (a) 4mm (b) 6mm (c) 38.
8mm
(d)
lOmm
In a slider-bar mechanism, when does the connecting rod have zero angular velocity? (a)
39.
When crank angle
= 0°
(b) When crank angle
= 90°
(c) When crank angle = 45° (d) Never A disc of mass m is attached to a spring of stiffuess k as shown in the figure. The disc rolls without slipping on a horizontal surface. The natural frequency of vibration of the system is
I~
2n: m
(b) _I
2n:
fJ m
(d) I~ I~ 2n: 3m 2n: 2m For a four bar linkage in toggle position, the value of mechanical advantage is (a) 0.0 (b) 0.5 (c) 1.0 (d) 00 What will the normal circular pitch and axial pitch of helical gear if circular pitch is 15 mm and helix angle is 30° (a) 13mmand39mm (b) 26mmand39mm (c) 26mmand 13mm (d) 13mand26mm The speed of an engine varies from 210 rad/s to rad/s. During cycle the change in kinetic energy is found to be 400 Nm. The inertia ofthe flywheel in kgnr' is (a) 0.10 (b) 0.20 (c) 0.30 (d) 0.40 If first and last gear having teeth 30 and 50 respectively of a simple gear train, what will be the train value and speed ratio gear respectively if first gear is driving gear (a) 3/5 and 5/3 (b) 3/5 and 4/5 (c) 5/3 and 3/5 (d) 4/5 and 3/5 The centre of gravity ofthe coupler link in a 4-bar mechanism would experience (a) no acceleration (b) only linear acceleration (c) only angular acceleration (d) both linear and angular accelerations In a four-bar linkage, S denotes the shortest link length, L is the longest link length, P and Q are the lengths of other two links. At least one of the three moving links will rotate by 360° if (c)
lIKes (1) av lKes(1)2av
Badboys2 36.
A-63
47.
W
S+LSP+Q
~
S+L>P+Q
(c)
S+PSL+Q
(d)
S + P > L+ Q
An involute pinion and gear are in mesh. Ifboth have the same size of addendum, then there will be an interference between the (a) tip of the gear teeth and flank of pinion (b) tip of the pinion and flank of gear (c) flanks of both gear and pinion (d) tip of both gear and pinion. ABCD is a four-bar mechanism in which AB = 30 em and CD = 45 em. AB and CD are both perpendicular to fixed link AD, as shown in the figure. Ifvelocity ofB at this condition is V, then velocity of C is
c
o, C)
I
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Theory of Machines and Machine Design
A-64
(a) y
(b) Iy
(c) 2_y
(d)
2
48.
49.
3.y
3 The transmission angle is maximum when the crank angle with the fixed link is (a) ff (b) 90° (c) 180° (d) 270° In the given figure, ABCD is a four-bar mechanism. At the instant shown,AB and CD are vertical and BC is horizontaL AB is shorter than CD by 30 cm, AB is rotating at 5 radls and CD is rotating at 2 rad/s. The length of AB is 4
B
c
A
57. The tangential force transmitted (in newton) is (a) 3552 (b) 2611 (c) 1776 (d) 1305 58. Tooth interference in an external involute spur gear pair can be reduced by (a) decreasing centre distance between gear pair (b) decreasing module (c) decreasing pressure angle (d) increasing number of gear teeth 59. Two identical ball bearings P and Q are operating at loads 30 kN and 45 kN respectively. The ratio of the life of bearing P to the life of bearing Q is (a) 8 1116 (b) 27/8 (c) 9/4 (d) 3/2 60. Match the following criteria of material failure, under biaxial stress a 1 and a2 and yield stress ay, with their corresponding graphic representations. List I List II o 2 P.
o
50.
(a) 10cm (b) 20cm (c) 30cm (d) 50cm A link OP is 0.5 m long and rotate about point O. It has a slider at permit B. Centripetal acceleration ofP relative to 0 is 8 m/sec'. The sliding velocityof slider relative to P is 2 mI sec. The magnitude of Coriolis component of acceleration
Maximum normalstress criterion
1.
cry
Badboys2(a) IS
51.
52.
53.
54.
55.
lti m/sec' (b) 8 m/sec/ (c) 32 m/sec' (d) Data insufficient Which one of the following is a criterion in the design of hydrodynamic journal bearings? (a) Sommerfield number (b) Rating life (c) Specific dynamic capacity (d) Rotation factor A cylindrical shaft is subjected to an alternating stress of 100 MPa. Fatigue strength to sustain 1000 cycle is 490 MPa. If the corrected endurance strength is 70 MPa, estimated shaft life will be (a) 1071 cycle (b) 15000 cycle (c) 281914 cycle (d) 928643 cycle 20° full-depth involute profiled 19-tooth pinion and 37tooth gear are in mesh. Ifthe module is 5 mm, the centre distance between the gear pair will be (a) 140 mm (b) 150 mm (c) 280 mm (d) 300 mm The resultant force on the contacting gear tooth in newton is (a) 77.23 (b) 212.20 (c) 225.81 (d) 289.43 A ball bearing operating at a load F has 8000 h oflife. The life of the bearing, in hour, when the load is doubled to 2F IS
(a) 8000 (b) 6000 (c) 4000 (d) 1000 56. Given that the tooth geometry factor is 0.32 and the combined effect of dynamic load and allied factors intensifying the stress is 1.5, the minimum allowable stress (in MPa) for the gear material is (a) 242.0 (b) 166.5 (c) 121.0 (d) 74.0
Q.
Maximum-distortionenergy criterion
2.
cr ".
cry
Y
- cry cr2
R.
Maximum-shear stress criterion
3.
cr1
- cry - cry
61.
62.
63.
64.
(a) P-2, Q-1, R-3 (b) P-3, Q-2, R-1 (c) P-2, Q-3, R-1 (d) P-3, Q-1, R-2 A solid circular shaft needs to be designed to transmit a torque of 50 N-m. If the allowable shear stress of the material is 140 MPa, assuming a factor of safety of 2, the minimum allowable design diameter in mm is (a) 8 (b) 16 (c) 24 (d) 32 Stress concentration in cyclic loading is more serious in (a) ductile materials (b) brittle materials (c) equally serious in both cases (d) depends on other factors Feather keys are generally (a) tight in shaft and loose in hub (b) loose in shaft and tight in hub (c) tight in both shaft and hub (d) loose in both shaft and hub For a parallel load on a fillet weld of equal legs, the plane of maximum shear occurs at (a) 22.5° (b) 30° (c) 45° (d) 60°
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Theory of Machines and Machine Design 65. The silver bearings are used almost exclusively in aircraft engines due to their excellent (a) fatigue strength (b) wear resistance (c) corrosive resistance (d) None of these 66. When a shaft rotates in anti-clockwise direction at slow speed in a bearings, then it will (a) have contact at the lowest point of bearing (b) move towards right of the bearing making metal to metal contact (c) move towards left of the bearing making metal to metal contact (d) move towards right of the bearing making no metal to metal contact 67. The most efficient riveted joint possible is one which would be as strong in tension, shear and bearing as the original plates to be joined but this can never be achieved because (a) rivets can not made with same material (b) rivets are weak in compression (c) there should be atleast one hole in the plate reducing its strength (d) clearance is present between the plate and the rivet 68. To resist breaking of the plate in front of the rivet, we make the distance from the centre of the rivet to the edge of the plate at least (a) 1.5 d (b) 2.5 d (c) 2d (d) 3d 69. The uniform pressure theory as compared to the uniform wear theory gives (a) higher frictional torque (b) lower frictional torque (c) either lower or high frictional torque (d) None of these 70. The limiting wear load of spur gear is proportional to (where Ep = Young's modules of pinion material, Eg = Young's modulus of gear material.
A-65
75.
76.
77.
78.
79.
(a)
Badboys2
(a)
(c)
80.
81.
(c) 71. American standard thread have the angle equal to (a) 55° (b) 60° (c) 29° (d) 58° 72. For overhauling which of the following condition is satisfied? (a) L2 = 1500 x 8 = 12000 hours
225. (b) Given: Safe stress (as) = 25.2 MN/m2 density (p) = 7g/cm3 Safe stress (a J = p x maximum periphered velocity (v2) as = p x v2 =_7_xl06 1000
xu2
2 25.2 xl000 u =---7
u = ..)3600 = 60 m / s 227. (c) Given: Number ofteeth (T) = 60
203. (a) Given, Initially, wI = w
Module (m) = 6 mm Number ofteeth on pinion (T p) = 20
Life = LI
Considering the relation, Lw3=c
160 940 = 0.17
wI
1000 x 60 x 3000 = (4900)3 2000 x 60 x L2 9800
Life=L,
111+ 112
2(510-430) 510+ 430
25.2xl06
After half of load, w 2 =-
112-111
2( 112-111)
111+ 112 2
Speed(N1)= 100rpm (Life) (LI) time duration = 3000 hours
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.
224. (d) Coefficient of fluctuation of speed = ......:..:....___:~ 11m
w 2
Centre distance (D) = m(T + Tp ) = 6 (60 + 20) 2 2 6x80 480 =--=-=240mm 2 2
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rrIII~ll)ll'l~ I~Nf.INI~I~IIINf. THERMODYNAMICS
Surrounding
~system
In the subject of thermodynamics, the inter-relationship among heat, work and system properties are studied. It is also called as the conceptual science of entropy and energy.
(Thermodynamic
Some Thermodynamical Terms in brief (i) Thermodynamic system: A thermodynamical system is an assembly oflarge number of particles which can be described by thermodynamic variables like pressure (P), volume (V), temperature (1). (ii) Surroundings: Everything outside the system which can have a direct effect on the system is called surroundings. The gas cylinder in the kitchen is the thermodynamic system and the relevant part ofthe kitchen is the surroundings. (iii) An adiabatic wall: The wall which prevent the passage of matter and energy. (iv) Diathermic wall: It prevent the passage of matter but allow the passage of energy. An aluminium can is an example of a container whose walls are diathermic. (v) Closed and open system: In a closed system, energy may transfer the boundaries of system but mass does not cross the boundary, while in open system, both mass and energy transfer across the boundary of the system. (vi) An isolated system: In this type of system neither the mass nor the energy can be exchanged with the surroundings. (vii) Equation of state: The relationship between the pressure, volume and temperature of the thermodynamical system is called equation of state. (viii) Properties : A property of a system is any abusable characteristic of the given system various properties of the system depend on the state of the system not on how that state have been reached. (xi) Intensive property of a system or those properties whose values does not depend upon the mass of the system. Eg: Pressure, temperature, viscosity etc., while extensive properties depend upon the mass of the system. Eg: Length, volume etc. (x) Equilibrium: A system is said to be in thermodynamic equilibrium when it does not lead to change its properties (macroscopic) and make balance with its surroundings. There, a system in mechanical, thermal and chemical equilibrium is said to be in thermodynamic equilibrium.
A thermodynamic system is described as a kind of a region available in space and this region is concentrated for the purpose of analysing a problem. The system is considered to be separated from surroundings (external to system) by the boundary of the system. The nature of the boundary may be real or imaginary and it is considered to be flexible i.e., it can change its shape or size. Ifwe combine a system and its surroundings, then it constitutes the universe.
system)
Types of thermodynamic systems: There are three types ofthermodynamic
(a)
systems:
Closed system: A thermodynamic system in which mass is not transferred across system boundary but energy may be transferred in and out ofthe system, is known as closed system. Mass in the piston - cylinder arrangement is the example of a closed system.
(b)
Open system: The open system is defined as a system in which mass as well as energy can be transferred with its surroundings. Open systems are most common. The region where analysis ofthe system is performed is known to be a control volume and the boundary of control volume is known as control surface. Eg: Air compressor
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THERMODYNAMIC SYSTEM
boundary
lnputmass stem boundary Input mass
Exit mass Surroundings Exit energy
(Open system) (c)
Isolated system: In an isolated system, no mass and no energy is transferred across system boundary. ~ystem
boundary
~ Surroundings (No mass transfer No energy transfer)
(Isolated system)
ZEROTH LAW OF THERMODYNAMICS
If objects A and B are separately in thermal equilibrium with a third object C then objects A and B are in thermal equilibrium with each other. Zeroth law of thermodynamics introduces thermodynamic quantity called temperature. Two objects (or systems) are said to be in thermal equilibrium iftheir temperatures are the same. In measuring the temperature of a body, it is important that the thermometer be in the thermal equilibrium with the body whose temperature is to be measured.
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Thermal Engineerging
A-84
FIRST LAW OF THERMODYNAMICS The first law of thermodynamics is based on conservation of energy. According to this law heat Q supplied to a system is equal to the sum ofthe change in internal energyfal.I) and work done by the system (W). Thus we can write Q = ~U+W More about First Law of Thermodynamics 1. Heat supplied to the system taken as positive and heat given by the system taken as negative. 2. It makes no different between heat and work. It does not indicate that why the whole of heat energy cannot be converted into work. 3. Heat and work depend on the initial and final states but on the path also. The change in internal energy depends only on initial and final states of the system. 4. The work done by the system against constant pressure P is W = P~V. So the first law of thermodynamics can be written as Q = /).u + p/).v . 5. Differential form of the first law; dQ = dU+dW or ~ = dU+NV SECOND LAW OF THERMODYNAMICS (i)
Kelvin - Plank Statement: It is impossible to construct an engine that can convert heat completely into work without producing any other effect.According to the statement the efficiency of any heat engine always be less than 100%. Clausius Statement: For a self acting machine, it is impossible to transfer heat from a colder body to a hotter body without the aid of external agency.
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ENTROPY Entropy is the another thermodynamical variable which many times very useful to understand the system. Entropy is related to the disorder or randomness in the system. Tounderstand this, let us consider two systems as shown in Fig.
Intensive and Extensive properties (a) Intensive properties: Intensive properties are those properties which does not depend on the mass available in the system. Eg : temperature, pressure, etc. (b) Extensive properties: Extensive properties are those properties which depends on the mass available in the system. Eg : Volume,energy,etc. Some terms like specificvolume, specific energy etc. come under the category of specific extensive properties. Thermodynamic equilibrium A systemis said to be in equilibriumwhen there is no driving forces within the system after isolation ofthe system from its surroundings. A system is said to be in thermodynamic equilibrium it satisfies the following three kinds of equilibrium: (a) Mechanical equilibrium (b) Thermal equilibrium (c) Chemicalequilibrium Internal energy or energy of the system The internal energy of the thermodynamic system is regarded as the combination of all kinds/forms of energy of the system. These all forms of energy include kinetic energy, potential energy vibrational energy, rotational energy etc. If dU is the internal energy of the system, then, dU=MC~T where, M = mass in kg, C = specific heat - capacity, ~T = change in temperature. Energy can also be considered as a property of a thermodynamic system. Consider a system that undergoes a change from state 'A' to state 'B' and the systemundergoes a cyclic process. A
f
~B
--~V
System 1 System 2 If SI and S2 are the entropies of the system I and 2 respectively at any temperature, then SI < S2. (i) Entropy is not a conserved quantity. (ii) Entropy can be created but cannot be destroyed. (iii) Entropy of the universe always increases. If a system at temperature T is supplied a small amount of heat ~Q, then change in entropy of the system can be defined as M=
~Q
T
for constant T
For a system with variable T, we have
(Total work done)eyeIe = (Total heat) cycle WI+W2=QI +Q2 QI-WI =W2-Q2 I~Q=dU+ 8wl Specific heat of constant volume (C v) .
It IS defined as the rate of change of internal energy with
respect to temperature keeping the volume as constant. C =(dU) V dT p Specific heat of constant pressure (C ) It is defined as the rate of change of errthalpy with respect to temperature keeping the pressure as constant.
sf dQ ~S
= Sf -
s, =
fT
s,
The second law of thermodynamics may be stated in terms of entropy as: It is impossible to have a process in which the entropy of an isolated system is decreased.
C =(dH) p dT v THERMODYNAMICAL PROCESSES Any process may have own equation of state, but each thermodynamical process must obeyPV = nRT.
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Thermal Engineerging 1.
A-85
Isobaric Process: Ifa thermodynamic system undergoes physical change at constant pressure, then the process is called isobaric.
p ......------to--~
p ......... :-~
(-~Vr
00
p
P
M
B
(v) Work done: W = P~V= 0 (vi) First law ofthermodynamics in ischoric process Q ~U+W=~U+O or Q ~U
........ -...........,
»c.sr
3.
'W=-P~V
W = +Pav
'---~----~-
....v
V Expansion
Compression
(i)
Isobaric process obeys Charle's law, VOC T
(ii)
dP Slope of P ~ V curve, dV = O.
Isothermal Process: A thermodynamical process in which pressure and volume of the system change at constant temperature, is called isothermal process. p
(iii) Specific heat at constant pressure C;=
5R
7R
p
2 for monoatomic and Cs= Tfordiatomic
(iv) Bulk modulus of elasticity: As P is constant, M and
=0
M
(-A:f
B
Badboys2 (v) Work done:
(i)
An isothermal process obeys Boyle's law PV = Constant. (ii) The wall of the container must be perfectlyconducting so that free exchange of heat between the system and surroundings can take place. (iii) The process must be very slow, so as to provide sufficient time for the exchange of heat. (iv) Slope of P - V curve: For isothermal process PV = Constant After differentiating w.r. t. volume, we get
W = P~V=nR~T (vi) First law ofthermodynamics in isobaric process Q= ~U+W
= ~U+P~V
= ~U+nR~T
= nCv ~T + nR~T = n( Cv + R)~T = nCp~T (vii) Examples:Boilingof water and freezingofwater at constant pressure etc. 2. Isochoric or Isometric Process: A thermodynamical process in which volume ofthe system remain constant, is called isochoric process.
P+V dP dV
dP -P V or tan O = dV (v) Specific heat at constant temperature: As~T=O, or
T
j
W=O
An isochoric process obeysGay - Lussac' sLaw, P oc T
dP (ii) Slope of P - V curve, dV
=
4.
(vi) First law ofthermodynamics in isothermal process. As ~T 0, :. ~U=O Q ~U+W=O+W or Q W Adiabatic Process: An adiabatic process is one in which pressure, volume and temperature ofthe systemchange but heat will not exchange between system and surroundings. p
00
(iii) Specific heat at constant volume Cv
=
-P
V
C =
o'-----------·v (i)
0
3R 5R 2 formonoatomicand Cv = 2 fordiatomic
(iv) Bulk modulus of elasticity : As Vis constant, ~V= 0
p
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Thermal Engineerging
A-86
(i)
Adiabatic process must be sudden, so that heat does not get time to exchange between system and surroundings. (ii) The walls of the container must be perfectlyinsulated. (iii) Adiabatic relation between P and V PVY =k (iv) Adiabatic relation between Vand T & P and T For one mole of gas PV=RT,
= k,
(Rnv
k
y
Isothermal expansion: If Qj is the heat absorbed from the source and WI is the work done, then, QI
2.
we get
W2 =
3.
Q2 ~
nR(lj - Tf ) y-I
nR(Ii - T2) y-I
W3~nRT2fn(~)
~ nRT2fn(~:J (As flU
=
=
0)
~ -nRT2fn(~J
k RY = another constant
or
AU ~ 0)
J
P
4.
Q
nflT
Adiabatic compression: If W4 is the work done during the adiabatic compression, then W4
0
= nflT = 0
(vi) First law ofthermodynamics in adiabatic process Q L1U+ W As Q 0, :. Sll=> W or UI-Uj -W .. UI Uj-W P-V Diagram Representing Four Different Processes
nR(T2 -Ii) y-I
nR(lj - Tf) y-1
-nR
(Ii- T2) y-I
Net work done in the whole cycle W =Wj+W2+W3+W4
(V
-- n R'T'II -LJ~n - 2J + nR(Ii -T2) Vi y-I
(V
II - 3J -nR__:__:_----='-'(Ii -T2) n RT 2~n V4 y-I
In the adiabatic expansion B ---+ C
...p
v:
1', y-1
1 2
500K 400K
300K
1. 3.
(As
RT
V
Specific heat:C
nR1Jfn(~ )
Isothermal compression: If Q2 is the heat reject to the sink and W3 is the work done during the process, then
k
Badboys2(v)
WI ~
Adiabatic expansion: If W2 is the work done during the adiabatic expansion, then
k R = new constant
or
~
nR1Jfn(~
V
or P=
Substituting in PVf
Also
RT
1.
T Vy-1 2 3
or
...(ii)
Similarly in the adiabatic compression D ---+ A
L------------___.v Isobaric Process 2. Isothermal Process Adiabatic Process 4. Isochoric Process
T Vy-1 2 4
1', v,y-l 1 1
...(iii)
CARNOT CYCLE
or
Camot cycle has four operations. Thermodynamic coordinates after each operation are shown in Fig. Initially at A coordinates are r; Vj,Tj.
From equations (ii) and (iii) , we have
p
...
or
V2
5_
V3 VIV3
V2V4
V2
Also
VI
V4
...(iv)
V3 V4
Efficiency of carnot engine Workdonebyengine (W) 11 Heat absorbed by engine from source ({1)
o-------------+v
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Thermal Engineerging
A-87
nR[Tjfn( ~ )-T2fn(~)] nRTjfn(~: ) As
r, - T2
----
11
_ 1 --T2
11
Enthalpy: Enthalpy is regarded as the total energy of a thermodynamic system. It is defined as the sum of internal energy and product of pressure and volume. It is an intensive property of the system. It is describe as: H=U+PV or h= u+ pv where, H = Total enthalpy U = Total internal energy P = Pressure, V = Volume(Total) h = specific enthalpy u = specific internal energy v= specific volume Enthalpy is also considered as a function of temperature for the case of perfect gases. Hence, it can be written as : L1H=MCpL1T where, L1H= H2- HI = enthalpy difference L1T = T2- T I = temperature difference C, = specific heat at constant pressure Dryness fraction (X) : Dryness fraction is defined as the ratio of the mass of vapour or dry saturated steam to the total mass of wet steam or mass of mixture (water in saturated form in liquid region as well as vapour region).
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My Dryness fraction (X) = My + ML . where, My= mass of steam or vapour ML= mass of Liquid (saturated) Drynessfractionis utilized to calculatethe quantity ofliquid or vapour phase within the mixture. The value of dryness fraction varies between 0 and 1. It also determines the quality ofthe steam.
can never be realised because dissipative forces cannot be completelyeliminated. Irreversible Process Any process which cannot be retraced in the reverse direction exactly is called an irreversible process. Most of the processes occurring in the nature are irreversible processes. Examples: (i) Diffusion of gases. (ii) Dissolution of salt in water. (iii) Rusting of iron. (iv) Sudden expansion or contraction ofa gas.
AVAILABILITY AND REVERSIBILITY Available Energy The sources of energy can be divided into two groups (l) High grade energy (2) Low grade energy The conversion of high grade energy to shaft work is exempt from the limitations of the second law, while conversion of low grade energy is subject to them. High Grade Energy (1) Mechanicalwork (2) Electricalenergy (3) Water power
Low Grade Energy (1) Heat or thermalenergy (2) Heat derived fromnuclear
fissionor :fusion (3) Heat derived from combustion
of fossilfuels
(4) Windpower (5) Kineticenergyof a jet (6) Tidalpower
Available Energy Referred to a Cycle. The maximum work outputobtainablefrom a certain heat input in a cyclic heat engine is called the Available Energy (A.E.), or the available part ofthe energy supplied. The minimum energy that has to be rejected to the sink by the second law is called the Unavailable Energy (U.E.), or the unavailablepart of the energy supplied. AE.+U.E. or AE. = Ql - U.E. For the given r. and T2,
11rev. =
T
1- T~
REVERSIBLE AND IRREVERSIBLE PROCESSES Reversible Process Any processwhich can be made to proceedin the reverse direction by variation in its conditions such that any change occurring in anypart ofthe directprocessis exactlyreversedinthe corresponding part of reverse process is called a reversible process. Examples: (i) An infinitesimally slow compression and expansion of an ideal gas at constant temperature. (ii) The process of gradual compression and extension of an elastic spring is approximately reversible. (iii) A workingsubstancetaken alongthe completeCarnot's cycle. (iv) The process of electrolysis is reversible if the resistance offered by the electrolyte is negligibly small. A complete reversible process is an idealised concept as it
Available and unavailable energy in a cycle. For a given T}, 11rev. will increase with the decrease of Tj. The lowestpracticable temperature of heat rejection is the temperature of the surroundings, To. u.E. = Ql- Wmax
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Thermal Engineerging
A-88
hrrnx
Wmax
..
TO 1-Tl
and
(1-~~)QI
AE. Qxy- To(Sy- Sx) or u.E. Qxy- Wmax or u.E. = To(S, - Sx) The unavailable energy is thus the product of the lowest temperature of heat rejection, and the change of entropy of the system during the process of supplying heat. Wmax
Availability of a Given System It is the maximum useful work (total work minuspdV work) that is obtainable in a process in which the system comes to equilibrium with its surroundings. It depends on the state of both the system and surroundings. Let U, S, and V be the initial values ofthe internal energy,entropy, and volume of a systemand Uo,So,and Votheir [mal values when the system has come to equilibrium with its environment. The system exchanges, heat only with the environment, and the process may be either reversible or irreversible, the useful work obtained in the process W ::;;(U-ToS+poV)-(Uo- ToSo+PoVo) Let = U - ToS+ PoV where is the availability function and is a compositeproperty of both the system and its environment, with U, S, and V being properties of the system at some equilibrium state, and Toand Po the temperature and pressure of the environment. (In the Gibbs function, G = U - TS + P V,T, and p refer to the system). The decrease in the availability function in a process in which the system comes to equilibrium with its environment is -o=(U - ToS+ PoV)-(Uo- ToSo+PoVo) .. W::;;-o Thus the useful work is equal to or less than the decrease in the availability function.
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Irreversibility of the Process
total volume of the gas. Therefore volume of the gas is equal to volume of the vessel. (4) The molecules of gases are in a state of random motion, i.e., they are constantlymoving with all possible velocitieslying between zero and infinity in all possible directions. (5) Normally no force acts between the molecules. Hence they move in straight line with constant speeds. (6) The molecules collide with one another and also with the walls ofthe container and change there direction and speed due to collision. These collisions are perfectly elastic i.e., there is no loss of kinetic energy in these collisions. (7) The molecules do not exert any force of attraction or repulsion on each other except during collision. So, the molecules do not posses any potential energy.Their energy is wholly kinetic. (8) The collisions are instantaneous i.e., the time spent by a molecule in a collision is very small as comparedto the time elapsed between two consecutive collisions. (9) Though the moleculesare constantlymoving from oneplace to another,the averagenumber ofmoleculesper unit volume of the gas remains constant. (10) The molecules inside the vessel keep on moving continuously in all possible directions, the distribution of molecules in the whole vessel remains uniform. (11) The mass of a molecule is negligibly small and the speed is very large, there is no effect of gravity on the motion of the molecules. Ifthis effect were there, the density of the gas would have been greater at the bottom of the vessel.
Equation of State or Ideal Gas Equation The equation which relates the pressure (P), volume (V) and temperature (T) of the given state of an ideal gas is known as ideal gas equation or equation of state. i.e., PV = nRT where R = universal gas constant Numerical value ofR = 8.31joule mol-1 kelvirr ' n = no. of moles of gas
Behaviour of Real Gases
The actual work done bya system is always lessthan the idealized reversible work, and the difference between the two is called the irreversibility ofthe process. 1= Wmax- W This is also sometimesreferredto as 'degradation' or 'dissipation' . For a non-flow process between the equilibrium states, when the system exchangs heat only with the environment .. I~O I = To [(1\S)system + (~S)SUlT.J Similarly, for steady flow process, I = To(1\Ssystem + ~SSUlT) The same expression for irreversibility applies to both flow and non-tlowprocesses. The quantity To(~Ssystem + 1\SSillT) represents an increase in unavailable energy (or energy).
The gases actually found in nature are called real gases. 1. Real gases do not obey gas laws 2. These gases do not obey the ideal gas equation PV=nRT 3. A real gas behaves as ideal gas most closely at low pressure and high temperature. 4. Equation of state for real gases is given by Vander waal's equation
BEHAVIOUR OF IDEAL AND REAL GASES
According to first law ofthermodynamics, heat given to a system (~Q) is equal to the sum of increase in its internal energy (~U) and the work done (~W) by the system against the surroundings. i.e., 1\Q= ~U + 1\W Heat (~Q) and work done (~W) are the path functionsbut internal energy (~U) is the point function.
Behaviour of Ideal Gases The behaviour of ideal gases is based on the following assumptions of kinetic theory of gases : (1) All the molecules of a gas are identical. The molecules of different gases are different. (2) The molecules are rigid and perfectlyelastic spheres of very smalldiameter. (3) Gas molecules occupy very small space. The actual volume occupied by the molecule is very small compared to the
(p+ :2)tV
-nb) =
nRT
Here a and b are Constant called Vander waal's constant.
ANALYSIS OF THERMODYNAMIC CYCLES RELATED TO ENERGY CONVERSION
Work Let us consider a gas or liquid contained in a cylinder equipped with a movable piston, as shown in Fig. Supposethat the cylinder has a cross-sectional area A and the pressure exerted by the gas at the piston is P.
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Thermal Engineerging
A-89
(iii) If the closed loop is traced in the anticlockwise direction, the expansion curve lies below the compression curve (Wx V
L___~A _
_.v
W= area A RCA
Fig.(i)
Fig. (ii)
W= - area A RCT>F.FA
Fig.(iii)
PROPERTIES OF PURE SUBSTANCES 1.
It is a single substance and has a uniform composition. It
V;
2. 3.
has constant chemical composition through its mass. It has a same colour, taste and texture. It has a fixed melting point and boiling point.
P(Vf -VJ =Pi1V
Cyclic Process and Non-cyclic Process If a system having gone through a change, returns to its initial state then process is called a cyclic process. If system does not return to its initial state, the process is called non-cyclic process.
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'-----!-f-;-'
JiV = area A RC[)F.A
{J
f PdV
If the pressure remain constant while volume changes, then the work done W =
p
!'
p
p
Types of Pure Substances Two different types of pure substances are : (i) Element: An element is a substance which cannot be split up into two or more simpler substances by usual chemical methods of applying heat, lighting or electric energy, e.g., hydrogen, oxygen, sodium, chlorine etc. (ii) Compound: A compound is a substance made up of two or more elements chemically combined in a fixed ratio by weight e.g. H20 (water), NaCI (sodium chloride) etc. P-T DIAGRAM OF A PURE SUBSTANCE
L----~--
..v
(a) cyclic process
L-------
...
v
(b) Non-cyclic process
Work done in Cyclic Process Suppose gas expands from initial state A to final state B via the pathAXB. p
o'----,:r:c-) -------'-c'--.v The work done in this expansion Wx = + areaAXBCDA N ow gas returns to its initial state B via path B YA. Work done during this compression Wy -area BYADCB The net work done W Wx+ Wy areaAXBCDA -areaBYADCB +areaAXBYA Thus for a cyclic process (i) Work done in complete cycle is equal to the area ofthe loop representing the cycle. (ii) Ifthe closed loop is traced in the clockwise direction, the expansion curve lies above the compression curve. (Wx >Wy), the area ofloop is positive.
If the heating of ice at - 10°C to stream at 250°C at the constant pressure of 1atm is considered 1-2 is solid (ice) heating, 2-3 is melting of ice at O°C,3-4 is the liquid heating, 4-5 is the vaporization ofwater at 100°C, and 5-6 is the heating in the vapour state. The process may be reversed from state 6 to state 1 upon cooling. The curve passing through the 2, 3 points is called the fusion curve and the curve passing through the 4, 5 points (which indicated the vaporization or condensation at different temperature and pressure) is called the vaporization curve. The vapour pressure of a solid is measured at different temperatures, and these are plotted as a sublimation curve. These three curves meet as the tripple point as shown in the figure. The slopes of sublimation curve and vaporization curves for all substance are positive and slope of the fusion curve for more substance is positive but for water, it is negative. The triple point of water is at4.58 mm ofHg and 273.16 K whereas that of CO2 is at 3885 mm ofHg and 216.55 K.So when solid CO2 ( dry ice) is exposed to 1 atm pressure, it gets transformed into vapour, absorbing the latent heat of sublimation from surroundings. Phase equilibrium diagram on P-T coordinates.
