Interference fit is a method of joining parts together by creating a joint where the two parts are slightly different in size, creating friction that holds them together. The strength of the bond can be controlled by adjusting the amount of interference. Thermal expansion can also be used to create an interference fit. Proper fit is important for applications such as bearings and shafts to prevent misalignment and excessive vibration.
An interference fit, also called a press fit or shrink fit, is a type of connection method used to join parts in products or structures together. In this type of joint, the two parts to be connected are slightly different in size and this discrepancy holds the pieces together. This fastening method is common in many manufacturing and assembly processes. When you use an interference fit, you create rigid, semi-permanent bonds between components.
The location of the interference fit is called a joint and is the area where two parts are joined together. Making the joint position of the first part slightly larger than that of the second part creates an interference fit that joins the two parts together. Since the joints of the two parts are not the same size, they must be pressed together hard. 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 benefits of an interference fit is that many times no adhesives or screws are needed to hold the parts together.
Increasing or decreasing the interference fit in the joint can control the strength of the bond or the strength with which the parts are held together. Changes in the amount of interference are achieved by making the dimensions where the two parts are joined larger or smaller. The amount of interference between the parts, along with the material of the parts, largely determines 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, the parts can simply be pressed together by hand. As the amount of interference increases or the material of the parts becomes less pliable, more force will be required to join them together. Large joining forces may require a hydraulic press or some type of machine that creates a mechanical advantage, such as a shaft press, to force one part into the other.
Another way to create an interference fit is with thermal expansion. Solid materials generally expand as they get hotter and contract as their temperature decreases. As a result, the parts can be heated or chilled prior to assembly so that they can join easily. The parts are then allowed to return to their normal temperature, creating the bond between the two. A good practice with this method is to make sure that the pieces being joined are made of the same or a similar material so that they grow and shrink in unison as the temperature changes. Because different materials have different thermal expansion and contraction characteristics, changes in temperature can cause the bond to become too tight or too loose.
Common applications of an interference fit include inserting various shapes into holes and shafts in bearings. An improper fit can cause misalignment between parts, slippage or, in the case of moving parts, excessive vibration. Proper fit is especially critical in the case of a shaft in a bearing or pair; many times this setup is used to transmit torque and if the fit is not right, the device will not work properly.
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