설계실 에어컨이 꺼지면 기계가 잠긴다? 열팽창 여유 설계법

A precision machine assembled perfectly at room temperature seizes or develops noise after extended operation? Thermal expansion likely wasn't accounted for. All machinery heats during operation, and different materials expand at different rates.

1. Thermal Expansion Coefficients (α) by Material

• Carbon steel (SS400, S45C): α ≈ 12 × 10⁻⁶ /°C
• Stainless steel (SUS304): α ≈ 17 × 10⁻⁶ /°C
• Aluminum (A6061): α ≈ 23 × 10⁻⁶ /°C
• Gray cast iron (FC200): α ≈ 11 × 10⁻⁶ /°C

2. Thermal Expansion Formula

ΔL = α × L₀ × ΔT

• Example: Steel shaft 1,000mm, operating temp rise 80°C → ΔL = 12×10⁻⁶ × 1000 × 80 = 0.96mm
• Aluminum housing 500mm, same conditions → ΔL = 23×10⁻⁶ × 500 × 80 = 0.92mm

3. Dissimilar Material Trap

When a steel shaft runs in an aluminum housing, differential expansion creates problems.

• Aluminum expands more: At operating temperature, clearance increases; upon cooling, it tightens. Calculate fit at both extremes.
• Practical tip: For dissimilar material fits, verify clearance/interference at both maximum and minimum operating temperatures.

4. Long Shafts and Sliding Joints

• Long shafts (≥1m): Fix one bearing (locating) and allow axial float at the other (floating) to absorb thermal growth. Fixing both ends causes buckling.
• Piping: Use expansion joints or loop piping. Per meter of pipe at ΔT=80°C: steel ≈ 1.0mm, SUS ≈ 1.4mm expansion.