Figure P9.29 shows a Geneva indexing mechanism used, for example, on the indexing heads of machine tools. Each time the driving arm makes one revolution, the Geneva wheel (for the four-slot design illustrated) turns 90°. The arm supports a rolling cylindrical hardened-steel pin that fits into the Geneva wheel slots. The pin is to have the same length and diameter. The wheel is made of heat-treated alloy cast iron (E = 140 GPa, v = 0.25). For a design contact stress of 700 MPa, determine the smallest acceptable pin diameter if the design overload torque (normal torque times safety factor) applied to the arm is 60 N m.