Shrink fitting uses temperature to change the dimensions of parts so that components fit and hold together. This joining technique is used in sub-assemblies to create a tight, interference fit between inner and outer components so that relative motion won’t occur or force can be transmitted from one part to another. Common shrink fitting applications include shafts in wheels and rotors, liners in engine cylinders, bearings and bushings in housings, and valve seats and valve guides in engine blocks.
Compared to traditional fastening or press fitting and assembly methods, shrink fitting provides important advantages. For example, parts that are shrink fit require less component machining since there’s no need for turning to produce keyways or threads. Assembly is also simplified because there are no keys to fit, screws to turn, or welds to make. Unlike pressing, shrink fitting won’t put excessive pressure on parts either. That’s especially important with shafts, where pressure-induced distortion can lead to run-out.
Induction Shrink Fitting vs. Cryogenic Shrink Fitting
Traditionally, shrink fitting has involved heating a cylindrical outer component with an open-end or hole so that this opening expands enough to accommodate an inner part with a smaller diameter. Known as induction shrink fitting, this technique relies upon thermal expansion, a fractional increase in a material’s size in response to an increase in temperature. Yet, size isn’t the only thing that can change. With metal parts, the application of heat can alter structural, electrical, and magnetic properties.
Cryogenic shrink fitting uses cooling instead of heating for minimal (if any) metallurgical changes. Unlike induction-based methods, cryogenic treatment relies upon thermal contraction, a reduction in a material’s size in response to a decrease in temperature. Importantly, these dimensional changes can be held to very tight tolerances – and for parts of various sizes. With Nitrofreeze® cryogenic shrink fitting, the parts we’ve processed have ranged from miniature components to large-diameter pipes.
How Compression Shrink Fitting Works
Cryogenic shrink fitting, or compression shrink fitting as it’s also called, cools the smaller part (the insert) to a very low temperature. The cryogen, the substance used to produce this temperature, is typically carbon dioxide (solid or liquid) or liquid nitrogen. The time that’s required for cooling varies and is a function of many factors, including the part’s material. With some metals, processing before or after cryogenic treatment may be required. For example, steel may be subject to additional transformation when exposed to cryogenic temperatures.
For especially demanding applications, such as those that require the tightest tolerances, compression shrink fitting may require cooling the inner part and heating the outer part. A bushing inside of a housing offers an example. The inner part, the precision bushing, is cryogenically treated. The housing, the outer part, is heated. After the bushing has reached equilibrium and the housing has been heated, the parts are fitted together to create an assembly. Because heating the housing can induce thermal stresses, it’s important to choose a compression shrink fitter with the right experience and expertise.
Choose Nitrofreeze® Cryogenic Shrink Fitting
Nitrofreeze® of Worcester, Massachusetts (USA) has been providing shrink fitting services since our inception in 2002. We’ve completed hundreds of compression shrink fitting jobs for U.S. Department of Defense contractors, research laboratories, and multinational corporations in the transportation industry, including air, railroad, and trucking companies. Whether you need only cryogenic shrink fitting or both heating and cooling, we’re ready to help. Depending on what you need, we can shrink fit your parts in-house or at your facility. To learn more, contact Nitrofreeze® at the phone number and email below.
(508) 459-7447 x 109 | mailto:firstname.lastname@example.org