Interference fits, commonly referred to as press fits and shrink fits, are traditional methods for joining cylindrical parts. In an interference fit, the inner part—such as a shaft, bearing or bushing—is machined to be bigger than the mating hole of the outer part—such as a gear or housing. The larger part is then forced into the smaller part. Both parts deform slightly to fit together and extremely high friction results between them. The friction is so great that even large amounts of torque cannot separate the parts. They operate as one.
For maximum joint strength and optimal performance, interference fits must be highly precise. Engineers go to great pains to calculate and achieve the appropriate allowance that will provide maximum friction between the parts. This dimensional precision increases both the cost and the time required for component production.