The shrink-fit concept is used in manufacture of spark plugs, where a compliant layer is used to seal the metal base to the ceramic insulator. In this case, the metal base is heated and formed around the ceramic, with an interface material (a low modulus material such as aluminium) forming a tight joint as it cools.
To maintain an interference fit during service, it is important that the part operates over a small temperature range, or that the ceramic and metal components have similar thermal expansions.
Another example of shrink fitting is in use of zirconia in metal extrusion dies. In this case the ceramic is shrink fitted into the metal holder which is then mounted to the extrusion apparatus.
The key to successful shrink fitting is to avoid a design where the ceramic is only partially inside the metal retainer. When this happens, high tensile stresses can occur in the ceramic where it extends from the metal component. These stresses can cause failure during assembly or later when combined with normal operating stresses.
In summary, the advantages and disadvantages of mechanical attachment are:
- actual fixing is quick, cheap and simple;
- little operator training is required.
- cost of manufacturing parts is high;
- complex machining of parts may be required;
- joints have relatively poor strength, fatigue and vibration resistance;
- joints are not leak tight.