The shaft is supported by bearings at each end, rotates at 500 rpm, and is made of SAE 1137 Cold Drawn Steel. It has a bearing ID of 20mm, dimension a of 54 mm, diameter D of 40 mm, and radius R of 2 mm. The document asks to calculate: (1) the actual estimated endurance strength (s'n) of the material, (2) the factor of safety (N) at dimension a=53mm including stress concentration, and (3) the factor of safety (N) at the load.