The maximum efficiency of a square threaded screw is
A
$$\frac{{1 - \sin \varphi }}{{1 + \sin \varphi }}$$
B
$$\frac{{1 + \sin \varphi }}{{1 - \sin \varphi }}$$
C
$$\frac{{1 - \sin \varphi }}{{1 + \cos \varphi }}$$
D
$$\frac{{1 + \cos \varphi }}{{1 - \sin \varphi }}$$
Correct Answer: $$\frac{{1 - \sin \varphi }}{{1 + \sin \varphi }}$$