Reference / Fits
Bearing Seat Fits for Motor Housings
The bore limits come straight out of ISO 286. The part that catches people out is what happens to those limits once the motor is hot.
Written by the sourcing desk that reads your RFQLast reviewed How we write these
Work out your bore
Enter the nominal bearing seat diameter and how hot the housing runs. Everything is computed in your browser and nothing is sent anywhere.
How to read the result
The bore limits are the sizes the shop has to hit at 20 °C on the inspection bench. The second number is the one that decides whether the bearing stays put in service.
As the housing warms it grows faster than the steel outer ring inside it. The difference is pure geometry: the gap between the two materials widens by the difference in expansion coefficient, times the diameter, times the temperature rise. Nothing about the machining changes it.
If that differential is a large fraction of your tolerance band, the room temperature fit is not really the design question. Either the outer ring is clamped or preloaded so it cannot move, or the fit at 20 °C has to be chosen knowing what it becomes at 90 °C.
This is the single most common reason a motor that passed inspection is noisy after a month in the field.
What the shaft gets
H6, H7 and H8 across the full range
Fundamental tolerance in micrometres. An H grade hole starts at the nominal size and runs plus, so a 62 mm H7 bore is 62.000 to 62.030 mm. Bands are read as over the first figure, up to and including the second.
| Nominal size (mm) | H6 | H7 | H8 | Typical motor use |
|---|---|---|---|---|
| over 0 to 3 | 0 / +6 | 0 / +10 | 0 / +14 | Small fan, appliance and servo bearings |
| over 3 to 6 | 0 / +8 | 0 / +12 | 0 / +18 | Small fan, appliance and servo bearings |
| over 6 to 10 | 0 / +9 | 0 / +15 | 0 / +22 | Small fan, appliance and servo bearings |
| over 10 to 18 | 0 / +11 | 0 / +18 | 0 / +27 | Small fan, appliance and servo bearings |
| over 18 to 30 | 0 / +13 | 0 / +21 | 0 / +33 | Small fan, appliance and servo bearings |
| over 30 to 50 | 0 / +16 | 0 / +25 | 0 / +39 | Most industrial and traction motor seats |
| over 50 to 80 | 0 / +19 | 0 / +30 | 0 / +46 | Most industrial and traction motor seats |
| over 80 to 120 | 0 / +22 | 0 / +35 | 0 / +54 | Most industrial and traction motor seats |
| over 120 to 180 | 0 / +25 | 0 / +40 | 0 / +63 | Large frame and generator seats |
| over 180 to 250 | 0 / +29 | 0 / +46 | 0 / +72 | Large frame and generator seats |
| over 250 to 315 | 0 / +32 | 0 / +52 | 0 / +81 | Large frame and generator seats |
| over 315 to 400 | 0 / +36 | 0 / +57 | 0 / +89 | Large frame and generator seats |
| over 400 to 500 | 0 / +40 | 0 / +63 | 0 / +97 | Large frame and generator seats |
Values are the ISO 286-1 fundamental tolerance grades IT6, IT7 and IT8. For an H grade hole the lower deviation is zero by definition, so the table is the complete answer for H6, H7 and H8.
What the fit class does not tell you
A drawing that says 62 H7 and nothing else is still an incomplete drawing for a bearing seat. Four things sit outside the fit class and each of them can put a good bore out of service.
Roundness
A bore can be inside an H7 band on every diameter you measure and still be oval. The outer ring takes the shape of the bore it is pressed into, and an oval seat makes an oval raceway. If roundness matters, call it out separately. ISO 1101 gives you the symbol for it.
The shoulder
The face the bearing seats against has to be square to the bore. A perpendicularity callout on that face is worth more than one grade of bore tolerance, because a cocked outer ring loads the raceway at an angle no fit class can correct.
Surface finish
Around Ra 0.8 or better is ordinary for a bearing seat. A rough bore effectively loses interference as the peaks flatten during assembly, so the fit you measured is not the fit you end up with.
Which ring rotates
The rule the bearing makers apply is that the ring which rotates relative to the load direction gets the interference fit, and the ring that stays still relative to it gets the clearance fit. For a normal motor, that is an interference fit on the shaft and a clearance fit in the housing. If your application turns that around, for example a stationary shaft with a rotating housing, the fits swap and everything on this page applies to the other part.
A worked example
A 6205 bearing has a 52 mm outside diameter. In an aluminium end bell that runs at 85 °C:
- H7 at 52 mm is 0 to +30 micrometres, so the bore is 52.000 to 52.030 mm at 20 °C
- The aluminium bore grows about 39 micrometres more than the outer ring does over a 65 K rise
- That differential is larger than the entire H7 band
So the honest reading is that at temperature this seat is a clearance fit whatever you print on the drawing. The design answer is axial clamping or a wave spring, not a tighter bore, and the drawing should say which. A shop that machines motor parts will usually ask about this before it quotes. That question is a good sign.
Questions about fits
Which housing fit does a standard electric motor use?
For a rotating shaft with a stationary outer ring, which covers almost every electric motor, the bearing makers put a clearance fit in the housing and an interference fit on the shaft. H7 is the usual housing bore. H8 appears on light duty and light alloy housings, and J7 is used where the outer ring has to be able to move axially, for example against a preload spring.
Why does an aluminium housing need a different fit from a cast iron one?
Aluminium grows about twice as fast as the bearing steel it holds. At a 70 K rise a 62 mm aluminium bore opens roughly 50 micrometres more than the outer ring does, which is more than the whole H7 tolerance band. Cast iron and steel housings sit close to the bearing steel and barely move relative to it. The calculator on this page gives the number for your diameter and temperature.
Can a shop hold H6 on a bore?
Many can, but it changes how the part is made and inspected. H6 usually means a dedicated finish boring pass, a bore gauge at the machine, a check on every piece and a part that is temperature stable before it is measured. H7 is ordinary work. If the bearing table says H7 is right for your load case, asking for H6 buys inspection you do not need.
Why does this page not print k6 and m6 deviations?
Because published sources disagree on them and we are not willing to print a number we cannot verify. H grade holes are definitional in ISO 286, so they can be computed exactly. The interference classes need the fundamental deviation tables, and the right place to read those is the table for the bearing you are actually fitting.
Standards and references
- ISO 286-1, Geometrical product specifications (GPS). ISO code system for tolerances on linear sizes. Defines the IT grades and the H hole deviations used above.
- ISO 1101, Geometrical product specifications (GPS). Geometrical tolerancing. Roundness, perpendicularity and runout callouts.
- SKF, Tolerances and resultant fits. Source for which class goes on the shaft and which on the housing.
- Engineering fit, Wikipedia. Background on the hole basis and shaft basis systems.
Figures on this page are typical ranges drawn from the standards above and from ordinary shop practice. They are not a specification for your part. The shop that quotes you confirms what it can hold on your geometry.