Robin A. Rhodes
Cryogenic Institute of New England, Inc.
rrhodes@nitrofreeze.com
Shrink fitting, (or “compression fitting” as it is sometimes called), is a method used to insert a pin or bushing into a housing or other assembly requiring an extremely tight tolerance fit. It can be used as an alternative to conventional press fitting, or more likely, to permit a mechanical fit that otherwise could not be accomplished via the mechanical force of press fitting.
In application, the insert is cooled via exposure to a cryogen, typically carbon dioxide (solid or liquid) or liquid nitrogen in order to reduce its size through the contraction usually associated with reduced temperatures. A companion operation of heating the housing (or other part) that receives the insert is conducted to enlarge the opening by taking advantage of the expansion usually associated with increased temperatures.
While it is not always necessary to use both heating and cooling in combination, the most demanding applications, including those with the tightest tolerances, often require this multi-step approach.
Care should be taken when using any cryogen and consideration of the material and relative masses needs to be considered carefully. In addition, because steels may be subject to additional transformation when exposed to cryogenic temperatures, additional processing either before or after may be warranted. Heating of metals (for expansion) should also be controlled and not induce unnecessary thermal stress on the component. For these reasons, many companies and individuals seek expert advice or contract with an experienced service provider for their compression shrink fitting requirements.
