What is induction shrink fitting?
Shrink fitting assembles two parts with an interference fit: the mating dimension of one part is smaller than the other at the same temperature. Heating the outer part makes it expand through thermal expansion, so it slips over the inner part, and as it cools it contracts and grips. Induction heating is a natural fit because it heats metal quickly, locally and without contact.
How thermal expansion creates the fit
Most metals expand when heated. The growth of a dimension depends on its starting size, the temperature rise and the material’s coefficient of thermal expansion. A bore heated enough grows beyond the diameter of the mating shaft, which lets the parts be assembled by hand or fixture. When the heated part returns to equilibrium, the interference between the parts creates the contact pressure that holds them together.
Specific temperatures, allowable interference and heating times depend on the materials, sizes and design requirements, so they must come from the part specification and testing.
Why heat with induction?
Induction generates heat inside the workpiece, so only the part that must expand is heated, and cycle times and repeatability are good. Fluxtrol’s induction heating course lists shrink fitting among applications of middle-frequency induction (3 to 50 kHz, along with surface hardening, brazing, small forging and tempering), and among applications of small solid-state power supplies (20 to 500 kHz, 1 to 25 kW).
Where flux control and coil design come in
The coil has to heat the outer component evenly enough for a uniform bore expansion, without overheating the mating part or nearby fixtures. These are the same problems Fluxtrol addresses in other applications: soft magnetic composite flux controllers concentrate the field in the heated zone and limit stray fields, and simulation lets engineers test a coil before it is built. Fluxtrol’s published shrink fitting work is limited, so we evaluate each application individually.
Shrink fitting in related research
- Bi-metal bushings. In work with IFUM, steel and aluminum cylinders joined by shrink fitting were induction heated and forged. The paper observes that shrink-fitted workpieces lack perfect contact, which allows higher temperature gradients.
- Residual stress. Fluxtrol authors note that internal stresses from induction processing may be positive, as in shrink fitting, or detrimental, as in deformation and cracking.
Talk to us about your assembly
If you are developing an induction shrink fitting station, contact Fluxtrol to review the coil, frequency and flux control options. You can also explore induction coil design and our free induction heating course .
Fluxtrol SMCs for shrink fitting coils
Shrink fitting in Fluxtrol research
Thermomechanical processing for bi-metal bearing bushings
Two concentric cylinders, 20MnCr5 steel and AA-6082 aluminum, were joined by shrink fitting and then induction heated and closed-die forged. The study modeled and tested the heating behavior.
- Shrink-fitted workpieces do not have perfect contact, so higher temperature gradients are possible
- Joining zone examined metallographically
Stress and distortion evolution during induction case hardening of tube
Notes that internal stresses from induction processing can be beneficial, as in shrink fitting, or detrimental, as in cracks and deformation.
- Coupled electromagnetic, thermal, structural and stress simulation