1
F.. U's3:m I',2.,3__.,a-.n-e-
-T
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Thermal Engineerging
A-90 T-s diagram for a pure substance Consider heating of the system of 1 kg of ice at-5°C to steam at 250°C. The pressure being maintained constant at 1 atm. Entropy increases of the system in different regimes of heating. 2500C
f
(i)
6_
l00'C ---------------
Entropy increase of ice as it is heated from -5°C to O°C at 1 atm. (Cpice = 2.093 kJ/kg-K) ilSl = S2- Sl dQ
=j-= T
T2=273 me dT
j
_p
T1=268
T
273
= mcp
en 268
Fig. (a) p-v- T surface for water which expands a freezing
273
= 1 x 2.093
en 268
Badboys2 (ii) Entropy
= 0.0398 kJ/-K increase of ice as it melts into water at O°C (latent heat offusion of ice = 334.96 kJ kg) ilS2 = S3- S2
mL
334.96
... (wherem = lkg) Entropy increase of water as it is heated from O°C to 100°C
(c Pwater
= 4.187 kJ/kg-K)
T3
ilS3 = S4- S3 = m cp
= 1 x 4.187t'n (iv)
en T2
(:~) p ~T
This equation forms the basis of the h-s diagram of a pure substance, also called the Mollier diagram. The slope of the constant pressure curve on the enthalpy-entropy diagram is equal to the absolute temperature. When this slope is constant, the temperature remains constant. Iftemperature increases, slope of the isobar increases. The constant pressure curve for different pressure can be drawn on the h-s diagram as shown in the figure. States 2,3,4 and 5 are saturation curves.
373 273 = 1.305 kJ/-K
Entropy increase of water as it is vaporized at 100°C, absorbing the latent heat of vaporization (2257 kJ/kg) ilS4 = S5- S4
mL
=T= (v)
Fig. (b) p-v- T surface of a substance which contracts on freezing h-s diagram or Mollier diagram for a pure substance. From the first and second laws of thermodynamics, the following property relations are obtained: Tds= dh-vdp or
= T = -----ri3 = 1.232 kJ/-K (iii)
Fig. (a) shows a substance like water that expand up freezing. Fig. (b) shows substances other than water which contract upon freezing. Any point on the p-v- T surface represents an equilibrium state of the substance. The triple point line when projected to the p- T plane becomes a point.
2257 273 =6.05kJ/kg-K
... (wherem=
1 kg)
Entropy increase of vapour as it is heated from 100°C to 250°C at 1atm.
--7
523
= 1 x 2.093
en 373
=0.706kJ/-K
p-v- T surface for the pure substance. The relation between pressure, specific volume and temperature can be understood with the help ofP-v- T diagram.
(s) Entropy
Figure shows the phase equilibrium diagram of a pure substance on the h-s co-ordinates indicating the saturated solid line, saturated liquid lines and saturated vapour line, the various phases and the transition (liquid + vapour or solid + liquid or solid + vapour) zone.
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Thermal Engineerging
A-91
STEAM TABLES In steam table, properties of water are arranged as a function of pressure and temperature. Saturates steam: Temperature table Specific volume, m3/kg
Internal Energy KJIKG
Temp.
Pressure
Sat
Sat
Sat
°C
kPa,MPa
Liquid
Vapour
Liquid
T
P
Vr
Vg
0.01
0.6113
0.001000 206.132
5
Enthalpy KJIKg
Entropy KJIKgK
Sat.
Sat
Sat
Evap.
Vapour
Liquid
Evap. Vapour Liquid
Or
Org
ug
hr
0.00
2375.3
2375.3
0.00
2501.3 2501.3 0.0000 9.1562
9.1562
0.8721
0.001000 147.118 20.97
2361.3
2382.2
20.98
2489.6 2510.5 0.0761 8.9496
9.0257
10
1.2276
0.001000 106.377 41.99
2347.2
2389.2
41.99
2477.7 2519.7 0.1510 8.7498
8.9007
15
1.7051
0.001001 77.925
62.98
2333.1
2396.0
62.98
2465.9 2528.9 0.2245 8.5569
8.7813
20
2.3385
0.001002 57.790
83.94
23319
2402.9
83.94
2454.1 2538.1 0.2966 8.3706
8.6671
25
3.1691
0.001003 43.359
104.86 2304.9
2409.8
104.87 2442.3 2547.2 0.3673 8.1905
8.5579
30
4.2461
0.001004 32.893
125.77 2290.8
2416.6
125.77 2430.5 2556.2 0.4369 8.0164
8.4533
35
5.6280
0.001006 25.216
146.65 2276.7
2423.4
146.66 2418.6 2565.3 0.5052 7.8478
8.3530
40
7.3837
0.001008 19.523
167.53 2262.6
2430.1
167.54 2406.7 2574.3 0.5724 7.6845
8.2569
45
9.5934
0.001010 15.258
188.41 2248.4
2436.8
188.42 2394.8 2583.2 0.6386 7.5261
8.1647
50
12.350
0.001012 12.032 209.30 2234.2
2443.5
209.31
2382.7 2592.1 0.7037 7.3725
8.0762
55
15.758
0.001015
9.568
230.19 2219.9
2450.1
230.20
2370.7 2600.9 0.7679 7.2234
7.9912
60
19.941
0.001017
7.671
251.09 2205.5
2456.6
251.11
2358.5 2609.6 0.8311 7.0784
7.9095
65
25.033
0.001020
6.197
272.00 2191.1
2463.1
272.03
2346.2 2618.2 0.8934 6.9375
7.8309
70
31.188
0.001023
5.042
292.93 2176.6
2469.5
292.96
2333.8 2626.8 0.9548 6.8004
7.7552
75
38.578
0.001026
4.131
313.87 2162.0
2475.9
313.91
2321.4 2635.3
1.0154 6.6670
7.6824
80
47.390
0.001029
3.407
334.84 2147.4
2482.2
334.88
2308.8 2643.7
1.0752 6.5369
7.6121
85
57.834
0.001032
2.828
355.82 2132.6
2488.4
355.88
2296.0 2651.9
1.1342 6.4102
7.5444
90
70.139
0.001036
2.361
376.82 2117.7
2494.5
376.90
2283.2 2660.1
1.1924 6.2866
7.4790
95
84.554
0.001040
1.982
397.86 2102.7
2500.6
397.94
2270.2 2668.1
1.2500 6.1659
7.4158
100
0.10135
0.001044 1.6729 418.91 2087.6
2506.5
419.02
2257.0 2676.0
1.3068 6.0480
7.3548
Badboys2
hrg
hg
Sat
Sf
Sat Evap.
Vapour
Srg
Sg
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Thermal Engineerging
A-92
..
N
::I
~ 00
e
~
>-== ~ == ;;.-
~
"0 ...: 0; == oS' 00
toll
,;;
== 00
.. ::I
e
~ == ;;;..
toll
-=
'O, p> 5 In reciprocating compressor, at initial point ofsuction and final point of compression a little higher value of pressure is required to open the inlet and outlet value respectively. Heat is positive when added to system, is in exact differential path function and boundary phenomenon. Work is negative, inexact differential, path function and transient phenomenon. Heat generated by bulb = 100 x 24 x 60 x 60 J = 8.64 x 106 J . . Heat dissipated = (L x v) x [Cy (T - 20)] .. 100 x 24 x 60 x 60=(1.20 x 3) x 2.5 x 3 x Cy(T -20) 0.32 x 106 = Cy (T - 20) = 1000 x 1.004 (T -20) => T=338.72°C First of all, process (1-3) is adiabatic, means a vertical line in T-8 diagram. As given figure is clockwise for (1-2-3) so from Figures 1 and 2, clockwise (1-2-3) will be selected. Under steady-state flow conditions, d W=-dH+dQ ...(i) Also, in reversible process, T. d 8 = dH - Vdp ... (ii) =>- Vd P = - dH + T. d8 From Eqs. (i) and (ii), we get dW=-Vdp Integrating both sides, we get W=-jVdp
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Thermal Engineerging
A-130
233. (c)
234. (b)
Enthalpy of balloon is given by H=U+Pr Initially balloon kept in insulated and evacuatedroom. => No heat transfer from outside. ~e = 0 Also, gas does not have to do any work against any external pressure. => ~ W=O From 1stlaw, => ~Q=~U+~W ~U=O Further, between initial and final states, total energy or enthalpy remains same for the gas. The change in pressure and volume is suchthat their product remains constant. Hence, h also remains constant. dH=dU +d(pVO) dH = dU + pdV + Vdp
f dH = f dq + f Vdp
T2=396.30K W
8.314 x(300- 396.30) 1.67-1 W
~kf;+~p.E+ W =-fVdp
kmol = 1194J/mol Imol=Mg=40 g For 40 g, W=1194
239. (d)
240. (a)
2
~Ssystem+ ~Ssurrounding~ 0 AL= Vs(Sweftvolume)= 0.0259m3, Pem =? N=2200rpm 1 For 4-stroke diesel engine, K = "2
1
235. (a)
1194 For 1kg W= --xl000 , 40 =29.7 kJ/kg In every case, entropy of universe is always positive. (L\S)universe~ 0
W=-fVdp
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= 1194__!!_
m
~=Q+fVdp Q-~=-fVdp
R(TI-T2) y-l
m
P=950kW=950
Work is done on the system.
Pem xVs xNx950xl03 =
= 8.31kJ 237. (b)
Due to internal friction produced in irreversible process, entropy of the system increases. From Clausius inequality, Cyclic integral of di < 0 for irreversible process
io'w
P = Pem xALxNxK 60
V2 Wisothermal= PIVIenVI x x 6 0.030 -0.8 0.015 10 en 0.015
236. (a)
x
60
1 2
950 xl 03 x 60 x 2 Pem = 0.0259 x 2200 =2 x 106 Pa=2MPa 241. (a)
H1
Ql + Q2 + Q3 + ....+ Qn < 0 Tl T2 T3 Tn For process I, 2500 _ 2500 = -1.042 1200 800 For process II,
238. (a)
2000 _ 2000 = -1.5 800 500 Process II is more irreversible than process I. In adiabatic process,
HI =2800kJ/kg H2= 1800kJ/kg Work done = HI - H2= (2800 -1800) kJ/kg = 1000kJ/kg Then, specific steam consumptionm
W = PIVI - P2V2 y-l
3600 = 1000 =3.60kg/kW-h
y-l
_]_= T2
(RL]r P2
=> 300 = T2
0.67 (~)1.67 = (0.5)0.4012 2
242.(d)
d=80mm Strokelength L = 2 x Crank radian =2 x60= 120mm Then, swept volume Vs= A x L = ~d2 xL 4
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Thermal Engineerging
A-131 258. (d)
= ~ x (80)2 x 120 244. (b)
two systems which are equal in temperature to a third system, they are equal in temperature to each other '.
=603cm3 We have dQ = dU + pdU mRTdV or T dS = mCvdT + --V-dS = 8+mR dV V (as isothermal process, dT = 8) or
263. (b)
fIdS=mRfdVI V
or
Accordingly when 50 cc of water at 25°C are mixed with 150cc of water at 25°C, the resulting temperature ofthe mixture will be 25°C, Same analogy applies to situations in (b) and (c). However,this argument is not valid when water and sulphuric acid, initially at the sametemperature,are mixed.Heretemperaturewill rise due to chemical reaction - the change is often violent. For reversed Carnot cycle, COP =
TL h-TH
For a fixed value of TH' as TL increases,COP also increases but not linearly. In fact COP decreases with increasing differencebetween operating temperatures.
~S=mR£n V2 VI ( PI' ~S=mR£nlp2)
According to zeroth law of thermodynamics, "when
(asPIVI =P2V2)
264. (d)
11=
WTurbine QSuppIied
(2732-335) 0.64 = 11T --'----3-6-08---'245. (a) 11T=0.96 266. (b,d) Signs of work for the four cases are given below (a) 0 (b) '-' ve (c) 0 (d) '-' ve
246. (a)
Badboys2
and 8=-5 KW m= 1kg/s 247. (c) In thermodynamics, energy or available energy of a system in the maximum useful work possible during a process that brings a system into equilibrium with surroundings (heat reservoir).
267. (b)
x2 =
n2 = 0.625 nl +n2
~S = -
256. (d)
257. (a)
R (0.375 £n 0.375 + 0.625 £n 0.625)
= 0.66 R = 5.49 Jik
L....-------------+S
254. (d)
0 .375
14 n2 =-=0.625 22.4
3
The object of the regenerative feed heating cycle is to supply the working fluid to the boiler at same state between 2 and 2' (rather than at state 2) there by increasing the average temperature of heat addition to the cycle. du = 8Q- 8W Since du is the property and it is exact differential so 8Q- 8W is the exact differential. Here we have to find out the work done an the air in the cylinder. work = change in volume due to piston displacement x pressure inside the piston = 0.0045 x 0.075 x 80 x 103 =27 joule. In throttling process enthalpy remains constant. h, =h2 1000= 800+x (2800- 800) x=O.1
R (nl£nxl + n2 £n X2)
8.4 nl =--= 22.4
252. (c)
T
~S = -
269. (c)
1 221 2 pV =-mnc = -.-mnc 332 or
3p = _!_ mnc2 = E 2
or
270. (c)
2
V
2E
p=-
3
3
1
p
l~
i,, ,, , ,, ,,, ,, ,,
:2 ,,, ,, ,, ,, ,,, ,,
Badboys2
Thermal Engineerging
A-132 279. (b)
P3 > PI
272. (a)
Wgas < 0 As we know that slope of isothermal process in PV diagram is less than slope of adiabatic process in PV diagram. Thats why P3 > PI and from the process it is clear that work done is negative. Cp = 0.98, = 0.7638 PI =20bar, T3 = 1500k P2 = lbar, 11=0.94
Incorporation ofreheater in a steam power plant always increases dryness fraction of steam at condenser inlet and always increases specific work output.
280. (b) End of combustion
c,
p
~ Exhaust valve open
I
11= Cp =~=1.28305 0.7638
c,
Patm
11-1 0.28305 T4 = (P4 , ----;- => = (20) 1.28305 T3 lp3) 1500 1
l
Intake
L...-----1f----------+----.v
T4 = 29047434k. 11= T3 - T4' T3 - T4
Badboys2 274. (c)
=> 0.94 =
1500- T4 1500-2904.7434
~suniverse = ~Ssyst + DSsurrounding
. P2 =R IogJ-=8.314 PI
0
(Throtteled)
I
~----;------------------;--~v
0.1
og0.5
TDC BDC Air-standard auto cycle with four reversible processes 1-2; isentropic compression 2-3; V = constant heat addition 3-4; isentropic expansion 4-1; V = constant heat rejection From the first figure, it can be seen that intake and exhaust are not constant volume processes.
~suniverse= 13.38 kJ/k Gases become cool during Joule Thomson's expansion only if they are below a certain temperature called inversion temperature T I: The inversion temperature is the characteristic of each gas. It is related to the Van der Waals' constants 'a' and 'b' by the relation
p
T _ 2a
281. (d)
1 - R.b 276. (a)
3
T'4= 2820.45 Work w= Cp (T3-T'4) w= 0.98(1500 - 2820.45) w= 1294.049 kJ/kg. ~ssurrounding = ~Su = ~Ssys
275. (d)
BDC
TDC p
Pressure constant heat addition and pressure constant heat removal are Brayton cycles. Constant
temperature
heat addition
and constant
temperature heat removal are Carnot cycles Pressure constant heat addition and pressure constant heat removal are Rankine cycles.
v
Volume constant heat addition and volume constant heat removal are Otto cycles. 277. (d)
F or a given saturation pressure, iftemperature is lower than the saturation temperature liquid or compressed
then it is subcooled
liquid. For 150 bar pressure
saturation temperature is 342.24. But as temperature is lower than that, thus it is compressed liquid at 45°C, specific enthalpy would be 188.45 kJ/kg.
Given, PI = 100 kPa Tl =27+273=300K Heat supplied (process 2-3) Qs = 1500 kJ/kg Heat rejected (process 4-1 ) QR = 700 kJ/kg Gas constant for air, R = 0.287 kJ/kg-K
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Thermal Engineerging
A-133 V3- v2 = 0.05 (VI - v2)
.. VI Compression ratio, r = 10 = V2
(:~ -I)=O.OS[ :~ -I]
Now, mean effective pressure is given by Work done Pmean = ----Swept volume V4 Vt Now -=-=10 'V3 V2 => VI = 10V2 Also swept volume VS=VI-V2 => VS=0.9VI Initiallyfor air
~diesel = 1- (r
...(i)
" _ nRT _lxO.287x300 vt - ~ 100
285. (c)
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288. (b)
The air standard diesel cycle is less efficient than the Otto cycle, given the same compression ratio and heat addition. However, it is more efficient than the Otto cycle with the same peak pressure and heat addition.
302. (d)
3
Qs - QR
W 800 Pmean = Vs = 0.7749 = 1032.39kPa
For same compression ratio and the same heat supplied, otto cycle is most efficient and diesel cycle is least efficient. In practice, however, the compression ratio of the Diesel engine ranges between 14 and 25 whereas that of the otto engine between 6 and 12. Because of its higher efficiency than the otto engine.
TI =300k T3 =6 TI T3 = 1800k we know that for maximum work output T2T4=TIT3
COP = Refrigeration effect = ~ = 2.33 Work done 1.5 200
293. (c)
300-200
2
T2 = ~TtT3
T L = Lower temperature T H = Higher temperature
RE
2
Power= --=COP 295. (c)
1t
Vs =-d
4
2
2 1t
L = -(10)
4
JTIT3 T4 = JTtT3 T3 =
=lkW 2
x15=1177.5cm
T2
= .)1800 x 300 = 734.84 k t
3 ~=(T2Jr-t v2 TI
r = 1+ ~ = 1+ 1177.5 = 1 +5.99 ~7 Vc 196.3
W
1l=-=1--Qs
1
r= (~ )r-I =9.39.
1
(r)y-l 303. (b)
~=1-
1800
(7iA-1
t
W
Pm=Vs ll+n
W = 973.44 kJ/kg 297. (b)
Vs = ~(25)2 (37.5) = 18398.43 em3 4 vc = v2 = 1500 em3 r = 1+~ = 1+ 18398.43 = 13.26 Vc 1500
= 60.5
301. (a) =0.861 m3/kg
.. Vs=0.9x 0.861 =0.7749m3/kg Work done in cycle W = Heat supplied - Heat rejected = = 1500 - 700 = 800 kJ/kg
=>
)~-Il~(ct_~; l
1 1(l.61)l.4_1] lldiesel = 1- (13.26)OAll.4 x 0.01
PIVl =nRTI
..
rc -1 = 0.05 [12.26] rc = 1.61
h
W
W
= - = ----Qs mcv(T3-T2)
_ W +n - mR~T --
y-1
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Thermal Engineerging
A-134 more efficient.
w = 11+nmR.1T = 11+nM>vc y-1
:.
(Y-1) 308. (d)
11DieseI>11DuaI> 110tto
11= 1__
1_=
M
.1n
-xl00= n
Vs =(r-l)vc Pm
=
11+n(.1p)vc
-(I-r)-xl00 r
= (r-1)-x
-...;,...:=........:...__~-=----
M
6-5 = (1.4-1)x-5-x100
11+n(M» = (y-l)(r-l)
310. (c)
(r)~-l=1- (r)~-I[r(:=~)]
I I I
I I --4--------1 >I P 11 V' I 2
sr-1 = r( s - 1) sr-I - r( s - 1) = 0 Efficiency of ideal regenerative cycle is exactly equal to that of the corresponding Camot cycle. Hence it is
maximum,
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307. (b)
=0.08 x 100=8%
11otto= 11diesel
1-
306. (d)
100
r
(Y-1)(r-1)vc
304. (a)
l-(r)y-1
(r)Y-I
70% V2 =1 v2' = 1 +0.7 (r-l) =0.7r +0.3 v2' = 1 +0.3 (r-l) =0.3r+0.7
Following figures shows cycles with same maximum pressure and same maximum temperature. In this case, otto cycle has to be limited to lower compression ratio to fulfil the condition that point 3 is to be a common state for both cycles. T -S diagram shown that both cycles will reject the same amount of heat.
3
4
11=1---
=1 V
=1.7
1.3
1.7
0.3r+0.7 r=4.68
P
V---+
.'. =(2.6)1.3
0.7r+0.3
1
(r)Y-I 1
--1-:-4-:---:-1 = 0.46 = 46%
(4.68) .
311. (c)
vc=O.OOI m3 Ys
7t 2 3 = -x 0.200 x 0.250m
4
= 1-[ S Thermal efficiency= 1
30%
lIx ~=(_!i_J v2 P2
2
v,1 11
0.001+~xO.2002 Qrejected = 1 Constant Qsup plied Qsupplied
Thus the cycle with greater heat addition Qsuppliedis
312. (c)
]1.4-1 =58.2%
0.001
Given PI = 1 bar P3 =40bar r=5
xO.250
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Thermal Engineerging
A-135
320. (b)
i.e., 0-3 Humidification and steam injection - temperature increases and w increases to i.e., 0-5 Humidification and water injection - temperature decreases but w increases i.e., 04 On a psychrometric chart 75% RH
P2 =Pl( 11=1---
:J 1
(r r'
Qs
0.025
=1.(5)14 =9.51 bar
= 1---
1
(5)°.4
=0.4746
R(T3-T2) r-1
= Cv (T3 - TI) = _____..:.._--=----~
321. (a)
v2 (P3 - P2) (40 - 9.51) x vc = = 76.255vc r-1 1.4-1 1+~=5
322. (b)
Vc
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Vs
= 4vc
W 11=-
30°
On a psychrometric chart Constant relative humidity lines are uphill curve not straight, to the right. Constant WBT lines straight downhill to the right. Constant specificvolumedownhill straight to the right. Constant enthalpy lines are not coincident to WBT. P=2u(v-u)(1 +cos velocitygradient du is less => highly viscous dy fluid. du If Il is low => velocity gradient dy is high => easy to flow fluid.
Adhesive forces are attractive forces between the molecular ofa liquid/fluid and the molecular of a solid boundary surface in contact. • Property of a liquid. • The basic cause of surface tension is the presence of cohesive forces. • It is a property by virtue of which liquids want to mnimize their surface area upto maximum extent.
Icr=~IN/m Wetting and Non-Wetting Liquids •
It is the mutual property of liquid-surface.
•
If adhesion »» cohesion, Liquid wets the surface. If cohesion > > > > adhesion, No wetting For wetting, angle of contact (8) should be acute and for non-wetting angle of contact (8) should be obtuse. For pure water 8 = 0°. For Mercury-glass, 8 = 130° to 140°.
Kinematic Viscosity (v) It is expressed as the ratio of dynamic viscosity (u) and density of fluid (p).
v=1:
p Units SI ~ m2/s CgS ~ Stokes/cmvs 1 stokes = 1Q-4 m2/s Effect of temperature and pressure on viscosity: • Viscosity of liquids decrease but that of gases increase with increase in temperature. • In ordinary situations, effect of pressure on viscosity is not so significant but in case of some oils, viscosity increase with increase in pressure.
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RHEOLOGY It is the branch of science in which we study about different types of fluids Bingham plastic
•
• •
CAPILLARITY When a tube of very fine diameter is immersed in a liquid, there will be rise or fall of liquid level in the tube depending upon whether the liquid is wetting with the tube or non-wetting. The rise or fall ofliquid level in the tube is a phenomenon known as capillarity. h : rise of liquid level in tube o : surface tension r : radius of capillary tube p : density of liquid 8 : angle of contact 2 c cos 8 h=--pgr
Rheopactic
'"'" ~~--....0:::
For an annular capillaryhaving external radius r2and inner radius r.,
0 For neutral equilibrium GM = 0 For unstable equilibrium GM < 0
Buoyancy
I
a h
leg
2 =
A =
a4 12 a2
When the bodies are immersed partially or fully in a fluid, the resultant hydrostatic force acts on the body in the vertical upward direction. This force is known as upthrust or buoyant force. FB : buoyant force FB = pgV V = volume of the fluid displaced by body
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Fluid Mechanics and Machinery
A-140
Centre of Buoyancy It is the point at which upthrust or buoyant force is acting on the body and is exactly same as the centre of gravity of displaced fluid.
Floatation For floatation of body, the density of the body must be equal to or less than density ofliquid i.e. ps s p density of density of solid liquid NOTE: For a completely submerged body, the centre of buoyancy doesn't change. However, for a floating body the centre of buoyancy changes when the orientation of body changes.
FLUIDKINEMATICS • •
There are two approaches to kinematics of a fluid flow i.e. Lagragian approach and Eularian approach. In classical fluid mechanics, Eularian approach is considered.
Different Types of Flow 1. Steadyflow If the properties in the flow are not changing with respect to time, such a flow is known a steady flow.
Badboys2 2. Uniformflow
•
Continuity equation: If states if no fluid in added/removed from the pipe in any length then mass passing across different reactions will be equal. Mathematically, for reaction (1 - 1) and (2 - 2), PlA1V1= P2A2V2 2
2
for incompressible fluid, A1V1= A2V2•
Continuity equation in cartesian - co-ordinates ~ (pu) + ~ (p v) + ~ (pw) + ap
ax
ay
Incompressibleflow If the density of the fluid doesn't change with respect to
pressure, the flow is known as incompressible flow.
4.
•
•
5.
Rotationaland Irrotationalflow If the fluid particles are rotating about their centre of mass, the flow is known as rotational flow. If the fluid particles aren't rotating about their centre of mass, the flow is known as irrotational flow. Laminar and turbulent flow: In Laminar flow, individual particles move in a zig-zag way. For Reynold's number (R ). If Re < 2000, flow in laminar If Re > 4000, flow in turbulent If 2000 < Re < 4000, flow may be laminar/turbulent Rate of flow / Discharge (Q): Q = Area x Average velocity Q=AxV
Internal and External flows : ---j. In case of an internal flow, it is surrounded or bounded by solid boundaries. Due to these solid boundaries the development of boundary layer is restricted. E.g: Flow through pipe ---j. In case of external flow, the fluid flows over the bodies which are immersed in an un-bounded fluid and hence the boundary layer develops freely in single direction. Eg : flows over air foild, turbine blades etc.
at
=
0
Acceleration of A Fluid Particle ~
A
A
A
V = u i-i v j-r w k ~
~
ev a=at
~
av ev =u-+v-+wax ay
~
ev az
J.. Convective acceleration
If the properties (velocityat any given time) is not changing with respect to space, such a flow is known as uniform flow.
3.
az
au au a =u-+v-+w-+-
ax ay av av a =u-+v-+w-+Y ax ay aw aw a =u-+v-+w-+x
z
ax
ay
au
~
+
av at
J.. L-...J temporal or local acceleration
au
az
at av av az at aw aw az at
a=~a2x+a2y+a2z For uniform flow, convective acceleration = 0 For steady flow
H _1
local/temporal acceleration = 0 For steady and uniform flow, total acceleration = 0 Consider a tank as shown in figure For the figure, convective acceleration = 0 temporal acceleration = 0 (if H is constant) temporal acceleration =1= 0 (if H is verying)
_
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Fluid Mechanics and Machinery
A-141
Stream Line It is an imaginary line drawn in such a waythat the tangent drawn
at any point on this line gives the direction of velocity vector of the fluid particle at that point. Equation of streamline in differential form
dy Id -~I
= Y= dx dx v Slope of equipotential line
where
Stream Function (w) •
It is defined only for 2D flows and is a function of space and time.
a\jJ ay a\jJ
PATH LINE
U= --
It is the actual path traced by a fluid particle.
V=-
ax
STREAK LINE It is the locus of all fluid particles at a moment which have passed
•
through a given point.
Rotational components in flow
V = ui = v} wz
There is no boundation on equation.
VORTICITY
Badboys2 It is defined as double of angular velocity. (Circulation per unit of enclosed area) Vorticity = 20)
as it satisfies continuiting
Equistream Line It is a line obtained byjoining points having same stream function values.
1 (av au 1 =-l---) 2 ax ay
where Wz is the net rotation of fluid particle about its own centre of mass. If'w, = 0 => flow is irrotational If'w, *- 0 => flow is rotational
\jJ
dy =~ dx u Slope of equistream line ( dY) dx
x ( dY) tjl=constant
dx
\jI=constant
=_~
x~
V
U
= (- 1)
:. Equistream and Equipotential lines are orthogonal to each other.
Cauchy-Riemann EqD In irrotational flows,
m
CIRCULATION
u= _ ax =_ a.v ~ ay B
IB
ax
= a.v1... (I) ay
It is defined as the line integral of velocity vector along a closed
loop.
r=~v.dr F = (Vorticity)Area
Velocity Potential Function (cj» • •
Velocity potential function q, is a function of space and time. It is defined in such a way q, that u v
aq, ax aq, =-ay aq, =--
w =--
az
•
where u, v and w are the components of velocity vector in x, yand z direction. q, only exists in irrotational flow. For this, q, must satisfy laplace equation i.e.
1'12 q, = 01 Equipotential Line It is a line joining the points having same potential function values.
v=-:=~
~ 1:=-~1
... (2)
Equations (1) and (2) are known as Cauchy-Riemann equations. FLUID DYNAMICS The following types of energies are involved in fluid dynamics. (a) Kinetic energy: Kinetic energy is defined as the energy which is because of motion of the body. (b) Potential energy : Potential energy is defined as the energy due to elevation of the body above the specified I arbitrary datum. (c) Pressure Energy : Pressure energy is defined as the energy due to pressure above datum (d) Internal energy: Internal energy is defined as the energy related with the inter-molecular altratiction of forces or internal state of matter. It can be stored as nuclear energy, thermal energy, chemical energy etc. Some expressions regarding above energies kinetic energy (k.E) = ..!..mv2 2 where, m = mass of the body, v = velocity of the body (b) Potential Energy (P.E) = mgH where, m = mass of the body H = elevation of the body from datum g = 9.8 m/s? (c) Pressure energy: Pressure energy (PEnergy) = VH (a)
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Fluid Mechanics and Machinery
A-142
DifferentKindsofHeads (a) Head: It is described as the amount of energy per unit weight. (b)
Kinetic head: It is defined as the kinetic energy per unit weight. kimetre . h ead =
kinetic energy weight of the body
-------=::..::...._-
P V2 The Bernoulli's Eq=in such a casecan be written as-1 + -21 + Zt pg g P2
vi
=-+-+Z2 +hf pg 2g where hf : head losses encountered as the fluid flows from point 1 to 2.
Applications of Bernoulli's Equation mg 2
(c)
kinetic Head = ~ 2g Potential head : It is defined as the potential energy per unit weight. . Ih d Potential energy Potentia ea = ------=..::...weight of the body mgH mg Potential head = H (d) Pressure Head: It is defined as the fluid pressure per unit specific weight. fluid pressure P =specificweight pg (e) Total head: It is defined as the sum of kinetic head, potential head and pressure head. Total head = kinetic head + potential head + Pressure head Pressure head =
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v2 HT=-+H+2g
(f)
v=~2gH Flow sensors
Flow Measurement Devices Venturimeter
P
pg
It is a highly accurate device used for measurement of discharge.
Euler's Equation of Motion The • • •
Following are the applications of Bernoulli's equation given below: (a) Sizing of pumps: In case of pumps, kinetic energy is converted into pressure energy according to Bernoulli's eqaution. (b) Ejectors: In ejectors, pressure energy of the fluid is converted into velocity energy for the purpose of entraining suction fluid. The mixed fluid is recompressed by converting velocity enegry into pressure energy. This process is based on Bernoulli's equation. (c) Pitot tube: It is utilized for the purpose of measuring the fluid flow velocity. The principle of pitot tube is based on the Bernoulli's equation. (d) Carburetor: Carburetor also works on the basis of Bernocelli's equation. When the velocity of air is increased, it lowers the static pressure and increases the value of dynamic pressure. (e) Siphon: A siphon is a device used for the purpose of removing a liquid from its container. The velocity expression is given as following:
CD
Euler's equation considers the following assumptions Flow is irrotational Flow is laminar Flow is invicid.
