A methodology for determining the reliability of mechanical components is given. The necessary design data are pointed out. Complex fatigue research machines are described, which are generating the required fatigue strength data in terms of cycles to failure and endurance strength. The test loading is that of alternating bending combined with steady torque. The design data obtained are presented. The design by reliability methodology and data are applied to the design of an actual alternator rotor inner shaft, and the results are compared with those obtained by conventional design procedure.