← MES Jr. Grade
Past Questions — 2015
The maximum magnitude of the unbalanced force along the perpendicular to the line of stroke is known as:
A)
hammer blow
B)
tractive force
C)
swaying couple
D)
none of these
In locomotive balancing, the unbalanced force acting perpendicular to the line of stroke is called the hammer blow. It is the maximum vertical dynamic load on the rail due to the rotating and reciprocating parts. Tractive force is the driving force along the line of stroke, and swaying couple refers to the couple causing lateral sway, not the perpendicular unbalanced force.
The type of thread preferred for power screw is
A)
ACME threads
B)
square threads
C)
buttress threads
D)
all of these
Square threads are generally preferred for power screws because they have the highest efficiency among common thread forms. Their thread faces are perpendicular to the axis, so there is minimal radial or bursting force and less frictional loss compared to ACME or buttress threads.
Why others are not the best choice:
- ACME threads: stronger and easier to manufacture than square threads, but slightly less efficient due to the thread angle.
- Buttress threads: used when load acts heavily in one direction only, so they are special-purpose rather than generally preferred.
- All of these: incorrect because only square threads are classically preferred for maximum efficiency in power screws.
The minimum number of teeth on a 20° standard involute pinion to mesh with a rack to avoid interference is given by
A)
15
B)
17
C)
18
D)
27
For a 20° standard involute gear, the minimum number of teeth on a pinion meshing with a rack to avoid interference is 17. This is a standard result in gear design. A smaller number of teeth would cause the involute profile to interfere with the rack during meshing.
The radius of gyration of a disc type flywheel of diameter D is
A)
D
B)
D/2
C)
D/3
D)
D/4
For a disc-type flywheel, the mass is assumed to be concentrated at the rim for maximum moment of inertia. The radius of gyration k is defined by I = mk^2. For a solid disc, the standard radius of gyration about its central axis is k = r/\sqrt{2}, but in flywheel MCQs this question usually refers to the equivalent radius of gyration of a disc-type flywheel as the mean radius, which is D/2 among the given options. Hence the correct option is D/2. The other options D, D/3, and D/4 do not match the standard relation used in such exam questions.
In a solid circular shaft subjected to pure torsional moment, the ratio of maximum shear stress to maximum normal stress at any point is:
A)
2
B)
1
C)
2/3
D)
1/2
In a solid circular shaft under pure torsion, the stress at a point is pure shear stress only. So the maximum shear stress is τmax, and the principal normal stresses developed are +τmax and -τmax. Hence the maximum normal stress in magnitude is also τmax. Therefore, the ratio of maximum shear stress to maximum normal stress is τmax/τmax = 1. So option (b) is correct. Other options are incorrect because they do not match the stress transformation result for pure shear.
Maximum shear stress in a Mohr’s circle is
A)
equal to diameter of the Mohr’s circle
B)
equal to radius of the Mohr’s circle
C)
less than the radius of the Mohr’s circle
D)
more than the radius of the Mohr’s circle
In Mohr’s circle for plane stress, the maximum shear stress is represented by the maximum vertical distance from the center of the circle to its outermost point. This value is exactly the radius of the circle. Hence, maximum shear stress = radius of Mohr’s circle.
Why others are incorrect:
(a) Diameter is twice the radius, so it is too large.
(c) It is not less than the radius; it is exactly equal.
(d) It is not more than the radius; it is exactly equal.
The bearing characteristic number of a journal bearing depends upon
A)
length, width and load
B)
length, width and speed
C)
viscosity, speed and load
D)
viscosity, speed and bearing pressure
The bearing characteristic number for a journal bearing is the Sommerfeld number, commonly written as S ∝ (ηN/P) × (r/c)^2, where η = viscosity of lubricant, N = speed, and P = bearing pressure (load per unit projected area). Thus, it depends directly on viscosity and speed, and inversely on bearing pressure.
Why others are incorrect:
(a) Length, width and load: these are not the main variables in the characteristic number itself.
(b) Length, width and speed: viscosity and pressure are essential, so this is incorrect.
(c) Viscosity, speed and load: the correct term is bearing pressure, not just total load.
Hence, the correct option is (d).
Critical damping is a function of
A)
mass and stiffness
B)
mass and damping coefficient
C)
stiffness and natural frequency
D)
stiffness and damping coefficient
For a single degree of freedom system, the critical damping coefficient is c_c = 2mω_n. Since ω_n = √(k/m), substituting gives c_c = 2√(km). Thus, critical damping depends on mass (m) and stiffness (k). इसलिए correct option is (a).
Why others are incorrect:
(b) Critical damping itself is not a function of the existing damping coefficient; rather, it is the reference value used to compare actual damping.
(c) Though c_c can be written using natural frequency, ω_n itself depends on mass and stiffness; the standard direct dependence is on m and k.
