Interference fit
What is an interference fit?
An interference fit is a type of fit in which two machined parts are joined so that there is a deliberate oversize, or interference, between them. In practice, this means that the shaft diameter is slightly larger than the diameter of the hole it fits into. When the parts are assembled—either by pressing them together or by using a temperature differential—contact pressure forms between the surfaces, allowing torque and axial forces to be transmitted without separate fastening elements. The interference fit is classified in the ISO fit system and differs from a clearance fit and a transition fit in that the connection is permanently press-fit.
Interference fit in machining and dimensioning
In the machining industry, interference fits are designed using a tolerance system. Typically, tolerance classes and fit pairs according to ISO 286 are used, such as H7/p6, H7/s6, or H7/u6, depending on the required pressing force and the load involved. The hole basis system is the most common, where the hole is held to, for example, H7 tolerance, and the shaft’s tolerance zone is varied to achieve the desired interference.
The amount of interference is specified to micrometer precision, depending on factors such as material, diameter, operating temperature, and the torque to be transmitted. Too little interference can lead to slipping, while too much can increase assembly forces and cause detrimental stresses or deformation. For this reason, the machinist must also consider surface roughness, measuring methods, and coefficients of thermal expansion during the design and manufacturing stages.
Methods for achieving interference fits
Interference fits can be achieved in several ways, depending on the size of the part, material, and production volumes.
In a thermal fit, the thermal expansion of materials is utilized. Typically, the part with the hole is heated, for example, by induction heating or in an oven, so its diameter temporarily increases. Alternatively, the shaft can be cooled, for example, with dry ice or liquid nitrogen. When the temperature equalizes, the parts press tightly together, forming a strong joint. This is also known as a shrink fit. In machine shops, this method is used, for instance, to secure cutting tools into machine tool holders using heat-shrink chucks.
In a press fit, parts are pressed together using a hydraulic or mechanical press. This method is common in machine shops, especially for installing bearings, gears, and bushings. The pressing creates a controlled elastic deformation that provides the required contact pressure.
Load transmission capability of an interference fit
The key feature of an interference fit is its ability to transmit torque and axial loads without keys, splines, or other mechanical locking devices. Torque transfer is based on friction force, which is the product of contact pressure and coefficient of friction. The higher the interference and contact area, the greater the load transmission capability.
In the machining industry, interference fits are often used as an alternative to keyways when a uniform stress distribution is desired without notch effects. For larger loads, an interference fit and a keyway may also be combined for added security.
Applications of interference fits in machine shops
Interference fits are common in machined assemblies where strength, precision, and vibration resistance are required. Typical applications include mounting bearings on shafts or in housings, fastening gears and pulleys to shafts, and various bushing and hub connections. Interference fits are also a key joining method in electric motors, transmissions, and pump assemblies.
In heavy-duty machining industries, such as process and energy technology equipment, interference fits are often used for large-diameter connections, where a thermal fit is usually the most practical solution.
Advantages and limitations of interference fits
The main advantage of an interference fit is its simple structure. The connection does not require screws, nuts, or welding, which reduces the number of components and possible failure points. In addition, the load is distributed evenly across the entire contact surface, which improves fatigue strength when properly dimensioned.
The limitation is the high requirement for manufacturing accuracy. Machining, measurement, and surface roughness must be well controlled to achieve the planned interference. Disassembly can be difficult and may require heat treatment or special tools. Excessive interference can cause cracks, plastic deformation, or residual stresses that weaken the long-term durability of the structure.
Summary
An interference fit is a widely used joining method in the machining industry, where parts are connected by controlled interference without separate fastening elements. When properly dimensioned and machined, it enables efficient torque and load transmission, good concentricity, and high reliability in demanding machining and assembly applications.
