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Previous years ISRO Mechanical Questions and Answers


2011.1 Match List I with List II and select the correct answer
List I                                              List II
A. Pelton wheel (single jet)           1. Medium discharge, low head
B. Francis Turbine                         2. High discharge, low head
C. Kaplan Turbine                         3. Medium discharge, medium head
                                                       4. Low discharge, high head
    A     B     C
a) 4      3      2
b) 1      3      4
c) 4      1      3
d) 1      2      3

Answer
a) A-4, B-3, C-2

Explanation
Comparison of hydro-electric turbines

2011.6. If the discharge of a centrifugal pump is throttled then its suction lift
a) Decreases
b) First increases and then decreases
c) Remains unchanged
d) Increases

Answer
d) Increases

Explanation
Centrifugal pumps are unique because they can provide very high flow rates (much higher than most positive displacement pumps) and because their flow rate varies considerably with changes in the Total Dynamic Head (TDH) of the particular piping system. This allows the flow rate from centrifugal pumps to be "throttled" considerably with a simple valve placed into the discharge piping, without causing excessive pressure buildup in the piping or requiring a pressure relief valve. Therefore, centrifugal pumps can cover a very wide range of liquid pumping applications.Throttling centrifugal pumps with a discharge valve is not as energy-efficient as using a Variable Frequency Drive (VFD) to slow the pump/motor speed down, but it is much less expensive to install. Of course, throttling a centrifugal pump's flow rate has certain limits. Capacity can be regulated at constant speed by throttling.
Because of throttling operation, flow rate of centrifugal pump will decrease. Pump "suction" is from the same source as before so static pressure in the suction side is same. As liquid level inside sump is same as before, hydro static pressure is also same. Now only variables to calculate the static suction pressure of the pump are velocity head (kinetic energy) loss and frictional loss. Due to decrease in flow rate velocity head loss as well as frictional loss decreases, by applying the Bernoulli's equation, pump static suction pressure will increase.


2011.17. Pitch diameter is equal to the product of
a) Circular pitch and number of teeth
b) Working depth and number of teeth
c) Clearance and number of teeth
d) Module and number of teeth

Answer
d) Module and number of teeth

Explanation
Circular pitch - The distance between the corresponding points of adjacent gears on the reference diameter.
Working depth - The depth of engagement of two gears
Clearance - Distance between the root circle of a gear and the addendum circle of its mate.
Module - Since it is impractical to calculate circular pitch with irrational numbers, mechanical engineers usually use a scaling factor that replaces it with a regular value instead. This is known as the module or modulus of the wheel and is simply defined as m=p/π where m is the module and p the circular pitch.

Let pitch diameter be d
Circular pitch x number of teeth = circumference of circle with diameter d
Working depth x number of teeth = no meaningful quantity
Clearance x number of teeth = no meaningful quantity
Module x number of teeth = m x N = p x N/π = circumference of circle with diameter d /π = d

2011.18 The tension in the cable supporting a lift moving upwards is twice the tension when the lift moves downwards. What is the acceleration of the lift?
a) g/4
b) g/3
c) g/2
d) g

Answer
b) g/3

Explanation
Let the acceleration of the lift be a
Tension in the cable when the lift is moving upwards = Tup = m(g+a)
Tension in the cable when the lift is moving downwards = Tdown = m(g-a)
Given Tup = 2Tdown
m(g+a) = 2m(g-a)
g+a = 2g - 2a
3a = g
a = g/3

2011. 19. Whirling speed of the shaft with the natural frequency of its
     a) Transverse vibration
     b) Longitudinal vibration
     c) Torsional vibration
     d) Coupled bending torsional vibration
Answer
a) Transverse vibration

2011. 33. Tungsten in high speed steel provides
     a) Hot hardness
     b) Toughness
     c) Wear resistance
     d) Sharp cutting edge
Answer
a) Hot hardness
Explanation
Tools employed for machining are made of high speed tools.
Carbon - Forms carbides, increase wear resistance, is responsible for the basic matrix hardness
Tungsten & Molybdenum - Improves red hardness, the ability of steel upon heating to red heat to retain hardness and durability. M50(name of alloy) which does not contain tungsten, is used for consumer twist drills and woodworking tools. Alloys with high tungsten content (>10%) are used to make high performance broaches (toothed tool to remove material)
Vanadium - A high volume of hard vanadium carbides provide high wear resistance.

