What term describes the situation when the hole is smaller than the shaft requiring pressure to fit the parts together?

The term that describes a situation when the hole is smaller than the shaft, necessitating pressure to fit the parts together, is known as interference fit. In engineering, an interference fit (also referred to as a press fit) occurs when two parts are designed to fit tightly together without any clearance. This fitting method ensures that the parts remain securely mated under operational conditions, which is achieved by using the elastic deformation of the materials.

When the hole is intentionally made smaller than the diameter of the shaft, external force is needed to assemble the two components. This design technique is often employed in applications where strong connections are critical, such as in gears or bearings, to prevent relative motion between the components once they are assembled.

The other terms do not accurately describe this specific situation. Negative tolerance refers to a condition where the actual dimension is less than the nominal dimension but does not specify the context of fitting parts together. Negative allowance relates to the permissible limits of variation in a dimension but lacks the implication of the tight fit required. Negative fits can signify an overall category of fit but are not as descriptive as the specific interference fit in practical applications. Hence, "interference of metal" aligns directly with the requirement of force-fit assembly due to size discrepancy.