1.A disk clutch is being proposed for an industrial application in which the power-input shaft supplies 12 kw steadily at 650 rpm. The patented friction lining material, which is to be bonded to one or more annular disk surfaces, is to be brought into contact with stiff steel disks to actuate the clutch. The outside diameter of the clutch disks must be no larger than 125 mm, and it is desired to configure the annular friction surfaces so that the inner diameter is about 2/3 of the outer diameter. The coefficient of friction between the patented friction lining material and steel is and the maximum-allowable contact pressure is . What is the minimum number of friction surfaces required for the clutch to function properly?