Shrink Fit Calculator | Interference Fit Guide

Shrink fitting is a method of joining a shaft and a hub through the use of thermal expansion and the contraction of the material. Both the shaft and the hub is manufacture with the shaft having a larger diameter than the hub. The difference in those two diameters is referred to as an interference.

Because the shaft has a diameter larger than the hub, the shaft will not fit into the hub at room temperature. In order to join the two component, though, the temperature of either the shaft or the hub must be changed. Heating the hub will allow the hub to expand, its bore will become larger, and the shaft can be inserted into the hub.

How Shrink Fitting Joins a Shaft and a Hub

Once the hub returns to room temperature, though, the hub will contract and lock the shaft into the hub. Interference is a critical factor in determining how the two components will join. Too little interference will result in a loose joint between the two components; too much interference will require too much force to assemble the components, which may lead to the hub cracking or the metal galling upon assembly.

For instance, light interference is often around 0.0005 inches times the diameter of the component, such as a pulley. Heavy interference can be 0.002 inches or more for component like gears. This interference will determine the pressure that is applied to the shaft; the pressure on the shaft will allow for the transmission of torque between the two components.

The materials of the components are another important factor in the shrink fitting process. Each material has what is referred to as a coefficient of thermal expansion; the rate at which a material expand with heat. Steel is a material with a predictable coefficient of thermal expansion; it is often used for both the shaft and the hub.

Aluminum has a higher coefficient of thermal expansion than steel, which means that aluminum will expand at a more faster rate than steel if the aluminum component is heated. Thus, the temperature of an aluminum component hub should be lower than that of a steel component to avoid damaging the aluminum. Cast iron has a lower coefficient of thermal expansion than steel, while bronze has a higher coefficient of thermal expansion than steel.

Thus, different temperatures are used for these two materials then are used for steel. There are different methods of assembling the components. One method is to heat the hub alone.

Another method is to chill the shaft alone. Each method is useful for different type of components, heating the hub alone is useful for components that may be damaged by excessive heat. Finally, both methods can also be used at the same time; heating the hub and chilling the shaft.

This last method is helpful in that it requires a lower change of temperature for the components. Additionally, a light oil film can be used on the components to reduce the force that the press must apply. The pressure between the components can be calculated with the interference between the components and the properties of the materials.

For instance, stiff materials like steel will have more resistance to deformation than soft materials like aluminum. Additionally, the thickness of the hub walls will impact the amount of pressure that acts on the components; the thicker the walls of the hub, the more pressure that will act upon the components. Additionally, the contact length between the components can be calculated to determine the force that will act on the components.

If the force is underestimated, the individuals may get stuck in the assembly process. To account for this, many individuals would of apply a safety allowance of 10 to 20 percent to the calculated force requirement. Common error when shrink fitting components include overheating the hubs; this can lead to the warping of the component.

Additionally, if the component is heated with an uneven heat source, such as a torch, hot spots can form on the component. These hot spots may cause the components to not be fitted in an even manner. In either case, the size of the hub bore should be measured while the component is hot, as the bore will contract with the cooling of the component.

Additionally, the temperature of the ambient environment in which the components are assembled should be considered. If the environment is cold, the components will reach their room temperature more quickly than if the environment is warm. If problems are encountered during the shrink fitting of the components, the interference between the components can be checked.

If the interference is negative, that means that the temperatures of the components are too low to create an effective interference fit. High forces during the fitting of the components may be due to the thick walls of the hub or the lack of dryness of the component surface; thinning the hub or adding lubrication to the component may help resolve this problem. Additionally, if the thickness of the hub walls are less than 10 percent of the diameter of the hub, it is possible that the component will crack; thus, the thickness of the hub walls should be verified prior to heating the component.

In general, shrink fitting the components requires the management of the thermal delta between the components, and that the technician heats the hub in an even manner to ensure even shrink fitting of the components.