What is a fit, and why do drawings carry tolerances?
The relationship you get when a shaft goes into a hole is called a fit. You cannot machine both parts at exactly Ø50.000 mm — the machine, tool wear and temperature always move the size by a few microns. That is why the drawing puts a mark next to the size, such as Ø50 H7/g6. The mark says: keep the hole between these two sizes and the shaft between those two sizes, and the parts will seat the same way every time.
The result falls into one of three groups: the part turns freely, it is sometimes tight and sometimes loose, or it only goes on under force. Enter a diameter and two tolerance classes below and the tool works out the limit sizes for the hole and the shaft, the clearance or interference between them, and which of the three groups the fit belongs to.
ISO 286 Hole and Shaft Fit Calculator
Nominal diameter 1–500 mm. The hole side follows the H basic-hole system (lower deviation zero).
HOLE H7
SHAFT g6
Where the tolerance zones sit relative to the nominal size (the center line is nominal)
How to read H7/g6
Take Ø50 H7/g6 apart piece by piece:
- Ø50 — the nominal size. Both the hole and the shaft have a basic diameter of 50 mm.
- H — the position of the hole tolerance. A capital letter always describes the hole. H means the zone starts right at nominal: the hole can never be smaller than 50 mm.
- 7 — how accurate the hole must be (the IT grade). The bigger the number, the wider the tolerance and the easier the job. At Ø50, IT7 is 25 microns and IT11 is 160 microns.
- g — the position of the shaft tolerance. A small letter always describes the shaft. g puts the zone just below nominal, so the shaft always comes out a few microns undersize.
- 6 — how accurate the shaft must be. At Ø50, IT6 is 16 microns.
In short, the letter says where the zone sits and the number says how wide it is. H on the hole and h on the shaft both touch the nominal line; as the letter moves back through the alphabet (g, f, e) the shaft gets thinner, and as it moves forward (k, m, n, p) the shaft gets thicker.
The three types of fit and where they are used
| Type of fit | What happens | Assembly | Example fits and typical use |
|---|---|---|---|
| Clearance | Even the largest shaft is smaller than the tightest hole, so there is always a gap. | Goes on by hand, turns freely | H11/h9 rough assembly · H8/f7 lubricated plain bearing · H7/g6 guided sliding shaft · H7/h6 close location, almost no play |
| Transition | Depending on the actual part sizes you get either a small gap or a small interference. | Soft mallet or light press | H7/js6 cover that is taken off regularly · H7/k6 keyed pulley, gear, bearing seat · H7/n6 hub that is rarely removed |
| Interference (press) | Even the smallest shaft is larger than the widest hole, so it always seats under force. | Press, or heat the outer part and chill the shaft | H7/p6 bushings and sleeves · H7/n6 permanently seated hub (borderline) |
Bearing seat fit guide
The question that comes up most often on the shop floor is: "What should I turn the bearing journal to?" There is one rule behind the answer and it is easy to remember:
The reasoning is simple. If the rotating ring is loose, it slowly creeps in its seat. That creep wears the seating surface, heats it up and eventually eats into the shaft. If the stationary ring is fitted tight, there is nowhere left for thermal growth to go and the bearing binds. That is why one side is tight and the other is left free.
Which ring is rotating?
- Shaft rotates, load direction fixed (electric motors, gearboxes, fan shafts — by far the most common case): the inner ring rotates relative to the load → tight fit on the shaft, the outer ring is stationary → free fit in the housing.
- Shaft stationary, housing rotates (conveyor drums, wheel hubs, idler pulleys): the outer ring rotates → tight fit in the housing, the inner ring is stationary → free fit on the shaft.
Common selection table
| Condition | Load | Shaft (journal) | Housing bore |
|---|---|---|---|
| Shaft rotates, load direction fixed | Light (below roughly 6% of the bearing rating) | j5 · js5 | H7 |
| Normal | k5 · k6 | H7 | |
| Heavy, shock loaded | m5 · m6 · n6 | H7 | |
| Shaft stationary, housing rotates | Normal | g6 · h6 | K7 · M7 |
| Heavy | g6 · h6 | M7 · N7 | |
| Bearing runs hot, must float axially | — | h6 | G7 · H8 |
This tool covers the H basic-hole system, so it does not calculate the G7, K7, M7 and N7 housing bores in the table. Use the bearing manufacturer's tables for those classes.
The point that gets missed: a bearing's own tolerance is not ISO 286
This is the single biggest source of confusion in the workshop. A bearing's bore and outside diameter are toleranced to ISO 492, and that tolerance runs from zero downwards. In other words the bore of a nominally Ø50 bearing is never larger than 50 mm — it is always slightly smaller.
The consequence: when you machine the shaft to k6, the actual interference is greater than what this tool shows against an H7 hole, because the bearing bore can be below even the smallest H7 size. This tool gives you the correct size range for the shaft; to work out the real interference on the bearing side you need the Δdmp tolerance the bearing manufacturer publishes for that size.
What to check on a bearing seat besides the diameter
- Surface roughness: On an interference fit the peaks of a rough surface are flattened during assembly and the interference drops. Bearing journals are usually asked for Ra 0.8 µm or better, and a ground surface is preferred.
- Roundness and alignment: The diameter can be right and the journal still oval or tapered, which loads the bearing unevenly. The shoulder being square to the shaft matters just as much.
- Measuring temperature: The standard measuring temperature is 20 °C. If you measure a part fresh off the machine you will read it oversize, and it will shrink as it cools.
- Measuring instrument: A caliper is good for about 0.02–0.03 mm. The whole tolerance of Ø50 k6 is 16 microns — you cannot check that with a caliper; you need a micrometer or a comparator.
Which process holds which tolerance?
| Grade | Tolerance at Ø50 | What produces it |
|---|---|---|
| IT5 – IT6 | 11 – 16 µm | Grinding, honing, very accurate boring. Hard to hold with ordinary turning or milling. |
| IT7 | 25 µm | Reaming, precision boring, or a light finish pass on a well set up lathe. A drill will not hold it. |
| IT8 | 39 µm | Reaming or normal finish turning. |
| IT9 – IT10 | 62 – 100 µm | Normal turning and milling. Form-tap holes sit in this band. |
| IT11 | 160 µm | Holes made with a standard drill, rough assembly locations. |
When you see H7 on a drawing, do not just drill the hole and move on. H7 is a finishing class: drill undersize first, then bring it to size with a reamer or a boring bar. The same applies to g6 or k6 on a shaft — leave a finish allowance instead of going straight to size in one pass, and measure before the last cut. To work out the limit sizes of a single hole from its IT grade, use the Hole Tolerances: IT Grades Calculator.