In those cases where operational complexities negate the use of a cored mains frequency approach, the standard RF or MF induction heater can be used. This type of unit uses turns of copper tube wound into an electromagnetic coil. There are no cores required, the coil needs to simply surround or be inserted into the part to be heated this makes automating the process straightforward. A further advantage is the ability to not only shrink fit parts but also remove them.
The RF and MF heaters used for induction shrink fitting vary in power from a few kilowatts to many megawatts and depending on the component geometry/diameter/cross section can vary in frequency from 1 kHz to 200 kHz, although the majority of applications use the range between 1 kHz and 100 kHz.
In general terms, it is best to use the lowest practical frequency and a low power density when undertaking shrink fitting as this will generally provide more evenly distributed heat. The exception to this rule is when using heat to remove parts from shafts. In these cases it is often best to shock the component with a rapid heat, this also has the advantage of shortening the time cycle and preventing heat build up in the shaft which can lead to problems with both parts expanding.
In order to select the correct power it is necessary to first calculate the thermal energy required to raise the material to the required temperature in the time allotted. This can be done using the heat content of the material which is normal expressed in kW hours per tonne, the weight of metal to be processed and the time cycle. Once this has been established other factors such as radiated losses from the component, coil losses and other system losses need to be factored in. Traditionally this process involved lengthy and complex calculations in conjunction with a mixture of practical experience and empirical formula. Modern techniques use finite element analysis and other computer-aided manufacturing techniques, however as with all such methods a thorough working knowledge of the induction heating process is still required. When deciding on the correct approach it is often necessary to consider the overall size and thermal conductivity of the work-piece and its expansion characteristics in order to ensure that enough soak time is allowed to create an even heat throughout the component.