Use of cryogenics for compression heat shrink fit

Cryogenic shrink connection, (or “compression connection” as it is sometimes called), is a method used to insert a pin or bushing into a housing or other assembly that requires an extremely tight tolerance fit. It can be used as an alternative to conventional press fit, or more likely, to allow mechanical adjustment that could not otherwise be achieved by the mechanical force of the press fit.

In application, the insert is cooled by exposure to a cryogen, typically carbon dioxide (solid or liquid) or liquid nitrogen to reduce its size through shrinkage normally associated with reduced temperatures. A complementary operation of heating the housing (or other part) that receives the insert is carried out to enlarge the opening taking advantage of the expansion generally associated with the increase in temperature.

While it is not always necessary to use heating and cooling in combination, the most demanding applications, including those with the tightest tolerances, often require this multi-step approach.

There are formulas that can be used to predict the rate of expansion and negative expansion (contraction) that materials will experience based on their thermal change. However, it should be noted that they are often based on measurements taken within a small range of temperatures close to ambient. As temperature changes move to extremes, whether hot or cold, the rate of expansion slows and the usefulness of the mathematical model as a predictor decreases.

In addition, care must be taken when using any cryogen and the material and relative masses must be carefully considered. Also, because steels can be subject to further transformation when exposed to cryogenic temperatures, further processing may be warranted sooner or later. Heating of metals (for expansion) must also be controlled and not induce unnecessary heat stress on the component or change its hardness or temper.

The Cryogenic Institute of New England, Inc., located in Worcester, MA, has successfully performed numerous shrink-fit work. These range from very large tubes, ranging in size over 24 inches in diameter, to very intricate miniaturized parts. Some of the clients that use the service include large multinational companies, US Department of Defense contractors, Research Labs, transportation companies, including trucking, air, and rail, to name a few.

Since time and transportation costs are often a consideration when using shrink fitting in an industrial application, several customers are looking to bring capacity in-house. This can be achieved with what is usually a modest investment in equipment, a local source of the cryogen of choice, and adequate safety equipment and training. Experts are available to help you set up an internal operation if that is what makes sense to you.