I~
+ V dv + g dz
= 0 I~
Throat (minimum cross-sectional area)
Euler's Eqn for steady flow
Integrating the above equation. We obtain Bernoulli's equation
P
CD
Converging section 2gh QTH = A2Al A2 _ A2
V2
- + - + Z= constant (Head form) pg 2g 1 P + - pV2 + pgz = constant 2 For Bernoulli's equation, there are two more assumptions i.e. • flow is steady • flow is incompressible Under the five assumptions stated above, the summation of all energies (Pressure, Kinetic and Potential) per unit volume remains constant at each and every point in a flow. BERNOULLI'S EQUATION FOR REAL FLUID In real fluids, viscous shear stresses are present due to which energy is not conserved.
1
Diverging section
2
Q = Cd QTH Coefficientof discharge (it's value vasics between 0.96 0.98)
Cd=~ h : piezometric head difference between 1 and 2 hL : head loss
Orifice meter : The principle of an orifice meter is same as that of venturimeter. In this type, the cross-section of the flowing stream is reduced while passing through the orifice, the value of velocity head is increased at the expense of pressure head. Bernoulli's equation
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Fluid Mechanics and Machinery provides a basis for the purpose of correlation maintained between increase in velocity head with the decrease in pressure head. An orifice meter is also termed as pipe orifice or orifice plate, In can be easily installed in the pipeline. A thin circular plate along with a hole in it is placed in the orifice meter. The diameter 1 of an orifice meter is generally kept"2 times the pipe diameter orifice meter is most commonly used for the purpose of measuring the flow offluid in pipes having fluids of single phase. ~2
I'
Vena confracta
:rl~~e
A-143
Working principle: A pitot tube consists of a tube which points directly into the flow of fluid. The liquid flows up the tube and after attaining equilibrium, the liquid is reached at a height above the free surface of the water stream. Now, neglecting friction, Po- P = Hpg where, Po = stagnation pressure P = static pressure Velocity (v) = ~2gH Flow Through Pipe Bends • The main aim of this chapter is to determine the forces. • The pipe bend is horizontal. Hence, there wouldbe no effect of weight. Consider a pipe bend as shown,
Direction of flow -+-1---+
Differential manometer
Fluid (system)
Orifice Meter Discharge is given as :
Fluid
Cc·~d2J2gH
Badboys2
Q=
~
or Q-CdAo~2gH where,
P1 A1
Cd = (coefficient of discharge)
l-C~(~r A = Area of cross-section of orifice meter D = diameter of pipe at section (1) d = diameter of pipe at section (2) Cc = contraction coefficient of water jet ~ Cd (coefficient of discharge) depends upon the Reynold's number (Rc) Pitot tube: A pitot tube is a device which is used for the purpose of measuring the velocity of fluid flow. It has a wide applicability such as for calculating the speed of air of an aircraft, speed of water of boat and also for measuring the velocities of liquid, air or gas in various industrius applications. A pitot tube is utilized for measuring the local velocity at a point in the flow stream.
Pitot tube
Fx, Fy are the horizontal and vertical forces acting on the fluid element. By momentum equation, Fxand Fycan be found PIAl - P2A2cos e + Fx=mV2 cos e - mVI Fy- P2A2sin
e = mV2sin e
F = \jIF2x + F2 Y Vortex flows •
When a certain mass of fluid is rotating with respect to some different axis, such a flow is known as Vortex flow. • There are 2 types of vortex flow (i) Free vortex (ii) Forced vortex FREE VORTEX • No external torque is required. Hence angular momentum remains conserved. 1 V: velocity Vocr r: radius FORCED VORTEX • External torque is required to maintain its angular velocity at a constant value. w= constant V o: r NOTE: • Free vortex flows are irrotational flows and thus, Bernoulli's equation can be applied.
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Fluid Mechanics and Machinery
A-144
•
Forced vortex flows are rotational flows and hence, Bernoulli's equation cannot be applied.
(ii) Exit Losses
Fundamental Equation of Vortex Flows dp = pw2 r dr - pg dZ General equation and can be applied between any two points For free surface, dp = 0 => pw? r dr = pg dZ Integrating the above equation we get,
Iz = • •
V12
h.=
2g
(iii) Losses Due to Sudden contraction
w:t I
A pipe is a closed contour which carries fluid under pressure. When fluid flows through pipe, it encounters losses. These losses can be broadly categorized into (i) Major losses (ii) Minor losses
CD
Major Losses •
h.=
These losses are due to friction. The losses are evaluated by Darcy-Weishback Equation.
vi [_1 _1]2 2g Cc
A2
CC=A
fLV2 ...(1)
2gd
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f: friction factors L : length of pipe
V : velocity in pipe of fluid d : diameter of pipe f= 4 f friction coefficient The above equation (1) is valid for both laminar and turbulent flow.
3
If'C, is not given, hj= 0.5 Vll2g Head loss occurs after Venacontracta as boundry layer separation occurs.
(iv) Entrance Losses
NOTE: Head loss is independent of pipe orientation. It depends only on details of the flow through the duct. For fully developed laminar flow, f= 64/Re where Re : Reynold's No.
O.5V2 h--f2g BEND LOSSES
pVD Re=-J..l
KV2 h--f- 2g
V = velocity D = diameter m = dynamic Viscosity
K = Constant which depends upon angle of bend and its radius of curvature.
Minor Losses • •
Bernoulli's Equation, momentum Eq" are used to determine these losses. The magnitude of minor losses is very loss.
FLOW THROUGH BRANCHED PIPES
Pipes in Series •
In series, discharge (Q) remains same but head is divided.
(i) Losses Due to Sudden Enlargement ,~
~:
N_ CD I
®
......_
-'
121 ~
13, d31 I : I
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Fluid Mechanics and Machinery Q=Q] =Q2=Q3 h-= (hd]
=>
2g
+ L2Vj + L3V}]
d1
Shear between fluid layers e = Il du/dy(x-dir.)
Entrance Length
+ (hf)2 + (hfh
h=~[LIV?
A-145 •
d2
d3
Dupit's Equation A pipe ofuniform diameter is said to be equivalent to compound pipe if it carries same discharge and encounters same losses.
The distance in downstream from the entrance to the location at which fully developed flow begins is called L entrance length for laminar flow in pipes. ~
= 0.06 R,
L, = entrance length D = diameter of pipe Steady Laminar Flow in Circular Pipes
~ ! --! -~---PIPES IN PARALLEL •
~P+.?Pdx "[:
In parallel arrangement, discharge gets divided.
Q=Q1 +Q2
(hf)1 = (hfh SYPHON
• Siphon is a long bend pipe used in carrying water from a Badboys2 reservoir at higher level to another reservoir at lower level.
•
shear stress R: radius of pipe M : dynamic viscosity of fluid
1" :
9+~---t-~• •
The height point of siphon is called summit. No section of the pipe will be more than 7.6 m above the hydraulic gradient line. When absolute pressure of water becomes less than 2.7 m gases come out from water and get collected at the summit thereby providing an obstruction to flow.
POWER TRANSMISSION THROUGH PIPE \J
ap
ax : pressure
gradient
u : velocity at a distance 'r ' from cente
t=(-:H u = - _1_
(ap)
(R 2 _ r2) 4M from above expression of 'u', we can conclude that velocity is varying parabolically.
ax
1 ( = - 4M
Umax
ap) ax
2
'1
l-2-) U max
Umax
.fi ,= U = U
P1 - P2 P theoretical = pQgH P actual = pQ (H - hf) where hf are the head losses in pipe. pQ (H - hf) 11= pQgH
=
i.e. average velocity equals the local
32MUL D2
LAMINAR FLOW BETWEEN
TWO PARALLEL PLATES
Case I : One plate is moving with a velocity of 'U' while the other is stationary.
--r-bj
= ~ I·
d~t-'--::I;;?::-
Laminar Flow in Pipes At low velocity of real fluids, viscosity is dominant. The flow of fluid takes place in form oflaminar. This laminated flow is known as laminar. No slip at boundary Flow is rotational No mixing of fluid layers
-u
(ap)
Uy 1 u=--b 2M
ax
(by-y
2
)
Case II : When both plates are at rest
Features of Laminar Flow • • •
( 1t R
velocity. Pressure drop (P, - P2) in a given finite length 'L'
H
•
2
R ,Q =
u=-2 at r = R/
for maximum efficiency 1he
Ox
head losses remain same but
u
=-
2~ (:)
(by - y2) (Poiseuille
flow)
Badboys2
Fluid Mechanics and Machinery
A-146
= __ 1
u
_
U
(ap)b
2
8J.l ax
max
•
large velocity gradients existing in it. Velocity within the boundary layer increases from zero to main stream velocity asumptically.
2
=3 Umax
TURBULENT FLOWS •
•
•
Boundary layer
In turbul ent flow, there is continuous mixing of fluid particles and hence velocity fluctuates continuously. u' and v' are fluctuating components of velocity
2 ( du ,2 1: =
1: = turbulent
shear stress
I: mixing length, 1= 0.4 y, y is distance from pipe wall Mixing length is the length in transverse direction where in fluid particles after colliding loose excess momentum and reach the momentum as of local environment.
•
Umax
- U
= 5.75 10glO (R/y)
---=..:..:,:=..:....___
V*
V. : Shear velocity V. u
(
y,
=t
Badboys2 • V* = 5.75 10glO ly') •
y' = 0' I 107 (for smooth pipes) y' = Kl30 (for rough pipes) Reynold's condition for rough & smooth pipes
• •
ReR < 4
y=o
au
y=O
-=0
a2u
ay2
ax
•
o IX X 1/2 'x' is the distance from leading edge of the plate. As x increase, boundary layer thickness increases. The transition from laminar to turbulent flow is decided by Reynold's No. R, ::;5 x 105 => flow is laminar R, > 6 x 105 => flow is turbulent
•
> O.
Displacement thickness (0) 0*
0'
=
1 o
(ll- ~ ') dy UOC)
Momentum thickness (8)
=> transition
8=
Thickness of Laminar Sublayer (0')
1
~(ll-
~)'
o UOC)
UOC)
dy
Energy thickness (OE)
11.6v
8
=--
fo
V*
II-
u (
OE = -U
Hydrodynamically Rough and Smooth Boundaries
87 < 0.25
ap
For separation of boundary layer,
In turbulent flow in pipes, average velocity equals local velocity at y = 0.223 R.
K
=0
ay
o : boundary layer thickness UOC) : free stream velocity Nominal thickness is the thickness of boundary layer for which J.l= 0.99 UOC) In case ofa converging flow (aPlax = - ve), the boundary layer growth is retarded.
=> smooth pipe
From Nikuradsee's
u = 0.99Uo
=> rough pipe
4 < ReR < 100 •
u=O
•
v ReR > 100
Conditions
y= 0 y=o at
ldy)
pu'v' = P I
Boundry
OC)
experiment,
Shape factor (H)
=> smooth boundary
=
u2 -2
,
j dy
Uoc>
0* e
Von Karman's Momentum Integral Equation Assumptions K
87 > 6
=> rough boundary
K 6 .. 0.25 < - < => transition
•
Flow is 2D, incompressible
•
-=0
dP
~
d8
dx
pU~
dx
0'
BOUNDARY LAYER THEORY • •
The concept of boundary layer was first introduced by L. Prandtl. Boundary layer is a layer in the vicinity of the surface with
and steady
where 8 : momentum thickness 1:0 : plate shear stress p : density UOC) : free stream velocity
Drag force (FD)
Badboys2
Fluid Mechanics and Machinery It is the force exerted by the fluid in a direction parallel to relative motion. A zero angle of incidence, of the plate the drag force is due to shear force. C _-fx _
A-147 (b) Medium head and small quantity of flowing water (c) Low head and larger quantity of flowing water Based on the specific speed of the turbine (a) Low specific speed turbine (specific speed < 60) (b) Medium specific speed turbine (specific speed : 60 to 400) (c) High specific speed turbine (specific speed: above 400)
~
to
1 2 -pU 2
00
CD = average drag coefficient Cfx = local drag coefficient For air flow over a flat plate, velocity (U) and boundary layer thickness (8) can be expressed as
~ =Hi)-Hir
Basic Definitions of Hydraulic Turbines ~ Impluse turbine : In this type, only kinetic energy is available at the inlet of turbine. Eg : Pelton wheel turbine
~
Reaction turbine : In this type, kinetic energy and pressure energy both available at the inlet of turbine. Eg : kaplan turbine, Francis turbine Radial flow turbine : In this type, the flowing of water is in the radial direction through the runner. Inward radial flowturbine: In this type, the flowing of water is from outward to inward radially. Outward radial flowturbine : In this type, the flowing of water is from inward to outward radially. Axial flow turbines : In this type, the flowing of water is through the runner along the direction parallel to the rotational axis of the runner. Mixed flow turbine : In this type, the flowing of water is through the runner in radial direction but leaves in the direction parallel to the rotational axis of the runner. Yangential flow turbine : In this type, the flowing of water is along the tangent of the runner.
~ ~
4.64 x
8 = ~Rex
~
TURBOMACHINERY
~
The conversion of energy carried by water into electrical energy is carried out by the turbo-generator. In this a rotating turbine driven by the water and connected by a common shaft to the rotor of a generator. Any turbine consists of a set of curved blades designed to deflect the water in such a way that it gives up as much as possible of its energy. The blades and their support structure make up the turbine runner, and the water is directed on to this either by channels and guide vanes or through a jet, depending on the type of turbine. The efficiency of any turbomachine
Badboys2
~ ~
ComparisonbetweenImpulseTurbineand ReactionTurbine
P[Power output)
11 = ---~--~=--..:....._-1000 x Q x g x H(Power input) where, Q = flow rate of the falling water the number of cubic metres per second g = acceleration due to gravity H = effective head Mass of a cubic metre of fresh water = 1000 kg .. mass falling per second = 1000 x Q
Classification of Hydraulic Turbines The hydraulic turbines are classified based on the following basis: ~ Based on the type of energy at inlet (a) Impulse turbines (b) Reaction turbines ~ Based on the direction of flowing water (a) Tangential flow turbines (b) Axial flow turbines (c) Radial flow turbines • Inward radial flow turbines • Outward radial flow turbined (d) Mined flow turbines ~ Based on the Head of water and water quantity available (a) High head and small quantity of flowing water
Reaction turbine (i) A part of energy offluid is converted into kinetic energy before entering the fluid into turbine.
(ii) There are no losses in (ii) There are losses in flow flow regulations regulations (iii) The whole unit is placed above the tailrace
(iii) The whole unit is submerged in water below tailrace (iv) Blades are in acting (iv) Blades are in acting mode only when they are mode at all the time in front ofno:zzle
Hydraulic Turbines In hydraulic turbines, the conversion of hydraulic energy into mechanical energy takes place. This mechanical energy is utilized for running an electrical generator which is directly connected with the shaft of the hydraulic turbine. Thus, finally, the conversion of mechanical into electrical energy takes place.
Impulse turbine (i) In this, the conversion of potential energy into kinetic energy takes place by nozzle before entering to turbine
Pelton Wheel Turbine The pelton wheel is an impulse turbine. The Pelton wheel turbine with water flow from moving cup (b) and actual motion of water and cup (c) are shown in fig. below. (8)
(b):) ~)
(e);::
v=Q
water
Fig. : Pelton wheel turbine: (a) vertical section ; (b) water flow as seen from moving cup; (c) actual motion of water and cup The volume rate of flow Q corresponding to head H
Badboys2
Fluid Mechanics and Machinery
A-148
Vw = velocity of whirl at outlet
Q= A~(2gH)
2
u = peripheral velocity
where A = area ofthe jet g = acceleration due to gravity. The input power to the turbine P
Vr1 = relative velocity at inlet Vr2 = relative velocity at outlet
= 1000 x Q x g x H = 1000 x A~2gH x g x H
VI = absolute velocity at inlet V 2 = absolute velocity at outlet
(.: Q = A~2gH ) Specific Speed Ns
=
1] x ~
2
P
r;;
H x"H
Where,
11 = rate of rotation (in rpm) H = effective head (in m) P = available power (in kW) The range of specific speed for pelton wheel is 10-80 Main parts of a pelton turbine : The following are the main parts are given of a pelton turbine: (a) Nozzle and spear: Spear controls the amount of water that strikes the buckets. (b) Runner: It consists of circular shaped disk. On the periphery of this circular disk, number of buckets are fixed evenly, buckets have the shape of hemi - spherical cup and divided by the splitter which divides the water jet into two parts, Runner is made up of cast iron or stainless steel etc. (c) Casing: Itacts as cover and prevents the water splashing. It is made up of cast iron and steel etc. (d) Breaking jet: It strikes the back of vane and utilized for stopping runner in a very short duration oftime. Velocity triangle for Pelton wheel :
Vf2 = velocity of flow at outlet Efficiencies : (a) Hydraulic efficiency : It is defined as the ratio of work done / second by jet of water to the input energy / second.
l1H
*
u
Vu
2
2
p
)1
V?
Maximum hydraulic efficiency (l1max.) (b)
(c)
(d)
=
1+ coso 2
Mechanical efficiency : It is described as the ratio of power available at the shaft and the power produced by the wheel. l1mech.
Badboys2
IE
2U(VWI ± VW2 ) = -....:....._-:----....:....
Pshaft
= Q
H
P W W The value of mechanical efficiency varies between 0.97 to 0.99. Volumetric efficiency : It is defined as the ratio of volume of water actually strikes the buckets and total volume of water supplied by the jet to the turbine. Overall efficiency : It is defined as the product of hydraulic efficiency (l1H)' mechanical efficiency (l1mech) and volumemetric efficiency (11 vol). 110 = l1H x l1mech x 11vol.
VU1
Some important formulas : Gross head = HG = difference between head race and tail race Net head = Hnet = ~ - hf- h
Francis Turbine Francis turbines are by far the most common type in presentday medium or large-scale plants. They are used in installations where the head is as low as two metres or as high as 300. These are radial-flow turbines. Francis turbine is completely submerged, it can run equally well with its axis horizontal or vertical. Francis turbine is shown in figure below
fLv2 where, hf
=--
2gdp
here, f = frictional factor L = Length of penstock v = mean velocity in penstock d, = diameter of penstock h = height of nozzle above the tail race ~
Work done/second
= (VWI ± VW2 ).u pavl
~
work done / weight
= (VWl ± VW2 ).g
u
where, Vw = velocity of whirl at inlet 1
Fig. : Francis turbine: (a) cut-away diagram; (b) flow across guide vanes and runner Francis turbines are most efficient when the blades are moving nearly as fast as the water, so high heads imply high speeds of
Badboys2
Fluid Mechanics and Machinery
A-149
rotation. T he range
~ 0f
W.D/s = pQd[VW1Ul ± VW2U2]
l
~) speerifiIC speeds, ( Ns = n x ~~)
for •
Francis turbine is 70-500.
Main parts of a Francis turbine : (a) Penstock: It is a tube of large diameter through which (b)
(c)
(d) (e)
Work done per second: (W.D/s)
water from dams reaches to the inlet of the turbine. Spiral casing : It is a closed passage. The diameter of the spiral casing is decreases along the flowing direction. The area of spiral casing is maximum at inlet and minimum (nearly zero) at outlet. Guide vanes : It is an aerofoil like shape vane which is fixed between two rings and a part of pressure energy is converted into kinetic energy by guide vanes. Runner: It is connected to the shaft of the turbine. Draft tube: It is defined as tube which expands gradually and it discharges water passing through the runner to the tail race.
For radial discharge,
VW2 = 0 , then
W.D/s = pQdVwI ul
~
Hydraulic efficiency : It is defined as the ratio of workdone per second on the runner and the energy at inlet/ second. 11H =
~
~
VWIUl±VW2·U2 gH
Mechanical efficiency : It is defined as the power to the power developed by the runner. as 11mech. Volumetric efficiency (11vol): It is defined quantity of fluid working on the runner to the of fluid supplied.
ratio of shaft It is denoted as the actual total quantity
Actual fluid quantity
Velocity triangle of Francis turbine :
11vol . = T ota I flU1·d quantity . ~
Badboys2
Overall efficiency (110): It is defined as the ratio of shaft power to the input power. It may also be defined as the product of hydraulic efficiency, mechanical efficiency and volumetric efficiency. 110
=
shaft power Input power
or 110 = 11H x 11mech. x 11vol.
KAPLAN TURBINES Kaplan turbine is a axial flow or propeller type turbine which has adjustable blades. It is an inward flow reaction turbine, i.e., the working fluid changes pressure as it moves through the turbine and gives up its energy. Axial-flow turbine and runner of kaplan-turbine shown in figure below.
Some important formulas : ~
Net head (H) :
•
At the exit of penstock,
• •
At the exit of draft tube
.
P , H = -- P -
v2
2g
Zl --
v2
2g
p
v2
P
2g
At the exit of draft tube when the position of draft tube is at the tail race. H=--
•
p
+-
At the exit of pen stock when the position of draft tube is at the tail race. H=-+z.+-
•
P
H =-+zl
Fig. : A Propeller or axial-flow turbine Main parts of a kaplan turbine: (a) Scroll casing: It is the casing in which guiding the water (b)
v2
2g
If the velocity at the exit of draft tube is negligible, then Net head, at the exit of penstock.
H=(~+Z -+-+-=0
D~
Pl~
oc"":""'_-----'-
6.
a
at D = 0
(ii) Fully developed the ~ D - 0 ; properties are not
+ dv ~ fluid flow
1
where,
Flow is steady y (i.e.)
v-u (c) ~
(y)k
= 5.75 10glO
+ 3.75
for v= u
(f)
5.75 + 10glO .1_ R
=
+ 3.75 = 0
0.223 => y = 0.223 R
Badboys2
Fluid Mechanics and Machinery 29.
(a)
A-169
Consider an element of disc at a distance r and having width dr.
dr
,r,\"
\~:\ \\" 1\('",,:, \,I,\,:, \\:~ Linear velocity at this radius = reo du Shear stress = Mdy torque = shear stress x area x r =
't
2 7tr dr x r
du
= M- 2m2dr
~p
dy assuming that gap h is small so that velocity distribution may be assumed linear dy
rro h
dT = M- 2m2dr
Badboys2
~p L
h
J2 7tMCOr3dr = M7tdco
d/2
o
4
h
'to =
dy
= 6.30
x
10-4 m2/s
= 7.91
4
x
105
rx;
= 5 ..{~
- 1.328 CD----
_
~RcL
2
45.
= 5
l x 0.15 X 10-4 6
10-3 m = 7.91 mm
x
x 105 (for middle
1.328 -2 - 1 ~4x 105 .
FD = 2 x 1 x 1 x
IfH is the head causing the flow, then V2 6LV2 H= _2 +--' 2g 2gD
x
point of plate)
10-3
6V2
Cf--
2
1226(6)2 = 2 x 1 x 1 x 2.1 x 10-3 x --2 = 92.69 x 10-3 N (a) Kinematic viscosity, U = 2.25 dia of pipe, d = 20 em Rate of flow = 1.5 liters/sec Now to find the flow we must know the reynolds number
Re=So
dM -=0 d6
6xl
6xO.5 Rex= 0.15 X 10-4 = 2
=~d2V
'4
pD3
6= 5~lx0.15xl0-4
7td2V; g 4 from continuity equation
4
64000M2
pDoD2
Hence the boundary layer is laminar over the entire length of the plate.
W
= ~D2V
flow, maximum
32M X 2000M2
VL
w 39. (b) Momentum of issuing jet is M = -QV2 g
Q
In laminar
42. (b) R, = --;- = 0.15 X 10-4
du du Shear stress = fl = pv dy dy = 129.3 x 6.30 x 10-4 x 0.25 (Asp = 129.3) = 0.02036 kg/rrr'
M=
be maximum so V
pD3
· gra dilent = -du = 0.25 m/ sec meter 31. (b) Ve Iocrty U
L should
16000M2
32h
Kinematic Viscosity,
be maximum
pVD attained when -= 2000 M
27tMCO r3dr
= --
'to to
should be maximum. velocity will be
rco h
du
T=
For
,
Now
U=
,
vd v 15000
(~)x(20)2
Re =
47.75 em/sec.
ud 20x47.75 - = v 2.25
= 424.4
Badboys2
Fluid Mechanics and Machinery
A-170
48.
R, = 424.4, means Reynoldsnumber ofthis flow is less then 2000(424.4< 2000) Hence the flow is "Laminar" (a) Given here, = 2xy, considering the following relation, -a a\jl =>4= -=-ax ay
62.
then,
65.
a\jl (a a\jl '1 _ a => - I --) -ax -" ay' ax ax a= ~(2xy) ax ax Similarly, a= 2x = ay
_ a\jl ax
On integrating, a\jl = 0+ ay
-
67.
= 2y = _ a\jl ay
C'
f a\jl = -2x => ax
2 \jI
= - 2x + C (y) z
Badboys2 49. (a)
2y2 _2x2 2y2 (y) = -2 +cl> lthen, 'I' = --+-+Cl
2
2
= y2 _ x2 + c1 or y2 - x2 + constant If flow in 2D, continuity equation becomes,
\jI
ay av ax ay So, for (i), u = x2 cos y, V = - 2x sin y
-+- =0
ay av ax + ay = 2xcosy-2xcosy
=0
au av for (ii), ax + ay = 1 - 1 = 0 au av 2yt for (iii) ax + ay = yt-T = yt - yt = 0 au av 1 1 for (iv) -+= -+x--= x ax ay x x 57.
H _ _E__13.6x103 x60x10-2 x9.81 - pg 1000x 9.81
=8.16m = 816 em (a) Given: Depth (h) = 5 km = 5 x 1000 = 5000 m Specific gravity = 1.3 P = pgh = 1.3 x 9.8 x 5000 P=63700Pa Valueof'P' IS closed to 63765 Pa' (b) Weight of wood piece (m) = 5 kg Weight of wood piece = weight of liquid displaced S = 60% of volume (v) x 1000 =~xl000 100 S = 600 v 513 v=-=-m 600 120 density of wooden piece (p] = Mass(m) Volume(v)
(y)
f c'(y) = f 2y C
(b) Let H = height of water column,
(c) Given: Pressure intensity (P) = 1.006 MN/m2 Specific gravity = 1.025, density (p) = 1.025 x 103 Let 'H' be the depth of point below water surface in sea we know that, P=pgH 6
H = _E_ = 1.006x10 = 100.04m ~ 100 m pg 1025x9.8 58. (b) Given: specific gravity of oil = 0.7 Pressure (p) = 0.14 kgf I emDensity of oil (Poil) = 0.7 x 1000 = 700 kg/m' H=_E_= 0.14x9.81 pg 700x9.81 0.14 x 9.81 x 10000 700 x9.81 1400 = 700 = 2 m of oil.
5 =-=600kg/m 1 120
3
Specific gravity of wood =
density of wood piece d . f ensity 0 water
=~=0.6 1000 69. (d) Hydrostatic force on vertical walls, PI = pgHI (i) P2 = pgll, (ii) Here, heights of vertical walls, HI =H2=4m then
_!l_ = pgHl =!!L 'P2
pgH2
H2
_!1_=i=l P2 4 70. (b) From Newton's Law of viscosity, (J.!) Shear stress (r) = J.!x velocity gradient du 't = J.!xdy 't
N
J.!= du =-2 _
m
dy
S
J.!= J:£_s = [MLr2][T] m2 [L2] J.!= [ML- 1 T- 1] 72. (d) Given: Atmospheric Pressure (Patm)= 1.03kg! em? Vapourpressure (Pv) = 0.03 kg I cm2 Air pressure (Pa) = ? P = Patm-P v = 1.03-0.03 = 1kg/ern74. (b) (fauge pressure (Pg) = 21 bar Atmospheric pressure (PatIn)= 1.013bar Absolute pressure (Pab)= 't Pab= Pg + Patm = 21 + 1.013 =22.013 bar
Badboys2
Fluid Mechanics and Machinery 75. (a) Given: diameter of pipe (D) = 20 em kinematic viscosity (v) = 0.0101 stoke Reynold'snumber (Re)= 2320 Let v = velocity of flowing water,
A-171 1
= +2
94.
Re = vD = vx20 v 0.0101
76.
2320 = v x 20 =? v = 2320 x 0.0101 0.0101 20 v = 1.1716 em/s ~ 1.117 cm/s (a) Given: Mass ofliquid = 5 tonnes = 5 x 103 kg volume= 10m3
Re= vD Il P
. .. mass of liquid mass density of liquid (p) = ---~volume
= 5 x 103 = 5 x 102 78.
10 = 500 kg/m' (b) Given: diameter of glass tube (d) = 3 mm surface tension (crT)= 0.0736 N/m contact angle for water (a) = 0° 4crT cosa Then capilary rise (H) = wd 4 x 0.0736 x cos 0°
4 x 0.0736 x 1
kg-f xd 9.81 x 10-6 x 3 = 10.4 mm (approx.) (c) Initial volume(vI) = 20 m3 Initial presssure (P1) = 100Pa Final volume (v ) = 40 m3 Final pressure 2)= 50 Pa Change in volume (dv) = V2- VI = 40 - 20 = 20 m3 Change in pressure (dP) = PI - P2= 100- 50 = 50 Pa change in pressure Bulk modulus of elasticity (k) = V oume lumetrifIC stram .
80. Badboys2
(p
82.
86.
93.
= dP =~=50Pa dv 20 vI 20 (c) Given: Gauge Pressure (Pg) = 25 bar Atmospheric pressure (Patm) = 1.03 g=9.81 m/s2 Absolute pressure (Pabs) = Pg + P t =25+ 1.03 am = 26.03 bar (b) Given: Side of the cube ~a)= 5 em = 5 x 10-2m Volume of cube (v) = (a) =(5 x 10-2)3 = 125x l0-6m3 Buoyant force acting on the cube =v.pg = 125x 10---6 x1000x 10 (Assuming g = 10m/s2) = 125x 10-2 = 1.25 N (a) Average velocity (Vavg) =
Maximum velocity (Vmax) Vavg. Vmax.
ap (Rf
ax =
-R~) 81l
ap(Rf -R~) ax 41l
(b) Given: kinematic viscosity (v) = 0.25 stokes diameter of pipe (D) = 10em for a critical velocity, Reynold's number should be between 2000 and 4000 . PvD Reynold's number ( Re) = -Il
= vD v 2
96.
2000= vxl0x10 =?V= 2000xO.25 =0.5m/s 0.25 IOx102 (a) Given: specific gravity = 0.85 Viscosity (u) = 3.8 poise N
= 0.38-2 s m
Diameter (D) = 5 cm flow velocity (v) = 2 m/s density (p) = 0.85 x 1000 = 850 kg/m'' pvD Reynold's number (Re) =-Il 850 x 2 x 5 x 10-2 0.38 = 223.7 = 224 104. (a) Given: fluid velocity (v) = 20 m/s pipe diameter (d) = 1 m dynamic density (p) = 0.150 kg/m' fluid viscosityru) = 0.0000122 pvd 0.15x20x1 Reynold's number ( Re ) = = ---Il 0.0000122 =2458901.6 ~ 245902 105. (b) Given: Average velocity (Vavg) = 5m/s 1 pipe radius (R) = 10 em = 10m = O.1m 1
another radius (r) = 5 cm = 20 m = 0.05 m According to velocity distribution, Umax. =2V avg. =2x5=10m/s
Vavg =Umax [1- ~:] =1+
(~~O~n
= 10[1- 0~~~5]= 10[0.75] Vavg. = 7.5 m/s 124. (a) Given: discharge (Qd) = 0.05 m3/s, f= 0.0025 Specific gravity = 0.7 diameter of pipe (d) = 0.2 m length of pipe (L) = 1000m Considering the following formula for head loss, H _ 4fLv2 L2gd for discharge (Qd) = Area x velocity
Badboys2
Fluid Mechanics and Machinery
A-I72
149. (b) Given: wheel speed (N) = 600 rpm 005=Axv= .
=~d2 4
Speed ratio (k) = 0.44 net head (H) = 300 m
xv
0.05 = ~(0.2)2 xv 4 4 x 0.05
v=
Speed ratio (k) =
H ow,
dl ea
oss
u _ 1tDN
1.59m/s
1tx (0.2)2 N
b
,,2gH where,
(H) 4xO.0025xl000x(I.59)2 L = 2xl0x0.2
-
k=
60
1tDN 60~2gH
= 6.32 m (which is near to 6.44 m)
125. (b)
0.44 =
4fL V2 Head loss (HL) = --2gd HL ocv2 HLI vr HLI --=-:::::>--=--=HL2 v~ HL2 HL2 = 4
X
(v)2
1
(2v)2
4
159. (c)
HLI = 4 times
127. (b) Given: MaximuI? velocity (vmax) = 6 m/s We know that the Ratio Maximum velocity ( v max.) _ Mean velocity (v mean)
Badboys2 134. (a)
vmax.
