Radial clearance explained: C2, CN, C3, C4 and C5
Why radial clearance decides noise and running temperature in small motors, how C3 differs from the normal group, and the verified ISO 5753-1 ranges up to C5.
What radial clearance is
Radial internal clearance is the total radial movement possible between the inner and outer rings of an unmounted bearing. It is a small number, in micrometres, and it exists because the raceway geometry must leave room for the balls, fits and thermal expansion.
Clearance changes during assembly: an interference fit compresses the inner ring, a hot housing expands the outer ring, and shaft and housing run at different temperatures. The catalogue value is a starting point, not the running value.
Why clearance matters in electric motors
Too little clearance loads the balls continuously: friction and temperature rise, high-frequency vibration appears, and a bearing that was quiet on the test bench becomes audible once the motor is hot. At the limit the clearance is used up and the bearing runs preloaded.
Too much clearance has different symptoms: the shaft loses stiffness and location accuracy, the balls accelerate across the raceway, and a low-speed motor may rattle at start-up. Small motors with tight air gaps and low noise targets sit where both errors are audible, so clearance is worth deciding deliberately.
The normal group and C3
The normal group is the catalogue default, written CN in the ISO tables and C0 by many Chinese makers. C3 is one step wider, C2 one step narrower; C4 and C5 continue wider.
C3 appears on many motor drawings for a mechanical reason: when both rings are interference-fitted and the housing runs hotter than the shaft, running clearance falls below the unmounted value, and C3 gives part back. Where fits are light and the assembly runs cool, the normal group is enough; C2 is used when the least possible play is wanted.
Clearance group is not a quality grade. A C3 bearing is not better or worse than a C0 bearing; it is the right or wrong group for a given fit and temperature.
Verified clearance ranges
The table reproduces verified ranges for radial contact groove ball bearings with a cylindrical bore, unmounted, at zero measuring load, in micrometres. The rows cover the bore range of this site, 3 mm to 20 mm, plus the next row.
| Bore d (mm) | C2 (µm) | CN (µm) | C3 (µm) | C4 (µm) |
|---|---|---|---|---|
| over 2.5 to 6 | 0–7 | 2–13 | 8–23 | 14–29 |
| over 6 to 10 | 0–7 | 2–13 | 8–23 | 14–29 |
| over 10 to 18 | 0–9 | 3–18 | 11–25 | 18–33 |
| over 18 to 24 | 0–10 | 5–20 | 13–28 | 20–36 |
Table values: ISO 5753-1:2009 Table 1 and GB/T 4604.1. These theoretical ranges at zero measuring load define each group's acceptance window. They are not a single target value: the purchase specification names the group, and production must lie inside the range.
The zero-load condition is not how the bearing is measured. With a measuring load applied, the reading includes elastic deflection, so it is larger than the table value. The correction comes from the standard tables: typically, for bore d of 18 mm or less, the measuring load is about 24.5 N and the C3 correction about 4 µm. Use the current edition of the standard, not this summary.
How clearance is measured
Clearance is measured on an unmounted bearing with a dedicated gauge, following GB/T 25769 and ISO 5753-1. The bearing is clean and at a stable temperature; one ring is fixed, the measuring load is applied radially to the other, and a dial indicator reads the relative movement. Readings are repeated at several angular positions, corrected for elastic deflection, and compared with the group range.
Results from different measuring loads, instruments or temperatures are not directly comparable, so a supplier should quote the range and the method, not just "C3". The measurement is also made before mounting: clearance checked after pressing onto a shaft shows assembled clearance, which is diagnostic, not a group verification.
Fits, temperature and running clearance
The running clearance is the table value minus the losses created by the installation:
- Interference on the shaft reduces clearance by roughly the amount of the interference.
- An interference fit in the housing acts on the outer ring in the same way.
- A shaft hotter than the housing expands more and takes up further clearance.
- At higher speeds, load and centrifugal effects add their own contribution.
Clearance and fit tolerances therefore have to be chosen together: a tight fit with the normal group may leave almost no running clearance in a hot motor, while the same fit with C3 may be correct. For unusual applications the supplier needs the fits, the temperature difference and the running speed, and the result is confirmed by a sample test.
Choosing a group
- Start from the shaft and housing fits you intend.
- Estimate the clearance lost to interference and temperature.
- Compare the remainder with the table range for the proposed group.
- Write the group on the drawing and the purchase order, and confirm on a sample.
Changing the clearance group does not change boundary dimensions. The same 6202 is 15 x 35 x 11 mm in CN, C3 or C4, so the group has to be written into the specification; it will not be inferred from the size.
Sources
- ISO 5753-1:2009, internal clearance, part 1, radial internal clearance.
- GB/T 4604.1, internal clearance, part 1, radial internal clearance for radial bearings.
- GB/T 25769, measuring methods for radial internal clearance.