v = belt velocity. Speed ratio N a N b = r b r a ( 100 − s) 100. (when pulley b is the driver) Arc of contact ( rarb ): θ a = 180 ° + 2 sin − − 1 ( r a − r b) L. θ b = 180 ° − 2 sin − − 1 ( r a − r b) L. Tension ratio for belt about to slip: For pulley ‘a’. F 1 F 2 = e μ θ a