- 2
1000 x 9.8 x 0.013 x 32
2
6 3 2x6 --=-:::::>vmean =--=4m/s vmean 2 m Given: mean diameter of runner (DQ1)= 200 mm Least diameter of jet (dL) = 1 em = 10 mm
. . ( ) Dm 200 jet ratio m =-=-=20 dL 10
1
Number of buckets (Nb) = 15+2m
1000x 6 = 0.679 = 67.91 == 69% (approx.) over all efficiency (110) is nears to the value 69%. 173. (a) Diameter of pipe (d) = 2 decimeter, Length (L) = 5
km Average speed of water (v) = 2m/s con stan t head (H) = 5 m Darcy's friction factor (t) = 0.01 Let Pabs= absolute discharge pressure at pump exit. Pabs 4tLv HL (head Loss) = pg = 2gd
20
= 15+2
= 15 + 10 = 25 Number of buckets = 15 + 0.5 m = 15 + 0.5 x 18
= 15 + 9 = 24 138. (a) Given: Power developed (P) = 3000 kw
m3/s
Overall efficiency of turbine
('10) = (
Ql Nl -=-:::::>--=-Q2 N2 p ) pgQdH 1000
P xl000 pgQdH
2
p _ 4fLv2p 4 x 0.01 x 5000 x (2)2 x 1000 abs 2d 2 x 0.2 Pbs = 5.503 bar 177. (a) (ftven: Initial parameters: Q1 = 1200 m3/s, N1 = 1000 rpm Final parameters: Q2 = ? , N2 = 1500 rpm for centrifugal pump, Q a: N
136. (a) Given: jet ratio (m) = 18
Head (H) = 5 m discharge (Qd) = 75
0.44 x 60.J2 x 9.8 x 300 D=-------3.14 x 600 = 1.07 m or 1.08 (approx.) Water lifted I s (Qd) = 0.013 m3/s depth (h) = 32 m Power consumption (P ) = 6 kw water density (p) = 1O~Okg/m-' pgQd h overall efficiency (110) = 1000 x Pc
i
=i
vmean
3.14 x D x 600
60·J2 x 9.8 x 300
3000 x 1000 1000x9.8x75x5
= 0.815 ~ 0.82 ~ 82% 142. (b) Given: Area of jet of water (A) = 0.002 m2 Striking velocity (v) = 15 mls blade velocity (vb) = 6 m/s Force exerted on the blades = p A v (v- Vb) = 1000 x 0.002 x 15 (15 - 6) = 270 N
1200
1000
Q2
1500
_ 1200 x 1500 Q2 1000
1800
m3/s
179. (a) Difference of mercury level (H) = 10 mm = 10 x 1O-3m
180. (b)
density of water (Pwater)= 1000 kg/m ' density of mercury (Phg)= 13600 kg/m ' gauge pressure (pg) = (PHg- Pwater)gH =(13600-1000) x 9.8 x 10 x 10-3 = 1234.8 Pa~ 1236 Pa(approx.) As we know that, Force = Pressure x Area
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Fluid Mechanics
and Machinery
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=dxgxHxA =dgHA 181. (a) Given: kinematic viscosity of air = 1.6 x 10-5 m2/s considering the following relation, Il a: (T)1I2 ... (i) p o:
T1
...(ii)
(T)3/2 we get, kinematic viscosity at 70° C = 2.2 x 10-5 m2/s VOC
187. (d) Given u = ')..xy3 - x2 y, v= xy2
FA = Pressure x Area = w x ~(2d)2 x 2h = 2whnd2 4 Case II : When container place with its large diameter upwards: then force (FB) will be : FB = Pressure x Area 2
=wx~(d)2 4
-"43Y 4
For the case of incompressible
flow,
FA
x2h=
wnd h 2
2whnd2 = 4
FB
wnd2h
2 204. (c) Given: Velocity distribution is given as :
au+av=o
ax ay au ax = ')..y3 -2xy, au ay = 2yx
~=(x..JI/7 = 3y
3
UO
8
displacement thickness (8*) is given as :
')..y3- 2xj + 2yx - 3y3 = 0 =0
')..y3 - 3y
y5 (')..- 3) = 0 ')..-3=0 ')..=3 190. (b) Given: diameter of pipe (d) = 0.04 m line velocity (vrnax) = 1.5 mls Ratio of average velocity with maximum velocity is 2. Then, Vavg: Vmax = 2: 1
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1 [7 817]8 8 1 [7S"x8
8
[Y]o- 81/7 S"y
vrnax 1.5 vavg. =-2-=T=0.75m/s
=8-
7
=8-S"8
8
4
=0.0004nm2
height (H) = 10m flow rate (Q) = 11m3Is
discharge (Qd) = A x vavg.
Losses due to friction and others (HLf) = 5m
=O.0004nx 0.75 =0 0003nm3/s .
3n
= --m 10000
Pumping power (p)= PgQ(H+HLf 1000
3
Is
195. (b) Considering Bernoulli's equation, between section (A) and section (B), 2 PYA Pv~ PA +--+pgzA = PB +--+pgzB 2 2 Here, PA=PB=P, YA=O'ZA =HI' ZB=(H-h)
y2 Hence, P + 0 + pgH = P +E..___!!_+ pg(H - h)
2
y2 pgH = E..___!!_+ pg(H - h)
2
y2 pgH = E..___!!_ + pgH - pgh
2
y2 pgh = E..___!!_
2
2gh =v~
199. (c)
8/7]
1/7
224. (a) Given:
Area (A) = ~d2 = ~(0.04)2
4
0
YB =figh Case I : When container placed with its large diameter down ward: then force (FA)will be :
)kw
= 1000 x 9.8 x O.l (10 + 5) kw 1000 P = 14.7 kw 226. (a) For maximum efficiency, jet velocity = 2 x wheel speed =2v 228. (a) Given: Head (H) = 405 m speed (N) = 400 rpm ~ = 0.45 Speed of wheel
(Il)
ndN rtd x 400 = -6-0 = -6-0-
20 u=-nd
3
....(i)
and also u = kn ~2gH = 0.45 •../2 x 9.81 x 405 u = 40.1 mls ....(ii) equation (i) and (ii), 20 - x n x d = 40.l
3
d = 40.1 x 3 = 1.92 m 20 x 3.14
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1)llf)I)'Jf~rl'If)~ 1~~f.I~I~I~11I~f. STEEL Iron contains carbon in two forms: (free form) and (combined form). But in steel, carbon is present in chemically combined form which is limited up to 1.5%. Beyond this percentage of carbon, categorized into cast iron. Or we may say steel is a mixture of iron and chemically combined carbon from 0.15%-1.5%. Some other elements are also present in steel like sulphur, silicon, phosphorous and manganese etc. Classification of Steel: These steel are known as plain carbon steel. According to percentage of carbon, it may be classified as under: Dead Mild Steel - below 0.15% carbon. Mild Steel (low carbon steel) - carbon from 0.15%-0.3%. Medium Carbon Steel- 0.3%-0.8% carbon. High Carbon Steel- 0.8%-1.5% carbon.
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Classification of Steel according Manufacturing Process: (a) Killed Steel: It is a well de-oxidised steel and the steel has been completely deoxidised by the addition of an agent such as silicon or aluminium, before casting, so that there is practically no evolution of gas during solidification. Killed steels are characterised by a high degree of chemical homogeneity and freedom from porosity. The main disadvantage of killed steels is that it suffers from deep pipe shrinkage defects. This steel is denoted by 'K'. (b) Semi-Killed Steel: It is a secondary level de-oxidised steel than the killed steel and does not show the degree of properties like killed steel. Most of steel carrying carbon 0.15% to 0.25% comes in this category. Generally it is free from blow holes and having a sound outer surface. It is most widely used in structural work. (c) Rimmed Steel: Generally dead mild steel or we may say steels having 0.5% carbon are rimmed and partially deoxidised. Due to rimmed, it consists a good surface finish. It is mostly used in rolling, deep drawing and spinning, etc. It is denoted by 'R'. Capped steels: Capped steel starts as rimmed steel but part way through the solidification the ingot is capped. This can be done by literally covering the ingot mold or by adding a deoxidizing agent. The top of the ingot then forms into a solid layer of steel, but the rim of the rest of the ingot is thinner than in rimmed steel. Also there is less segregation of impurities. The yield of rimmed and capped
steel is slightly better than that of semi-killed steel. Effect of Alloying Elements on Steel Alloying Effect on Steel Element Chromium It promotes hardness, toughness and corrosion resistance. Silicon Improves elasticity,magnetic permeability and decrease hysteresis losses. Nickel Improves corrosion resistance, toughness, ductility, deep hardness and tensile strength. Cobalt Improves toughness, hardness, tensile strength and thermal resistance.s Manganese Minimise the bad effect of sulphur and increase strength and toughness also. Tungsten Increases toughness, hardness, shock resistance, wear resistivity and red hot hardness, etc. Molybdenum Improves thermal resistance, wear resistance, red hot hardness and hardness etc. Vanadium Promotes elastic limit, shock resistance, ductility and tensile strength etc. Titanium Promotes hardness. Niobium Decrease hardness and promotes fine gram growth, impact strength and ductility etc. Aluminium It acts as a de-oxidizer and promotes fine growths Copper It Increase corrosion resistance and strength etc. Boron It improves hardenability. Steel Alloys: Along with the carbon, all steels may be alloyed by mixing some other elements in various proportions to improve following most common properties of steel. Some of them are given below: (a) To improve hardness, toughness, wear resistance, corrosion resistance, ductility and red hot hardness, etc. (b) Sometimes alloying is done to improve grain structure. Classification: Steel alloys may be classified into many types on the basis of different properties. Some of them are given below: (a) Internal Structure: On the basis of internal structure steel alloys.
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Production Engineering (b) According to Application: Structural steel, Tool steel and Special Alloys steel. (c) Principle Alloying Element: Nickel steel, Manganese steel, Tungsten steel and Chromium steel etc.
Special Steel Alloys Stainless Steel: It is alloy of steel containing chromium as principal alloying element along with other elements like Nickel and Manganese, etc. Generally, chromium present in stainless steel is about 12%. The chromium present in stainless steel reacts with oxygen present in atmosphere and makes a strong layer of chromium oxide which is a highly corrosive resistant in nature. On the basis of structure, stainless steel may be classified into following: (a) Austenitic Stainless Steel: Austenitic steel (not temperable): Cr= 16.5 - 26%, Ni = 7 - 25%, Mo if used 1.5 - 4.5%, C = max 0.07%. It contains about 10%-12% chromium, 7%10% Nickel, 2% Manganese and 1%-2% Silicon and some other elements in minor quantity like Molybdenum and Titanium etc. Its hardness and strength may be improved by cold working only, not by any heat treatment etc. It is highly corrosion resistant and non-magnetic in nature. These alloys are highly resistant to many acids including hot and cold nitric acid and at temperature above 1200°F, are stronger and scale-less than any other plain chromium alloys. It consists good ductility and weldability, etc.
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(b) Martensitic Stainless Steel: Martensitic steel (temperable) : Cr = 12 - 18%, Mo ifused = 1.3 - 2%, C = max 0.25%. It contains 10%-14% chromium along with 0.08%-1.5% carbon and some other elements. The carbon dissolves in austenite which when quenched, provides martensitic structure. It consist comparatively less corrosion resistivity and good strain resistivity. It responds good for heat treatment. (c) Ferritic Stainless Steel: Ferritic steel (partial temperable) Cr = 12 -30%, Mo if used = 1.3 - 2.5%, C = max 0.08%. It contains about 12%-18% and 25%-30% chromium without any other major alloying elements. Sometimes (1%-15%) manganese and upto 1% silicon is added. Generally, low carbon steel is employed to make ferritic stainless steel. It consists poorer ductility and formability along with good weldability having good corrosion resistivity. It is mostly used in utensils, cutlery, surgical instruments and furnace parts, etc.
High Speed Steel (H.S.S.): It is a well known tool steel and possesses the best combination of all properties. Ferritic austenitic steel ( partial temeperable) : Cr = 17 - 27%, Ni = 4 6%, Mo ifused = 1.3 - 2%, C = max 0.10%. Which are essential for a good cutting tool for working at higher speed. These are hardness, toughness, wear resistance, hot hardness. High speed and cutting feed may result in production of high temperature at job and tool steel. So, it requires to be retain its properties like hardness and toughness etc. at generated high temperature. This property of retaining hardness and toughness etc. at elevated temperature is known as red hot hardness. High speed steels belongs to the Fe-C-X multicomponent alloy system where X represents chromium, tungsten, molybdenum, vanadium and cobalt. Generally, the X component is present in
A-175 excess of 7%, along with more than 0.60% carbon. High speed steel may use with almost 2-3 times higher cutting speed than high carbon cutting tool. High speed steel may retain its hardness upto 600°C approximately. According to the alloying elements, high speed steel may be divided into following: (a) Plain High Speed Steel: It contains 18% tungsten, 4% chromium, 1% vanadium, 0.7% carbon with rest percentage of iron(Fe). It consists good red hot hardness, wear and shock resistivity. It is commonly used for making cutting tools for lathe machines, planner machines, shaper machines and drilling machines, etc. Such HSS tool could machine (tunn) mild steel jobs at speed only up to 20 - 30 mlmin. (b) Cobalt High Speed Steel: It contains about 20% tungsten, 12% cobalt, 4% chromium, 2% vanadium, 0.8% carbon and rest iron. It improves red hardness and retention of hardness of the matrix. (c) Vanadium High Speed Steel: It is simply plain high speed steel containing higher percentage of vanadium which provides better abrassive resistance than plain high speed steel. It forms special carbides of supreme hardness, increase high temperature wear resistance, retention of hardness and high temperature strength of the matrix. (d) Molybdenum High Speed Steel: It contains 6% molybdenum, 6% tungsten, 4% chromium, 2% vanadium and rest iron. It shows better cutting properties. It improves red hardness, retention of hardness and high temperature strength of the matrix, form special carbides of great hardness. Tungsten High Speed Steel: It contains 0.73% carbon, 18% tungsten, 4% chromium, 1% vanadium andrestiron(Fe).1t improves red hardness, retention of hardness and high temperature strength of the matrix, form special carbides of great hardness.
HEATlREAlMENTPROCESS PRACnCE
USED IN ENGINEERING
Heat treatment is an operation or combination of operations involving heating at a specific rate, soaking at a temperature for a period of time and cooling at some specified rate. The aim is to obtain a desired microstructure to achieve certain predetermined properties (physical, mechanical, magnetic or electrical).
The important principle of heat treatment are as follows: (a) Phase transformation during heating. (b) Effect of cooling rate on structural changes during cooling. (c) Effect of carbon content and alloying elements.
Objectives of heat treatment (heat treatment processes): (a) To increase strength, harness and wear resistance (bulk hardening, surface hardening). (b) To increase ductility, toughness and softness (tempering, recrystallization, annealing). (c) To obtain fine grain size (recrystallization annealing, full annealing, normalizing). (d) To remove internal stresses induced by differential deformation by cold working, non -uniform cooling from high temperature during casting and welding (stress relief annealing).
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(e) Toimprove surface properties (surface hardening, corrosion resistance-stabilizing treatment and high temperature resistance-precipitation hardening, surface treatment). (t) To improve cutting properties of tool steels (hardening and tempering). (g) To improve magnetic and electrical properties (hardening, phase transformation). (h) Toimprovemachinability(fullannealing and normalizing). The process of heat treatment may be classified into following types:(a) Annealing: Annealing is basicallyknown as metal softening process in which the metal heated upto its critical temperature or 30°-50°C above its critical temperature and then allows to cool at a specific rate like in full annealing process metal allowed to get cool in furnace. Normally at the rate of 10°-30°C per hour decrement of temperature of furnace. Annealed is done for one of the following purpose:(a) To reduce hardness. (b) To relive internal stresses. (c) To improve machinability. (d) To facilitate further cold working by restoring ductility. (e) To produce the necessary microstructure having desired mechanical, magnetic and other properties. Types of annealing process: (a) Full annealing: It is defined as the steel to austenite phase and then cooling slowly through the transformation range when applied to steel. Full annealing is called as annealing. (b) Box annealing: Annealing a metal or alloy in a sealed container under condition that minimize oxidation. The material is packed with cast iron chips, burnt charcoal. It is also called as black annealing or pot annealing. (c) Bright annealing: Annealing in a protective medium is to prevent surface discoloration is called bright annealing. The protective medium is obtained by the use of an inert gas, such gas, argon or nitrogen or by using reducing gas atmosphere. (b) Normalizing: Normalizing: Steel is normalized by heating 50 to 60°C (90 to 110°F) into the austenite-phase field at temperatures somewhat higher than those used by annealing, followed by cooling at a medium rate. For carbon steels and low-alloy steels, normalizing means air cooling. Many steels are normalized to establish a uniform microstructure and grain size. The faster cooling rate during normalizing results in a much finer microstructure, which is harder and stronger than the coarser microstructure produced by full annealing. Steel is normalized to refine grain size, make its structure more uniform, make it more responsive to hardening, and to improve machinability. When steel is heated to a high temperature, the carbon can readily diffuse, resulting in a reasonably uniform composition from one area to the next. The steel is then more homogeneous and will respond to
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Production Engineering the heat treatment more uniformly. The properties of normalized steels depend on their chemical composition and the cooling rate, with the cooling rate being a function of the size of the part. Although there can be a considerable variation in the hardness and strengths of normalized steels, the structure usually contains a fine microstructure. This process is almost similar to annealing except in this process metal is heated 40°-50°C above its critical temperature and holding time is very shorter than annealing like (15 minutes) and then cooled down at room temperature in still air. This process improves impact strength of metal and removed internal stress of metal. It also increase mechanical properties of metal like softness, mechanibility and refine grain structure of metal. (c) Hardening: In this process, metal is heated 30°-50°C more than its critical temperature and hold at that temperature up to a specific time, then cooled rapidly by quenching in water, oil or salt bath. This process increase hardness of metal. (d) Spheroidizing: Spheroidizing: - To produce steel in its softest possible condition with minimum hardness and maximum ductility, it can be spheroidized by heating just above or just below the A 1 eutectoid temperature and then holding at that temperature for an extended period of time. Spheroidizing can also be conducted by cyclic processing, in which the temperature of the steel is cycled above and below the A 1 line. This process breaks down lamellar structure into small pieces that form small spheroids through diffusion in a continuous matrix. Surface tension causes the carbide particles to develop a spherical shape. Because a fine initial carbide size acceleratessperoidization, the steel is often normalized prior to spheroidizing. This is the process of producing a structure in which the cementite is in a spheroidal distribution. If the steel is heated slowly to a temperature just below the critical range and held for a prolonged period of time, then structure will be obtained the globular structure obtained given improved machinability of steel. (e) Tempering: This process may be defined as opposite of hardening. In this process, hardened steel is re-heated below critical temperature and allows to cool as slow rate which increase the softness and decrease the hardness and brittleness. This process increases toughness and ductility of steel. This process enables transformation of some martensite into ferrite and cementite.Tempering is used to reach specific values of mechanical properties, to relieve quenching stresses, and to ensure dimensional stability. It usually follows quenching from above the upper critical temperature; however, tempering is also used to relieve the stresses and reduce the hardness developed during welding and to relieve stresses induced by forming and machining. The exact amount ofmartensite transformed into ferrite and cementite will depend upon the temperature to which the metal is re-heated. When the hardened steel is reheated to a temperature between 100°-200°C, then
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Production Engineering carbon is precipitated out from martensite to form a carbide called epsilon. This leads to the restoration of BCC structure in the matrix. Further, heating between 200°C350°C enables the structure to transformation to ferrite and cementite. Classification of Tempering: The tempering process may be classified as given below:(i) Low temperature tempering: In this tempering process, steel is re-heated after hardening between temperature range 150°C-200°C. This process mostly used for tempering carbon tool steel, low alloy steel, surface hardened parts and measuring tools, etc. This process increase toughness and ductility and reduce internal stress. (ii) Medium temperature tempering: In this process, hardened steel is re-heated between temperature range 300°-450°C and retained at this temperature for a specific time and then allowed to cooled down at room temperature. In this process, martensite and austenite transformed into secondary troosite, which causes increase toughness and reduction of hardness. This process also improves ductility but reduce its strength. This process is used for the steel which supported to used with impact load like hammer, coils and cheals, etc. (iii) High temperature tempering: In this process, hardened steel is re-heated between temperature range 500°-650°C, then holding for a certain time and after which allows to cooled down at room temperature. This process removes internal stress completely and provide a micro structure which have good strength and toughness. This process is mostly used for crank shafts, gears and connecting rods etc.
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(f) Case Hardening: This process is also known as Carburising. In this process, the hardness is increased only at outer surface. In this process, steel is heated upto red hot and then immersed into high carbon reason which causes a production of surface having high carbon contents. This results increase hardness surface. The carbon is infused into surface of steel by diffusion from carbon monoxide gas at elevated temperature ranges between 870°-950°C due to having carbon contents at surface of steel. The hardness increased up to limited depth of steel and below this outer surface, a soft and tough core maintained automatically. Case hardening is any of several processes applicable to steel that change the chemical composition ofthe surface layer by absorption of carbon, nitrogen, or a mixture of the two and, by diffusion, create a concentration gradient on the surface. The processes commonly used are carburizing and quench hardening, cyaniding, nitriding, and carbonitriding. (g) Nitriding: It is also a case hardening but carbon is replaced by Nitrogen. It provides equal advantage for both carbon-alloy steel and other alloys steel. It can be used for
A-177
complete hardening and selected portion hardening. Even heat treated parts can be skin hardened through this process.Alloyshaving Chromium,Aluminium, Molybdenum and Vanadium responds best by this process. This process can be achieved by using gas nitriding and salt bath nitriding. But in both process, steel is heated below critical temperature. But this process is comparatively slower than other hardening process. This process is mostly used for hardening drills remains and milling, cutting tool in which the hardness at this shank is normally required less than this cutting edge. (h) Cyaniding: This is also a case hardening process which
is used for low and medium carbon steels. In this process, carbon and nitrogen absorbed at surface which cause the hardness at the surface only. In this process, steel is heated in between range of 800°-950°C temperature in a molten salt bath. The types and proportion of cyanide salt in preparing the molten salt bath depends upon the amount of carbon contents needed at the metal surface. Mostly a mixture of sodium cyanide, sodium chloride and sodium carbonate is used in equal ratio. The hardness induced in the case of metal is due to the formation of compounds of nitrogen and carbon absorbed at surface. In this process, a low temperature is used normally 120°1500C. (i) Induction Hardening: Induction hardening is a form of
heat treatment in which a metal part is heated by induction heating and then quenched. The quenched metal undergoes a martensitic transformation, increasing the hardness and brittleness of the part. Induction hardening is used to selectively harden areas of a part or assembly without affecting the properties of the part as a whole. Induction heating is a non-contact heating process which utilises the principle of electromagnetic induction to produce heat inside the surface layer of a work-piece. By placing a conductive material into a strong alternating magnetic field electrical current can be made to flow in the material thereby creating heat due to the I2R losses in the material. In magnetic materials, further heat is generated below the curie- point due to hysteresis losses. The current generated flows predominantly in the surface layer, the depth of this layer being dictated by the frequency ofthe alternating field, the surface power density, the permeability of the material, the heat time and the diameter of the bar or material thickness. By quenching this heated layer in water, oil or a polymer based quench the surface layer is altered to form a martensitic structure which is harder than the base metal. Itis fastest method of hardening in which metal contain medium or high cast are hardened by this process. In this process, metal are placed under a high frequency (2000 cycles/sec) alternating current. When this high frequency current is passed through the metal, the carbon
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Production Engineering
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contents present in metal itself increases its hardness after passing the current. The metal itself is quenched into liquid bath. This process takes 1-5 seconds only. This can be done on a specific area and to whole components normally automobile parts like crank shaft, gears and tappet pins are hardened by this process. The depth of hardness and degree of hardness depends upon the voltage and frequency of current. In hardening of large component, slower frequency current is used. (j) Flame Hardening: In this process, a high intensity oxy-acetylene flame is used to heat the steel. After heating steel above critical temperature steel is quenched to air or water bath. Jet can be used but this process is limited with medium and high carbon steel. This process can be made manual or fully computerised and automatic. Flame hardening consists of austenitizing the surface of steel by heating with an oxyacetylene or oxyhydrogen torch and quenching immediately. A hard surface layer of martensite forms over a softer interior core. (k) Laser Beam Hardening: It is a surface hardening process and almost similar to flame and induction hardening. In this process, medium and high carbon steel is coated with absorbtive media like Zinc or Manganese Phosphate and then a Laser Beam is passed through that which causes production of heat inside the metal and after passing the laser beam, metal is quenched into water or oil bath. It is a faster method and can be easily done on complete or localised area of metal. This process can be manual or fully automatic. (I) Heat Treatment of Non-Ferrous Metal: The mainly used heat treatment process for Non-ferrous metal is strain hardening, dispersion hardening but most popular method is age hardening / precipitation hardening. Age Hardening: In this process, non-ferrous alloys are heated into a single phase solid solution. On account of their decreasing solid solubilitywith lowing the temperature their structure is transformed into two distinct phase. After which these metal allows to cooled down at rapid rate which caused structure is a super-saturated solid solution. When this alloy metal is heating at a predetermined temperature again the solute atoms precipitate of supersaturated solid solution. This process results in increasing hardness. This one of the reason due to which this process is known as precipitation hardening also.
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Defects in Heat Treatment Process: Due to some reasons, some defects may arise during heat treatment process. (a) Oxidation: If during heat treatment process, atmosphere is oxidising then oxidation may occurs, which can be prevented by controlling heating atmosphere or using carburesing agents. (b) Cracks: Sometime, cracks in metal during quenching, this may happens due to having unproper designs of object, too much hardness or delay in tempering after quenching. This defectmay be controlled by improving
(c)
(d)
(e)
(f)
part design, maintain proper hardness and doing tempering on proper timing. Soft Spots: This defect may arise due to localised de-carburisation, heterogeneous initial structure or formation of bubbles during quenching process. This defect may be controlled by quenching properly in suitable solution and ensuring the heterogeneous structure of metal. Coarse Grain Structure Formation: This defect may arise due to heating at elevated temperature to a long period then specified. This can be controlled by heating at specified temperature up to proper time. Shape Distortion: This defect caused by non-uniform heating. This can be controlled by heating gradually up to specified temperature. Holes Formation: This defect is caused due to bubble formation during quenching which can be controlled by carefully quenching and using specified quenching media / solution.
METAL CASTING Metal casting is a process in which molten metal is poured (in liquid state) into a mould. There molten metal acquiresthe desired shape and size. Which is made previously in the mould after some time when metal gets solidified it is removed from mould. Casting is the oldestmethod of shaping metal and non-metals. In earlier time most popular casting method was "Sand Casting" in which desired shape article is pressed in to sand and when the article removed from sand it leaves an impression or cavity in the sand. Which is exactly according to the shape of article after removal of article molten metal is poured in this cavity formed in the sand. The article used to make cavity in sand is known as pattern and the cavitymade in sand is known as mould. Advantage of Casting 1. Casting is a cheap, fast and economicalmethod ofproducing any shape of metal and Non-metals. 2. Large and heavy structures can be made easily by casting method. 3. For identicalmass productioncasting is verysuitablemethod. 4. Dueto productionofminimum scrap,wastageofraw material isminimised. 5. Complex shape can be made easily by casting method with low production cost and in less time investment. 6. Casting is suitable for metal, non-metal and alloys. 7. Insertion of any objectof same material or dissimilar metal is easier in casting method. 8. Some mechanical properties achieved in casting process are distinct from any other manufacturing method. Someimportant terms (A) Mould. (B) Pattern (C) Core. (A) Mould: It may be defined as a shape made up of sand, Die Steel, Ceramic, and rubber etc. in which desired cavity is produced with the help of suitable pattern. According to the material used, in making cavity, the material can represent the mould's name like, ifsand is prime material then it will be known as sand mould, and rubber mould if rubber is prime material in masking mould. Sand mould may further be classified in following types :
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Production Engineering (i) Green sand mould :- The mold contains well prepared mixture of sand, water (moisture) and binder (clay), as name resemble green is not actually green colour but normally natural sand used in wet condition having suitable percentage of moisture and clay. (ii) Skin dried mould:- It is more expensive mould having additional binding material with Green sand, which enables it less collapsibility, but higher finishing and produce better dimensional accuracy. This additional bonding material used in this mould is dried by using torch etc. (iii) Dry- Sand Mould :- It is mould silica sand which is mixed with organic binder and baked in suitable ovan. Where its moisture content is reduced due to which it provides lower collaspibility. These moulds are used for better dimensional accuracy because its formation is more time consuming. Where as additional heat and bonding material, involvement causes reduction in production quantity and increase in production cost. (iv) No-Bake Mould:- The sand is mixed with liquid resin and allowed to get hardened at room temperature.
(v)
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sand casting process for most ferrous and non-ferrous metal in which un-bonded sand is held in the flask with a vacuum. The pattern is specially vented so that a vacuum can be pulled through it. A heat softened thin sheet (0.003 to 0.008 Inch) of plastic film is draped over the pattern and a vacuum is drawn (27-53 KPA). A special vacuum forming flask is placed over the pattern and is filled with a free-flowing sand. The sand is vibrated to compact the sand and a sprue and pouring cup are formed in the cope. Another sheet of plastic is placed over the top of the sand in the flask and a vacuum is drawn through the special flask, this hardens and strengthens the un-bonded sand. The vacuum is then released on the pattern and the cope is removed. The drag is made in same way then molten metal is poured, white cope and drag are kept under a vacuum because plastic vaporises but the vacuum keeps the shape of sand till the metal gets solidified. After which vacuum is turned off and the sand runs freely, releasing the casting.
permeability. No - Moisture generated defect.
S. Better life of pattern because sand did not touch the pattern surface.
Disadvantage: 1. Lowers, the production rate. 2.
Patterns may be classified according to the following factors: (a) (b) (c) (d)
(i)
Shape and size and casting Number of casting to be made Method of moulding to be used Parameters involved in the moulding operations
Solidpattern :-
Solid patterns are made in single piece having simple geometrical dimensions, it is easy to fabricate having separately defined parting line, runner and Gate etc.
(ii)
Split pattern :-
Vacuum Moulding:- (V-Process) is a variation ofthe
Advantage of Vaccum Moulding Process: 1. Produced very Good Surface finish. 2. Cost of bonding material is eliminated. 3. No- Production of toxic fumes and provide excellent 4.
A-179 etc. These patterns are made slightly over size, for over weight, material so that extra metal can be used for matching etc. Most commonly used patterns are listed below.
Takes more time hence increases production cost.
(B) Pattern: Pattern may be defined as a solid hollow shaped item used to make cavity in the mould or we can say the replica of shape what we desire to cast patterns are made by various metals and non-metals depending upon the requirement like, wood, wax, aluminium, ferrous and ceramics
)
()
()
f
,...,.....
:':;c
pattern
~~."
pattern
~g
When model have difficult geometrical dimensions then patterns are made in two parts that meet along the parting line of mould using two separate pieces allows the mould cavities in the cope and drag to be made separately and the parting line already determined.
(iii) Match Plate Pattern :A match plate pattern is similar to a split pattern except that each half of the pattern is attached to opposite sides ofa single plate. The plate is usually made up of wooden material. This pattern design ensures proper alignment of mold cavities in cope and drag and the runner system can be included on the match plate. Match plate patterns are used for larger production.
r-
Cope Pattern
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(iii) Machining Allowance: It is also known as finishing
Fig. (cope and drag pattern) (iv) Cope and drag pattern :A cope and drag pattern is similar to a match plate pattern, except that each half ofthe pattern is attached to a separate plate and the mould halves are made independently just as with match plate pattern. This match plate helps in proper alignments of mould cavities in the cope, drag and runner, etc. Match plate patterns are used for larger production and often used when the process is automated. Cope pattern
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allowance. After casting process every casting needs some machining or finishing operations in which a considerable amount of material needs to be removed from casting surface to compensate the loss of material from the surface of casting, some additional amount of material is provided in addition of draft allowance, this percentage of extra material over casting surface is known as machining allowance. This allowance is provided both inside walls and out side walls of castings. (iv) Shake Allowance: Before withdrawal of pattern from mould, the pattern is wrapped all around the faces to enlarge the mould cavity slightly which facilitates its safe removal and causes the enlargement in mould size. So it is desirable that the original pattern dimensions should be reduced to account for this increase in dimensions or we can say that shake allowance is provided in (-ve) to the original size of pattern. (v) Distortion Allowance: The tendency of distortion is not common in all castings. Only castings which have an irregular shape and some such design that the construction is not uniform through out will distort during cooling on account of setting up of thermal stresses in them. Such an effect can be easily seen in some dome shaped or 'U' shaped castings. To eliminate this defect an opposite distortion is provided in the pattern, so that the effect is neutralised and the desired casting can be achieved.
