Machine Design Questions and Answers – Screw Jack

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1. How many types of jacks are there which can be used in a screw jack?
a) 2
b) 3
c) 4
d) 5

Explanation: Hydraulic jack and mechanical jack.

2. Which of the following does not form the important part of the screw jack?
a) Frame
b) Nut
c) Cup
d) Coupling

Explanation: There is no specific requirement of coupling in the screw jack.

3. The transverse shear stress at the root of the threads in the nut can be given by?(symbols have their usual meaning, z=number of threads in nut)
a) 4W/πdz²
b) W/πdtz
c) 4W/πtd²
d) None of the mentioned

Explanation: At root of threads, the area parallel to direction of force is considered which is equal to circumference x thickness x no. of threads.
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4. What type of friction in cup design is recommended for the set screw?
a) Sliding
b) Rolling
c) Static
d) None of the mentioned

Explanation: Sliding friction, the total force require to raise the load is quite large as compared to that required in rolling friction. Hence length of handle in some cases becomes extremely large and thus it is impractical to use cups with sliding friction.

5. A differential screw is defined as a mechanical device consisting of two screws connected in parallel.
a) True
b) False

Explanation: The two screws are connected in series.

6. In a differential screw, two screws are connected in series such that the resultant motion is the summation of the motion of the two screws.
a) True
b) False

Explanation: The resultant motion is the difference of the individual motion of two screws.

7. A compound screw consist of two screws in parallel such that the resultant motion is the summation of the individual motion of screws.
a) True
b) False

Explanation: The two screws are connected in series with the resultant motion being the sum of individual motion.

8. What is the output from differential screws when pitch of the two screws is 12mm and 8mm? Also 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) 13200 N-mm
b) 15200 N-mm
c) 19200 N-mm
d) 17200 N-mm

Explanation: 120 x 300=5k+2.5k or k=4800N. Output=k x (12-8) or output=19200N-mm.

9. 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%

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

10. Efficiency of conventional power screw is greater than the efficiency of recirculating ball screw.
a) True
b) False

Explanation: In recirculating ball screw, sliding friction is replaced by rolling friction and efficiency increases from 40% to 90%.

11. 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