Question 3 of 5: Clearance, transition and interference fits

Question 3: Clearance, transition and interference fits (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

A fit describes the relationship between the toleranced size ranges of two mating parts, conventionally a hole and a shaft, both toleranced about the same basic (nominal) size using the ISO/ASME limits-and-fits system (hole-basis system, symbol H for the hole).

Clearance fit. The shaft’s tolerance zone lies entirely below the hole’s, so the shaft is always smaller than the hole for every combination of permitted sizes — the parts can always be assembled and will always have some air gap and can move or rotate relative to each other. Application: a rotating shaft in a plain bearing/bushing (e.g. H7/g6), where free rotation with a thin, predictable oil-film gap is required.

Transition fit. The two tolerance zones overlap, so depending on where each individual part lands within its own tolerance, the assembled pair may have either a small clearance or a small interference — the fit is not guaranteed either way. Application: a dowel pin or a gear locating on a shaft (e.g. H7/k6), where accurate location/alignment is needed but the assembly must still be feasible without excessive press force, and may be light enough to disassemble for maintenance.

Interference fit. The shaft’s tolerance zone lies entirely above the hole’s, so the shaft is always larger than the hole for every permitted combination — the parts must be pressed or shrink-fitted together and, once assembled, transmit load through the resulting radial pressure at the interface rather than through a separate fastener. Application: a bearing outer race pressed into a housing bore, or a gear hub shrink-fitted onto a shaft to transmit torque without a key.