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With shrink fitting of mechanical precision processings we refer to the operation of union/assembly of two complementary components, with the aim to define an interlocking connection; usually, when talking about shrink fitting, it refers to shafts and joining elements and gears, for example a shaft with its nut or screw, or with rolls and shells.
There are two types of shrink fitting, hot and cold, depending on the temperature at which the process takes place.
Heat shrinking is used when the thermal expansion of the joint position is necessary, to perform the coupling and overcome the dimensional interference of the components; the expansion occurs in a uniform way, thus avoiding colder areas that would prevent or impair stable coupling.
Essential is, therefore, an accurate control of the temperature, which will change depending on the geometry, size and weight, and the materials of the two components, as too elevated temperatures could lead to a structural change of the components.
During this type of procedure, the client is always invited to attend, providing drawings for the assembly of the complete group and the interferences.
Cold shrink fitting, or cryogenic shrinking, usually occurs at about -196 °C, by immersing the pieces to be processed in liquid nitrogen contained in a special container; in contact with liquid nitrogen, the piece cools and contracts, while the liquid is boiling, only to calm down when the piece and liquid nitrogen reach the same temperature. This technique is used for the single or series production of parts, when shafts must be assembled with toothed wheels or levers, or when there is a need to have solid interference positions, and may concern components formed from ferrous and non-ferrous metals.
In these procedures, the geometries and the structure of the component materials remain unchanged, oxidation layers are avoided, and it is a simple and fast practice, requiring low investments.
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