Choosing a Fits-and-Tolerance Scheme for a Shaft-Hub Assembly
A workshop must produce 200 shaft-and-hub assemblies for a small gearbox. The shaft nominal diameter is 40 mm. The hub must locate accurately on the shaft and transmit a light torque through a key, but must also be removable for maintenance with a hand press, not a hydraulic press. Available reamers and ground shaft stock limit practical tolerances to IT7 for the hole and IT6 for the shaft. The trainee proposes an interference fit (H7/p6) because "it will never slip". The supervisor points out that the maintenance procedure requires disassembly in the field with a simple puller, and that the key already carries the torque. The workshop must select a fit and specify the limits for both parts.
1. Evaluate the H7/p6 proposal against the functional requirements and recommend a more suitable fit from the basic hole system.
Read the worked answer
H7/p6 is a force/interference fit. It would grip the shaft firmly and transmit torque by friction, but it requires a press or heat for assembly and a heavy puller or press for removal. The stated requirement is hand-press removal in the field, and the key already transmits the torque. So the interference is unnecessary and actively harmful to maintainability.
Better choice: a transition or close-clearance location fit in the basic hole system, e.g. H7/k6 (transition, light press, locates well) or H7/js6 (transition, near-zero clearance). If hand-press only is required and the key does all torque work, H7/k6 gives reliable concentric location with a light press fit that a simple puller can break.
Reasoning: the fit must (a) locate the hub concentrically, (b) not require a hydraulic press, (c) not rely on friction for torque. H7/k6 satisfies all three; H7/p6 fails (b) and is over-specified for (c).
2. For the chosen H7/k6 fit at 40 mm, state the nature of the limits (hole and shaft) and explain how the fit behaves at its extremes.
Read the worked answer
For a 40 mm H7 hole, the tolerance zone starts at the basic size and goes positive: lower deviation 0, upper deviation +25 µm (IT7 = 25 µm at this size range). So hole limits are 40.000 / 40.025 mm.
For a 40 mm k6 shaft, the zone straddles zero: the fundamental deviation for k at this size is about +2 µm, and IT6 = 16 µm, giving 40.002 / 40.018 mm.
Behaviour at extremes:
- Maximum clearance = max hole − min shaft = 40.025 − 40.002 = 0.023 mm.
- Maximum interference = min hole − max shaft = 40.000 − 40.018 = −0.018 mm (i.e. 0.018 mm interference).
So the fit ranges from 0.023 mm clearance to 0.018 mm interference - a true transition fit. It locates the hub accurately, assembles with a light press or mallet, and a simple puller can break the small interference. The key carries torque, so friction is not relied upon. The technician should confirm the exact deviation values against the standard tolerance table before issuing the drawing, since values vary slightly by size range.