Choosing a fit instead of guessing in the table
Why the bore is almost always the basis
The hole-basis system puts the bore on H and varies the shaft. The reason is tooling cost. A bore is made with a reamer, a broach or a boring head in fixed steps, while a shaft can be turned and ground to any size. Nail down the bore and adapt the shaft, and you need one tool instead of ten.
Shaft basis exists too, and it belongs wherever one length of drawn bar carries several components: conveyors, agricultural machinery, anything built on bright steel from stock. Then the shaft is h and the bores take the letters. If you mix both systems in one assembly, say so on the drawing, or production will call.
The number after the letter is the tolerance grade, and it decides the price. IT7 in the bore means reaming, IT6 on the shaft usually means grinding. One grade finer is rarely a small change; it is normally a different process.
Four questions that lead to the pairing
Instead of leafing through tables, a short chain of decisions helps. It rules out candidates in seconds.
- Do the parts move relative to each other? If yes, it is a clearance fit and the only remaining question is how much clearance. Plain bearing bush, guide pin, thrust washer.
- Does the joint have to centre? Then the clearance must not exceed the runout you are asking for. That rules out generous clearance fits immediately.
- Will the joint be taken apart again? An interference fit that service is expected to break annually is a design fault. A transition fit plus a retaining feature is usually the better answer.
- Does the joint transmit torque? If it does, and without a key, there is no way around an interference fit, and the tolerance is only where the calculation starts.
The pairings you actually need
All figures for Ø 30 mm, hole basis, bore H7 running from 0 to +21 µm.
| Pairing | Clearance or interference | Typical use |
|---|---|---|
| H8/f7 | 20 … 74 µm clearance | plain bearings with an oil film, larger shafts, uncritical guides |
| H7/f7 | 20 … 62 µm clearance | sliding fit with reliable lubrication, bearing bushes |
| H7/g6 | 7 … 41 µm clearance | close-running guides, pistons, slides |
| H7/h6 | 0 … 34 µm clearance | location without motion, assembled by hand, cover spigots |
| H7/k6 | 15 µm interference … 19 µm clearance | transition fit, locates well, light press to assemble |
| H7/n6 | 28 µm interference … 6 µm clearance | tight transition fit, gear on a shaft with a key |
| H7/p6 | 35 … 1 µm interference | press fit, bearing inner ring, bush in a housing |
Calculated with the ISO fits calculator to DIN ISO 286. Other nominal sizes give other numbers.
That last sentence matters more than it looks. The same H7/g6 gives 7 to 41 µm of clearance at Ø 30 mm and 10 to 59 µm at Ø 60 mm. Tolerance zones grow with nominal size, and not linearly. Copy a clearance figure from an old drawing onto a larger part and you have moved half its functional behaviour.
What the fit designation does not cover
Form
Two dimensions inside tolerance can still refuse to go together. A bore with 20 µm of roundness error touches at two points and has air next to them, even though every two-point measurement is correct. Interference fits and bearing seats therefore need a cylindricity tolerance, customarily about half the size tolerance. Without one, the fit is a promise about two points.
Surface
Assembling an interference fit flattens the roughness peaks. Part of the calculated interference is gone afterwards. DIN 7190 estimates the smoothing at roughly 0.8 times the sum of the peak-to-valley heights of both parts. With Rz 6.3 on the shaft and Rz 10 in the bore that is around 13 µm taken off the interference. For H7/p6 at Ø 30 with a minimum interference of 1 µm, nothing is left. More on this in the article on Ra, Rz and the fit.
Temperature
Fit values apply at 20 °C. A steel gear on a steel shaft keeps its seat at 80 °C as well; a steel gear in an aluminium housing does not. As soon as two materials meet, the fit is a function of operating temperature.
Transmissible torque
An interference fit transmits torque through friction at the joint. How much depends on the actual interference, the joint diameter, the joint length and the coefficient of friction, not on the combination of letters. Writing P7 on the drawing and hoping is not a calculation.
Three cases from practice
Rolling bearings
Rolling bearings follow their own logic, because the rings are already toleranced and enter the calculation as a standard part. The rule: the rotating ring gets interference, the stationary one may have clearance. With a rotating shaft and a stationary housing that means k6 or m6 on the shaft and H7 in the housing. If the housing rotates instead, it reverses.
Gear on a shaft
With a key the gear does not need an interference fit, it needs clean location without tilt. H7/n6 or H7/k6 are the usual answers, depending on whether it has to come apart again. The key transmits the torque, the fit keeps the gear square to the axis. Choose a clearance fit here and you get flank wear on one side.
Dowel pins
A parallel pin to ISO 2338 is supplied to m6 and the bore is reamed H7. That gives a transition fit. The pin locates, but without extra retention it takes no pull-out load. If the pin is meant to stay put for good, use an H7 bore and a grooved pin, or plan an ejection hole.
The price of the last ten micrometres
IT7 in a bore is reaming and passes unnoticed. IT6 on a shaft means grinding in most workshops: another setup, another machine, another pass. IT5 means fine grinding with an inspection report, and that is a statement rather than a tolerance.
So before every tight fit it is worth asking whether the function really needs it. Often what looks like a size problem is a location problem, and a runout tolerance on the right datum axis solves it more cheaply than one grade less.
Frequently asked questions
What exactly does H7/g6 mean?
H7 is the bore, g6 the shaft. The capital letter always refers to the internal feature, the lower-case one to the external feature. H means the lower deviation of the bore is zero; g places the shaft entirely below nominal size. The result is a clearance fit, at Ø 30 mm with 7 to 41 µm of clearance.
When transition fit and when interference fit?
A transition fit such as k6 or n6 locates the part, goes together with a press or gentle heating and comes apart again. It transmits no meaningful torque, so a key or a pin is needed for that. An interference fit such as p6 or r6 transmits torque through the pressure in the joint, is hard to assemble without heating and is practically impossible to separate without damage.
Do fits apply to plastic parts as well?
The limit deviations to ISO 286 apply regardless of material; the function does not. Plastics creep under sustained load, absorb moisture and expand five to ten times as much as steel. A calculated interference fit in POM can lose its grip after a few weeks under load. For injection-moulded parts, ISO 20457 is the more suitable basis.
Why do old drawings sometimes show only a number?
Before the ISO system, designations such as sliding fit, running fit or press fit were common, sometimes with company-specific number codes. There is no direct translation, because the old names described the function rather than the tolerance zone. When redrawing old parts, the only reliable route is to measure the clearance of an existing assembly and find the ISO pairing that matches.