Colour Coding in Pattern
._______ Drag Pattern Fig. Match Plate Pattern
Design of Pattern Pattern as we know very well a master/ shape used to make cavities in mould of desired shape and size. During pattern designing we have to keep the following parameter in mind as given under, like material selection for pattern making. C patterns are made from wood, aluminium, plastic, rubber, ceramics and Iron etc. In general, pattern making process involves drawing making of desired object, to be made by casting along with addition of various allowance measurements with the dimensions. Most of the dimensional allowances to be added in pattern making are listed below: (i) Shrinkage Allowance: Shrinkage on solidification is the reduction in volume caused when metal loses temperature after casting. The shrinkage allowance is provided to compensate the reduction in volumetric dimensions. Aluminium permissible shrinkage allowance is 0.013 mm- 0.01 mm. (ii) Draft Allowance: At the time of withdrawing the pattern from the sand mould. It may damage the edge etc. so for making withdrawn easy, all patterns are given a slight taper on all vertical surface i.e. the surfaces parallel to the direction of their withdrawal from the mould. The taper is known as draft allowance.
Although colour coding is not accepted but the most commonly used coding are given below. (i) Red ~ machining surface (ii) Black ~ un-machining surface (iii) Yell ow ~ core prints (iv) Red strips on yellow base ~ Seats for loose pieces. (v) Black strips on yellow base ~ Stop ofts. (vi) No - colour ~ parting surface. (C) Core: Core is generally made up of sand having bonding resin in proper quantity these core's are used for making hollow section inside the casting. A good core must have following properties. (a) It should have good permeability, so that gas can easily escape during casting process. (b) It should be made good refractory material so that it can withstand the high temperature and pressure of flow of molten metal. (c) It should have high collapsibility i.e. it should be able to disintegrate quickly after solidification of casting metal. (d) The binding material or core material should not produce additional gases during casting process.
Classification ofCore:(i) (ii) (iii)
Horizontal Core Vertical core Balanced core
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hanging over core Wire core Core molding: Cores are made separately in a core box made up of wood or metal. cores are made by two ways (i) manually by hand and (ii) by using core making machines.
Characteristics of cores: (i) Permeability: Cores are made more permeable than the mold to achieve, good permeability. Coarse sand & fine sand in a specific quantity are mixed with molasses. (ii) Collapsibility:- Core should possess good collapsibility so that it can be easily removed from the casting after solidification without making any damage to the casting. (iii) Strength:- Core should possess enough strength so that it should not be de-shaped during placing in mold or during the molten metal pouring. (iv) Thermal Stability:Core material should have good thermal stability so that it can withstand the high temperature during casting process.
SOLIDIFICATION AND COOLING In this process molten metal loses heat to the surrounding atmosphere and changes its state from liquid to solid, if conductivity of mould is higher it acts as the center of nucleation and crystal growth commences from the mold and extends towards the center. We can say, solidification occurs by nucleation of minute crystals or grains, which then grow under the influence of crystallographic and thermal conditions. The size ofthese grains get affected by the composition of alloy and its cooling rate. During solidification heat is being extracted from the molten metal as soon as it enters the mold. This heat is called super heat. The latent heat of fusion is also evolved during solidification and it must be transferred to the surrounding mold before complete solidifications can be achieved. Thus there are three stages of cooling i.e. liquid-solid and solid
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Solidification (i)
A-1Sl can be controlled by providing risers or heads. These are attached to the casting at the right location so that they can continuously supply hot molten metal to the shrinking casting untill it is completely solidified. Delivery of molten metal is mostly accomplished by Gating System. Where as reserve metal is supplied by risers or heads. Both functions can be served by either of two. Hence no clear - cut distraction can be made.
Classification: (i)
Parting Gate
(ii) Branch Gating (iii) Step Gate (iv) Horizontal Gating
Design of gating system: The following formulas should be kept in mind while designing of gating system.
(a) Bernoulli's equation p
v2
pg
2g
- = - + h = constant Where, p = pressure, v = velocity of liquid h = head, p = density of liquid (b) ContinuityLaw: Flowrate Qr=A, VI =A2 V2 where, A = Area of cross - section V = Velocity ofliquid (molter n metal)
Time taken for pouring: volume of mould cavity Ag ~2gH
Pouring time (t) =
Where, A_g_ = area of gate
Design or Sprees: Area of ratio (R)= ~
= ~:
Properties
Fluidity: The ability of filling all parts of mold cavity is known as fluidity.
(ii) Hot cracking: During cooling process a part of casting may be placed under tension and these tensile stresses are greater when the metal is weak and thus ultimately metal gets cracks. Ifthere is a relatively large reduction in temperature during subsequent solidifications, thermal contraction may cause cracking. (iii) Effect of Inocculation: It is a process in which the properties and structures of casting are enchanced by adding another material (metal or non-metal) to the molten metal before pounng.
RISER AND GATING DESIGN Riser is a cavity made in mold to compensate the shrinkage arises
where, A3 = area of sprees at bottom Al = area of sprees at tope
Some most important formulas used: (a) Time taken to pour (t )
Volume of mould cavity Ag~2gH
in casting and acts as a reservoir of molten metal.
Gating: Gating design must control the phenomenon in such a way that no part of the casting is isolated from active feed channels during the entire freezing cycle, it is reffered to as a directional solidification. The degree ofprogressive solidification
A3 (b) Aspiration effect: A2 (c) Solidification time:
~
= ~h;
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where, c = constant, V = volume, A = surface area (d) Relative freezing time (Rp) R) (e) Volumeratio ( V
(A/V) casting = (A IV)river
Vriver .
=V
castmg
(f) Caini's formula: R p = --
a
RY-b
+c
where, a = Freezing characteristic constant b = Contraction ration from liquid to solid c = Relative freezing rate of river and casting CLASSIFICA nON OF CASTING (a) Sand Casting: In this process a cavity is made in a sand mold by using desired pattern and then after molten metal poured into mould. Which is after solidification known as casting. There are two main types of sand used for moulding Green Sand and dry sand. In green sand un-burned sand mixed with proper amount of clay as it binds and moistens and when the sand is mixed with binding material other than clay and moisture is known as Dry Sand. Application of Sand Casting: 1. It is mostly used for cheapest casting process to maintain low production cost. 2. Complex geometrical shape can be easily made by the process. 3. Sand casting method is used for producing very heavy parts like fly wheel of power press, Railway wheel etc. 4. Many large structures are produced by this method like engine blocks, engine manifolds cylinder heads and transmission cases etc.
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Steps involved in sand casting: (i) Mould making: - In the process expendable sand is packed around the pattern, which is a replica of the external shape of the casting when the pattern is removed, the cavity that will form is used for casting. Any internal feature of casting that cannot be made by pattern that is made by separate cores. (ii) Clamping: - Once the mould has been made, it must be prepared for the pouring of molten metal. So the surface of the mould cavity is first lubricated to facilitate the removal of the casting, then the cores are positioned and the mould halves are closed and securely clamped together. It is essential that the mould halves remains securely closed to prevent the loss of any material. (iii) Pouring: This process involves pouring of molten metal in to mould in such a way that all section of mould fills properly. This can be checked by rising level of molten metal in the risers. (iv) Cooling: This process involves cooling of molten metal
inside the mould, often pouring after cooling process when molten metal gets solidified casting comes out after breaking the mold. Trimming: During casting process some extra material remains attached with casting, this excess material removed from casting is known as trimming. (b) Die casting: - In this process cope and drag are replaced with metal die. Molten metal is poured into cavity, made in metal dies. (c) Pressure - Die Casting: - In this process molten metal poured in metal die along with a specific pressure. This pressure application enhance casting finishing and increase production rate. (d) Slush Casting: - In this method molten metal is poured into the mould and began to solidify at the cavity surface. When the amount of solidified material is equal to the desired wall thickness, the remaining slush is poured out the mould. As a result slush casting is used to produce hollow part without usmg core. (e) Plaster Mold Casting - In this method sand is replaced with plaster of paris is rest the process is similar to sand casting method. (f) Investment Casting: - In this method a mould is made of ceramic by using a wax pattern. When molten metal is poured into mould wax get melted and replaced by molten metal. It is mostly used for casting of (S.S), Aluminium alloy and magnesium alloys etc. (g) Centrifugal Casting: - In this process mold kept rotating at high speed and molten metal poured from centre of axis of mould. Then molten metal due to its moment of inertia moves towards inner wall of moving mould and due to light weight of impurities present in molten metal segregated and collected near the axis of rotation, which enables to make more pure casting having higher accuracy and lowest impurities. (h) Continuous Casting: - In continuous casting process molten metal is poured from a specific height in a vertical mould. This vertical mould kept cooling facilities so that the casting continuously cooled down. This process is mostly used for casting pipes, rod and sheet of brass, bronze copper, aluminium and Iron etc. (i) Shell mould casting: - This process is similar to sand casting method except the molten metal is poured into an mold having thin walled shell created from applying a sand resin mixture around a pattern. The pattern used in this method can be reuse to make many mold. This process is mostly used for casting carbon steel, alloy steel etc. CASTING DEFECfS: (i) Un - filled section: - This happens due to insufficient metal pouring at lower temperature than required. (ii) Blow holes or porosity: - This defect happens, if molting temperature is too high and non -uniform cooling on the permeability of molding sand is low. (iii) Shrinkage: - Some time after solidification the casting gets reduced in size at surface or internally which is known as shrinkage defect. Normally it happens due to improper
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cooling rate, improper gating, rising and type of material also. (iv) Hot tears: - Too much shrinkage mostly causes cracks internally and on external surface known as hot tears. It happens due to improper cooling, and over ramming of molding sand, etc. (v) Mis-Run: - When molten metal fails to reach at every section of mold then some sections remains un-filled known as misrun. (vi) Cold shut: - When molten metal comes from two or more paths into the mould and during meeting these different flow if not fuse together properly is known as cold shut. (vii) Inclusions: - Any un-wanted metallic / non-metallic waste present in casting is known as inclusion thus inclusions may be slag of sand oxides or gases etc. (viii)Cuts and washes: - These defects occurs due to erosion of sand from the mould or core surface by molten metal. (ix) Shot metal: - This defect appears in the form of small metal shots embedded in the casting which are exposed on the fractured surface of the latter. It happens when the molten metal is poured into mould particularly when its temperature is relatively lower. It may splash the small particle separated from the main stream during the spray and thrown ahead and solidified quickly to form the shots.
HEAT FLOW RATE DURING Badboys2
(i)
SOLIDIFICATION
Heat flows from the hoter portion to cooler portion ofthe casting.
Rate of heat flow per unit Area ~
C = Specific heat of molten metal 8p = Molten metal pouring temperature 8f= Cooling temperature of metal 80 = Initial temperature of mould
1
=- k
( :)
(a)
Hot forging: Hot forging may be defined as a process in which metal is heated up to its plastic state and then a suitable external pressure is applied to achieve desired shape and size. The deformation of shape of metal depends on the type of force applied on it. If the force is applied along its length the cross-section will increase on the cost ofreduction of its length. Similarly if the force is applied against its length the length will increase and the cross-sectional area will decrease. Forging may be used to bend the work piece. Without change its length along with using suitable dieand punch etc. In forging process external force may be applied by hand hammer, power operated hammers, and presses etc. If the force applied bymannually by hammers this process is known as smithy process. Classification of forging: Forging may be classified into following types (a) Upsetting (b) Drawing out or drawing down (c) Bending (d) Setting down (e) Forge Welding Up setting: In this process cross-section of work piece is increased with corresponding reduction in its length.
ky/ hrm' ~~~ force application
Where k = Thermal conductivity in KJlhrmk°. dt dx = thermal gradient in units of temperature
(T) and
distance (x). if metal is cooling against a large mold wall and heat flow is normal to the mold surface thickness (x) of solid metal deposited will be proportional to the square root oftime (t) or x =
K}
.Jt
Solidification time
o:
Volume ( S ~ Ar
unace
2 ea J
.: K = Constant
Where Pm = metal density P =densityofmolten metal L = latent heat ofliquid metal.
a = Thermal diffusityofmould Cm = Specific heat of mould
K = pc
Metal attains plastic state when an external force is applied along its length accross of its cross-section. Which results in increase in its dimensions at right angle to the direction of applied force with a corresponding reduction in its length, parallel to the direction of applied force. Normally bar stocks are used for being jumped by a desired amount so that this part can be given a desired shape through the jumped further operations. The jumping operation can be performed in any localised area i.e., the particular part in the bar shape, where said increase in cross-section is desired is heated till it acquires a plastic state. Than the length which do not required to be jumped cooled abruptly by quenching in water, and the hot portion is placed under suitable load. This operation may carried out manually ifthe work peice is small enough to handle and when heavy force is required (such as in large work peice) heavy hammer is used called as sledge hammer. The objects of cooling the bar length, which is not to be jumped out is two fold. Firstly to localised the reduction in length to the desired extent and secondly to prevent the bar from bending during up setting due to heavy blows.
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(b)
(c)
Drawing out or drawing down: This process is exactly a reverse process to that of up setting orjumping in the sense that, contrary to the latter, it is employed when a reduction in thickness or width on is desired with a corresponding increase in its length. In this process specific shape oftools also required to achieve the desire shape, known as pair of sewageand fullerthe selectionof the abovetools is governed by the shape of the cross-section of the stock, the Rod or bar heated up to pre-determined length to the plastic state followedby the cooling of the unwanted length for drawing by sudden quenching in water. If the bar is of rectangular or square cross section it is laid flat on the anvit face and hammered by the peen of cross-peen hammers by the limit. Ifthe reduction is to be done both in width as well as in thickness the operation is repeated by turning the bar at 90°. The desired result can be more quickly achieved by keeping the bar on the edge or hom of the anvit and then drawing. Bending: Bending of bars, flats and other simillar stock material is usually done in smithy shop, this can be done to produce different types of bent shapes such as angle, ovals and circles etc. Any desired angle or curvature can be made through this operation. For making a right angle bend that particular portion of stock, which is to be subjected to bending, is heated and jumped on the outer surface. This provides an extra material at that particular place which compensates for the elongation ofthe outer surface due to hammering during bending. This operationis carried out on the edge of a rectangular block. After bending, the outside bulging is finished by means of a flatter and the inside by means of a set hammer, this process can be made by mannually or by using forging. Machine along with jigs and fixture. Setting down: In the operationthrough which the rounding ofa comer is removed, to make it square, by means ofa set hammer. By putting the face of the set hammer over the round portion, formed by fullering or bending ofthe comer and hammering it at the top reduction in thickness takes place resulting in a sharp and square comer. Finishing is the operation through which the un-evenness of a flat surfaceis removed bymeans of a flatter or a set hammer and round stems are smoothened to the correct shape and required size by means of sewage after the job has been shaped roughly to the finished size through other operations. Forge welding: In this process two peice of simillar metals are heated properly up to sufficientwelding heat andjoined together by application of external heat, two important considerations are always made in order to get a sound weldedjoint. (i) Proper end preparation of the metal peices to be joint (ii) Rising the temperature of the prepared ends to the correctweldingheat. The surfacesto bejoined together should be quite clean i.e., they should be free from scale, dirt or ash. Otherwise this presence will lead to the failureof thejoint. In additionto this a fluxis applied on the hot metal which helps in over coming the above difficulties. This flux usually stand for wrought iron
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(d)
(e)
and borax for mild steelmelts at high temperature and form slag containing the iron oxide, ash etc. This slag forms a layer over the hot surface thus preventing it from comming in further contact with air. Which the result, further oxidation of iron is checked. There are three types ofweldedjoints in common which are • Butt weld: When two bare are made tojoin end to end bywelding, such that joint formation is at right angles to the lengths of the work piece it is known" as butt weld. • Scarf weld: This is alsoknown as lap weld. It is known so for the reason that the ends of the metal pieces to be joined are made to overlap each other and then hammered. Thus the weldformationis at an inclination with the top and bottom faces of the joined pieces. Also due to the distinct end preparation it is easy to apply correct pressure by hand hammering in proper direction. • "V" weld: It is also known with so many names e.g. split, splice or fork etc. It is employedwhere a highly strong weldedjoint is neededparticularlyin heavyworkwhere the greater thickness ofthe job enables the formation of =v: easily, to ensure perfect joining of metals the scarf of one piece should be made rough byproviding steps on it.
(i) b..tt VIA9Id
(ii) scerf VIA9Id
(iii) V-\I\9Id
Differentweldingjoin HOT EXTRUSION The process of extrusion consists of corresponding a metal inside a chamber to force it out through a small opening called die.Any plastic material can be extrudedsuccessfully. Most of the process used for extruding metals are hydraulically operated horizontal presses. A large number of extruded shapes are in common use, such as tubes, rods, structural shapes and lead covered cables. The principle of operation are the same for both hot and cold extrusion, and choice of one ofthese is governed by factors like the metal to be extruded, thickness of extruded section size of raw material being used, capacity of press, and type of product etc.billets of 125mm to 175mm in diameter and 300 to 675 mm length are in general used as raw material forextrusions of steel needs adequate lubrication around the billet. This is done by providing a coating of fine glass powder over the surface of hot billet. The process of extrusion suits best to the non ferrous metals and alloys although some steel alloys like stain less steel are also extruded. The extrusion process can be classified as follows (1) Direct or forward extrusion (2) Indirect or backward extrusion
Direction Extrusion: As shownin the figurebelow,In this process billetofraw metal to be extrudedis heatedto its forgingtemperature and forced in the machine chamber, this force push forward the billet and billet passed through the die. The length of extruded
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Production Engineering part will depend upon the size of the billet and cross-section of the die. The extruded part is then cut to the required length. The over hanging extruded length is fed is to a long support called the run out table.
A-1S5 The rod is fed up to stops through straightening rolls, cut to size and pushed into the header die. The rod is gripped in the die and punch operates on the projected part to apply pressure and form the head. The bending operation may be completed in a single or two strokes. Automatic machines for producing bolts and screw are also available in which all the operations like cutting stock to size, shank extrusion, heading trimming and threading etc. are performed simultaneously to produce finished components. The processis also successfullyadoptedfor producing rivets and nails. COLD EXTRUSION
Fig. Forwarded extrusion process It is a usual practice to leavethe last nearly 10% length of billet as un-extruded. This portion is known as discard which contains the surface impurities of billet. Indirect Extrusion: As shown in followingfigure ram or plunger used is hollow type, and as it pressed the billet against the back wall of the close chamber, the metal is extruded back in to the plunger. As the billet does not move inside the chamber, there is not friction between them. As such, less force is needed in this method in compressionto the directextrusion.Amore complicated type of equipment is required because plunger becomes weak due to the reduction in the effective area of cross-section and difficulty is exprienced in supporting the over heating extruded part.
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The most common cold extrusions process is impact extrusion, in which soft and ductile metal is used to formed various product like tubes for tooth paste, lotions, shaving cream, paints and condenser cans etc. The raw material used is in slug form having been either turned from a bar or punched out of a strip. The operation is performed with the help of a punch and a die. The prepared slug is in the die and struck from top by the punch operating at high pressure and speed. The metal flows up along the surface ofthe punch, forming a cup shaped component.When the punch moves up. Compressed air is used to separate the component from the punch. In the mean while a fresh slug is fed into the die. The production rate is quite high about 60 components per minute. Mostlywall thickness produced from 0.7 mm to 0.1 mm but only soft and ductile material can produced bythis processlike lead,tin, aluminium,zinc, and respectivealloys etc. Uniform diamensions, low scrap production and high production capacity is main advantage ofthis process. Although Die and punch are used in like drawing process but its high production rate, and tolerance of the order of ± 0.762 mm up to 12.7 mm diameter and± 0.127 mm upto 25 mm dia can be easily obtained. WELDING
Fig. BackwardExtrusions Advantage and limitations of hot extrusion 1. Due to application of higher pressure a very dense structure is produced. 2. Better surface finish is produced having higher dimensional tolerence. 3. Low tool cost involves and fast in production rate. 4. Most suitable for production of parts having uniform crosssection having fine surface finish and high dimensional accuracy. 5. Excessesive length object is creak problem in handling the extruded rod during extrusion.
Welding is a process ofjoining two or more than two similar or dissimilar metals together with or without use of pressure, and filler materials. Without use of external heat we get success in welding of Gold and Silver only till today but in future the use of temperature in welding may be reduced considerably. Welding process may be classified as follows: (i)
Homogeneous welding: In this method two similar metals are joined together by welding and use of filler of same material ifrequired. For example, mild steel with mild steel welding this process is also known as autogenous welding.
(ii)
Hetrogeneous welding: In this method, welding is done with two dissimilar metals and the filler metal used in this processis usuallykept, oflow meltingpoint than the parental metals. For example copper and brass, mild steel and cast iron etc.
COLD FORGING This is a cold up-setting process adopted for large scale production of small cold up set parts from wire stock. A few examples of such parts are small bolts rivets, screws, pins nails and small machine parts. Small balls for ball bearings are also made by this method. The machine, tool, and dies are almost simillar as in hot forging.
Classification Pressure (a)
fo Welding According
to Application
of
Non-pressure welding/fusion welding: In this process of weldingthe temperature ofjoining edge of metals are heated
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up tomeltingpoint andwhen it startsto meltthe fillermaterial is filled between joints. For example-Gas welding, Arc welding, Electric beam welding and Thermit welding etc. (b)
Pressure welding: In this process of welding two edge to be joined are heated up to their plastic state and then sufficientpressure is applied till the weld is performed. But no-fillermaterial is used commonlyin there weldingprocess. For example-Forge welding, Resistance welding etc.
Classification
of Welding on the Basis of Heat Source
(a)
Chemical welding: According to chemical method, heat is produced by oxidation or may be burning of coal and gas etc. Heat is also produced by chemical reaction of two or more salts together. For example iron oxide and aluminium powder produced heat by chemical reaction. This method of heat generation are employed in forge welding, gas welding and thermit welding.
(b)
Electric welding: These proceses use electrical energy to produce heat required to melt the work piece. Electrical energy based joining process may further classified as follows: (i)
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Electric arc welding: In an open circuit when resistance of air gap between two terminals of conductors is less than the quantity of current/voltage carrying acrossthem, the electrons willjump from one terminal to another. This is calledjumping of electrons and due to this arc a high temperature generates at bothterminals which is about 3700°C-4000°C.
(ii) Resistance welding: In this method heat is produced when sufficient quantity of current is passed through a conductor having proper resistances. For example spot welding, projection welding etc. (iii) Induction welding: In this method heat is producedby
use of high frequency current to produce sufficient eddy current in the workpiece to be weld. (c)
Mechanical method: This method is rarely used in modem practice because the heat production in this method is very low as compared to energy applied as heat produced by friction or heavy blow/impact load etc.
CLASSIFICATION OF RESISTANCE WELDING (A) Spot welding: Welding machine used in this type of welding consists of two cylindrical pointed electrodes, out of them one electrode is kept fixed and other electrode is movable. Movable +-- Electrode
Spot welding
The work piece is to be weld placed between these electrodes and a high ampere current is passed through them for a limited time till the metal get fused at the place of welding and then applied sufficientpressurewhich make completethe weldingjoint. This process is mostly used in thin metal sheet welding like domestic utensils, cabinets like structure etc. Important
Factors Related to Spot Welding
1.
Welding pressure control: For good welding a sufficient pressure application for enough time is very important and this pressure applied for welding is known as welding pressure. This pressure should be applied on job on accurate time of plastic state of metal. Time for pressure application depends upon the thickness and properties of metals to be welded. 2. Time management: It is the total time consumed while completing different stages of welding and it is known as cycle time also. This cycle time must be adjusted in such a manner so that the metal should acquire sufficient plastic stage required for good welding and cut the supply automatically after completing the heating stage. Time control may be managed by different methods like das pot circuit breaker, electronic circuit etc. 3. Surface penetration: The surface to be welded should be free from all/dust or any un-wanted materials. So that the penetration of welding joint should be max, min and weld can make proper. 4. Electrodes: The electrodes must possess mainly these characteristics i.e. high electrical and thermal conductivity and it should have sufficient mechanical strength to with stand high pressure to which they are subjected.These are made water cooled. The surface of electrodes must be easily cleanable so that the resistance between the surface of electrodes and work metal should be kept minimum. Electrodes are mainly made up of copper alloys with molybdenum and tungsten. Spot welding process may further be classified into following types depending on their application. (a) Rocker arm type: The machineusedin spotweldingprocess consists of one fixed electrode and other movable electrode which is mounted on a rocker arm and moved in up and down direction by mechanical arrangements. In some machine mechanicalarrangement is poweredwith hydraulic system to make more automated system. (b) Press type: In this types machines are used in heavy or thick sheet welding and movable electrode is operated electrically or by compressed air. (c) Portable Guns: In many places it is not feasibleto transport hence for that purpose a portable machine is required. This Portable Gun carries two electrodes and the transformer is supported generally on over head rails. Mainly it is used in automobile industry. (B) Seam Welding
Fig. Spot welding
Itis series of closely spaced or single line spot welding. The weld shape for individual spot may be of any shape like round or rectangular. In this process, two circular disc shaped electrodes
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are used out of which one is kept moving and other kept only movable but not joined with any moving arrangement. The work piece is placed between these electrodes and current is passed through these electrodes. This current carrying electrode when rotates the work piece moved forward and a continuous line of spot welding performed.
In resistance butt welding the metal to be weld should have equal cross section and properly faced at their respective ends. For welding wires and rods up to 12.7 mm diameter the machine may be used as spring operated and for larger diameter high pressure is mostly applied hydrauliceley or pneumatically. Resistance butt welding may be used to increase length of pipes, rodes, wires and bars of highly conductive material like copper, brass and aluminium etc.
(E) Resistance Flash Welding It is almost similar to resistance butt welding except that it is operated comparatively on less current. In this method, current is switched on before abutting the ends of bar etc. and then the movable clamp is transported towards the fixed clamp containing another metal piece maintaing small gap between both mating ends.
Continuous line of welding joint
Fig.Line diagram of Seam welding The machine used in seam welding is almost similar to spot welding except it contains circular disc shaped electrodes attached with revolving mechanism between two circular disc like electrodes one powered by rotating force is known as drive and another kept movable is known as driven. The pressure applied on driving wheel electrode by hydraulically or phenumatically. The seam welding is mostly used for metal having sufficient electrical resistivity. For example mild steel, tin plates and many dissimilar metals like steel with brass and bronze.
Badboys2 Seam welding
may be further classified as circular longitudinaltype, universaltypeandportabletype.
type,
Work Piece Fixed clamp
Movable clamp
Fig. Resistance flash welding
(C) Projection Welding This welding is almost similar to spot welding except of having any projection on both faces of electrodes. So it is most effectively used in mass production of multi point spot welding in single stroke as desired projection.
Due to this small gap, a flash developed between the ends which produce a high heat at both ends and metal at both ends gets melted, after this melting sufficient pressure is applied on movable clamp and both ends get fused and welding joint gets completed.
(D) Resistance Butt Welding
The flash developed at the ends of work piece only on a small part of it, so comparatively less electric current consumed. It is more faster process then the resistance butt welding and no facing at ends of metal required in this method. During welding, slug and remaining molten metal comes out from the weld joint, so weld joint made by this method is more stronger than resistance butt welding joint.
Resistance butt welding has similar working principle of welding as in spot welding except that electrodes are in clamp shape in which one clamp is fixed type and another is movable type. The job to be weld are normally bars, pipes, wires etc. One piece to be weld kept in fixed clamp and other clamped in movable clamp. Both metal pieces are faced (finished at ends) properly. Then movable clamp containing working metal (steel pipe) is so adjusted that both ends meet together which are to be welded. After properly meeting ends of metal , the current is switched to till corresponding ends of metals are reached to the fusion point.
These resistance butt and flash welding processes are limited on the capacity of clamping size of welding machine and the material coming out from the joints need extra machining etc. which may increase production cost of welding.
PERCUSSION WELDING This method of welding involves the use of stored electrical energy either in reactors, capacitors or storage batteries etc.
i
-:
Movable clamp
Work Piece
/
Fixed clamp
Fig. Resistance Butt welding
In percussion welding the heat for welding is secured simultaneously over the complete area of abutting surface from an arc produced by rapid discharge of stored electrical energy followed immediately by application of pressure. Percussion welding permits welding harden steels without affecting heat treatment and dissimilar metals can be weld successfully like steel with Mg etc.
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Production Engineering
A-ISS
MUL TI IMPULSE WELDING This method is also known as pulsating welding, it is applicable to spot welding, seam welding and butt welding processes. etc. Pulsating welding process consists of applying the current in a series of impulses which may be a fraction of cycle or no. of cycles. This process has certain advantage, for example more thicker materials can be easily welded with same equipments increase electrode life and spettering of welding reduced considerably.
welding due to development of magnetic field around. It generally happens in D.C. arc welding due to having fixed polarity. Arc blow generally occurs in three directions forward, backward and side.
7.
Arc crater: It may be defined as the penerat ions of arc in base metal, it depends upon arc length, electrode width and thickness of base metal.
8.
Spatter: Molten metal dispersed around the welding beads in small drops form is known as spatters.
ELECfRIC ARC WELDING
9.
In this method no external pressure is applied, only the metal to be welded are heated up to welding temperature and a pool of molten metal fills the gap in between the joints, then these joints allow to cool in air and weld get completed.
Chipping: Removing the spatters and slage etc. formed on welding bead on metal surface during welding is known as chipping. The slage is formed as a by-product due to use of coated electrode in welding process.
10.
Edge preparation: For making different types ofjoint, some
In some types of electric arc welding an additional filler material is applied known as electrode and heat is produced by electric Arc about 3400°C. At initial stage electrode requires potential about 60 - 100 volt and in running condition when a regular arc is produced, it requires only 15 - 45 volt normally to maintain the welding operation.
Important 1.
side of work metal has to be grinded in specific shape and size. The grinding at edge/side of work piece is known as edge preparation.
11. Weaving of electrodes: This term is related with forward motion of welding electrode on the surface of welding plane. Weaving means tilting of electrode simultaneously along with forward motion of electrode. This is used for increasing width of deposition of molten metal over weld.
Terms Related with Electric Arc Welding
Open circuit voltage: This voltage may be called the voltage at electrode when no Arc is formed and machine is in switched on condition generally it remainsant 60 -100 volt.
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Motion of Electrode
Arc voltage: This voltage may be defined as the voltage of an electrode on electrode when regular Arc is formed during welding operation. Arc voltage i.e.
3.
= Cathode drop + volumn drop + Anod drop Fig. Different weaving styles
V=V epa +V +V
Duty cycle: Duty cycle is the time duration up to which that specific machine can supply a specific current and voltage for a specific time duration without making any hazard to a welding machine.
4.
As per requirement of joint, there are different weaving styles as shown in above figure. 12.
electrode tip from work metal during welding process. Actually it is better known by practices about the correct length of Arc. The distance between the electrode tip and work metal depends upon the voltage and current used for various welding process. Normally about 3 mm distance is assumed as correct distance, less than 2 mm is counted as short Arc length and above 3 mm upto 6 mm is known as long Arc length.
Power factor: It is the relation in between the current used and total current supplied to machine. Power factor =
5.
Current used
kW
Current supplied
kVA
Polarity: This term is mainly associated with D.C. arc welding because, D.C. current has fixed polarity i.e. + ve and - ve terminal and for the A.C. they interchanged at every cycle. It may be classified as follows.
13.
Straight polarity: Work piece made positive terminal and the electrode is made negative terminal, it is used for more thick plates etc.
