What Is Khe hở C3? — Bearing Glossary

Radial internal clearance C3 is the bearing internal clearance group larger than the standard CN (Normal) class, defined by ISO 5753-1 and used to compensate for thermal expansion, interference fits, or heavy loads under demanding operating conditions.

Internal clearance is the relative displacement between inner and outer rings before installation. The C3 group has clearance values approximately 8–15 µm larger than CN, depending on bearing size. After mounting onto the shaft and into the housing, this clearance contracts — the target is an operating clearance near zero or slightly positive (5–10 µm) at stable operating temperature. See the complete clearance selection guide at bearing internal clearance and browse deep groove ball bearings in C3 clearance class.

Selecting the wrong clearance group is one of the most common causes of premature bearing failure. CN clearance in high-temperature applications goes negative as the bearing heats up, creating excessive contact and increased ball load — resulting in surface spalling within the first few hundred hours.

When C3 Is Required

C3 becomes necessary in three main situations. First, continuous operating temperatures above 60°C — every additional 10°C reduces clearance by approximately 2–3 µm. Second, interference fits on the inner ring ≥ k5/m5 — an interference of 20–40 µm directly consumes internal clearance. Third, high vibration or impact loads such as crushers and air compressors.

A practical example: bearing 6308 C3 (d = 40, D = 90, B = 23 mm, C = 32.5 kN) mounted on a centrifugal pump shaft. A φ40 shaft with k5 tolerance creates 18 µm interference — initial CN clearance of 13–28 µm is almost entirely consumed. C3 has initial clearance of 23–41 µm; after mounting, 5–23 µm remains, sufficient to absorb an additional 5–8 µm from thermal expansion.