(d) It does not depend on the actual damping coefficient.
The point of contraflexure can exist in a
A)
simply supported beam
B)
cantilever beam
C)
beam fixed at both the ends
D)
beam with overhangs with no loads on the overhanging sections
A point of contraflexure is a point in a beam where the bending moment changes sign, i.e., from sagging to hogging or vice versa. This happens only when both positive and negative bending moments occur in the same beam. In a simply supported beam, bending moment is usually of one sign only under normal loading, so no contraflexure occurs. In a cantilever beam, the bending moment also remains of one sign throughout. In a beam fixed at both ends, point of contraflexure may occur depending on loading, but the standard case asked here is the overhanging beam, where negative moment develops near supports and positive moment in the span, creating a clear point of contraflexure. Hence the correct answer is option (d).
Hydrostatic bearing is most suitable for carrying very heavy loads with
A)
very high speed
B)
very slow speed
C)
high speed
D)
slow speed
A hydrostatic bearing supports the load using externally pressurized lubricant, so the oil film exists even at zero or very low speed. Therefore, it is especially suitable for carrying very heavy loads at very slow speed.
Why others are incorrect:
(a) very high speed and (c) high speed: these are more typical situations where hydrodynamic effects become important due to motion-generated pressure.
(d) slow speed is close, but the standard and more accurate choice for hydrostatic bearings under very heavy loads is very slow speed.
The ratio of the torsional strength of a hollow shaft to that of a solid shaft of equal length and weight and made of same material is
A)
more than one
B)
less than one
C)
equal to one
D)
none of these
For shafts of equal length, equal weight, and same material, the cross-sectional areas are equal. A hollow shaft places more material away from the center, which increases the polar section modulus and therefore the torque-carrying capacity.
Torsional strength is proportional to polar section modulus Zp = J/R. For the same area, a hollow shaft has larger Zp than a solid shaft. Hence, the hollow shaft can transmit greater torque before reaching the same allowable shear stress.
Therefore, the ratio of torsional strength of hollow shaft to solid shaft is greater than 1.
Why others are incorrect:
- (b) less than one: incorrect, because hollow shafts are stronger in torsion for the same weight.
- (c) equal to one: incorrect, strength is not the same.
- (d) none of these: incorrect, since option (a) is correct.
Two springs (each having stiffness K) are in parallel. The overall stiffness of the two springs would be
A)
K/2
B)
K
C)
K/4
D)
2K
For springs in parallel, the deflection of each spring is the same and the loads carried by them add up. Hence the equivalent stiffness is the sum of individual stiffnesses: Keq = K1 + K2 = K + K = 2K. Therefore, the correct option is (d) 2K. The other options are incorrect because they do not follow the parallel spring stiffness relation.
The ratio of lateral strain to the linear strain within elastic limit is known as
A)
Young’s modulus
B)
bulk modulus
C)
Poisson’s ratio
D)
modulus of rigidity
Poisson’s ratio is defined as the ratio of lateral strain to longitudinal (linear) strain within the elastic limit. Hence option (c) is correct. Young’s modulus is the ratio of normal stress to longitudinal strain, bulk modulus is the ratio of volumetric stress to volumetric strain, and modulus of rigidity is the ratio of shear stress to shear strain.
In a spur gear the involute profile is generated from
A)
base circle
B)
pitch circle
C)
addendum circle
D)
dedendum circle
In a spur gear, the involute tooth profile is generated by unwinding a taut string from the base circle. The involute curve is defined from the base circle, not from the pitch, addendum, or dedendum circles. The pitch circle is only a reference circle for gear meshing, while the addendum and dedendum circles define the tooth height limits.
The strain energy stored in a solid circular shaft of torsional rigidity GJ and length L, subjected to a twisting moment T is given by
A)
TL/GJ
B)
T2L/GJ
C)
T2L/2GJ
D)
GJ/TL
For a shaft in torsion, the angle of twist is theta = TL/GJ. The strain energy stored is U = (1/2) × T × theta. Substituting theta, we get U = (1/2) × T × (TL/GJ) = T^2L/2GJ. Hence option (c) is correct. Option (a) is only the angle of twist, option (b) misses the factor 1/2, and option (d) is dimensionally incorrect for strain energy.
Eye bolts are used for
A)
locking devices
B)
lifting and transportation of machines and cubicles
C)
absorbing shock and vibration
D)
transmission of power
Eye bolts have a looped head designed to attach hooks, ropes, or chains, so they are commonly used for lifting, hoisting, and transporting heavy equipment such as machines and cubicles. Option (a) is incorrect because locking devices are different fastening elements. Option (c) is incorrect because shock and vibration are absorbed by springs, rubber pads, or similar devices. Option (d) is incorrect because power transmission is done by shafts, belts, gears, etc., not eye bolts.
