How Shrink Fits Work
A shrink fit uses thermal expansion to assemble interference fit parts without pressing. You heat the hub (or cool the shaft) until the bore expands enough to slip over the shaft. When the hub cools back to ambient temperature, it contracts and grips the shaft with tremendous force.
Thermal Expansion
All metals expand when heated. The amount of expansion depends on three factors: the original dimension, the coefficient of thermal expansion (CTE), and the temperature change. The formula is simple: ΔL = L × α × ΔT.
Assembly Clearance
You need more than just enough expansion to overcome the interference. Add assembly clearance (typically 0.002″ to 0.005″) so you have time to position the hub before it cools and seizes. More clearance means more time to work, but requires higher temperature.
Pro tip: Use an oven for uniform heating. Torches create hot spots that cause uneven expansion and can damage the material. For aluminum, keep below 400°F to avoid temper changes.
Material Considerations
Aluminum expands nearly twice as much as steel per degree. This means aluminum hubs need lower temperatures for the same interference. But it also means aluminum hubs lose their grip faster in high-temperature environments.