1. Diameter D₃ – perform static analysis (eqn. 18-12) – max. moment and torque
2. Diameter D₂ – perform static analysis (eqn. 18-12) – high moment and max. torque
3. Diameter D₁ – assume based on diameter D₂, between 0.7 to 0.8 times D₂ – negligible moment and zero torque
4. Diameter D₄ – perform static analysis (eqn. 18-12) – high moment and max. torque
5. Diameter D₅ – perform static analysis (eqn. 18-12) – high/moderate moment and max. torque
6. Diameter D₆ – perform static analysis (eqn. 18-12) – negligible moment but max. torque

• First identify all critical locations where fatigue analyses are necessary to determine the diameters
• Critical locations are locations on the shaft where there is a combination of a stress concentration with a high bending moment
• Stress concentration locations are essentially those locations where abrupt changes in the cross-sectional area (case 1) occur or where there is non-uniformity in the cross-sectional area (case 2)

1. Diameter D₁ at the fillet – ignore fatigue analysis – case 1, but low moment
2. Diameter D₂ on the keyway – perform fatigue analysis (eqn. 18-39) – case 2 and high moment
3. Diameter D₂ at the fillet – perform fatigue analysis (eqn. 18-39) – case 1 and high moment
4. Diameter D₃ – no fatigue – no stress concentration, though max. moment
5. Diameter D₄ at fillet – perform fatigue analysis (eqn. 18-39) – case 1 and high moment
6. Diameter D₅ at fillet – perform fatigue analysis (eqn. 18-39) – case 1 and high moment
7. Diameter D₆ at fillet – ignore fatigue analysis – case 1, but low moment
8. Diameter D₆ on keyway – ignore fatigue analysis – case 2, but low moment

Note: In eqn (18-39), use K_f=1 in the numerator, but the actual value in the calculation of S_e