Induction Shrink Fitting
Thermal expansion assembly and disassembly of interference-fit components — bearings, gears, rings, and sleeves — without flame, distortion, or metallurgical damage.
How It Works
The outer component (ring, bearing, sleeve) is heated to expand its bore by thermal expansion. The expansion is calculated as: ΔD = D × α × ΔT, where α is the coefficient of thermal expansion.
For steel (α ≈ 12 × 10⁻⁶ /°C), heating by 200°C gives approximately 0.24% expansion. A 100 mm bore expands by ~0.24 mm — sufficient for most interference fits which are typically 0.05–0.15 mm.
Induction is ideal because the heat is delivered fast (10–60 seconds) and only to the outer component. The inner shaft or housing stays cold and does not expand, maintaining the clearance needed for easy assembly. This is a critical advantage over oven heating, where everything heats up together.
For disassembly, the same principle works in reverse: heating the outer ring allows extraction from the shaft. Induction is the standard method for removing bearings in maintenance and overhaul operations.
Typical Parameters
| Component | Frequency | Power | Temp Range | Cycle Time |
|---|---|---|---|---|
| Small bearings (< 100 mm OD) | 10 – 50 kHz | 2 – 10 kW | 80 – 120°C | 10 – 30 s |
| Large bearings (100–500 mm OD) | 3 – 10 kHz | 10 – 50 kW | 80 – 150°C | 30 – 120 s |
| Gear rings (steel, 100–300 mm) | 3 – 30 kHz | 10 – 40 kW | 150 – 250°C | 20 – 60 s |
| Turbine discs / sleeves | 1 – 10 kHz | 20 – 100 kW | 150 – 300°C | 30 – 120 s |
| Motor rotor lamination stacks | 10 – 50 kHz | 5 – 20 kW | 200 – 350°C | 15 – 60 s |
| Disassembly (bearing removal) | 3 – 30 kHz | 5 – 30 kW | 80 – 150°C | 15 – 60 s |
Key Considerations
- Temperature limits are set by metallurgy — bearing steels (52100, SUJ2) begin to lose hardness above ~175°C. Tempering effects start at 150°C. Never exceed the bearing manufacturer’s maximum recommended temperature.
- Circumferential uniformity is critical to prevent oval distortion. Rotate the part during heating or use a multi-turn coil that surrounds the full circumference evenly.
- Thermal soak time after reaching surface temperature ensures the bore has actually expanded. Surface temperature alone may read high while the bore is still at a lower temperature.
- The assembly window is limited — once removed from the coil, the part begins cooling and contracting. Typical window is 5–30 seconds for large parts. Practice the assembly motion before heating.
- Blind-hole fits require extra expansion margin because heat conducts from the heated ring into the cold shaft during the assembly slide, accelerating contraction.
- Induction is 5–20× faster than oven heating and heats only the target component. No heating of adjacent seals, lubricant reservoirs, or housing structures that could be damaged by oven temperatures.
Common Coil Geometries
Multi-Turn Solenoid
Standard for rings and bearings. The part sits inside the coil for uniform circumferential heating. Coil ID sized 10–30 mm larger than the part OD.
Pancake Coil (Flat Spiral)
For heating one face of a flange or disc to achieve axial expansion. Useful when the part cannot be placed inside a solenoid due to geometry constraints.
Internal Coil
For expanding a bore from inside — used for very large housings where an external coil would be impractically large. The coil inserts into the bore and heats outward.