Bearing clearance C2, CN, C3, C4: when looser is better

Updated 2026-09-24

Radial internal clearance is the total free play between the rings before installation. It is specified, manufactured and measured before mounting — but the bearing actually runs on what is left after the fits and the temperature have consumed part of it. Choosing C2, CN, C3 or C4 is therefore a budget problem, not a quality preference.

What each class means

The classes are groups, not single numbers: the catalogue prints a µm range per bore size, and adjacent classes overlap slightly. Concrete values for a given model are on its specification page — e.g. the 6205 row, the sealed C3 variant and larger rollers such as 22220.

The clearance budget: how play disappears in service

Operating clearance = manufactured clearance
                    − inner-ring expansion from the interference fit
                    − outer-ring compression in the housing
                    − thermal expansion difference inner vs outer ring

1. Inner-ring interference fit stretches the inner ring over the shaft, reducing play.
2. Outer-ring interference (when used) compresses the outer ring, reducing play further.
3. Heat gradient — the inner ring runs hotter than the housing in most machines; the shaft expands radially toward the already-expanded outer ring.
4. Deflection under load closes additional angular play in the loaded zone (roller bearings, especially cylindrical rollers, are more sensitive here than deep-groove ball bearings).

If the result is at or below zero, the bearing is unintentionally preloaded: torque rises, temperature rises, grease life collapses and the failure mode usually looks like “bad bearing” even though the steel was fine.

The practical selection pattern

• Standard electric motors and general gearboxes with recommended fits: C3 — this is why a large share of sealed catalog rows are produced in C3, including most 6205-2RS/C3 inventory.
• Loose/transition fits or non-locating outer rings in simple housings: CN, sometimes C2.
• Large bore, heavy fits, hot inner ring (e.g. conveyor-pulley spherical rollers): C3 or C4 to the catalogue recommendation.
• Precision spindles, angular-contact pairs: clearance classes are replaced by preload entirely — see DB/DF/DT pairs and preload.
• Cylindrical rollers with NU/NJ sliding fits: clearance is often selected one group looser because axial displacement and thermal growth matter — see the NU/NJ/NUP guide.

How clearance interacts with seals and precision

• Sealed-for-life rows are frequently C3 because the factory grease fill and housing tolerances assume some thermal headroom — read the designation as a whole, as shown in 6205 vs 6205-2RS.
• Loose clearance is not high precision. C4 on a precision spindle increases vibration and destroys the locating accuracy; P5/P4 precision bearings are delivered in selected clearance groups matched to the intended preload.
• Do not “fix” a hot bearing with one looser class blindly. If a C3 bearing runs hot, the causes are usually fit, alignment, lubrication or load — moving to C4 without measuring masks the cause and shortens the next bearing too.

Field checklist before ordering

1. Note shaft and housing tolerance fields from the drawing (the fit table decides most cases).
2. Estimate the inner-minus-outer temperature differential in steady-state operation.
3. Start from the catalogue recommendation for that duty, then move one class only with documented reason.
4. Order the full designation — 6308 C3 and 6308 CN are different parts despite the identical 40×90×23 mm envelope.
5. For cross-brand substitutes, compare the clearance group and the fits originally designed — the cross reference tool flags this as a verify item.