fitclearance

Fit clearance is the intentional space between two mating parts in a mechanical assembly that allows relative motion or easy assembly. It is the difference between the actual size of one feature (such as a hole) and the mating feature (such as a shaft), within the specified tolerance zones. In practice, fits with positive clearance allow parts to slide or rotate freely, while fits with zero or negative clearance can require pressing or force to assemble.

Fits are commonly categorized as clearance fits, transition fits, or interference fits. A clearance fit maintains some positive clearance under worst-case tolerances, ensuring easy assembly and rotation. A transition fit may have either a small clearance or slight interference, depending on manufacturing variation. An interference fit yields no clearance and typically requires force or heat/cooling to assemble.

Clearance can be described as radial (half the difference between diameters) or diametral (the full difference in diameters). The design of fit clearance is guided by standards and tolerancing systems, such as the ISO system of limits and fits, which defines tolerance zones and how hole and shaft sizes relate to each other. The choice of fit affects performance, durability, lubrication, and ease of assembly.

Manufacturing and measurement practices influence fit clearance. Common manufacturing methods include turning, milling, grinding, and honing, while metrology tools like micrometers, calipers, and go/no-go gauges verify that actual clearances meet design requirements. Proper selection of clearance is essential for reliable operation of components such as bearings, pins, gears, and fasteners.