Every IT grade you tighten roughly doubles the machining effort on that feature. Yet we still receive drawings where a locating diameter that never sees a bearing carries h5. This guide walks through the three fits that solve 90% of drive-component cases — with the actual limit deviations and what they cost.
1. Why fit class drives cost
An IT7 diameter can usually be finished in one turning operation. IT6 needs grinding. IT5 needs grinding plus temperature-stabilised measurement — and a scrap rate that rises steeply with every micron you remove from the band. On a typical Ø30 shaft seat, moving from h7 to h5 multiplies the feature cost by a factor of three before a single quality document is written.
The engineering question is therefore never "how tight can we get it?" but "what does the mating part actually require?" A deep-groove ball bearing is happy on h6/j6. A labyrinth seal carrier is not a bearing — k7 locates it perfectly well at a fraction of the cost.
"The most expensive tolerance is the one nobody can justify in the design review."
— Werksnorm PW-QN 07, Design Review Guideline
2. The three fits that cover 90% of cases
For rotating shafts in general machine building, three shaft fits do almost all of the work: h6 for bearing seats and precision location, g6 where the part must slide during assembly or operation, and k6 where you want light interference to eliminate fretting under alternating load.
3. Limit deviations: the numbers
For a nominal diameter of 30 mm (the 18–30 mm ISO band boundary is at 30 inclusive), the limit deviations are:
| Fit (Ø30) | Upper dev. | Lower dev. | Typical use |
|---|---|---|---|
| h6 | 0 | −13 µm | Bearing seats, location |
| g6 | −7 µm | −20 µm | Sliding hubs, spline pilots |
| k6 | +15 µm | +2 µm | Light interference, anti-fretting |
| h7 (ref.) | 0 | −21 µm | Non-critical location |
Praxis-Tipp
If the mating part is bought (bearing, coupling, seal), copy the fit recommendation from its datasheet instead of deriving your own — the manufacturer has already validated it across the temperature range. Specify your own fit only for part pairs you design on both sides.
4. Shop-floor practice
In our production, every h6 seat is finish-ground and measured at 20.0 °C ± 0.2. The protocol reports the actual value, not just "pass" — so your assembly team knows whether a seat sits at the top or bottom of the band before they choose the bearing from their own tolerance stock. That single habit has eliminated thermal-mounting rework for three of our OEM customers.
5. Specification checklist
- Identify every diameter with a mating part — only these need an ISO 286 fit at all.
- Take the fit from the mating part's datasheet where one exists.
- For your own part pairs: h6/g6/k6 first; justify anything tighter in the design review.
- Add a concentricity requirement only between features that rotate against each other.
- Request actual-value measurement protocols for seats you thermally mount.
Dipl.-Ing. Markus Krüger
Head of Quality · 21 years at PRÄZIWERK · Guest lecturer, HS Düsseldorf
Markus leads the measuring room and writes the Werksnorm design guidelines. Questions on this article: m.krueger@praeziwerk.de
