축(Shaft) 설계의 3가지 함정 — 단차, 키홈, 응력 집중

1. Stress Concentration at Diameter Steps

• Diameter ratio (D/d): Large-to-small diameter ratio. Higher ratio = more stress concentration. Aim for D/d ≤ 1.5.
• Fillet radius (r): Always add a radius at step transitions. Guideline: r/d ≥ 0.1. For d=30mm: R3 minimum.
• Avoid undercuts: Grinding relief undercuts cause extreme stress concentration. If unavoidable, use arc undercuts (R≥1mm).

2. Keyway Stress Concentration

• Keyways reduce cross-sectional area and raise stress concentration factor (Kt) to 1.6–3.0.
• Placement: Keep keyways away from bearing supports and steps. Minimum distance = one shaft diameter.
• End geometry: End mill radius at keyway ends is unavoidable. Larger end radius (≥0.5× keyway width) reduces stress concentration.
• Alternative: For heavy/cyclic loads, consider splines or serrations instead of keyways.

3. Bearing Shoulder Design

• Optimal shoulder height: 3–5% of bearing bore diameter.
• Too tall: Difficult bearing removal, smaller fillet radius, higher stress concentration.
• Too short: Shoulder yields under axial bearing load.
• Key rule: Shoulder height h = bearing chamfer (r₁) + 1mm minimum clearance. Shaft fillet radius must always be smaller than bearing bore chamfer.

4. Material Selection

• General power transmission: S45C (heat-treated to HRC 28–32)
• High precision / high strength: SCM435, SCM440 (case-hardening/nitriding capable)
• Corrosion resistance: SUS420J2 (martensitic), SUS316 (non-magnetic)