Engineering Questions with Answers - Multiple Choice Questions

# Machine Kinematics MCQ – Efficiency of Self Locking Screws

1 - Question

Which of the following screw thread is adopted for power transmission in either direction?

Explanation: A square thread, is adapted for the transmission of power in either direction. This thread results in maximum efficiency and minimum radial or bursting pressure on the nut.

2 - Question

Multiple threads are used to secure
a) low efficiency
b) high efficiency

Explanation: The power screws with multiple threads such as double, triple etc. are employed when it is desired to secure a large lead with fine threads or high efficiency. Such type of threads are usually found in high speed actuators.

3 - Question

Screws used for power transmission should have
a) low efficiency
b) high efficiency
d) strong teeth

Explanation: None.

4 - Question

If α denotes the lead angle and φ, the angle of friction, then the efficiency of the screw is written as
a) tan(α − φ)/tanα
b) tanα/tan (α − φ)
c) tan(α + φ)/tanα
d) tanα/tan (α + φ)

Explanation: Efficiency, η = Ideal effort/Actual effort = tanα/tan (α + φ).

5 - Question

A screw jack has square threads and the lead angle of the thread is α. The screw jack will be self locking when the coefficient of friction (μ) is
a) μ > tan α
b) μ = sin α
c) μ = cot α
d) μ = cosec α

Explanation: A screw will be self locking if the friction angle is greater than helix angle or coefficient of friction is greater than tangent of helix angle i.e. μ or tan φ > tan α.

6 - Question

To ensure self locking in a screw jack, it is essential that the helix angle is
a) larger than friction angle
b) smaller than friction angle
c) equal to friction angle
d) such as to give maximum efficiency in lifting

Explanation: A screw will be self locking if the friction angle is greater than helix angle or coefficient of friction is greater than tangent of helix angle i.e. μ or tan φ > tan α.

7 - Question

A screw is said to be self locking screw, if its efficiency is
a) less than 50%
b) more than 50%
c) equal to 50%
d) none of the mentioned

Explanation: Efficiency of self locking screws is less than 1/2 or 50%. If the efficiency is more than 50%, then the screw is said to be overhauling.

8 - Question

A screw is said to be over hauling screw, if its efficiency is
a) less than 50%
b) more than 50%
c) equal to 50%
d) none of the mentioned

Explanation: Efficiency of self locking screws is less than 1/2 or 50%. If the efficiency is more than 50%, then the screw is said to be overhauling.

9 - Question

While designing a screw in a screw jack against buckling failure, the end conditions for the screw are taken as
a) both ends fixed
b) both ends hinged
c) one end fixed and other end hinged
d) one end fixed and other end free.

Explanation: For buckling failure, The screw is considered to be a strut with lower end fixed and load end free. For one end fixed and the other end free, C = 0.25.

10 - Question

The load cup of a screw jack is made separate from the head of the spindle to
a) enhance the load carrying capacity of the jack
b) reduce the effort needed for lifting the working load
c) reduce the value of frictional torque required to be countered for lifting the load
d) prevent the rotation of load being lifted