Induction shrink fitting
Induction shrink fitting refers to the use of induction heater technology to pre-heat metal components between 150 °C (302 °F) and 300 °C (572 °F) thereby causing them to expand and allow for the insertion or removal of another component. Typically the lower temperature range is used on metals such as aluminium and higher temperatures are used on metals such as low/medium carbon steels. The process avoids the changing of mechanical properties whilst allowing components to be worked. Metals typically expand in response to heating and contract on cooling; this dimensional response to temperature change is expressed as a coefficient of thermal expansion.
Induction heating is a non contact heating process which uses the principle of electromagnetism induction to produce heat in a work-piece. In this case thermal expansion is used in a mechanical application to fit parts over one another, e.g. a bushing can be fitted over a shaft by making its inner diameter slightly smaller than the diameter of the shaft, then heating it until it fits over the shaft, and allowing it to cool after it has been pushed over the shaft, thus achieving a 'shrink fit'. By placing a conductive material into a strong alternating magnetic field, electric current can be made to flow in the metal thereby creating heat due to the I2R losses in the material. The current generated flows predominantly in the surface layer. The depth of this layer being dictated by the frequency of the alternating field and the permeability of the material. Induction heaters for shrink fitting fall into two broad categories:
Often referred to as a bearing heater, the mains frequency unit employs standard transformer principles for its operation. An internal winding is wound around a laminated core similar to a standard mains transformer. The core is then passed through the work-piece and when the primary coil is energised, a magnetic flux is created around the core. The work-piece acts as a short circuit secondary of the transformer created, and due to the laws of induction, a current flows in the work-piece and heat is generated. The core is normally hinged or clamped in some way to allow loading or unloading, which is usually a manual operation. To cover variations in part diameter, the majority of units will have spare cores available which help to optimise performance. Once the part is heated to the correct temperature, assembly can take place either by hand or in the relevant jig or machine press.
...
Wikipedia