Discussion
A screw jack is said to be self-locking if its efficiency is :
Answer: B
A screw jack is self-locking if its efficiency is less than 50%.
A self-locking screw holds the load without an applied effort; the load cannot drive the screw back down. This happens when the friction angle \(\phi\) is at least the helix angle \(\alpha\). Working through the efficiency of the screw, \(\eta = \frac{\tan\alpha}{\tan(\alpha + \phi)}\), the condition \(\phi \ge \alpha\) keeps the efficiency below one half.
So efficiency of 100% is impossible with friction, more than 50% means the screw would overhaul under the load, and 'equal to 40%' is only one value inside the self-locking range, not the condition itself.
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