A "fit" is the relationship between a hole and the shaft that goes in it — whether it slides, locates or has to
be pressed. ISO 286 codifies fits as a hole letter + grade and a shaft letter + grade, like H7/g6.
Getting the fit right is the difference between a bearing that spins freely and one that seizes.
The three families
Clearance fits always have a gap — for parts that must move or be assembled by hand (sliding covers, loose pins). Interference fits always overlap — the shaft is bigger than the hole, so it must be pressed or shrunk in and won't come out (bearing races, gears on shafts). Transition fits sit in between — sometimes a slight gap, sometimes a slight overlap — for accurate location where you'll still use a keyway or fastener to transmit torque (dowels, locating spigots).
Hole-basis: keep the hole constant
Holes are harder to make to size than shafts (you finish a shaft by turning or grinding; you're stuck with the
reamer for a hole). So the standard approach is hole-basis: fix the hole at H (its
lower limit exactly on the nominal size) and vary the shaft to get the fit you want. That's why you see H7 holes
everywhere. Get exact limits for any combination on the Limits & Fits tool.
A shaft turns freely in its bore at Ø25 H7/g6. The design is scaled up and the same joint becomes Ø100 H7/g6. What happens to the minimum clearance?
Minimum clearance goes from 7 µm to 12 µm, and the maximum from 41 µm to 69 µm, because both the fundamental deviation and the IT grade widen with nominal size. A fit code names a behaviour, not a gap. H7/g6 promises that the parts will always slide and never press — it does not promise a number, and the number is what your oil film, your runout budget and your leak rate actually care about. Look it up every time; do not carry a remembered clearance across a size change.
Try it — the two tolerance zones
A shortlist that covers most work
H7/h6 — precise location, hand-slide, easily separated. H7/g6 — a running/sliding
fit with a small guaranteed clearance (bushings, spindles). H7/k6 — transition, light tap to
assemble (accurate location, use a key for torque). H7/n6 — tighter transition, needs a press.
H7/p6 or s6 — interference for press/shrink fits. There's a ready reference on the
fits chart.
Designing a press fit
For interference fits, the interference isn't the whole story — it creates a contact pressure that both grips the shaft and stresses the hub. Too little and it slips; too much and the hub cracks. Size it with the Press/Shrink Fit tool, and remember thermal expansion can loosen a shrink fit in service.