Test: Threaded Joints & Power Screws - 3 - Mechanical Engineering MCQ

Due to tightening bolt is always subjected to tensile stress.

If pitch is mentioned in the designation of th screw thread then, it is fine pitch otherwise coarse pitch.

Tensile force on each bolt

for design of bolt


d = 22.25 mm 30 mm (for safe design)

Acme thread is adopted for power transmission in either direction.

For.self locking
φ ≤ α (Friction angle > Helix angle)

(μ = tan φ)

If φ < α
i.e., friction angle is less then Helix angle, then torque require to lower the load will be negative i.e., no force is required to lower the load the ioad itself will begin to turn the screw and descend down, unless a restraining torque is applied this condition is known as overhauling.

Advantages of multiple threaded screw.
1. It provides large axial motion per revolution of the screw, this increases the travelling speed of the sliding member.
2. The efficiency of multithreaded screw is more than single threaded screw due to increase in helix angle.
Note that the mechanical advantage obtained with multiple start threaded screw is lower than that of single threaded screw therefore the effort required to raise a particular load will be more in case of multiple start screw then single start.

For self-locking, the maximum efficiency of screw should be 50%.
If efficiency is more than 50% the condition is known as overhauling.

For self locking
φ ≤ a (Friction angle > Helix angle)
tan φ > tan α

(μ = tanφ)

Since in power screw, the seif locking property is desirable and to achieve self locking property, the power screw are designed for low efficiency.

Square thread:
1. Square threads are less efficient than trapezoidal thread.
2. Square threads are difficult to manufacture. They are usually turned on lathe with single point cutting tool.
3. The strength of a screw depends upon the thread thickness at the core diameter since square thread have less thickness at core than trapezoidal this reduces the load carrying capacity.

Screw used for power transmission should have
1. High efficiency
2. High strength
3. High load carrying capacity
4. Smooth and noise less

Steel-screw, Phosphor Bronze-nut, Cast iron also used for nut in case of low speed.

η less than 50% so statement 1 correct α > φ statements 2 incorrect