Shrink Fitting in Engineering

In this article, we take a look at how the use of liquid nitrogen can enable versatile and reliable shrink fitting.

A Large Peg in a Small Hole

In engineering, an interference fit (also known as a friction fit) is a very tight fit formed between two mating parts with negative geometrical clearance – i.e., where the ‘inner’ part (i.e., a shaft) is larger than the hole it fits into. The huge amount of friction between interference-fitted parts results in an incredibly strong joint, suitable for use in hard-wearing industrial applications where force or torque will be transmitted through the joint. For example, in train wheelsets, the wheels are commonly joined to the axles via interference fit.¹

Interference fits can be achieved in two ways: via the application of force, or by exploiting thermal expansion and contraction effects in a process known as shrink fitting.

Creating an interference fit via the application of force in the form of cold pressing finds application in the joining of hubs, bearings, bushing and retainers. Even stronger interference fits can be achieved with hot pressing or via the application of extremely large forces: these methods can be used, for example, for the permanent coupling of gears and shafts. Achieving interference fits via the application of large forces has its limitations; however, these forces risk unwanted deformation to the components being joined.²

In addition, the tightest interference fits cannot be achieved simply by applying force. At the extreme end of the interference fitting spectrum lies a class of fits known (perhaps confusingly) as forced fits. Forced fits are high-interference fits that cannot be disassembled without risking the destruction of the parts involved. These super-secure fits can typically only be achieved via shrink fitting techniques.

The Shrink Fitting Process

Shrink fitting refers broadly to techniques in which thermal expansion and contraction effects are used to achieve an interference fit. Commonly, this involves the heating of the ‘outer’ part so that it undergoes thermal expansion. The two pieces can then be relatively easily fitted together. As the outer part returns to room temperature it contracts back to its original dimensions, forming an incredibly tight joint around the ‘inner’ part.

Shrink fitting can also be achieved via cryogenic means: cooling the ‘inner’ part using liquid nitrogen causes it to contract so it can be fitted into the ‘outer’ part, after which it expands as it returns to room temperature to form a tight interference fit. For many materials, cryogenically shrink fitting, rather than using heat, offers advantages as high temperatures can cause grain growth and loss of desirable mechanical properties in the heated part.³

Liquid nitrogen shrink fitting is therefore a vital process for safely creating very strong interference fits between parts, in cases where pressing or heating them may cause damage. This process is ideal for most components that require a precise and reliable interference fit, such as bearings, sockets and shafts. Current research applications for shrink fitting include nuclear reactor vessels, gear mechanisms and crank shafts in ships.^4–6

Liquid Nitrogen for Cryogenic Shrink Fitting

Compared to other cryogenic applications, shrink fitting requires a small volume of liquid nitrogen – typically less than 50 L per month – which doesn’t warrant the installation of a liquid nitrogen tank onsite. Air Products now offers a dewar-fill service to provide a versatile and low-cost solution for cryogenic shrink fitting applications. We offer 30 L or 50 L dewards for purchase or rental, and delivery of liquid nitrogen at a frequency to suit any requirements.

Click here to find out which dewar size and delivery schedule are right for your business.

References

1. Type A Wheelset Press-Fitting Machine - Railway Technology. https://www.railway-technology.com/contractors/bogies/railteco/pressreleases/type-wheelset-press-fitting-machine/.
2. Preferred Mechanical Tolerances Metric ISO 286 | Engineers Edge | www.engineersedge.com. https://www.engineersedge.com/manufacturing/preferred_mechanical_tolerances_metric_iso_286_13166.htm.
3. Król, R. & Siemiątkowski, Z. The analysis of shrink-fit connection – the methods of heating and the factors influencing the distribution of residual stresses. Heliyon 5, e02839 (2019).
4. Lee, J., Park, J. & Cho, Y. A novel ultrasonic NDE for shrink fit welded structures using interface waves. Ultrasonics 68, 1–7 (2016).
5. Eyercioglu, O., Kutuk, M. A. & Yilmaz, N. F. Shrink fit design for precision gear forging dies. Journal of Materials Processing Technology 209, 2186–2194 (2009).
6. Siemiątkowski, Z., Rucki, M. & Lavrynenko, S. INVESTIGATIONS ON THE MODELED SHRINK-FITTED JOINTS OF ASSEMBlED CRANKSHAFTS. Journal of Machine Construction and Maintenance 108, 33 (2018).

This information has been sourced, reviewed and adapted from materials provided by Air Products PLC.

For more information on this source, please visit Air Products PLC.