Machine Kinematics Multiple Choice Questions on “Efficiency of Self Locking Screws”.
1. Which of the following screw thread is adopted for power transmission in either direction?
a) Acme threads
b) Square threads
c) Buttress threads
d) Multiple threads
Answer: b
Clarification: 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. Multiple threads are used to secure
a) low efficiency
b) high efficiency
c) high load lifting capacity
d) high mechanical advantage
Answer: b
Clarification: 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. Screws used for power transmission should have
a) low efficiency
b) high efficiency
c) very fine threads
d) strong teeth
Answer: b
Clarification: None.
4. 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)
Answer: d
Clarification: Efficiency, η = Ideal effort/Actual effort = tanα/tan (α + φ).
5. 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
Answer: a
Clarification: 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. 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
Answer: b
Clarification: 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. 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
Answer: a
Clarification: 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. 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
Answer: b
Clarification: 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. 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.
Answer: d
Clarification: 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. 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
Answer: d
Clarification: 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.