Blow holes: It is a type of defect formed during welding process due to presence of any impurity or air bubbles or any space remains un-filled by molten metal during welding process.
14.
(ii) NegativepolaritylReverse polarity: Work piece is made negative terminal and electrode is made positive terminal, it is used for thin plates welding.
Buckling: It is also a type of defect. When work metal is twisted or deshaped in un-wanted direction during welding the process is known as buckling.
15.
Hard facing: Hard facing may be defined as the process of
(i)
The polarity have a considerable effect in welding because heat generated at positive terminal is much more than the negative terminal. Heat generated at positive terminal is about 2/3rd higher than negative terminal. 6.
Arc length: Arc length may be defined as the distance of
Arc blow: It may be defined as the deviation of arc during
hardening the surface by welding process. 16.
Heat affected zone: During welding process some time weld metal looks separated from work metal, it happens due to improper heating. This effect is called Heat affected zone or we may say the place had been effected by improper welding heat.
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Production Engineering 17. Padding: This is the process of making number oflayers of metal on a used part of metal to increase its dimensions.
18. Penetration: It may be known as depth of fusion during welding process.
19. Slag: When a flux coated electrode is used in welding process then a layer of flux material is collected over welding bead which contains the impurities of weld material. This layer is known as slag. It is removed by chipping of weld.
Classification of Electric Arc Welding (A) Metal arc welding: In this welding process the arc is made between work metal and electrode (may be bare or coated electrode). Base electrode is made up of same material but using it having certain disadvantages such as welded surface may be subjected to oxidation. To prevent the oxidation of welding surface, coated electrodes are used.
(B)
Carbon arc welding: This process is mainly employed with D.C. supply only due to having specified polarity in D.C. supply. A carbon electrode with negative polarity produce arc when close to work metal connected with positive polarity current. Straight polarity connections are made to prevent carrying over of carbon contents over metal surface during carbon electrode fusion. Otherwise deposition of carbon contents may result in a brittle and bad weld.
A-189 Metal Arc Welding The basic principle of metal arc welding is the development of electric arc between the metal electrode and work metal. The metal electrode (bare or coated) having sufficiently high ampere current when kept at proper distance to job an electric arc is developed or we can say the high ampere current value over come to the resistance offered by air gap between the electrode and job having different polarity of current. And a certain amount of electrons jump over the work metal surface from electrode which produced a high temperature near about 3400°C. This high temperature is utilised in melting the work metal up to molten stage at joining points and the electrode also melts simultaneously. Melting of work metal at joining make a pool of molten and alongwith in the molten filler metal cover this pool of metal. This covering of molten electrode over pool of molten metal is known as " welding bead".
Electrical Energy: Both A.c. and D.C. electrical energy are widely used in arc welding process. Both have some advantages and disadvantages which regulate the use of particular electrical energy for a specific welding. Use of electrical energy also depends on the material of work metal properties of material to be weld like thickness of metal etc.
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Carbon arc welding is mostly for steel sheet and casting holders consist of magnetic coil which guide the Arc. This welding process is operated manually or by machine or both.
Properties
of A.C. and D.C. Arc Welding
Properties
A.C. welding process
D.C. welding process
1.
Installation cost
Less initial cost investment.
Higher initial investment.
2.
Maintenance
Economical and easier.
Critical and costiler.
3.
Current value
Mostly suitable with higher current value
Better suitable with lower current value.
4.
Arc
In some cases it is comparatively difficult
Comparatively easier to develop an arc it
to develop an arc and maintaing of arc
consists almost every time problem of Arc
is little difficult than D.C. arc welding. It
blow etc. In D.C. welding process
consists very rarely problem of arc like
maintaining of arc is comparatively
arc blow etc.
easier.
S.No.
5.
Power supply
It is most preferred withA.C. mains supply
It is easily used with A.C. and any D.C. power supply also.
6.
Polarity
Its polarity is interchanged with every change
It has fixed polarity.
of cycle of power,
7.
8.
9.
Electrode
Bare electrodes are not suitable, so only flux
In this process bare and coated both types
coated electrodes are mostly used.
of electrodes, can be used easily.
Maintaining small arc is difficult, only
Maintaining of small arc is easier than
iron powder electrodes are exceptional.
A. C. Arc welding.
Welding
By this process welding of thin sheet is
Thin sheet can be easily welded by this
capabilities
difficult. Welding capability is limited up to
process. It has distinct polarities so it is
Arc length
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Production Engineering
A-190
to.
Welding distance
only ferrous metals generally due to change
easier to weld different metals also other
in polarity in every cycle.
than ferrous metal.
Voltagedrops are less as compared to D.C.
It has relatively more voltage drops so
supply at a distance from main supply. So for distance welding from power mains supply
welding is preferred to do at nearest to the D.C. mains supply.
A.C. welding is mostly preferred. GAS WELDING
It may also be considered under non-pressure fusion welding. The source of heat required for fusion of metal is achieved by flame of suitable gas combustion. It consists of a flow of any suitable gas under specific pressure which gives a flame after burning in presence of oxygen etc. Tools and Equipments In gas welding process different tools and equipments are used. Some of the mainly used are mentioned below: Welding torch - or blow pipe may be defined as the equipment designed for mixing oxygen and combustible gas (acetylene etc.) in required proportion and injecting for combustion and making flame or we may say that with this equipment we can acquire an adequate mixed proportion of oxygen and acetylene (in oxyacetylene gas welding) to develop a suitable flame for welding
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Classification: (a) According to pressure of acetylene gas (i) High pressure welding torch (ii) Low pressure welding torch (b) According to number of tips used with torch (i) Single tip welding torch (ii) Multiple tips welding torch (c) According to fuel used (i) Acetylene welding torch (ii) Hydrogen welding torch (d) According to application (i) Mannual welding torch (ii) Automatic welding torch Hose pipe: Itis used for supply gases from pressure regulator to welding torch. These are made up of rubber coating overthreaded net pipe. It should have sufficient strength, light in weight, economical, and non-reactive with gas which they tend to carry. These are fixed with welding torch with the hose pipe clamp. Pressure regulator: Itis a pressure controlling devices used for supply of desired pressure of gas to loose pipe connected with welding torch. Itis mounted directly over gas cylinders. Classification: (a)
(b)
Single stage regulator: Itregulates pressure of gas at one stage only. It has to be regulated from time to time as the internal pressure inside cylinder varies. Two stage regulator: It is desired to regulate pressure of gases at two stages. One is auto-controlled and other is
controlledbyextremelymounted screw.It consistsofa small storage chamber due to which the out going pressure is out of effect of pressure variation inside the cylinder. This type of regulator consists of pressure gauges mounted on regulator which shows the pressure of gas inside the cylinder and out going gas pressure. Acetylene gas purifier: These are used in low pressure acetylene gas generators. It is used for detecting impurities like sulphides and phosphomines etc from the acetylene gas to improve the properties of acetylene gas. Water seal or hydraulic back pressure value: It is used in low pressure acetylene generator system. It is mounted between welding torch and acetylene generating cylinders/tank. Important
Applications:
Itreduces the back fire hazards It works as non-return value against atmospheric air and oxygen when the pressure of acetylene gas is reduced than the atmospheric pressure inside the tank. Safety valve: It is a safety device used to provide safety against high pressure of gas than the recommended range. Welding table: It is used for placing jobs during welding operations. It is made up of mild steel and top is made by some refractorymaterial/refractory brisk etc. Welding torch lights: Itis an instrument which produces spark used for lightening weldingtorch. In practice, electronicgas lights are commonly used other gas welding equipments are welding goggles, apron, gloves, and wire brush etc. (i) (ii)
Gases used in welding process: 1.
2.
Oxygen (02) It does not go through combustion itself but very helpful in combustion process with different gases. It is storedin metalliccylindersat about 120 kg!em?in liquefied state. It is prepared by following two methods mainly (a) Byliquefication of air (b) By electrolysis of water Acetylene (C2H2): It is highly inflamable gas and produces about 3600°C temperature.
Production
method:
(a)
Combination of carbon and hydrogen: In this processtwo carbon electrodes are used to produce arc in presence of hydrogen gas which make C2H2 in which a little amount of methane and ethane gases are found.
(b)
Natural gas de-composition: It is most popular method
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Production Engineering
A-191 5.
producting acetylene gas in modern life. In this method natural gas is treated by electric arc which produces acetylene and hydrogen. (c)
By calcium carbide: "In this method calcium carbide is reacted with water as resultant acetylene gas and lime are produced. CaC2+ Calcium Acetylene
2~O Water
~
Ca(OH)2 + C2~ Lime
carbide The reactor vessels used for producing acetylene are called generator. According to pressure of generated gas, the generator may be classified as under (i) Low pressure Generator: Containing gas pressure of about 0.1 kg/cm-. (ii) Medium Pressure Generator: Containing gas pressure of about 0.1 to 1.5 kg/cm-. (iii) High Pressure Generator: Containing gas pressure of more than 1.5 kg/cm-. Properties of Acetylene: (i) It is colourless gas and lighter than air. (ii) It explodes at about 300°C itself in presence of oxygen. (iii) It has mild smell and having no harmful action to being but in more than 40% cases it creates problems in respiratory system. (iv) It can be converted, into liquid state at about 1°C temperature and 49 kg/ern? pressure. Properties of Hydrogen Gas: (i) It is highly inflamable gas and produces about 2400°C temperature. (ii) It is a colourless, odourless and tasteless gas. (iii) It is generally used for cutting and welding soft metal likealuminium, magnesium and lead etc. (iv) Retort gas: It is a mixture of number of in flam able gases produced by decomposition of oil at about 740°C in a retort. Natural Gas: It is a colourless and odourless gas which is a mixture of hydrocarbons and achieved from oil mines. Propane and butane: These are produced from oil refineries. Some other gases also used in gas welding process. For example coke oven gas, petrol or kerosene gas, argon and helium etc. Filler material or Electrode: Filler material may be defined as the material rod required to fill the gap between the metal in molten state. Dry various metal electrodes are used with different welding processes. Welding Rods
Applications Mild steel etc.
2. 3.
Low carbon steel (copper coated) High carbon steel Stainless steel
4.
Aluminium
L
For making hard weld etc. Stain less steel goods welding Aluminium goods welding
7.
Cast iron
Welding of copper made articles Mainly in gas welding and brazing etc. For cast iron welding
Properties: 1. It should be economical and easily available. 2. It should have low melting point than the filler metal. 3. It should have sufficient quality of dissolving impurities of molten metal and light inweight so that it can float above the welding metal in molten condition. 4. It should be easily removable after welding. 5. It should not produce any deflect in weld.
Flux
Application
1.
Borax (Na2B407)
2.
Cast iron flux
3.
Brazing flux
4.
Alumina
Used with mild steel and low carbon steel etc. Used with cast iron, high carbon steel, ferrous silicon and silver steel etc. Used with copper, brass and bronze etc. Aluminium and its alloys etc.
S.No.
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S.No.
6.
Copper silver alloy Brass
FLAME It is produced by combustion of gases and due to oxidation,
different temperature are achieved. A flame can be adjusted for different temperature range. So these different flames have a distinct role in gas welding process. Middle
:.:J- ~c:n_e_......~Outer ~~~:,-----~~) I, , Zone
--Inner Zo~~"'--
./
J
-i-----T-
./
- ----
....
Fig. National flame
Middle Zone
Outer Zone
Fig. Carburising flame
Inner Zone
r-----J< -----~~~ vy-----i
Middle Zone Outer Zone
Fig. Oxidising flame Classification
of Flame
1.
Neutral flame: It is achieved when acetylene and oxygen are used in equal quantity. It consists of only two specified parts of flame, one is inner and outer envelop. It is most widely used in gas welding, it produces above 3200°C temperature.
2.
Carburising flame: This flame can be achieved by increasing acetylene gas quantity in flame it consists three distinct flames and acetylene feather can be easily detected in this flame, it is generally used in hard facing, nickel, and monel welding etc.
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Production Engineering
A-192 3.
Oxidising flame: It can be achieved
by increasing percentage of oxygen in natural flame. It is generally used with brass welding.
Common Difficulties in Flame Formation 1.
2.
3.
4.
Breaking offlame: Looks like burning gas with maintaining some distance from tip of welding torch. It can be rectified by reducing pressure of gas etc. Flickering offlame: In this fault, flame shows flickering. It happens due to increase in moisture contents in acetylene and it can be removed by removing moisture contents from acetylene gas. Popping: In this fault as usual sound like pit-pit comes from welding torch. It can be rectified by regulating the pressure of gas. Back fire: In this fault flame disappear suddenly with an abnormal sound, it happens due to following reasons. (a) Using welding torch less than its recommended pressure (b) When tip of welding torch get two close to job (c) Over heating of tip etc.
WELDING METHODS 1.
Leftward welding: In this process most ofheat is absorbed by filler material rod so it is preferred in welding thin upto 6 mm thick sheet.
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SOLDERING AND BRAZING In this method of joining metals, particularly in the shape of sheet thin wire form, or thin wire with thin sheet like electronic part with PCB. In this method a low melting filler material is used and no fusion takes place in work piece. These filler metal used in this process is known as solder. These are made in various composition depending upon the application and requirement of strength of joint. Some important compositions are as follows:
1.
Tin 67% : lead 33%
2.
Tin 50% : lead 50%
3.
Tin 30% : lead 67%
In some process of soldering alloy of copper and zinc to which silver is also added sometimes is known as hard solder. Germal silver, used as a hard solder for steel in an alloy of copper, zinc and nickel, in general the classification of solder in the above two catagories is according to their melting point. Soft solders usually melt at a temperature below 350°C and hard solder above 600°C the operation performed by using a soft solder is known as soft soldering and when using a hard solder is known as hard soldering. In this process work piece is cleaned properly and than a solder ion tool is used in heated condition, which melts the solder and then a suitable flux is applied to joining point. This flux works to prevent the formation of oxidation. Normally zinc chloride is used as soldering flux. The soldering tool is made up in two types one is total iron made which is used by heated in furnace and another is copper tiped placed between electrical elements and the tip is heated electrically.
Brazing: Brazing is almost similar to the joining process of +- Work
Piece
e\O
/~\o~
o'~
~\~ev
2.
Fig. Leftward welding Rightward welding: In this technique most of heat offlame is absorbed by base metal so it is preferred in welding thick sheet generally 6 mm to 25 mm thick. Rest of flat, vertical, horizontal and overhead welding methods are similar as described in electric arc welding method. Welding
~
torch
.-V':'ork piece
Fig. Rightward welding
soldering except hard solder material is used in place of soft solder and work piece is heated up to red hot in brazing but in soldering process work piece remains cools only soldering material is melted and spreaded over the work piece to make soldering. But in brazing process work piece is heated up to red hot condition and then after hard solder material is allowed to melt with flux over the joint to be weld. So that solder material get melted and filled the small gap between the joint of work piece to be brazed.
SPECIAL WELDING TECHNIQUIES (a)
Some of the special welding techniques are given as follows : TIG (Tungsten Inert Gas welding): It is also known as Gas Tungsten Arc welding (GTAw). This process utilizes a non - consumable tungsten electrode that provides a very intense current to the welding arc. This welding arc provides the required heat to melt the metal. This electric arc is struck between a non consumable electrode and the metal work piece. The tungsten and weld puddle arc given a protective enviroment and also cooled with the help of an inert gas (eg. argon). A welding rod is also ted at joints alongwith filler material and melted with the base metal.
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Direction of travel
A-193
(iv) It requires minimum post weld cleaning Applications: It can be applied for deep groove welding of plates and castings. All commercial metals can be welded by this process. It also finds its application in automotive repair. MIG can also be incorporated into robotics. Some more applications are rebuilding equipment, overlay of wear resistant coating, welding pipes, reinforcement of the surface of a worn out rail road tracks. WELDING DEFECTS:
Copper backing bar
Gas Tungsten are (TIG) welding (GTAW) Advantages : 1. It produces, perfect, precise welds with suitable selection of proper welding rods and wires. 2. It has the capability to weld various metals. Most of the common metals or alloyslike mild steel, Stainless steel, titanium, aluminium and copper. 3. It uses a lesser amount of amperage as compared with other processes. 4. It is a clear welding process and does not leave any deposite over weld pead. 5. It has a high value of controlability 6. TIG welds are strong, ductile and resistant to corrosion. (b) MIG (Metal Inert Gas welding) : It is generally regarded as a high deposition rate welding process. In this process, consumable electrodes are used, which is generally in the form of coiled wire fed by a motor drive to argon shielded arc. Wire is consistently fed from a spool. A high value of current densities arc utilized. The diameter of wire is kept generallywithin the range of 0.80 mm to 2.30 mm. The consumable electrode in this process serves two purposes (i) its acts as a source for the arc column (ii) It also acts as the supply for the filler material. The shielding gas in this process, forms the arc plasma, stabilizes the arc on the metal being welded, shields the arc and molten weld pool, and allows smooth transfer of metal from the weld wire to molten weld pool.
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Metal Inert Gas (MIG) welding Adavantages : (i) It can work in all positions according to the need. (ii) it has a high deposition rate (iii) It requires less shilled labour
There are various types of welding defects which are given as follows:
Weld defects ~ (a) Undercut
(e) Lack of fusion
(b) Cracks
(f) Lack of penetration
(a) Slag inclusions: Various types of oxides, fluxes and electrode material are trapped in the welding zone. Dueto this trapping, inclusions are produced. These inclusion can be removed by grinding process or any other suitable mechanical process. (b) Under-cut: It can be defined as the notch which is formed due to the melting away the base/parent metal at the toe ofthe weld. It generally increases the stress and also reduces the fatigue strength of the material. It can be prevented by cleaning the metal before welding. It can be repaired with smaller electrode. (c) Porosity: Porosity is devloped when gas bubbles are entraped during cooling of weld pool. It is also devloped due to chemical reactions happened during welding. It can be controlling the welding speed. (d) Incomplete fusion: It is developed when the insufficient heat is provided and the travelling speed of weld torch or electrode is very fast. It is developed due to low amperage, steep electrode angle short arc gap, lach of pre-heat etc. It can be repaired by removing and rewelding. (e) Overlap: Overlaping in welding is caused due to improper welding technique, steep electrode angle and fast travel speed. It can be prevented by using a proper welding technique. (f) Underfill: It is developed when joint is not completely filled by with weld metal. It is caused by improper welding technique. It can prevented by applying proper welding technique for the weld type and position.
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Production Engineering
A-194 (g) Spatter: It is developed due to high power arc, magnetic arc blow and damp electrodes. It can be prevented by reducing arc power, arc length and by using dry electrodes. (h) Incomplete penetration: It occurs due to low amperage, low preheat, tight root opening, short arc length and fast tra vel speed. (i) cracks: The development of cracks results in the premature failure of the parts when they are subjected to dynamic loading conditions. There arc many types of cracks, some ofthem are given as: (a) Longitudinal cracks (b) transverse cracks (c) crater cracks (d) under bead cracks (e) toe cracks These cracks occur when the joint is at elevated temperature or after the solidification of weld metal. These can be prevented by altering the design in joint, altering the parameters, procedures, preheating the component etc.
(b) Magnetic particle testing: It is used to defect surface discontinuities in materials like iron, cobalt, nickel and their alloys. A magnetic field is produced into the component to be tested. The magnitization of the component can be done directly or indirectly. It the defects are present in the component after magnetization, then the defects will create a leakage field. After magnetization, iron particles are applied to the surface of the component. The particles will be attracted and aggregate near leakage fields, thus giving an indication of defect. It is used in gas pipe welding. (c) Ultrasonic testing: (UT) In this testing, ultrasonic waves are propagated in the component to be tested. The very short ultrasonic wave of frequencies ranging from 0.1- 15 MHz and upto 50 MHz are used for the purpose of defection of internal flows or cracks. In ultrasonic testing, electrical pulses are converted into mechenical vibrations and the returned mechanical vibrations arc converted into electrical pulses. Adevice called transducer converts electrical energy into mechanical vibrations. In this testing,a propr (Connected to ultrosonic machine) is passed over the surface of the component to be tested. As the wave travels through the materical, from the defective location, the wave get reflected. The transducer picks up the signals and CRT (cathode Ray Tube) screen records the pulse - height pattern. The spacing between pulses and height of pulses are interpreted for the purpose of finding the correct location of cracks in the component. (d) Radiographic testing: (RT) In this testing, the hidden flows are defected by using the ability of short wave length electromagnetic radiation to penetrate various materials. Radiographic Testing method reveals the surface and sub-surface defects.
(NOn NON - DESTRUCTIVE TESTING (FORWELDING) It is defined as the process of testing the welded components for discontineities, cracks, inclusions, spatters penetrations, undercuts, porosity etc. In this type of test, the component is not destructed and after testing the component, it can be further used. Some important kinds ofNDT (non-destructive testing) are given as : (a) liquid penetrant test: It is also known as Dye penetrant test or penetrant test. It is utilized for the purpose of detecting the surface detects, porosities, cracks etc in welding components. In this test, the material (component) is first cleaned and coating is applied with a fluorscent dye solutions. The excess solution after some time (dwell time) is removed. The bleedout is easily detected in visible dyes while fluorescent dyes are view with an ultrovoilet lamp.
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S. No.
Name of Machine
MACHINING Machining may be defined as a process of removing extra material from the work piece to achieve a desired shape and dimensions by using any cutting tool. Metal may be removed either in chips form or in fine powder form like metal removed form is tabulated as under:
Narne of operation to be carried out
Removed Metal form (Either Chip / Powder)
1
Lathe
Turning, Drilling, Inner turning, Threading and Taper turning, etc.
Metal removed in form of chips
2
Drill Machine
Drilling, Tapping, etc.
Chips
3
Shaper
Shaping
Chips
4
Milling Machine
5
Planer
6
Milling and Boaring, etc. Chips Planning, Turning, etc.
Chips
Broaching Machine
Broaching
Chips
7
Grinding Machine
Grinding
Powder
8
Polishing Machine
Polishing
Very fme powder
9
Buffing Machine
Buffing and Polishing, etc.
Very fme powder
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Production Engineering
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The common features of machining process are listed below:1. The material of tool should be harder than the work piece to be machined. 2. The tool should be strong enough and hold rigidity on a proper support so that it can withstand the heavy pressure during machinery. 3. The shape of cutting tool should be designed in such a manner that cutting edge produce maximum pressure on work piece. 4. There is always a relative motion oftool with regard to the work or that of the work with regard to the tool or both in relation to each other. Basic Elements of Machine Tool All machine tools do one similar work that of removal of material from work piece and all these machine tools have some common elements as given below:1. Frame Structure. 2. Slides and Guideways. 3. Spindles and Spindle bearing, etc. 4. Machine Tool Drive.
MACHINE TOOL CONTROLS On observing machine tools, we find that it contains many levers, hand wheels, stop switches, drivers etc. All of which are known as the control of machine tool which performs a specific function in every machine tool. All their controls specified are of the following types: 1. Mannual control. 2. Semi-automatic control. 3. Automatic control. 4. Numerical control.
Importantfactors requiredin today's scenarioasfollowing: (a) (b) (c)
Quick metal removal. High class surface finish with economic tooling cost. Minimum idle time of machining at lower power consumption. Cutting Action For cutting action, a relative motion between the tool and work piece is necessary. The relation motion between tool and work piece can be maintained either by keeping work piece stationary and moving to tool or by keeping tool stationary and moving work piece. The cutting action can be classified into following types:1. Orthogonal cutting and 2. Oblique cutting.
Work Piece ....---------.._
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)-
--, Movement
1-
-..- _.-.-.-.-.-.-.-- _o_._._._._.-c-.- _.-.-
---·-0-·-
\
--.
Work Piece Movement
-c-·
~ Cutting Tool Movement (a) Orthogonal Cutting
(b) Oblique Cutting
Turning on Lathe in Cutting Process
As shown in above figure, two types of tool shapes are used in orthogonal cutting process. We see that the cutting edge is rectangular and the turning face of work piece is made flat. This type of cutting is known as two-dimensional cutting. while in oblique cutting process, the tool's cutting edge is made like triangular / inclined. This process is known as three-dimensional cutting. CUTTING TOOLS Cutting tools may be defined as the tools required for cutting. The cutting tools used in power operated machines are commonly harder and having more red hot hardness than manually operated tools. These tools are designed to acquire more useful cutting using minimum power consumption. Properties of Good Cutting Tool Material 1. It should be tough enough and having good strength. 2. It should have good resistance against shock, wear, corrosion, cracking and creep, etc. 3. It should have good response for hardening, tempering and annealing, etc. 4. It should be economical and easily available. 5. It should have capability to retain these physical and mechanical properties at elevated temperature during
6.
machining operations. This property may be known as red hot hardness. It should be easily fabricated into tool shape.
Classification of Cutting Tools Cutting tools may be classified as follows on the basis of having number of cutting point / edges:-
1. Single Point Cutting Tools: These cutting tools contain only one cutting edge/point. For example, turning, parting and grooving tools for lathe machine, shaper tools and planer tools, etc.
2. Multi Point Cutting Tools: These cutting tools contain more than one cutting edge / points. For example, drill bit, broach and milling cutter, etc. On the basis of motion cutting may be broadly classified as follows:-
1. Linear or Reciprocating Motion Tools: For example, shaper tools, lathe tools and planer tools, etc.
2. RotaryMotion Tools:For example, drill bit, milling cutter, grinder wheels and honning tool, etc. Common Cutting ToolMaterials Depending upon their physical, chemical and mechanical properties, etc. some metal and alloys in common use are
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bronze and cast iron, etc. It can be employed for two times more speed than common High Speed Steel tools. 5. Cemented Carbide:These are generally used in sintered tips form made up of powder metrology process. These are directly manufactured into desired shape and size and mounted on suitable holders (either by brazing or by clamping, etc.). These holders are normally made by medium carbon steel. It gives better results than satellite and high speed steel. It can be used with four times more cutting speed than high speed steel tools and can retain its hardness up to 1200°C temperature. 6. Ceramics or Cemented Oxides: These are made by applying sintering process with aluminium oxides and boron nitride in powder form. It is also made up in readymade tips form. Which is used after mounted on a suitable tool holder (either by brazing or by fastening). These can easily retain their hardness up to 1200°C temperature and can work 2-3 times faster than tungsten carbide tips. Sometimes these ceramics give more satisfactory results in finishing, etc. than tungsten carbide, etc.
mentioned below:1. High Carbon Steel: High carbon steel shows different hardness with different percentage of carbon contents. It shows BHN hardness from 400-750 with different percentage of carbon. It contains carbon percentage 0.6%1.5%normally. But high carbon steel start losing its hardness above 200°C. So, its application is limited in slow moving / operating tools, hand tools and wood working machine tools, etc. For example, hammers, cold chisels, files, anvil, saws, screw drivers, center punch and razors, etc. 2. Diamond: Diamond is the hardest and brittle material but its use is limited due to its high cost. It consists great wear resistance but low shock resistance. So, it is used in slow speed cutting of hard materials like glass cutting tool, grinder wheel, dressing tool and other cutting tools, etc. 3. High SpeedSteel: It is most commonly known cutting tool material. It contains 18W, 4Cr, 1% V. In some tools, additional cobalt with 2%-15% is also added to increase its hardness up to 600°C. It contains sound ability to bear impact loading and perform intermittent cutting. 4. Stellite:It contains 40%-50% cobalt, 15%-35%chromium + 12.25%vanadium + 1%-4% carbon normally and it consists good shock resistance, wear resistance and hardness. Normally, it retains its hardness up to 920°C temperature and it is used for comparatively harder materials like hard
Cutting ToolGeometry The different angles provided in cutting tool also plays a significant role in machining process along with the material of tools. Here we give a sketch of single point cutting tool designed for different turning processes.
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~nd
Nose Radius ~ Side cutting ~ angle
cutting angle
Face _....3....._
Shank ---1
___;;""-L-
Top View
Side Rake angle
-:r
Top rake angle
~ Side Clearance angle
End relif angle
Front View Front clearing angle Side View Cutting Tool Angles
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Rake angle: The angle between face of tool and a plane parallel to its base. If this inclination is towards the shank, it is known as back rake angle or top rake angle and if measured along with side is known as side rake angle. These angles reduce the strength of tool's cutting edge. But along with reducing the strength, these angles also through away the chip from the cutting edge, which causes reduction of pressure on cutting edge of tool. Negative rake: When these angles are made in reverse direction to the above are known as negative rake angle. Obviously these angles strengthen the tools but reduce the keenness of cutting edge but these angles are used for extra hard surfaces and hardened steel parts, etc. and used generally carbide tips, etc. Lip angle: Lip angle may be defined as the angle between face and the flank of tool. As the lip angle increases, cutting edge will go stronger. It would be observed that since the clearance angle kept constant, this angle varies inverse to the rake angle. So, when the strong cutting edge is required like for harder material, rake angle is reduced and lip angle increased. Clearance angle: As the name resembles, this angle is made in tool to provide clearance between job and cutting edge of tool. If the angle is provided in side of cutting edge, it is known as side clearance angle and if this angle is given at front of tool it is known as front clearance angle. Relief angle: This angle formed between the flank of tool and a perpendicular line drawn from the cutting point to the base of the tool. Cutting angle: The total cutting angle of the tool is the angle formed between the tool face and a line through the point which is a tangent to the machined surface of the work at that point. Obviously, its correct value will depend upon the position of tool in which it is held in relation to the axis of the job.
The grains of metal in front of cutting edge of tool start elongation the line AB and continue to do so until they are completely deformed along CD. The region between ABCD is known as shear zone. Types of Chips Chips may be classified as given under:1. Discontinuous or Segmental chip. 2. Continuous chip. 3. Continuous chip with built-up edge. 1. Discontinuous Chip
Work Piece
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Discontinuous Chip Formation
CHIP FORMA nON Chip may be defined as a thin strip of metal removed from the work piece as the tool progressed into work piece. Like in lathe machine, where job is kept moving and a study tool advanced into it, the metal's thin strip removed from work piece due to its plastic deformation but as the length of chip increase a stress compress the chip and after a limit, this chip gets fractured and removed from work piece. The shearing of metal chip formation does not, however, occurs sharply along a straight line.
2.
These type of chips formed in small pieces as shown in figure. This type of chips are produced during machining of brittle material like cast iron and bronze, etc. In machining of brittle materials, shear plane gradually reduce until the value of compressive stress acting on the shear plane becomes too low to prevent rupture along with as the tool advance formed in work piece. At this stage, any further advancement of tool results in the fracture of metal ahead of it, that's why it results in production of segmented chips. In this type of chip formation, excessive load has to withstand by tool which results in poor surface finish of work piece. Continuous Chip Formation
Continuous Chip Work Piece Work Piece B
Continuous Chip Formation Chip Formation
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As shown in figure, the chip formed in a continuous ribbon form and breaks after a certain length. It happens when ductile material is machined. In this chip formation, minimum load forced on the tool's cutting edge. So, that a better finish is achieved and minimum wear and tear occur in tool edge.
Built-up Edge
3. Continuous Chip with built-up Edge This type of chip is generally formed during machining ductile material and a high friction exists at the chip tool interface. Due to high friction, a high temperature generates at melting point of chip and cutting edge of tool. Due to generation of high temperature, chip formed at high temperature. As the cutting proceeds, the chip flows over this edge and up along the face of tool. Periodically, a small amount of the built-up edge separates and leaves with the chip or embedded in the turned surface. Due to this, chip formed is not smooth. When the tool is operating with a built-up edge a short distance, back from the cutting edge, the wear takes the form of cratering of tool face caused by the extreme abrasion of chip. This type of chip formation may be reduced by using proper coolant.
Work Piece
Showing Built-up edge Due to built-up edge chip formation, surface finish achieved is rough and chance of production in crater on the surface of work piece. CUITING FORCE Cutting force is a very important factor in tool designing like we consider a lathe turning tool, it is a single point cutting tool. The force acting on the tool is the vector sum of three component cutting force mutually at right angle. The resultant cutting force is denoted by (R).
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Work Piece
-: _._._._._._._._._._
..
I
Tool where, Fn = force normal to machine surface Ff = force acting parallel to the axis of work piece F, = tangential force along work piece Out of these three components, force Ft is the largest and Fn the smallest. In case of orthogonal cutting, only two component force come into play since the value of Fn is zero in that case. In single point cutting turning process, the component Fn- Ff and F, can be easily determined with the help of suitable force dynometer. Thus resultant R can then be calculated from the following relationship:R
= '\jIF n2 + F f2 + F t2
_._._.-._._._._._._._
..
