A 15mm diameter shaft rotates in long fixed bearings 60 cm Apart. A rotor of mass 20 kg is mounted at the mid span. The mass centre of the rotor is 0.5 mm from the shaft axis. If the bending stress in the shaft is not to exceed 120 MPa, find the range of speed over which the shaft must not run. Take modulus of elasticity for the shaft material as 200 GN/m2.