NHPC JE'21 ME: Daily Practices Quiz. 23-Nov-2021

Quiz: Mechanical Engineering
Exam: NHPC JE
Topic: Miscellaneous
Each question carries 1 mark
Negative marking: 1/4 mark
Time: 6 Minutes

Q1. Percentage of work save in case of single acting pump and double acting pump when we use air vessel in reciprocating pump is respectively
(a) 84.8 % and 39.2%
(B) 39.2 % and 84.8%
(c) 39.2% in both
(d) 84.8 % in both

Q2. Air is flowing over a flat plate with a free steam velocity of 24 m/s, and its kinematic viscosity is 72×10^(-6) If a particular point, the Reynolds number is 30000, its location from the leading edge is.
(a) 0.05 m
(b) 0.07 m
(c) 0.08 m
(d) 0.09 m

Q3. A pitot static tube is used to measure
(a) Stagnation pressure
(b) Static pressure
(c) Dynamic pressure
(d) Difference between the static pressure and dynamic pressure

Q4. The constant pressure lines ……. on a Mollier diagram
(a) Are equispaced throughout
(b) First Converge and then diverge
(c) Diverge from left to right
(d) Diverge from right to left

Q5. The dynamic load on a gear is due to
1. inaccuracies of tooth spacing
2. irregularities in tooth profile
3. deflection of the teeth under load
4. type of service (i.e., intermittent, one shift per day, continuous per day)
Which of these statements are correct?
(a) 1, 2 and 3
(b) 2, 3 and 4
(c) 1, 3 and 4
(d) 1, 2 and 4

Q6. Consider the following statements:
A 20° stub tooth system is generally preferred in spur gears as it results in
1. Stronger teeth
2. Lesser number of teeth on the pinion
3. It is restrained to expand or contract freely
4. Reduction of interference
Which of these statements are correct?
(a) 1, 2 and 4
(b) 3 and 4
(c) 1 and 3
(d) 1, 2, 3 and 4

Solutions

S1. Ans. (a)
Sol. By use of air vessel in reciprocating pump, head loss due to friction in suction as well as in delivery pipe is reduced and by this reduction in head a considerable amount of work is saved.
And percentage of work save in case of single acting pump is 84.8 % and 39
2 % in double acting pump when we use air vessel in reciprocating pump.

S2. Ans.(d)
Sol.
Re=ux/ϑ
x=(30000×72×10^(-6))/24=0.09 m=9 cm

S3. Ans. (c)
Sol. A simple pitot tube measure stagnation pressure but a pitot static tube is used to measure dynamic pressure.

S4. Ans. (c)
Sol. Constant pressure lines diverge from left to right on Mollier diagram.
Mollier diagram is drawn between enthalpy (h) on y-axis and entropy (s) on x-axis.
The slope of constant pressure line on h-s curve is absolute temperature.

S5. Ans.(a)
Sol. The dynamic load on gears is due to the following factors
(i) In inaccuracies in tooth profile
(ii) Errors in tooth spacing
(iii) Runout of the gear
(iv) Gear mesh stiffness variation as the gear teeth pass through the meshing cycle
(v) Inertia of rotational masses
(vi) Deflection of teeth
(vii) Stiffness of the rotating parts

S6. Ans.(a)
Sol. A 20° stub tooth system is generally preferred in spur gears as it results in
1. Stronger teeth.
2. Lesser number of teeth on the pinion.
3. Smaller interference.
4. Transmits very high load.