Induction shrink fitting
Encyclopedia
Induction shrink fitting refers to the use of induction heater
Induction heater
An induction heater is a key piece of equipment used in all forms of induction heating. Typically an induction heater operates at either medium frequency or radio frequency ranges. Three main components form the basis of a modern induction heater including the power unit , the work head and the...

 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
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 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.

Process

Induction heating
Induction heating
Induction heating is the process of heating an electrically conducting object by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal...

 is a non contact heating process which utilises the principle of electromagnetism
Electromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...

 induction to produce heat in a work-piece. In this case thermal expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...

 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
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

, 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
Permeability (electromagnetism)
In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically...

 of the material. Induction heaters for shrink fitting fall into two broad categories:
  • Mains frequency units utilising magnetic cores (iron)
  • Solid state (electronics)
    Solid state (electronics)
    Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...

     MF and RF heaters

Mains frequency units utilising iron cores

Often referred to as a bearing heater, the mains frequency unit employs standard transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

 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
Magnetic flux
Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...

 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
Machine press
A machine press, commonly shortened to press, is a machine tool that changes the shape of a workpiece.-Servomechanism:A servomechanism press, also known as a servo press or a electro press, is a press driven by an AC servo motor. The torque produced is converted to a linear force via a ball screw....

.

Power consumption

Bearing heaters typically range from 1 kVA to 25 kVA and are used to heat parts from 1 to 650 kg (2.2 to 1,433 lb), dependent upon the application. The power required is a function of the weight, target temperature and cycle time to aid selection many manufacturers publish graphs and charts.

Industries and applications

  • Railway - gearboxes, wheels, transmissions
  • Machine tools - lathe gearboxes, mills
  • Steel works - roll bearings, roll neck rings
  • Power generation - various generator components


Due to the need to insert a core and also that to be effective, the core has to be in relatively close proximity to the bore of the part to be heated, there are many application in which the above bearing heater type approach is not feasible.

Solid state MF and RF heaters

In those cases where operational complexities negate the use of a cored mains frequency approach, the standard RF or MF induction heater can be utilised. This type of unit uses turns of copper tube wound into a 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 utilise the range between 1 kHz and 100 kHz.

In general terms, it is best to utilise 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
Thermal energy
Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....

 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 utilise finite element analysis and other computer-aided manufacturing
Computer-aided manufacturing
Computer-aided manufacturing is the use of computer software to control machine tools and related machinery in the manufacturing of workpieces. This is not the only definition for CAM, but it is the most common; CAM may also refer to the use of a computer to assist in all operations of a...

 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.

Output frequency

As shrink fitting requires a uniform heating of the component to be expanded, it is best to try to utilise the lowest practical frequency when approaching heating for shrink fitting. Again the exception to this rule can be when removing parts from shafts.

Industries and applications

There are a huge number of industries and applications which benefit from induction shrink fitting or removal utilising solid state RF and MF heaters. In practice, the methodology employed can vary from a simple manual approach where an operator assembles or disassembles the parts to fully automatic pneumatic and hydraulic press
Hydraulic press
A hydraulic is a machine using a hydraulic cylinder to generate a compressive force. It uses the hydraulic equivalenta mechanical lever, and was also known as a Bramah press after the inventor, Joseph Bramah, of England. He invented and was issued a patent on this press in 1795...

 arrangements.
  • Automotive starter rings onto flywheels
  • Timing gears to crankshafts
  • Motor stators into motor bodies
  • Motor shafts into stators
  • Removal and re-fitting of a gas turbine
    Gas turbine
    A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....

     impeller
    Impeller
    An impeller is a rotor inside a tube or conduit used to increase the pressure and flow of a fluid.- Impellers in pumps :...

  • Removal and re-fitting of hollow bolts in electrical generators
  • Assembly of high precision roller bearings
  • Shrinkfitting of 2-stroke crankshafts for ship engines

Advantages & disadvantages

Advantages:
  • Process controllability - Unlike a traditional electric or gas furnace the induction system requires no pre-heat cycle or controlled shutdown. The heat is available on demand. In addition to the benefits of rapid availability in the event of a downstream interruption to production, the power can be switched off thus saving energy.
  • Energy efficiency - Due to the heat being generated within the component energy transfer is extremely efficient. The induction heater heats only the part not the atmosphere around it.
  • Process consistency - The induction heating process produces extremely uniform consistent heat this often allows less heat to be used for a given process.
  • No naked flame - This allows induction heating to be used in a wide variety of applications in volatile environments in particular in petrochemical applications.


The main disadvantage of this process is that, in general, it is limited to components which have a cylindrical shape.

External links

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