A flange coupling is used to connect two 50.8-mm diameter shafts. The coupling is secured by four 16-mm diameter bolts in a 150 mm bolt circle. The shafts are connected to the coupling by a flat key having a width of 10 mm and a thickness of 8 mm. The allowable shearing stress for the bolts is 2.83 MPa while the allowable shearing and compressive stress for the key is 12 MPa and 31 MPa, respectively. Determine the safe length of the key in mm.