~_.
R
and in case of orthogonal cutting process, as stated that Fn is almost zero. So, value of R=
IF2f
'\j
+ F2 t
According to A.S.M.E. cutting manual, tangential cutting force will be as given below:Pt
=K --p K a TC Ld
where, P, = tangential cutting force ~ = constant depending upon the material Ka = constant depending upon the true rake angle of tool T = average chip thickness L = length of cutting edge in active engagement
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c and d are exponents depending upon the material being out. The variable T and L are introduced in order to embrace the nose angle. Nose radius feed per revolution and depth of cut.
(Ft cos $ + Fc sin $) sin $ bx t
Stress in Metal Cutting
Shear Strain
As we know that when tool applied a force on work piece and resulting chip formation, the chip production occurs due to stress and strain development. To compute the stress and strain developed on chip, we consider a single point cutting tool as given below:-
It has been defined as the deformation per unit length. In metal
cutting, the diagram for measuring shear strain is taken from a shear plane, we have AB
Shear Strain, y = CD =
AD+DB CD
= tan ($ - a) + cos $
= -----
cosu
sin $ cos ($ -
a) .
Work Done in Cutting
A Strain in Cutting The values are calculated for the conditions at the shear plane where the two normal force Fs and Ns are existing. Let, Fs = force across the shear plane As = area of shear plane $ = shear angle b = width of chip t = thickness of chip Fc = cutting force Ft = tangential force Fn = force normal to shear plane
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(Z) =
F AS s
(kg F/mm2)
and (as> (mean normal stress) =
F
__!!_
As
2
(kg F/mm )
where, Fs = Fc cos $ - Ft sin $ Fn = Ft cos $ + Fc sin $ Ao A = -.(where Ao = area of chip before removed) s sin o So, mean shear stress (t) =
Fs
A=
Fc cos$ - Ft sin $ bxt
s
The work done in cutting process may be calculated by adding work done in shearing and work done in overcoming friction arise. If W = total work done Ws = work done in shearing Wf = work done in overcome friction Wm = (work done in cutting + work spent in feeding) Ao = (cross-sectional area of chip before removal) Now, assuming that there is no work loss, then total work done must be equal to the work supplied, then total work done, we have W = Ws + Wf ... (1) Now, we assume that total work supplied is used in cutting but partly used in feeding the tool, then we have Wm = work consumed in cutting + work spent in feeding Wm = Fc x x Ft x feed velocity
v,
Now, assuming that the Ft is very minor in comparison of Fc. So, neglecting the feeding work, we have Wm = Fc x Vc ... (2) Assuming that there is no work loss, we have Wm = W ... (3) So, putting value in equation in (3), we have FcxVc=Ws+Wf ... (4) as we know, Ws = Fs x Vs (shear force x shear velocity) Wf = F x Vf (friction force x velocity of chip flow) then, FcxVc=FsxVs+FxVf ... (5) if the forces are taken in kg and velocity in metre per minute, the work done will be in kgf m/min. Then, W=
Total work done in cutting per unit time ... Volume of the metal removed m unit time
sine (Fc cos $ - Ft sin $) sin $ bxt and mean normal stress,
So, we have
Iw
=
Fe Ao
I
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Horse Power Calculation . . Work done in cutting / minute H.P. required for cuttmg = 4500 Power =
Fe x Ve H.P. 4500
... (1)
Fe x Vc kw 4500 x 1.36
... (2)
Source of Heat in Metal Cutting
4.
Engineering
face and therefore the chip does not get hardened. The chip separates from work piece at the shear plane. Accounting all above Lee and Shaffer's had developed a slip-line field for stress zone, in which no deformation would occur even if it is stressed to its field point. From all these, both of them had derived the following relationship: 1t
= - + a - 1 = 45° + a - 1 4 or we can say,
1+1-0,=45°1
... (1)
BASIC PRINCIPLES OF MACHINING
3 WorkPiece
Area (1) = Primary deformation area
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Area (3) = Tool work piece interface Assuming that all work done is converted into heat, then the heat generated we have (Q), where Wm = Fe x then we have,
v,
(i) Drilling: Drilling is the process or operation used for manufacturing circular holes. These holes are produced by a specific type of end cutting rotating tool which is generally termed as drill. The machine used for the purpose of drilling is known as drill machine. The operationsperformedby drill machine in addition to producing holes are tapping, reaming, boring, counter boring, spot facing etc. • Working principle: A large amount of force is exerted by the rotating edge of the drill on the workpiece and then the hole is produced. During driling operation, the metal is removed by shearing and extresion. ~-001
Tool (Drill) feed motion
IQ= Fe x Vel· EARNST-MERCHANT THEORY It is based on the principle of minimum energy consumption. It states that during cutting the metal, shear should occur in the direction in which the energy requirement for shearing is minimum. The other assumption made by them includes= 1. The behaviour of metal being machined is like that of an ideal plastic. 2. At the shear plane the shear stress is maximum is constant and independent of shear angle ( Partial face milling: In this type, the milling cutter is in overhanging position from one side of the work-piece. => End milling: In this type, the diameter of milling cutter is less than the width of the work piece. => Profile milling: In this type, outside periphery of the flat part of work-piece is cut.
SHAPING: (WORKING PRINCIPLE) It is described as a process in which metal is removed from metal work piece surface in horizontal, vertical and angular planes. In these operations, a single point cutting tool is utilized, which is held on the ram that provides a reciprocating motion to the tool. A single point cutting tool is clamped in the tool post which is mounted on the machine's ram. The motion ofthe ram is the reciprocating TO and FRO, which resulting the tool cuts the material in the forward stroke. There is no cutting during return or bachward stroke. Shaping operations are generally used for producing slots, grooves and keyways. It also produces contour of can cave or conven or a combination of these.
k = Return stroke time Cutting stroke time
As we know that,
Return stroke = K
x
cutting stroke time
ke 1000V Time taken to complete one double stroke, (T
=>kxT
c
=--
e
=--+-1000V T
_
2J
ke 1000V
e + ke
_ e(k + I)
2s - 1000V - 1000V
1000V
1
Now, N = _1_=
e(k + 1) 1000V
T2s
e(k
+ I)
Machining time: (T.J As we know that, Time taken to complete one double stroke (T 2s)' T
_ e(k+ 1) 1000V
2s -
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Let, b = breadth of work piece, (mm), f= feed rate (mm/double stroke) Now, Total number of double strokes needed to complete
b
(vertical surface)
(Horizontal surface)
(Inclined surface)
(Working principle) Classification of shaping machine: (i) Horizontal type (ii) Vertical type (iii) crank type (iv) Hydraulic type (v) Universal type Mechanisms used in shaping machines: (i) crank and slotled lever mechanism (ii) Hydraulic shaper mechanism (iii) Whitworth quick return mechanism Cutting speed: In is defined as the ratio oflength of cutting stroke to the time required by the cutting stroke. Let, V = cutting speed, m/min. N = Number of douple strokes ofthe ram/min. K = ratio of return time to cutting time 1= length of cutting stroke Time required by cutting stroke (T c) cutting stroke length (m) cutting speed (m / min)
T = c
e
the work =f"
Hence, Time taken to complete the cut b)
eb(k+l)
=> T2s x ( f" = 1000Vf (iv) Lathe machine (working principle) A Lathe is defmed as a machine tool on which work piece is rotated on its own axis for the purpose ofperforming various operations like cutting, knurling, turning, facing etc. In a lathe machine, the work piece is helded between the chucks which revolve. The tool post consists of a cutting tool which is fed against the work piece for required depth and also in required direction. The material from the work - piece is removed in the form of chips and the required shape is obtained. Some parts of a lathe: (a) Bed (b) Legs (c) Head stock (d) Tail stock (e) Gear- box (t) carriage
Workpiece \ ( ( \
Tool
I
/'"
V x 1000 Working Principle of Lathe machine
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Some operations performed on Lathe in brief: => Turning: In this operation, straight, curved and conical workpieces are produced. => Facing: In this operation, the flat surface is developed at the end of the work piece. => Boring: In this operation, a hole or a cylindrical cavity is entarged which are manufactured by another process => Threading: In this operations, threads are produced internally or externally => Knurling: In this operation, a regurlarly shaped roughness is developed on cylindrical surfaces. Machining properties / cutting parameters: => Feed: It is defined as the distance through which the cutting tool advances between two consecutive cuts. => Depth of cut : It is defined as the advancement of cutting tool into the job in a transverse direction => Cutting speed : It is defined as the speed through which the spindle rotates. (a)
. ( ) 7tDN Cuttmg speed V =--
1000
where, D = diameter of workpiece (rum) N = rotational speed (rpm) (b)
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Machining time: (T)
= _L_
FxN where, L = length of work-piece F = feed rate (mmlrev.) N = rotational speed (rpm) (D-d)
(c)
Depth of cut : (tc)
= -2-
where, d = diameter of work piece after machining D = diameter of work - piece before machining (d) Metal Removal Rate (MRP) = 7tD tc FN Types of Lathes : (a) Centre or Engine lathe (b) Bench lathe (c) Speed lathe (d) Tool room lathe (e) Automatic lathe (t) Turret lathe (g) Capstan lathe (h) Computer - controlled lathe Grinding: Grinding is a machining purpose used for the purpose of removal of the metal with the help of applying abrasives which are bonded to form a rotating wheel. It is generally utilized for good surface finishing, grinding of craks and burns etc. It can be utilized for flat, conical and cylindrical surfaces.
Engineering
Types of grinding machine/operations: The following are the grending machines: (a) Surface grinding (b) cylindrical or External grinding or centre - type grinding (c) Internal cylindrical grinding (d) centerless grinding (e) Form and profile grinding (t) Plange cut grinding => In surface grinding. It utilizes a rotating abrasive wheel for the purpose of removing material and thus resulting in a flat surface => In cylindrical grinding, It is utilized for the purpose of grinding cylindrical surfaces and work-piece shoulders. => In internal cylindrical grinding, It is used for the purpose of grinding the internal diameter ofthe work piece and also tapered holes => In form and profile grinding, the grinding wheel does not transverse the work-piece and having the exact shape as of the finished product. => In plunge - cut grinding, It is used to grind the work pieces having projections, multiple diameters or other irregular shapes. Various types of grinding wheel:
n
Wheel thickness
Grinding faces
IE
wheel diameter
I
I
I
)!
.--r---------r---,
Type 1 (straight)
'" L; r--? 1= Type 2 Rrecessed t (one side straight)
Diameter of Recessed
~
c
I I ~
Type 3 Recessed (Both sides straight) Grinding face
) Type 4 (Tapered face straight wheel)
Grinding face
Thickness
J,
t
LI...........__ _-----'--'II
O'----------r----r----O
Type (cylindrical or wheel ring)
~II~
Type 6 (straight cup wheel)
Grinding face
Type 7 (Flaring cup wheel)
Type 8 : Saucer wheel
Type 9 : Dish wheel WorkTable Grinding Principle
=> Type 1, Type 2 and Type 3 are utilized for cylindrical, internal centreless and surface grinding.
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=> Type 4 is usually used for thread grinding of a gear teeth.
=> Type 5 is utilized for producing flat surfaces. => Type 6 is utilized for grinding flat surfaces by applying grinding wheel. => Type 7 is utilized for the purpose of grinding tools. => Type 8 is utilized for the purpose of sharpening of circular or band saw. => Type 9 is utilized for the purpose of grinding various kinds of tools in the tool room. Characteristics of grinding wheel The performance of a grinding wheel depends on the following factors: (a) Abrasives: Abrasives are used due to its two main mechanical properties i.e. hardness and toughness. It also has a sharp edges. Some ofthe properties of abrasives are indentation, fracture r resistance, wear resistance etc. There are generally two types of abrasives which are as : => Natural abrasives: These are sand stone, corundum diamond and gasnet etc. => Synthetic abrasives : These are manufactured and have well defined properties of roughness and hardness. Eg : silicon carbide and aluminium oxide. (b) Bond: It has the property of adhesiveness. Due to this property, the abrasive grains are cemented together for the purpose offormation of grinding wheel. As per the demand, it serves the imparting of hardness or softness properties to the grinding wheel. Some bonds are given as follows: => vitrified bond => silicate bond c> shellac bond => Rubber bond => Oxy chloride bond c> Resinoid bond (c) Grit: It is also termed as grain size. After passing the materials through screens, the size of the grain grit is determined with the number of meshes / linear inch. It influences the stock removal rate and surface finish. Grain size selection depends upon the type of grinding, type of material; material removal rates (MRR) and required surface finish. (d) Wheel grade: The wheel grade is measured by the strength ofthe bonding material. These are generally two kinds of wheels used which are hard wheel (Strong bond and abrasive grains can with stand with larger forces) and soft wheels (ifthe material to be grinded is hard then the abrasives grains are wear out and resulting losing of sharp edges for cutting is lost, this process is known as glazing.) Selection of grinding wheel: The grinding wheels are selected depending upon the following given factors. (a) Material's properties (b) Required quality of surface finish (c) Accuracy in dimensions (d) Method ofgriding i.e. either dry or wet (e) Rigidity, size and machine type (t) Speed and feed of wheel
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A-205 (g) Type of grinding to be done Grinding wheel parameters : => Depth of cut : Itis defined as a thickness of the material removed through grinding wheel in a single transverse stroke. Depth of cut(Tc) d, d2
= (dl
~d2)
= diameter ofthe work-piece before grinding = diameter of the work-piece after grinding
=> Feed: Feed is described as the motion of the workpiece longitdinally per revolution in cyclindrical grinding. Feed (f) = k..> A (where A is constant) where, A = face width of wheel in mm = 0.4 to 0.6 (finish grinding) = 0.6 to 0.9 (Rough grinding) fxN
=> Work travel : work travel = --m/ .
1000
. mm.
where, N = Rotational speed (m/min).
MANUFACTURING PROCESSES IN BRIEF Manufacturing process is defined as the conversion raw material into finished or find product. Classification of manufacturing processes: (i) Primary shaping processes: ~ casting ~ Powder metallurgy ~ Plastic technology (ii) Forming processes: ~ Forging ~ Extresion ~ Rolling ~ Sheet metal working ~ Rotary swaging ~ Explosive forming ~ Electromagnetic forming (iii) Machining Processes : ~ Turning
~
of
Drilling
~ Milling ~ Grinding ~ Shaping and Planning ~ Non - Traditional machining such as : ultra sonic machining, Electro-chemical maching etc. (iv) Joining Processes ~ Pressure welding ~ Resistance welding ~ Diffusion welding ~ Soldering ~ Brozing (v) Surface finishing processes ~ Honing ~ Lapping ~ Electro-plating ~ Plastic coating ~ Metallic coating
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~ ~
Sanding Tumbling
COMMONLY USED MACHINES AND TOOLS: (i) Lathe machine: ~ Cylindrical turning : It involves the reduction of diameter of work-piecebyremoving material along the axis of work - piece from the cylindrical job's surface. ~ Taper turning: In this type, material is removed at an angle to the work-piece axis. And thus diameter of the workpiece is increased or decreased. ~ Eccentric turning: In this type, the axis of work-piece does not coincide with the main axis. ~ Knurling : In this type, a diamond shaped impression is embossed on the work piece. ~ Facing: In this type, flat surface is developed by machining the ends ofthe work-piece. ~ Parting - off: In this type, the work piece is cut after obtaining required shape and size. ~ Chamfering: In this type, the end ofthe work - piece is bevelled. (ii) Milling Machine: ~ Plain milling: In this type, a flat, horizontal surface is made paraller to the axis of rotation of plain milling cutter ~ Side milling : In this type, a flat vertical surface is developed on the side of work-piece with the help ofa side milling cutter. ~ Facemilling: In this type,facemilling cutter is utilized with rotating motion about a perpendicular axis to the work -piece. ~ End milling: In this type, a flat surface is developed. The developed flat surface may be horizontal, vertical or at an angle with the table. ~ Thread milling: In this type, threads are produced by utilizing a single or multiple thread milling cutter. ~ Form milling: In this type, irregular contours are generated with the help ofa form cutter. (iii) Drilling machine : ~ Drilling: In this type, a cylindrical hole is developed
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with a drill which is a cutting tool having cutting edges ~ Boring: In this type, the hole (pre-existing is enlarged by using drilling operation. ~ Reaming: In this type, a preexisting hole produced by drilling or boring is finished and sized. ~ Counter-boring: In this type, the pre existing drilled hole is enlarged cylindrically at the end of the hole. (iv) Shaper machine: ~ Horizontal surfaces : In this type, a flat surface is generated on a workpiece by holding it in a vise. ~ Vertical surfaces: In this type, the end of a workpiece, squaring up a component are produced. ~ Angular surfaces : In this type, an angular cut at an angle other than 90° with the horizontal or vertical plane. (v) Planer machine: ~ Horizontal surfaces: In this type, the tool is feeded crosswise for the purpose of completing the cut, while the work piece is provided a reciprocating motion along with the table. ~ Vertical surfaces: In this type, the tool is feeded down ward for the purpose of completion of the cut, while the work piece is provided reciprocating motion along with the table. ~ Angular surfaces : In this type, the tool is feeded at an angle for the purpose of completion ofthe cut, while the work - piece is provided reciprocating motion along with the table. (vi) Grindingmachine: ~ Cylindrical surfaces: In this type, cylindrical surfaces of a work piece are fmishedby utilizing cylindricalgrinders. ~ Tapered surfaces : In this type, tapered surfaces of a work piece are finished by using cylindrical grinders ~ Horizontal surfaces : In this type, the horizontal surfaces of work pieces are finished by utilizing the surface grinders. ~ Threaded surfaces: In this type, threads are produced by utilizing a thread grinding machine along with single or multiple rib wheels.
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EXERCISE 1.
For TIG welding, which ofthe following gases are used? (a) Hydrogen and carbon dioxide (b) Argon and helium (c) Argon and Neon (d) Hydrogen and oxygen 2. The pre-heating of parts to be welded and slow cooling of the welded structure will lead to reduction in : (a) residual stresses and incomplete penetration (b) cracking and incomplete fusion (c) cracking and residual stress (d) cracking and underfill Which one ofthe following is a solid state joining process? 3. (a) Gas-Tungsten arc welding (b) Resistance spot welding (c) Friction welding (d) Submerged arc welding 4. Arc stability is better with: (a) ACwelding (b) DC welding (c) Both (a) and (b) (d) None of these In which type ofwelding, molten metal is poured forjoining 5. the metals? (a) Arc welding (b) Thermit welding (c) MIG (d) llG The gases used in tungsten inert gas welding are: 6. (a) argon and helium (b) neon and helium (c) neon and argon (d) ozone and neon 7. Amount of current required in electric resistance welding is regulated by changing the: (a) Input supply (b) Primary turns of the trasnformers (c) Seondary turns of the transformers (d) All ofthese The material used for coating the electrode: 8. (a) Protective layer (b) Blinder (c) De- oxidiser (d) Flux 9. The electric resistance welding operates with: (a) Low current and high voltage (b) High current and low voltage (c) Low current and Low voltage (d) High current and High voltage 10. Fluxes are used in welding in order to protect the molten metal and the surfaces to be joined from: (a) oxidation (b) carburizing (c) unequal temperature distribution (d) distortion and warping 11. Twostainless steelfoilsof 0.1 mm thickness are to bejoined. Which of the following processes would be best suited? (a) Gas welding (b) TIGwelding (c) MIG welding (d) Plasma arc welding
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12. In oxy-acetylene welding: (a) Pressure is applied (b) Filler metal is applied (c) Both Pressure and filler metal arc applied (d) Neither pressure, nor filler metal is applied 13. What should be the size of weld in case of buss welded joint? (a) Twice the throat of weld (b) Half ofthe throat (c) Equal to the throat of weld (d) None of these. 14. Welding process in which two pieces to be joined are over - llaped and placed between two electrodes in known as : (a) percussion welding (b) spot welding (c) seam welding (d) projection welding 15. The abbriviation ERW in ERW pipes stands for: (a) electrically resistance welded (b) elastic reinforced with wire (c) extra reinforcement welded (d) electrically reinforced and welded 16. T - joint weld is used: (a) where longitudinal shear is present (b) where sever loading is encountered and the upper surface of both piece must be the same plane (c) To joint two pieces of metal in the same manner as rivet joint metals (d) Tojoin two pieces perpendicularly 17. Half corner weld is used: (a) where longitudinal shear is present (b) where sever loading is encountered (c) tojoin two pieces of metal in the same manner as rivet joint metals (d) none of these 18. The range of optimum pressure applied in electric resistance welding is given by : (a) 0-5MPa (b) 5-lOMPa (c) 10-25 MPa (d) 25 - 50 MPa 19. Electronic components are often joined by : (a) soldering (b) brazing (c) welding (d) adhesive 20. The method ofjoining two similar or dissimilarmetals using a special fussible alloy is : (a) Soldering (b) brazing (c) Arc welding (d) All of these 21. The taper provided on pattern for its easy and clean withdrawl from the mould is known as : (a) Taper allowance (b) Distortion allowance (c) Pattern allowance (d) draft allowance 22. Sand are graded according to their: (a) clay content (b) gram SIze (c) clay content and grain size (d) None of these
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Production Engineering
A-20S 23.
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Sweep pattern is used for moulding parts having: (a) Triangular shape (b) Elliptical shape (c) Uniform symmetrical shape (d) Complicated shapes having intricate details In foundaries, a square pan fitted with a wooden handle is known as: (a) Bellow (b) Slick (c) Shovel (d) Riddle An aluminium cube of 20 em side has to be cast along a cylinderical riser. If the volume shrinkage during solidification is 6%, then shrinkage volume of cube after solidification will be : (a) 400cm3 (b) 480cm3 (c) 500cm3 (d) 540cm3 With a solidification factor of 0.97 x 106 s/m-', the solidification time in (seconds) for spherical casting of200 mm diameter is : (a) 539 (b) 4311 (c) 1078 (d) 918 Hot chamber die-casting machines are used for alloys with
35.
(a) Law melting temperature (b) High melting temperature (c) Low thermal conductivity (d) low electric resistance Which of the following processes is commonly used to manufacture powder coated steel central heating radiators? (a) sand casting (b) Bending (c) Shaping (d) Press work In an orthogonal cutting process, the cutting force and thrust force are 1200 Nand 600 N respectively. It the rake angle of the tool is zero, then what will be the coefficient of friction in fool- chip interface?
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(a) 2 (c)
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112
(b)
-Ii u Ji
(d) Which one of the following cutting tool bits are made by powder metallurgy process? (a) carbon steel tool bits (b) Stellite tool bits (c) less tool bits (d) Tungsten carbide tool bits Which one of the following is a single point cuting tool? (a) hacksawblade (b) millingcutler (c) pasting tool (d) grinding wheel The lip angle of a single point cutting tool is : (a) 10°-30° (b) 300t060° (c) 50°-60° (d) 60°-80° A milling machine has a metal removal rate 25 cm3/min. for a steel work piece. The depth of cut is 4.5 mm and width of cut is 90 mm. Then the required table feed will be : (a) 61.7 mmlmin. (b) 51.7 mmlmin. (c) 65.4mm1min (d) 48.8mm1min. For cutting tool material, which is correct order of increasing hot hardness (a) H.S.S, carbide, diamond (b) Carbide, H. S. S, diamond (c) Diamond, carbide, H.S.S (d) Carbide, diamond, H.S.S
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Which ofthe following among the given options is a single point cutting tool? (a) Milling cutter (b) Hack saw blade (c) Turning tool (d) Grinding wheel Which process involves increasing ofthe cross - sectional area by pressing or hammering in a direction parallel to the original ingot axis? (a) up setting (b) Peening (d) Setting down (c) Swaging Which of the following is not a type of industrial forging? (a) Drop forging (b) Roll forging (c) Blast forging (d) upset forging Which of the following statement is correct? (a) Hot rolling produces a stronger shaft than cold rolling (b) Cold rolling produces a stronger shaft than hot rolling (c) Shafts are not made by rolling process (d) Angle of twist of shaft is inversely proportional to shaft diameter Which ofthe following is commonly used die material? (b) Molybdenum (a) Tungsten (c) Cast iron (d) Hot work tool steel Reaming operation can be performed on : (a) Drilling and milling machine (b) Lathe and drilling machine (c) Shaper and drilling machine (d) Shaper and milling machine In a drilling machine the metal is removed by : (a) shearing and extrusion (b) Extrusion (c) Shearing (d) shearing and compression Which is not the part of drilling machine (a) Spindle (b) Tool holder (c) Table (d) Cross-slide Lathe beds arc produced by which of the following production processes? (a) Rolling (b) casting (c) Drawing (d) Forging When work piece is fed in the same direction and that of the cutter tooth at the point of contact, that type of milling is known as: (a) Down milling (b) upmilling (c) slot milling (d) slab milling Disign of jigs and fixtures need careful attention to: (a) Idle time reduction (b) Disign for safety (c) Swarf clearance (d) All of these TheA.P.F (atomic Packing Factor) for BCC structure is: (a) 0.52 (b) 0.68 (c) 0.74 (d) 0.84 Which of the following surface hardening processes needs quenching? (a) Induction hardening (b) Flame hardining (c) Nitriding (d) case carburizing
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Production Engineering 48.
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In iron-carbonequilibriumdiagram,the x-axisis represented by: (a) carbon percentage (b) Temperature (c) Nickel percentage (d) None of these In annealing heat treatment process, the hypocutectoid steel is: (a) Heated from 40° C to 50° C above the critical temperature and then cooled slowly in the tumace. (b) Heat from 40° C to 50° C above the upper critical temperature and then cooled suddenly in a suitable coolingmedium (c) Heated from 40° C and 50° C below the critical temperature and then cooled in still air (d) Heated below or close to the lower critical tempeature and then cooled slowly. 18 : 8 stainless steel consists of: (a) 18%vanadium, 8% chromium (b) 18%chromium,8.1nickel (c) 18%tungsten, 8% nickel (d) 18%tungsten, 8% chromium On high rate of cooling, austenite converts into: (b) Ferrite (a) martensite (d) Pearlite (c) Ledeburite Which ofthe followingis correctfornormalazing operation? (a) It relieves internal stresses (b) It produces a uniform structure (c) After heating, the material is allowed to cool in atmosphere (d) The rate of cooling is slow The crystal structure of austenite is : (a) Simplecubic (SC) (b) Body centred cubic (BCC) (c) Face centered cubic (FCC) (d) Hexagonal closed packed (HCP) Austenite decomposes into territe and cementite at a temperature of: (a) 1148°C (b) 727°C (c) 1495°C (d) 1539°C Alloy steel as compared carbon steel is more (a) strong (b) tough (c) fatigue resistant (d) All of these Shock resistance of steel is increased by adding (a) Aluminium (b) Cobalt (c) Nickelchromium (d) Carbon Carbon steel is (a) produced by adding carbon in steel (b) an alloy of iron and carbon with varying quantities of phosphorus and sulpher (c) purer than the cast iron (d) None of these The raise yield point oflow carbon steel (a) Phosphorus is added (b) Silicon is added (c) Carbon is added (d) Sulphur is added Stress-concentration occurs when a body is subjected to (a) Extensive stress (b) reverse stress (c) fluctuating stress (d) non-uniform stress
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A-209
60. Diamondweight is expressedin terms of carats. One carat is equal to (a) 20mg (b) 200mg (c) 400mg (d) 1mg 61. When a body recovers its original dimensions on removing the load then it is called (a) plastic (b) brittle (c) elastic (d) None of these 62. Abilityofmaterial to under go large permanent deformations in tension is called (a) plasticity (b) stiffness (c) toughness (d) hardness 63. Shock resistance steel should have (a) high wear resistance (b) low wear resistance (c) toughness (d) low hardness 64. Essential gradient of any hardened steel is (a) carbon (b) pearlite (c) martensite (d) cementite 65. Steel containing 18% chromium and 8% nickel is called (a) austinitic stainless steel (b) ferritic stainless steel (c) martensitic stainless steel (d) None of these 66. Steel having combination 88.7% ferrite and 13%cementite is known as (a) martensite (b) austenite (c) pearlite (d) All of these 67. A metal which is brittle in tension can become ductile (a) in presence of notches (b) in presence of emprillement agents such as hydrogen (c) under hydrostatic condition (d) None of these 68. Etching solution used for medium and high carbon steel, pearlite steel and cast iron is (a) Nital- 2% RN03 is ethyl alcohol (b) 1% hydrofluoric acid in water (c) 50% NH2, OH and 50% water (d) picral- 5% pieric acid and ethyl alcohol 69. Steelcontaining 15to 20% nickel and 0.1% carbon is called (a) ferritic stainless steel (b) austenitic stainless steel (c) martensitic stainless steel (d) None of these 70. Chrome steel is widely used for (a) connecting rod (b) cutting tool (c) handtool (d) motor car crank shaft 71. Carbon steel castings are (a) easily weldable (b) tough and ductile (c) brittle (d) All of these 72. Vandium when added to steel it (a) decreases tensile strength (b) increases tensile strength (c) remain constant (d) None of these 73. High speed steel should have (a) wear resistance (b) hardness (c) toughness (d) both (a) and (b) 74. Alloy steel containing 36% Nickel is known as (a) Stainless steel (b) High speed steel (c) Die steel (d) HS.S.
