A rope drive transmits 800 h.p. from a pulley of effective diameter 4m, which runs at a speed of 90 r.p.m. The angle of lap is 1600, the angle of groove 450, the coefficient of friction 0.28, the weight of the rope 1.5 kgf / m, and the allowable tension in each rope 240 kgf. Find the number of ropes required.