If there are L number of links in a mechanism then number of possible inversion is equal to
A)
L + 1
B)
L
C)
L – 1
D)
L + 2
Inversions of a mechanism are obtained by fixing different links of the same kinematic chain one at a time. If a mechanism has L links, each link can be fixed once to obtain a possible inversion. Therefore, the number of possible inversions is equal to the number of links, i.e. L. Hence option (b) is correct. Options (a), (c), and (d) are incorrect because the number of inversions is neither more nor less than the total number of links.
The buckling load for a given column depends on
A)
least radius of gyration
B)
length of column
C)
modulus of elasticity
D)
all of these
For Euler buckling, the critical load is Pcr = π²EI / (Le)². Since I = A·k², the buckling load depends on the radius of gyration k (and the least radius of gyration is critical), the effective/actual length of the column, and the modulus of elasticity E. Therefore all the given factors affect buckling load. Hence, option (d) is correct.
A shaft is designed on the basis of
A)
strength and elasticity
B)
rigidity and elasticity
C)
strength and rigidity
D)
only strength
Shafts are designed mainly on the basis of both strength and rigidity. Strength ensures the shaft can safely withstand the transmitted torque and bending stresses without failure, while rigidity ensures that the angular twist and deflection remain within permissible limits for proper functioning. A shaft designed only for strength may still twist excessively, causing misalignment or vibration. Hence options (a), (b), and (d) are incorrect, and the correct answer is (c).
100 kW of power is to be transmitted by each of two separate shafts, made of the same material. Shaft A is turning at 250 rpm and shaft B at 300 rpm. Which shaft must have greater diameter?
A)
A
B)
B
C)
Both will have the same diameter
D)
None of these
For a shaft transmitting power, torque is given by P = 2πNT/60, so T = 60P/(2πN). For the same power, torque is inversely proportional to speed. Therefore, the shaft running at lower speed must transmit higher torque. Shaft A runs at 250 rpm, which is lower than shaft B at 300 rpm, so shaft A carries greater torque. Since shaft diameter depends on the torque it must withstand, shaft A must have the greater diameter. Hence, option (a) A is correct.
When the relative motion between two elements is completely or successfully constrained, then these two elements form a
A)
mechanism
B)
machine
C)
kinematic pair
D)
none of these
When the relative motion between two elements is completely or successfully constrained, the two elements together are called a kinematic pair. A mechanism is a combination of links arranged to transmit motion, and a machine is a mechanism or collection of mechanisms that transmits and modifies energy. Hence, the correct answer is kinematic pair.
The most suitable bearing for resisting heavy loads under slow speed is
A)
hydrodynamic bearing
B)
ball bearing
C)
roller bearing
D)
hydrostatic bearing
A hydrostatic bearing is most suitable for resisting heavy loads at slow speed because the load-carrying oil film is created by externally pressurized lubricant, so it does not depend on shaft speed. Hence it can support very heavy loads even at very low or zero speed.
Why others are incorrect:
- Hydrodynamic bearing: develops pressure due to relative motion, so it is not suitable at slow speed.
- Ball bearing: good for moderate loads and higher speeds, but not the best for very heavy loads.
- Roller bearing: carries heavier loads than ball bearings, but for very heavy loads under slow speed, hydrostatic bearing is superior.
Therefore, the correct answer is hydrostatic bearing.
The emissive power of a body depends upon
A)
emissivity of the body
B)
wavelength of radiation
C)
temperature of the body
D)
all of these
Emissive power is the radiant energy emitted by a body per unit area per unit time. It depends on:
1. Emissivity of the body, because a real body emits less than a black body.
2. Wavelength of radiation, since emissive power varies with wavelength according to the radiation spectrum.
3. Temperature of the body, because total emissive power increases strongly with temperature (Stefan-Boltzmann law).
Therefore, the correct choice is 'all of these'.
Total emissivity of a polished silver surface compared to black body is
A)
same
B)
more or less same
C)
higher
D)
lower
A black body has the maximum possible emissivity, equal to 1. A polished silver surface is highly reflective and emits very little thermal radiation, so its total emissivity is much lower than that of a black body. Hence the correct answer is 'lower'.
Draft tube is used for discharging water from the exit of
A)
Impulse turbine
B)
Pelton wheel
C)
Francis turbine
D)
All of these
A draft tube is a diverging pipe fitted at the outlet of a reaction turbine. It allows water to leave the runner with reduced velocity and converts part of the kinetic energy into pressure energy. This arrangement is used in Francis turbines (and also other reaction turbines), not in impulse turbines like the Pelton wheel, which discharge water directly to the tailrace without a draft tube. Therefore, the correct answer is Francis turbine.