A hot rolled shaft is subjected to torsional load that varies from 320 Nm clock wise to 120 Nm anti -clockwise and an applied bending moment at a critical section varies from 400 Nm to 200 Nm. The shaft is of uniform cross section. Determine the required shaft diameter. The material has an ultimate strength of 560 Mpa and yield strength of 420 Mpa. Assume factor of safty to be 1.8