Quiz: Mechanical Engineering
Exam: ISRO TECHNICAL ASSISTANT
Each question carries 1 mark
Negative marking: 1/3 mark
Time: 10 Minutes
Q1. Autofrettage is a method of
(a) joining thick cylinders
(b) Reliving stresses from thick cylinders
(c) Prestressing thick cylinders
(d) Increasing the life of thick cylinders
Q2. In circular arc cam with a flat faced reciprocating follower, the contact between the
follower and the nose of cam is only maintained if :
(a) External spring force is less than the inertia force of the follower
(b) External spring force is greater than the inertia force of the follower
(c) External spring force is zero
(d) The inertia force is more than the spring force
Q3. A governor is said to be isochronous when the equilibrium speed is
(a) variable for different radii of rotation of governor balls
(b) constant for all radii of rotation of the balls within the working range
(c) constant for particular radii of rotation of governor balls
(d) constant for only one radius of rotation of governor balls
Q4. The effect of increasing the stiffness of springs of centrifugal clutch is
(a) The decrease of engagement speed
(b) The increase of engagement speed
(c) The increase of frictional force at maximum speed
(d) None of the above
Q5. The saturation temperature of steam with increase in pressure increase
(b) rapidly first and then slowly
(c) slowly first and then rapidly
Q6. Carbonization of coal is the process of
(a) pulverizing coal in inert atmosphere
(b) heating wood in a limited supply of air at temperatures below 300°C
(c) strongly heating coal continuously for about 48 hours in the absence of air in a
(d) binding the pulverized coal into brackets
Q7. Superheating of steam is done at
(a) constant volume
(b) constant temperature
(c) constant pressure
(d) constant entropy
Q8. Fluid is a substance that
(a) cannot be subjected to shear forces
(b) always expands until it fills any container
(c) has the same shear stress at a point regardless of its motion
(d) cannot remain at rest under action of any shear force.
Q9. Match List-I with List-II and select the correct answer using the code given below the lists:
List – I
Radiation heat transfer
A B C D
(a) 3 1 2 4
(b) 2 4 3 1
(c) 3 4 2 1
(d) 2 1 3 4
Q10. Specific speed of a fluid machine
(a) refers to the speed of a machine of unit dimensions
(b) is a type-number representative of its performance
(c) is specific to the particular machine
(d) depends only upon the head under which the machine operates
Sol. Autofrettage: It is one of the oldest methods of prestressing (method of increasing pressure capacity) the cylinders. It is overloading the cylinder before it is put into service. In this method, a forged cylinder is subjected to a large internal pressure, so as to expand the internal diameter, causing overstrain. The internal pressure is applied is such a way that a portion of the cylinder at the inner surface is subjected to stresses in the plastic range. When the pressure is released, the portion of the cylinder at the outer surface contracts exerting the pressure on the portion at the inner surface which has undergone permanent deformation. As a result residual compressive stresses are induced at the inner surface and tensile stresses at the outer surface.
It is commonly used to gun barrels. It not only increases the pressure capacity of the cylinder, but also improves the endurance strength.
Sol. When the cam contact force goes to zero, the cam and follower will separate. Large enough spring force is necessary to keep cam and follower in contact throughout the entire rotation. External spring force should be greater than inertia force of the follower to maintain positive contact. If external spring force is too large, there is possibility of higher contact stress leading to surface failure.
Sol. Isochronous governor is a governor that keeps the speed of a prime mover constant at all loads. Also known as a static governor. A governor is said to be isochronous if equilibrium speed is constant for all the radii of rotation in the working range. Therefore, for an isochronous governor the speed range is zero and this type of governor shall maintain constant speed.
Sol. When the load is released from clutch lever, it again engages with the shaft. As the stiffness of spring will increases, the restoring force on clutch will increase and hence velocity of engagement will be more.
S5. Ans (b)
Sol. At atmospheric pressure the saturation temperature is 100°C. However, if the pressure is increased, this will allow the addition of more heat and an increase in temperature without a change of phase. Therefore, increasing the pressure effectively increases both the enthalpy of water, and the saturation temperature.
S6. Ans (c)
Sol. Coal carbonization is used for processing of coal to produce coke using metallurgical grade coal. Coal carbonization involves heating of coal in the absence of air.
S7. Ans (c)
Sol. Superheated steam is a steam at a temperature higher than its vaporization (boiling) point at the absolute pressure where the temperature is measured
S8. Ans (d)
Sol. A fluid is a substance that continuously deforms under an applied shear force.
(a) Heat exchangers : Effectiveness
(b) Turbulent flow : Eddy diffusivity
(c) Free convection : Nusselt number
(d) Radiation heat transfer : View factor
S10. Ans (b)
Sol. Specific speed is of a fluid machine is a type number representative of its performance.
In order to compare the performance of different pumps, it is necessary to have some term which will be common to all the centrifugal pump the term ordinarily used for this purpose is the specific speed. This is a type characteristic and can be used to predict the behavior of one pump based on tests of similar, but different sized pumps.
The specific speed is a characteristic of pumps which can be used as basis for comparison of the performance of different pump.