3-9. A disk clutch is to be used as an overload protection device for a conveyor system drive. The conveyor is driven by a 1200 rpm, 7.5 hp induction motor by means of a gearbox having a ratio r of 160. The disk clutch is set for an axial force F, of 275 lb by means of springs, and the brake lining has radial dimensions d = 2.5 inches, D=3.75 inches. The clutch is used as a coupling on the motor shaft between motor and gearbox. If the coefficient of friction for impending slip is f=0.4, at what percentage of rated motor torque will this clutch slip? Assume there are N = 4 disk interface surfaces.