What is true in respect of the self locking screw jack with square thread?
(I) The friction angle is more than the helix angle of the thread.
(II) The efficiency of the screw jack
should not be more than 50%

Correct option is C

​$$\begin{aligned}&\textbf{Statement (I): \textit{The friction angle is more than the helix angle of the thread.}} \\[0.5em]&\text{Friction angle } (\phi) \text{ is defined as:} \\&\phi = \tan^{-1}(\mu) \\[0.5em]&\text{Helix angle } (\alpha) \text{ of the square thread is the angle made by the thread profile with the horizontal axis.} \\[0.5em]&\text{For self-locking condition:} \\&\phi > \alpha \Rightarrow \tan^{-1}(\mu) > \alpha \\[0.5em]&\textbf{Statement (II): \textit{The efficiency of the screw jack should not be more than 50\%.}} \\[0.5em]&\text{Efficiency } (\eta) \text{ of a screw jack is given by:} \\&\eta = \frac{\tan \alpha}{\tan(\alpha + \phi)} \\[0.5em]&\text{A self-locking screw jack must have efficiency } \leq 50\%. \\&\text{If } \eta > 50\%, \text{ it enters the overhauling condition — not safe for jacks.}\end{aligned}$$​​