2011. 77. The image of the point (1,2,3) in the plane 2x+y+z = 13 is
     a) (5,4,5)
     b) (5,5,4)
     c) (3,3,4)
     d) (4,5,5)
Answer
c) (3,3,4)
Explanation
The image of a point in a plane is the point of intersection of the line perpendicular to the plane and containing the given point.
The direction cosines of the normal to the plane 2x+y+z=13 are (2,1,1). Equation of a line containing (1,2,3) and having direction cosines (2,1,1) is (x-1)/2 = (y-2)/1 = (z-3)/1 = t. Any point on this line is given by (2t+1,t+2,t+3).  As the image of the point lies in the plane it satisfies 2x+y+z=13. Substituting, 2(2t+1)+(t+2)+(t+3) = 13, 4t+2+t+2+t+3 = 13, 6t + 7 = 13, 6t = 6, t = 1
Hence the image is (2*1+1,1+2,1+3) = (3,3,4)

2014. 1. Shock resisting steel should have higher
     a) Stiffness
     b) Hardness
     c) Toughness
     d) Shear Strength
Answer
c) Toughness
Explanation
Toughness is the ability of the material to deform plastically and to absorb energy in the process before fracture. Toughness can be measured by the impact energy required to break a specimen.

2014. 10. Aluminium crystal structure is
     a) FCC
     b) BCC
     c) HCP
     d) None of these
Answer
a) FCC
Explanation
Classification of common metals based on crystal structure
BCC - Sodium, Potassium, Chromium, Manganese, Iron, Niobium, Molybdenum, Tungsten
FCC - Aluminium, Calcium, Nickel, Copper, Silver, Platinum, Gold, Lead
HCP - Beryllium, Magnesium, Titanium, Cobalt, Zinc, Zirconium, Cadmium

2014. 14. Grashof number

Explanation
It is the ratio of buoyancy to viscous force. In natural convection the Grashof number is the dimensional parameter that governs the fluid flow. The ratio of Grashof number to Reynolds number determines whether natural or forced convention dominates the flow.

2014. 15. Rayleigh number Ra is defined as (where Gr is Grashof number, Pr is Prandtl number and Re is Reynolds number)
    a) Ra = RePr
    b) Ra = GrRe
    c) Ra = GrPr
    d) Ra = GrPr/4
Answer
c) Ra = GrPr
Explanation
The magnitude of the Rayleigh number is a good indication as to whether the natural convection boundary layer is laminar or turbulent.

2014. 16. For two bodies having direct impact, co-efficient of restitution is given by
     a) velocity of approach/velocity of separation
     b) velocity of separation/velocity of approach
     c) Initial velocity of body/Final velocity of body
     d) None of the above
Answer
b) velocity of separation/velocity of approach
Explanation
If coefficient of restitution is 1, then the collision is perfectly elastic and kinetic energy is conserved.
If coefficient of restitution is 0, then the collision is perfectly inelastic and the objects stick together after collision.

2014. 17. A perfect frame is that which satisfies the equation (Assume n = No. of members and j = No. of joints)
     a) n = 3j-2
     b) n > 3(j-1)
     c) n < 3(j-1)
     d) n = 2j-3
Answer
d) n = 2j-3
Explanation
Triangle frame is the simplest perfect frame and it has 3 members and 3 joints.
Perfect frame with 4 joints has 5 members.
The only equation satisfying both the condition is d)

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