Efficiency of Self Locking Screws

Question 1
Marks : +2 | -2
Pass Ratio : 100%
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
μ > tan α
μ = sin α
μ = cot α
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.
Question 2
Marks : +2 | -2
Pass Ratio : 100%
The load cup of a screw jack is made separate from the head of the spindle to
enhance the load carrying capacity of the jack
reduce the effort needed for lifting the working load
reduce the value of frictional torque required to be countered for lifting the load
prevent the rotation of load being lifted
Explanation:
For the prevention of the rotation of load being lift, the load cup of a screw jack is made separate from the head of the spindle.
Question 3
Marks : +2 | -2
Pass Ratio : 100%
Multiple threads are used to secure
low efficiency
high efficiency
high load lifting capacity
high mechanical advantage
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.
Question 4
Marks : +2 | -2
Pass Ratio : 100%
Screws used for power transmission should have
low efficiency
high efficiency
very fine threads
strong teeth
Explanation:
None.
Question 5
Marks : +2 | -2
Pass Ratio : 100%
A screw is said to be self locking screw, if its efficiency is
less than 50%
more than 50%
equal to 50%
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.
Question 6
Marks : +2 | -2
Pass Ratio : 100%
While designing a screw in a screw jack against buckling failure, the end conditions for the screw are taken as
both ends fixed
both ends hinged
one end fixed and other end hinged
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.
Question 7
Marks : +2 | -2
Pass Ratio : 100%
If α denotes the lead angle and φ, the angle of friction, then the efficiency of the screw is written as
tan(α − φ)/tanα
tanα/tan (α − φ)
tan(α + φ)/tanα
tanα/tan (α + φ)
Explanation:
Efficiency, η = Ideal effort/Actual effort = tanα/tan (α + φ).
Question 8
Marks : +2 | -2
Pass Ratio : 100%
A screw is said to be over hauling screw, if its efficiency is
less than 50%
more than 50%
equal to 50%
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.
Question 9
Marks : +2 | -2
Pass Ratio : 100%
Which of the following screw thread is adopted for power transmission in either direction?
Acme threads
Square threads
Buttress threads
Multiple threads
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.
Question 10
Marks : +2 | -2
Pass Ratio : 100%
To ensure self locking in a screw jack, it is essential that the helix angle is
larger than friction angle
smaller than friction angle
equal to friction angle
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.