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

The question was asked during an online interview.

I want to ask this question from Efficiency of Self Locking Screws topic in section Friction of Machine Kinematics

Answer 1

Right option is (b) smaller than friction angle

To explain: 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.