An interference fit, also called a press fit or shrink fit, is a type of connection method used to join parts together in products or structures. In this type of joint, the two parts being connected are of slightly different sizes, and this discrepancy holds the pieces together. This method of attachment is common in many manufacturing and assembly processes. Stiff, semi-permanent bonds are created between the components when an interference fit is used.
The location of the interference fit is called a joint, and it is the area where two parts are joined together. Making the first part's joint location slightly larger than that of the second part creates an interference fit joining the two parts. Since the two parts' joints are not the same size, they must be pressed together with force. It is actually the friction created by the interference of the two parts at the joint that holds the two pieces together. One of the advantages of an interference fit is that many times no adhesives or screws are needed to hold the parts together.
Increasing or decreasing the interference at the joint can control the strength of the bond or how tightly the parts are held together. Changes in the amount of interference are accomplished by making the dimensions where the two parts are joined larger or smaller. The amount of interference between parts, along with the material of the parts, largely dictate the assembly method that will be used.
If the amount of interference between the two parts is not that great or their materials are flexible, then parts can simply be pressed together by hand. As the amount of interference increases or the material of the parts becomes less pliant, greater force will be required join them together. Large joining forces may require a hydraulic press or some type of machine that creates a mechanical advantage, such as an arbor press, to force one part into the other.
Another way to create an interference fit involves thermal expansion. Solid materials generally expand as they grow hotter and contract as their temperature decreases. As a result, parts may be heated or refrigerated before assembly so that they can go together easily. The parts are then allowed to return to their regular temperature, creating the bond between the two. A good practice with this method is to make sure that the pieces to be joined are made of the same or similar material so that they will grow and shrink in unison as the temperature changes. Since different materials have different thermal expansion and contraction characteristics, changes in the temperature may cause the bond to become too tight or too loose.
Common applications of an interference fit include inserting various shapes into holes and shafts into bearings. An improper fit can result in misalignment between parts, slippage, or in the case of moving parts, excess vibration. The correct fit is especially critical in the case of a shaft into a bearing or couple; many times this configuration is used to transmit torque and if the attachment is not correct, the device will not function properly.
