A shrink fitting is a very old technological solution. It has been successfully used for many years in many fields of technology. This type of joint is classified as a non-separable fit. The essence of this method is the interference fit, i.e. the hub bore of a gear wheel has a smaller diameter than the shaft on which the wheel will be mounted.
The assembly of such a fitting involves varying the temperature of one or both of the components to be assembled in order to obtain sufficient assembly clearance. Once the temperatures have equalized, a properly designed fitting is considered inseparable and its properties correspond to a monolithic part. On the calculation side, shrink fits are calculated in a similar way to press fits. The difference between these joints is that a correctly designed press fit can be disassembled nondestructively. The reason why shrinkage fit cannot be disassembled is due to two phenomena. The first is the occurrence of micro-scale joints between the two connected parts. In press fits these are partially smoothed during the assembly process. The second phenomenon is oxidation of the surface, which is prevented during press fitting by means of oil coating. Meanwhile, during shrink fitting, the oxide layer on the metal surface is already formed when one of the components is heated. After assembly, the oxidation process continues until all the oxygen in the joint is used up. Since the volume of the oxides is greater than that of the parent material, the interference is even greater.
In some areas of industry, shrink fitting is a basic technology. It has been used for many years in the production of firearm barrels, in railway wheel sets and in manufacturing of internal combustion engines, where it is applied for seating valve seats and guides in engine heads, and to fix starter gear rims on the flywheels. My opinion, based on my experience with documentation of gearboxes, is that this type of fitting is too rarely used in this area. It eliminates keyed or spline joints and provides superior load carrying capacity, similar to the monolithic parts. A technology without a need of notching mating slots in both parts is much less expensive. For heat-treated gears, the low tempering operation is ideal for mounting the wheel on the shaft without requiring additional energy consumption. Heat-treatment purists are sure to be outraged because the gear wheel should be tempered immediately after hardening. I dare not protest, but in the company where I worked for many years, shrink fitted gears had the bore ground after hardening and only then went into a low tempering completed with mounting on the shaft. The final operation was to grind the teeth, based on the previously ground bearing surfaces on the shaft. This ensured that the runout of the gears relative to the bearing locations was kept to a minimum. In spite of this technology with such non-state-of-the-art tempering, we had no quality problems with these wheels due to micro-cracks. If, however, an immediate post-hardening tempering is necessary for a certain material type, it is either necessary to use a double tempering or to check whether it is possible to assemble the fitting by just cooling the inner component.
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Probably it is the unavailability of literature and proven calculation methods that prevents a wider use of this type of fitting. However, there is some amount of knowledge, only scattered. I managed to gather some of it and supplement it with my own and my colleagues’ experience. Also, I have not noticed any more recent development of this method, while there were certain developments in technologies that can expand the area of its application. I have included some developmental suggestions in the concluding remarks of this paper. An example of a successful application of such a fitting in a drivetrain is the mining roadheader’s gearbox shown above, in which the output shaft has been converted from a monolithic shaft to a gear and shaft using a shrinkage fitting. Previous attempts to use a spline in this location had failed.