13. A line shaft ABCD, 9 metres long, has four pulleys A, B, C and D at equal distance apart. Power of 45 kW is being supplied to the shaft through the pulley C while the power is being taken off equally from the pulleys A, B and D. The shaft runs at 630 r.p.m. Calculate the most economical diameters for the various portions of the shaft so that the shear stress does not exceed 55 MPa. If the shear modulus is 85 GPa, determine the twist of the pulley D with respect to the pulley A.