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 use 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, electrical 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:
· Mains frequency units use magnetic cores (iron)
· Solid state (electronics) MF and RF heaters