$875.$ The shaft has a square$-$threaded screw with a lead of $9$ mm and a mean radius of $15$ mm$.$ If it is in contact with a plate gear having a mean radius of $20$ mm$,$ determine the resisting torque M on the gear which can be overcome if a torque of $7$ N $$ m is applied to the shaft. The coefficient of static friction between the gear and the screw is s$=0\mathrm{.}2.$ Neglect friction at the bearings located at A and B$.$