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Production Engineering
A-210
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Case hardening process is (a) carburizing (b) cynidity (c) nitridity (d) All of these Normalising of steel is done to (a) remove strains caused by cold working (b) refine grain structure (c) remove dislocation causedin the internal structure due to hot working. (d) All of these Steel containing pearlite and ferrite is (a) ductile (b) soft (c) hard (d) tough Percentage of carbon in carbon steel is (a) 0-1% (b) 0.1-1.5% (c) 1.5-4.2% (d) 1- 3% Cutting tools are manufactured by (a) High speed steel (b) Nickel steel (c) Chormesteel (d) None of these Silicon Steel is widely used in (a) electrical industry (b) connecting rod (c) cutting tool (d) All of these Steel containing 11- 14% chromium and 0.35% carbon is called (a) ferritic stainless steel (b) martensitic stainless steel (c) austenitic stainless steel (d) All of these Nitriding is a process for (a) softening (b) hardening (c) tempering (d) All of these Temperature at which the first tiny new grains appears is called (a) melting temperature (b) criticaltemperature (c) pointing temperature (d) recrystallinetemperature Annealing of steel is done to (a) improvemachinability (b) softeners of metal (c) release internal stress (d) All of these Machining properties of steel are improved by adding (a) Carbon (b) Chromimum (c) Silicon (d) Sulphur, lead and phosphorus To make low carbon steel tougher and harder (a) Carbon is added (b) Carbon reduced (c) Silicon added (d) Aluminium added Chilling heat treatment and alloy adding (a) decreases machinability (b) increase machinability (c) increase carbon percentage (d) None of these Ifsteel is cooled in still air, the structure obtained is called (a) sorbite (b) pearlite (c) toorsite (d) mortensite Heat treatment process used for castings is (a) hardnening (b) normalising (c) annealing (d) tempering
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90. Temperature at which the change starts on heating the steel is called (a) uppr critical temperature (b) point of recalesense (c) lower critical temperature (d) All of these 91. Heat treatment process used to soften the hardened steel is (a) annealing (b) hardening (c) tempering (d) quenching 92. Eutectoid based composition of carbon steel at room temperature is called (a) martensite (b) ferrite (c) comontite (d) pearlite 93. In steel, main alloy causing corrosion resistance is called (a) cobalt (b) vandium (c) carbon (d) chromium 94. Hardness of Steel depends on (a) Carbon percentage (b) Silicon percentage (c) Shape and distribution of carbide in iron (d) None of these 95. Advantage of austempering is (a) mere uniform microstructure is obtained (b) quenching eracts are avoided (c) None of these (d) All of the above 96. Delta iron exists in the temperature range of (a) 1400°C-1530°C (b) 768°Cto 900°C (c) 1400°C-1550°C (d) 350-786°C 97. Induction hardening have high (a) carbon percentage (b) cemiteteformation (c) power factor (d) frequency 98. Sorbite is obtained by (a) quenching of steel in oil (b) heating above its critical temperature (c) reduction of silicon percentage (d) annealing of steel 99. Temperature at which the changes end on heating the steel is called (a) uppercriticallimit (b) lowercriticallimit (c) melting point (d) point ofrecalesence 100. Heat treatment process is (a) hardening by quenching(b) annealing (c) tempering (d) All of these 101. Ifsteel is slowlycooled in furnace, the structure obtained is called (a) ferrite (b) sorbite (c) martensite (d) pearlite 102. Steel having combination of6.67% carbon and 93.33% of iron is known as (a) austenite (b) pearlite (c) cementite (d) martensite 103. By normalising of steel, its (a) ductility decrease (b) ultimate tensile strength increase (c) field point increases (d) All of the above
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Production Engineering 104. An alloy steel contains (a) more than 0.5% Mn and 0.5% Si (b) more than 0.15% Mn and 0.5% Si (c) less than 0.5% Mn and 0.15% Si (d) more than I%MnandO.05 Si 105. In carbon steel castings the percentage of (a) carbon between 1.5 - 2.5% (b) carbon below 1.7% (c) various carbon between 0.5 - 1.5% (d) more than 1.5% carbon 106. In steel as the percentage of carbon increase the following has decrease (a) ductility (b) tensile strength (c) hardness (d) toughness 107. Silicon steel is widely used in (a) chemical industry (b) mechanical parts making (c) electrical industry (d) die and puncher 108. Weld decay is the phenomenon found with (a) mild steel (b) wrought iron (c) cast iron (d) stainless steel 109. Annealing of white cast iron results in the production of (a) nodulariron (b) cementite (c) malleable iron (d) cast iron 11O. Solder is an alloy of (a) copper and tin (b) lead and copper (c) lead with zinc (d) lead and tin 111. The manufacturing process in which metal change its state from liquid to solid. (a) Casting (b) Machining (c) Forging (d) Turning 112. In which casting consumable pattern is used. (a) Sand casting (b) die-casting (c) PD.C (d) Investment casting 113. In case of Investment casting (a) wax pattern used (b) wooden pattern used (c) metallic pattern used (d) any of these can be used 114. The casting process by which hollow casting produced without using core is known as (a) Sand casting (b) Die casting (c) Centrifugal casting (d) Slush casting 115. For non sysmetric shape suitable casting method is (a) Sand casting (b) Slush casting (c) investment casting (d) all of these 116. The purpose of adding wood flour to foundry sand is to improve (a) collapsibility (b) strength (c) mouldability (d) all of these 117. Surface finish of casting depends upon (a) mold degassing (b) pattern fmish (c) casting process (d) all of these 118. Cores are used to make casting (a) Hollow (b) moresolid (c) more economic (d) moreweak
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A-211
119. Wood for pattern is considered dry when moisture content is (a) 5% (b) zero (c)
less than 15%
(d)
less than 30%
120. For steel casting following type of sand is better. (a) coarse grain (b) fine grain (c) medium grain (d) None of these 121. Trowel is (a) pointed tool (b) wooden hammer (c) tool used to repair corner (d) long, flat metal plate fitted with a wooden handle 122. Shrinkage allowance is made by providing (a) cores (b) taper in casting (c) addition in dimension of pattern (d) all of above 123. Casting process in which molten metal poured into mould under pressure is known as (a) sand casting (b) slush casting (c) vacuum casting (d) pressure die casting 124. Casting process in which mould kept revolving is known as (a) slush casting (b) vacuum casting (c) centrifugal casting (d) die casting 125. Facing sand used in foundary work comprises of (a) Silica and Clay (b) Clay, sand and water (c) Clay and abumina (d) Silica and aluminium 126. Accuracy of shell moulding is of the order of (a) O.oInvm (b) 0.1 nvm (c) 0.003m1mtoO.005m1m (d) None of these 127. Mark the most suitable material for die casting in the following (a) copper (b) Nickel (c) Steel (d) Cast iron 128. In general, the draft on casting is of the order of (a) 10-15m1m (b) 10-5m1m (c) 20-10mlm (d) 1-IOmlm 129. The purpose of riser in a casting process (a) act as feeding way in mould (b) act as reservoires (c) feed molten metal from basis to gate (d) None 130. Match plate pattern is (a) Green sand moulding (b) Pitmoulding (c) machining moulding (d) Pit moulding 131. For making ornaments and toys casting process used is (a) die casting (b) Investment casting (c) sand casting (d) slush casting 132. True centrifugal casting is used to get (a) chilled casting (b) accurate casting (c) dynamically balanced casting (d) Solid casting 133. Draft on pattern for casting is providing for (a) Sapteremoval from mould (b) adding shrinkage allowance (c) providing better finishing in casting (d) for machining allowance
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Production Engineering
A-212
134. The gate is provided in mould to (a) provide a reservoires (b) constant flow (c) feed mould according to rate of cooling (d) all of above 135. Sand slinger gives (a) better packing of sand (b) uniform sand density (c) better packing of sand near flask (d) none of above 136. As the size of casting increases, it is often better to use increasingly (a) Coarse grain (b) fine grain (c) mediumgrain (d) none of these 137. Black colour marking in pattern is used to indicate (a) machined surface (b) un-machined surface (c) parting surface (d) None 138. Loam Sand comprises of percentage of sand and mould (a) 10: 50 (b) 20 : 80 (c) 50 : 18 (d) 80:20 139. The ratio between the pattern shrinkage allowances of steel and iron is approx. (a) 2: 1 (b) 1: 1 (c) 1:2 (d) 1: 10 140. Sweep pattern is suitable for __ casting (a) small (b) medium (c) large (d) any of these 141. Fluidity is greatly influenced by the temperature of (a) tapping (b) melting (c) solidification (d) pouring 142. Chills are used in mould to (a) achieve directional solidification (b) reduce the possibility of blow holes (c) reduce freezing time (d) smoothens metal flow for reducing splatter 143. Which ofthe followingmaterialrequiresthe largestshrinkage allowance,while making a pattern for casting. (a) Aluminium (b) Brass (c) cast Iron (d) carbon steel 144. The height of the down - sprue is 175 mm and its cross sections area at the base is 200 mm-, the cross-sectional area ofthe horizontal runner is also 200 mm/. Assuming no losses the correct choice for the time (in second) required to fill a mould cavityof volume 106 mm-, will be (use g = 10m!
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(a) 2.67 (b) 8.45 (c) 26.72 (d) 84.50 145. Two castings of the same metal have the same surface are one casting is in the form of a sphere and the other is a cube. What is the ratio ofthe solidification time for the sphere to that of the cube. (a)
(c)
3 4 5 41t
6 (b)
1t
31t
(d)
8
146. Consumable patterns are made of (a) wax (b) polystyrene (c) ceramics (d) none of above 147. Limestone used in melting of cast iron acts as (a) flux (b) catalyst (c) alloying element (d) none of these 148. Electric indirect arc furnace is normally used for melting of (a) non-ferrous alloys (b) cast steel (c) ferrous alloys (d) all of these 149. The draw back with metallic patterns is (a) costly (b) heavy in weight (c) difficult to shape (d) all of these 150. There is no need of withdrawal of pattern from the mold if is used (a) solid pattern (b) split pattern (c) thermoplastic pattern (d) consumable pattern 151. Polystyrene used as consumable pattern material has a relative density of (a) 1.2-1.25 KN/m3 (b) 0.2-0.25 KN/m3. (c) 0.2-1.0 KN/m3 (d) all ofthese 152. In small castings which of the following allowance can be ignored (a) draft allowance (b) shrinkage allowance (c) matching allowance (d) rapping allowance 153. Small patterns are often used for (a) bends (b) pipework (c) drainage pelting (d) all ofthese 154. Permeability of sand decreases when (a) moisture percentage increases (b) compactness increases (c) bonding contents increases (d) all of above 155. Providing more than adequate machining allowance (a) increase machining cost (b) reduce machining cost (c) reduce casting weight (d) all of above 156. By compacting, sand density (a) increases (b) decreases (c) have no effect (d) None 157. Compacting of sand affects its (a) strength (b) permeability (c) density (d) all of these 158. The draft allowance to be provided on a pattern depends on (a) vertical length of pattern (b) intricacyofpattern (c) molding method (d) all of above 159. Contraction allowance in cast steel casting will be least for casting, having dimensions (a) upt0600mm (b) 600-1000mm (c) 1000-1800mm (d) above1800mm 160. Distortion in casting can be reduced by (a) modifying design (b) sufficientmachining allowance (c) improving foundary facility (d) all of above
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Production
Engineering
161. Clay content of green sand is usually (a) 5-10% (b) 18-30% (c) 5-30% (d) 10-50% 162. The water percentage in green sand is kept normally (a) 6-8% (b) 5-10010 (c) 10-20010 (d) 20-30% 163. Clay used for foundary sand should be (a) kaolinite (b) mont-morillonite (c) illite (d) all of these 165. Main contents of moulding sand are (a) Silica sand, clay and water (b) Silica sand, dust and carbon (c) Sand, coal powder and water (d) Green Sand and water 165. is used in magnesium moulding process. (a) boric sulphur (b) molasis (c) charcoal (d) all of these 166. Graphite is sprinkled on the surface of green sand mold to (a) exclude the burn out effect (b) minimize surface defects (c) improve surface finish (d) reduce the number of blow holes. 167. Hot tears in casting are caused due to (a) too much ramming of mold (b) grain size of sand (c) size of casting (d) rate of poring of molten metal 168. Rough surface may appears due to (a) large grain size sand (b) lowramming (c) high permeability (d) anyone of above 169. Scabs may be caused by (a) low permeability of sand (b) high moisture content of sand (c) intermittent running of molten metal over sand surface (d) all of the above 170. The advantage of shell moulding is (a) less sand requirement (b) dimensional accuracy (c) good finish (d) high productivity 171. Hardness of the mould is affected by (a) ramming of moulding sand (b) percentage of moisture (c) binder percentage (d) all of above 172. Blow holes in casting are due to (a) high moisture content of sand (b) low permeability of sand (c) excessive fine grains and gas producing ingredients (d) any of above
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A-213
173. Which of the following defect may occur due to improper design of gating system. (a) Cold sheets (b) mis-run (c) rough surface (d) all of these 174. Sprue are generally (a) uniform in size (b) tapered downwards (c) tapered upward (d) None 175. The design of gate should be able to (a) avoid erosion of cores and moulding cavity (b) prevent scum slag and eroded sand particles from entering the mould cavity (c) minimise turbulence and dross formation (d) all of above 176. In Magnesium alloy casting, normally solidification shrinkage is of (a) 1% (b) 2 % (c) 4 % (d) 10% 177. Solidification time for riser should be (a) less than that of casting (b) more than casting (c) same as casting (d) none of above 178. Forging of steel is done at a temperature of (a) 800°C (b) lO00°C (c) lO00°F (d) 1200°C 179. Process used for making Nut and Bolts is (a) hot piercing (b) upsetting (c) hot drawing (d) none of these 180. Process of shaping metal sheet by processing them against a desired shape is known as (a) upsetting (b) spinning (c) rolling (d) all of these 181. Porosity of metal is largely eliminated in _ (a) cold working (b) hot working (c) annealing (d) casting 182. Production of countours in flat blank is term as (a) piercing (b) punching (c) blanking (d) upsetting 183. Forging temperature used for plain carbon steel is (a) 800°C (b) lO00°C (c) 11OO°C (d) 1300°C 184. Gear shaping is related to (a) upsetting (b) hot (c) template (d) drawing 185. Mass production generally done by (a) Casting (b) Machining (c) Hobbing (d) All of these 186. Effect associated with cold forging is (a) shrinking (b) elongation (c) strain hardening (d) all of these 187. Crank shaft is made by (a) hot forming (b) coldforming (c) machining (d) casting
Badboys2
Production Engineering
A-214
188. For extrusion process (a)
199. Notching is the operation of
complex section are produced from bar stocks
(b) the strength of finished product is improved due to cold working (c)
Good surface finish and close tolerence is generated
(d)
all of these
189. Seam less tube can be produced by (a)
steam hammer forging (b)
piercing
(c)
casting
none of these
(d)
190. Process of extrusion is like (a)
a tooth paste coming from tube
(b)
air press from nozzle
(c)
both (a) and (b)
(d) none of these
(a)
removal of excess metal from the edge of strip to make it suitable for drawing without wrinkling
(b)
cutting in a single line across a part of the metal strip allow bending or forming in progressive die operation while the part remains attached to the strip
(c)
both (a) and (b)
(d)
none of these
200. Process consists of pushing the metal inside a chamber to force it out by high pressure through an orifice which is shaped to provide the desired form of the finished part, is called (a)
piercing
(b)
forging
(c)
extrusion
(d)
cold peening
201. Parts of circular cross section which are symmetrical about the axis of rotation are made by
191. Material good for extrusion is (a)
Low carbon steel
(b)
Cast iron
(a)
hot forging
(b)
(c)
S.S.
(d)
HS.S.
(c)
cold forging
(d) none of these
192. Upsetting or cold heading machine is a (a)
rolling process
(b)
extrusion process
(c)
forging process
(d)
none of these
Badboys2 193. The major problem in hot extrusion
is
(a)
design of punch
(b)
design of die
(c)
wear of punch
(d) wear and tear of die
194. Process of increasing the cross-section ofa bar and reducing its length is called (a)
drawing down
(b)
drifting
(c)
spinning
(d) upsetting
195. Cold working (a)
requires much higher pressure than hot working
(b)
increase hardness
(c)
distort grain structure
(d)
all of these
202. Mechanical properties of the metal improve in hot working due to (a)
grain growth
(b) recrystallisation
(c)
recovery of grains
(d) refmement of grain size
203. If there are bad effects of strain hardening on a cold formed part the part must be (a)
tempered
(b)
annealed
(c)
hardned
(d) normalised
204. A tooth paste tube can be produced by (a)
hollow backward extrusion
(b)
forging
(c)
solid forward extrusion
(d)
none of these
205. The true strain for a low carbon steel bar which is doubled in length by forging is
196. Cold working process can be applied on the component having diameters up to (a)
20mm
(b)
25mm
(c)
30mm
(d)
50mm
197. Which of the following is a gear finishing operation (a)
hobbing
(b)
milling
(c)
saving or burnishing
(d)
none of these
198. Roll forging (a)
causes a steadily applied pressure instead of impact force
(b)
is a forging method for reducing the making it longer
(c)
is used to force the end of a heated bar into a desired shape
(d) none of these
hot spinning
(a)
0.307
(b)
0.5
(c)
0.693
(d) 1
206. The process of hot extrusion is used to produce (a)
certain rods made of aluminium
(b)
steel pipes for domestic water supply
(c)
stainless steel tubes used in furniture
(d)
large size pi pes used in city water main s
207. Extrusion process can effectively Reduce the cost ofproduct through (a)
Saving in tooling cost
(b)
Saving in administrative
(c)
material saving
(d)
all of these
cost
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Production
Engineering
208. Hot press forging (a) causes a steadily pressure instead of impact force (b) is used to force the end of a heated bar into a desired shape (c) is a forging method for reducing the diameter of a bar and in the process making it layers (d) all of these 209. In hot working (a) annealing operation is not necessary (b) power repowerments are low (c) surface finish is good (d) grain refinement is possible 210. In a solid extrusion die, purpose of knock out pin is (a) shopping the part to extrude through the hose (b) ejecting the part after extrusion (c) allowing the job to have better surface finish (d) reducing the waste of material 211. In electric resistance welding, two copper electrodes used to cooled by (a) air (b) water (c) both (a) and (b) (d) None of these 212. An example of fusion welding is (a) Thermitwelding (b) Arc welding (c) Forge welding (d) Gas welding 213. Welding process in which flux is used in form of gannual is (a) D.C.Arc welding (b) Spot welding (c) Thermitwelding (d) SubmergedArc welding 214. In arc welding face shield used to protect eyes from (a) Spatters (b) Spark (c) Infra-red and ultraviolet rays (d) None of these 215. Gases used in tungsten gas welding are (a) Carbon dioxide and H2(b) CO2 and oxygen (c) Argon and helium (d) Acetylene and nitrogen 216. Open circuit voltage for Arc welding is of the order of (a) 20-40V (b) 10-20V (c) 40-50V (d) 40-95V 217. Welding of steel structure on site work of a building easily made by (a) Spot welding (b) Buttwelding (c) Arcwelding (d) Any of the above 218. Tig welding is preffered in followingmetal welding (a) Silver (b) Aluminium (c) Mild steel (d) All of these 219. In arc welding temperature generated is of the following order. (a) lOOO°C (b) 1800°C (c) 3500°C (d) More than 4000°C
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A-215 220. Temperature of oxy-hydrogen flame as compared to oxyacetylene flame is (a) less (b) more (c) same (d) depends on oxygen percentage 221. Oxidising flame is obtained by supplying (a) more oxygen and less volume of acetylene. (b) both oxygen and acetylene kept in same volume. (c) acetylene volume kept more than oxygen volume (d) None of these 222. Oxidising flameas comparedto neutral flame has inner core (a) shorter in size (b) less luminous (c) more luminous (d) both (a) and (b) 223. Maximum flametemperature occurs (a) at inner core of flame (b) at outer core of flame (c) attipofflame (d) next to the inner core 224. Maximum used flame in gas welding method is (a) oxidising (b) neutral (c) carburising (d) None of these 225. Strongest brazing joint is (a) Lapjoint (b) Buttwelding (c) Scrafwelding (d) None of these 226. Melting point of the filler metal in brazing should be above (a) 400°C (b) 420°C (c) 6(X)°C (d) 800°C 227. Seam welding is continuous (a) spot welding process (b) type of projection welding (c) multi-spot welding (d) None of these 228. Welding process preferred for cutting and welding for nonferrous metal is (a) MIGwelding (b) TIGwelding (c) Inert gas welding (d) None of these 229. The welding process in which electrode do not consumed is (a) MIG welding (b) TIGwelding (c) Argon welding (d) None of these 230. The welding process in which electrode get consumed is (a) MIGwelding (b) TIGwelding (c) Spot welding (d) None of these 231. Grey cast iron is usually welded by (a) resistance welding (b) gas welding (c) spot welding (d) arc welding 232. In arc welding using direct current amount of useful arc heat at the anode and cathode respectively are (a) two third of one third (b) One third and two third (c) equal (d) none of these 233. Multipoint welding process is (a) seam welding (b) spot welding (c) projection welding (d) percussion welding
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Production Engineering
A-216
234. Amount of current required in electrical resistance welding regulated by changing the (a) polarity (b) input supply (c) by altering no. ofturns of primary winding (d) by changing no. of turns of secondary winding 235. Welding of chromium molybdenum steels cannot use (a) Oxygen acetylene welding (b) Thermitwelding (c) Soldering (d) Electric arc welding 236. Spot-welding, projection welding and seam welding are classification of (a) Thermitwelding (b) Resistance welding (c) Arc welding (d) Spot welding 237. An arc is produced between a bare metal electrode and workin (a) D.C.welding (b) Submerged arc welding (c) Spot welding (d) None of these 238. In arc welding, two low welding speed results in (a) Excessivepilling up of weld metal (b) Electrode waistage (c) Over hauling without penetration edge (d) All of these 239. Fillers material is essentially used in (a) Spot welding (b) Gas welding (c) Seamwelding (d) Projection welding 240. Rate of welding steel by carburising flame as compared to neutral flame is (a) less (b) same (c) more (d) all of the above 241. Carburising flame is used to weld (a) Brass and bronze (b) Steel, and copper (c) Hard surfacing materials such as satellite (d) Any of above 242. Filler material is used in (a) Spot welding (b) Butt welding (c) Seamwelding (d) None of these 243. Cleaning of metal in electrical resistance welding is (a) important (b) not important (c) have no effect (d) none of these 244. An example of fusion welding is (a) Spot welding (b) Gas welding (c) Projection welding (d) All of these 245. Welding process using a pool of molten metal is (a) TIGwelding (b) MIGwelding (c) Submergedarc welding(d) None of these 246. In spot welding the electrode tip diameter (d) should be equal to
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(a) Less than (c)
j15i
.[t
(b).[t (d)
247. In arc welding current used is (a) AC. current at low frequency (b) AC. current at high frequency (c) D.C. current (d) All of these 248. An arc is produced between a bare metal electrode and the work in welding process known as (a) Gas welding (b) Submerged arc welding (c) D.C.welding (d) None of these 249. Seam welding used for metal sheets having thickness in the range (a) below3 mm (b) 3-5mm (c) 3-6mm (d) 0.025-3mm 250. In Arc welding, range of temperature generated at arc is (a) IOOO°C - 2000°C (b) 2000°C- 4000°C (c) 4000°C-6000°C (d) None of these 251. Projection welding is a (a) type of arc welding (b) type of continuous spot welding (c) type of gas welding (d) none of these 252. In resistance welding voltage used for heating is (a) below 10V (b) 10V (c) higher than 10V (d) None of these 253. In arc welding, penetration is minimum for (a) DCSP (b) OCRP (c) AC. (d) None of these 254. In electrical resistance welding, pressure applied varies in the range (a) 50-100kgF/cm2 (b) 100-150kgF/cm2 (c) 150-200 kg F/cm2 (d) 250-550kgF/cm2 255. Which of the following current is preferred for welding of non-ferrous metal by arc welding? (a) DC (b) AC. at high frequency (c) AC. at low frequency (d) None of these 256. Main criterion for electrode diameter selection is (a) Thickness of work piece (b) Typeof work piece metal (c) Welding pressure applied (d) Welding process applied 257. In projectionwelding diameterofthe projectionas compared to thickness of the sheet is approximately (a) same (b) half (c) double (d) 1.5times 258. Number of zones of heat generation in spot welding are (a) 1 (b) 2 (c) 3 (d) None of these 259. In spot welding tip of electrode made up of (a) Sinteredmetal (b) Carbide (c) Copper (d) Brass
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Production Engineering 260. Material used for coating the electrode is called (a) binder (b) oxidiser (c) flux (d) slag 261. In arc welding, arc is created between work piece and electrodes due to (a) type of current (b) electronsjumping from electrode to workpiece (c) high resistivity due to presence of air (d) none of these 262. During Arc welding with increaseof thickness ofmaterial to be welded, welding current have to (a) decrease (b) increase (c) remain constant (d) none of these 263. In resistance welding pressure released (a) after welds gets cool (b) when work gets heated (c) just after the weld completed (d) none of these 264. Welding process used for joining round bars is (a) Thermitwelding (b) Projection welding (c) Seamwelding (d) Butt welding 265. Welding in which the metals to be joined are heated to a molten state are allowed to solidify in presence of a filler materials is called (a) Spot welding (b) Fusion welding (c) D.C.welding (d) None of these 266. In a welding process, flux is used to (a) Permit perfect cohesion of metal (b) remove oxides of metal formed at high temperature (c) both (a) and (b) (d) none of these 267. In electrical resistance welding (a) Voltagekept high and current also high (b) Voltagekept high and current kept low (c) Voltagekept low and current kept high (d) None of these 268. In forehand gas welding operation, the angle between the rod and work piece is kept around (a) 15° (b) 10-20° (c) 30° (d) 45° 269. Material best weldable with itselfis (a) copper (b) aluminium (c) mild steel (d) all of these 270. Arc length in electric Arc welding is the distance between tip of the electrode and (a) work piece (b) bottom of crates (c) centre of crates (d) none of these 271. Oxygen to acetylene ratio in case ofneutral flame is (a) 0: 1 (b) 1:2 (c) 0.8:2 (d) 2: 1
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A-217 272. In MIG welding helium or argon is used in order to (a) act as flux (b) act as shielding medium (c) providing cooling effect(d) all of these 273. Oxygen to acetylene ratio in carburising flame is (a) 0.5: 1 (b) 0.9: 1 (c) 1: 1 (d) 1: 2 274. A lathe machine special in (a) Diameter oflathe (b) Gross weight of machine (c) Speed of lathe (d) Swing oflathe 275. Lathe machine bed made up of (a) alloys (b) cast iron (c) mild steel (d) prg Iron 276. Shanks of tapes drills are provided standard tapes known as (a) tapes shank (b) morse tapes (c) chapman tapes (d) None of these 277. The length of a hacksaw blade is measured from (a) extreme end to extreme end (b) centre of hole at one end to the center of holes at the other end (c) the formula L = 16 x width (d) None of the above 278. A plug gauge is used for measuring (a) out side bore (b) cylindrical bores (c) spherical holes (d) tapes bores 279. Standard milling arbores size is (a)
1"
(b)
1_!_"
(c)
_!_"
(d)
Anyone of above
2
4
280. Standard taper generally used on milling machine spindles is (a) Morsetaper (b) Shellr'sistaper (c) Champman taper (d) None of these 281. Sintered and tungsten carbides can be machined by (a) Conventional process (b) Grinding only (c) E.DM. (d) None 282. The binding material used in cemented carbide tool is (a) chromium (b) cobalt (c) sulpher (d) nickel 283. Discontinous chips are formed during machining by (a) mild Steel (b) aluminium (c) cast Iron (d) brass 284. Continous chips are formed while machining of (a) cast iron (b) mild steel (c) aluminium (d) None of these 285. To prevent tool from rubbing the work, angle provided on tool is (a) reliefangle (b) rake angle (c) clearance angle (d) None of these
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A-21S 286. In metal machining due to burnishing friction, heat is generated in the (a) friction zone (b) friction less zone (c) work-tool contact zone (d) None of these 287. A single point tool has (a) rake angle (b) cutting angle (c) clearance angle (d) None of these 288. Angle on which the strength of the tool depends is (a) cutting angle (b) lip angle (c) rake angle (d) clearence angle 289. Velocity oftool relative to workpiece is called (a) average velocity (b) cutting velocity (c) shear velocity (d) chip velocity 290. The angle provided to prevent rubbing between workpiece and cutting tool is known as (a) relief angle (b) rake angle (c) lip angle (d) None of these 291. Cutting tool used in lathe, shaper and planer is (a) Multi point cutting tool (b) Two point cutting tool (c) Single point cutting tool (d) Multi point cutting tool 292. Angle between the tool face and the ground and surface of fank is called (a) rake angle (b) lip angle (c) clearance angle (d) cutting angle 293. Velocity of tool along the tool face is called (a) Chip velocity (b) Cutting velocity (c) Shear velocity (d) None of these 294. The depth of cut depends upon (a) tool material (b) cutting speed (c) regidityofmachining tool (d) All of these 295. The metal in machining operation is removed by (a) distortion of metal (b) shearing the metal across a zone (c) tearing chips (d) cutting the metal across a zone 296. Tool life is most affected by (a) tool geometry (b) cutting speed (c) feed and depth (d) None of these 297. Tool signature (a) description of tool shape (b) the plane of tool (c) design and description of various angles provide on tool (d) brandlmodle none of tool 298. Tool signature comprised of (a) property of tool (b) speed of cutting tool (c) 7-various elements (d) 6-elements 299. Depth of cut for roughing operation as companied to finishing operation is (a) same (b) more (c) less (d) None of these
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Production Engineering 300. Cutting speed should be kept low while machining (a) Soft material (b) Regular shape material (c) Casting (d) All of above 301. The type of chip produced when cutting ductile material is (a) continous (b) discontinous (c) built up edge (d) None of these 302. The average cutting speed for machining a cast iron by a high speed tool steel tool is (a) 10m/min (b) 20m/min (c) 30m/min (d) None of these 303. Relief angle on high speed tools generally vary in the range (a) 0-5° (b) 5°_10° (c) 10°-20° (d) 20° to 30° 304. In metal machining, due to friction between the moving chip and the tool face, heat is generated in the (a) Shear zone (b) Friction zone (c) Work-tool contact zone (d) None of the above 305. Material having lowest cutting speed is (a) Bronze (b) Aluminium (c) High carbon steel (d) Cast iron 306. Cutting tools used on milling machining machine is (a) Single point (b) Double point (c) Multi point (d) Any of above 307. The cutting edge of the tool is perpendicular to the direction of tool travel in (a) oblique cutting (b) orthogonal cutting (c) both (a) and (b) (d) None of these 308. Orthogonal cutting system is also called (a) Single-dimensional cutting system (b) Two-dimensional cutting system (c) Three dimensional cutting system (d) Any of above 309. In metal cutting operations, chips are formed due to (a) stress deformation (b) shear deformation (c) sharpness of cutting edge (d) linear transformation 310. With increase of cutting speed, the built up edge made (a) larger in size (b) smaller (c) remains same (d) None of these 311. Cutting ratio is the ratio of (a) Chip thickness to depth of cut (b) Chip velocity to cutting velocity (c) Both (a) and (b) (d) None of the above 312. Continous chips formed when machining speed is (a) lower (b) constant (c) higher (d) None of these 313. Which of the following tool material has highest cutting speed? (a) HS.S. (b) Carbon steel (c) Tool steel (d) Carbide tools
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Production Engineering
A-219
314. Tool cutting forces, with increase in cutting speed (a) increase linearly (b) decrease linearly (c) remains constant (d) None of these 315. Chip breakers are provided on cutting tool is (a) for operator's safety (b) better finish (c) permit short ships (d) forminimizing heat generation 316. Maximum cutting angles are used for machining (a) cast iron (b) mild steel (c) aluminiumalloys (d) None of these 317. When radial force in cutting is two large will cause (a) better finish (b) poor finish (c) decrease tool life (d) increase tool life 318. Segmentedchips are formedwhile machining (a) softmaterial (b) tough material (c) brittlematerial (d) high speed steel 319. As cutting speed increase the built up edge (a) reduced (b) increase (c) becomelarger (d) None of these 320. In tool signature, the largest nose radius is indicated (a) in starting (b) at the end (c) in middle (d) All of these 321. In equation YIn = C, value ofn depends on (a) Material of workpiece (b) Material of tool (c) Cutting position (d) All of these 322. The relationship betweentool life (T) and cutting speed (Y) m/min is given as
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yn
(a)
v r-c
(b)
-=C
(c)
r
(d)
vr=-c
Y
=C
T
323. Chips are broken effectively due to which ofthe following property (a) Elasticity (b) Toughness (c) Workhardening (d) Stress produced 224. Continous chips are formed when machining (a) brittle metal (b) ductilemetal (c) high speed (d) All of these 325. Finishing obtained on workpiece mostly affected by (a) Cutting speed (b) Feed rake (c) Lubricant used (d) Depth of cut 326. Machinability tends to decrease with (a) increase in strain-hardening (b) increase in tensile strength (c) increase in carbon contents (d) None of these 327. Machinability tends to increase with (a) increase in hardness (b) decrease with decrease hardness (c) remain same as hardness varies (d) proper stress releaving and proper heat treatment
328. With high speed steel tools, the maximum safe operating temperature is in order of (a) below200°C (b) above300°C (c) 200°C (d) None of these 329. Best method of increasing the rate of removaling metal is (a) increase feed rate (b) increase depth of cut (c) increase speed of tool (d) increase cutting angle 330. In a cutting operation, the largest force is (a) Radial force (b) Longitudinal force (c) Tangential force (d) Force along shear plane 331. Metal in machining operation is removed by (a) distortion of material (b) shearing of metal (c) fracturing of metal (d) any of above 332. When material is ductile and cutting speed is slow then chips formed are (a) Continuous (b) Discontinuous (c) Powder shape (d) None of these 333. During machining process when ductile metal is cutting at medium speed then chips formed are (a) Continuous (b) Discontinuous (c) Continuous with built up edge (d) Power shape 334. Chip formedwhen Ductile Metalmachined with high speed (a) Continuous chips (b) Discontinuous chips (c) Continuous chips with built up edge (d) Fragmented chips with built up edge 335. Material having hight cutting speed is (a) Bronze (b) Aluminium (c) Cast Iron (d) High carbon steel 336. An angle provided between tool face and line tangent to the machined surface at cutting points called as (a) rake angle (b) lip angle (c) cutting angle (d) clearance angle 337. Angle provided in a single point cutting tool to control chip flow is (a) Siderake angle (b) End relief angle (c) Backrake angle (d) Sliderelief angle 338. Velocityof chip relative to work-piece is acting (a) Along the shear plane (b) Normal to shear plane (c) Normal to tool place (d) Along the tool face 339. The coefficient of friction between chip and tool can be reduced by reducing the (a) loweringrake angle (b) feed of tool (c) width of tool (d) dept of cut 340. In metal machining due to plastic deformation of metal maximum heat is generated in the (a) Friction zone (b) Shear zone (c) Point of contact of cutting tip and work piece (d) All of above
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