Machine Design Questions and Answers Part-20

1. What is the efficiency of differential screws when pitch of the two screws is 12mm and 8mm? The nut is rotated by applying a force of 120N at a radius of 300mm and the two screws remain stationary. The torque of raising and lowering for the two screws is 5k N-mm and 2.5k N-mm where k is the effective axial weight on the screw.
a) 6.48%
b) 8.48%
c) 23.1%
d) 42.8%

Answer: b
Explanation: 120 x 300=5k+2.5k or k=4800N. Output=k x (12-8) or output=19200N-mm. Efficiency=output/2πx120x300.

2. In which case is the wear more?
a) Conventional power screw
b) Recirculating power screw
c) Equal in both the cases
d) Cannot be determined

Answer: a
Explanation: In recirculating ball screw, there is a lubricant film between the contacting surfaces and hence lesser wear.

3. Resilience is defined as the property of material to absorb energy when deformed _________ and to release this energy when unloaded.
a) Elastically
b) Plastically
c) Up to fracture point
d) None of the listed

Answer: a
Explanation: It is the definition of resilience.

4. The shock absorbing capacity of bolt can be increased by making the shank diameter ________ the root diameter.
a) Lesser than
b) Equal to
c) Greater than
d) No effect

Answer: b
Explanation: If shank diameter is equal to the thread diameter than, the shank is subjected to higher stress and thus absorbs a great portion of strain energy and relieves the thread portion of high stress.

5. The shock absorbing capacity of bolts can be increased by.
a) Increasing the shank diameter
b) Decreasing the length of shank portion of bolt
c) Increase the shank diameter and decreasing the length of shank
d) None of the listed

Answer: d
Explanation: Shock absorbing capacity can be increased by decreasing the shank diameter or increasing the length of shank.

6. If the length of the shank is doubled, then strain energy absorbed by shank
a) Doubles
b) Remains same
c) Increases 4 time
d) Become half

Answer: a
Explanation: Strain energy absorbed by shank is linearly proportional to its length.

7. Which of the following help in accomplishing the locking?
a) Creating supplementary friction
b) Using special locking devices like split pins
c) By plastic deformation
d) All of the listed

Answer: d
Explanation: All the methods can be employed to lock the screw.

8. Coarse threads have helix angle _____ fine threads.
a) Equal to
b) Greater than
c) Lesser than
d) There is no relation

Answer: c
Explanation: Finer threads have lower helix angle.

9. Which threads have greater resistance to unscrewing?
a) Coarse
b) Fine
c) They have equal resistance
d) Cannot be determined

Answer: b
Explanation: Fine threads have greater resistance to unscrewing as a result of lower helix angle.

10. Fine threads have greater resistance to unscrewing as a result of lower helix angle.
a) Fine threads
b) Coarse Threads
c) Either of fine or coarse
d) None of the listed

Answer: a
Explanation: Fine threads have lesser helix angle and hence greater resistance for unscrewing and thus are recommended for fluctuating loads.