Internal clearance is the total distance one ring can move relative to the other. The figure that matters is not the clearance in the box but the clearance left after mounting and after the bearing reaches operating temperature.
The Groups
C2 is below normal, CN is normal and is usually unmarked, and C3, C4 and C5 are progressively greater than normal. The actual values depend on bore size, so the same group means different micrometres on a 30 millimetre bore and a 300 millimetre bore.
What Consumes Clearance
Two things, mainly. An interference fit on the inner ring expands it and closes the clearance, typically consuming a large part of the initial figure on a tight fit. And the temperature difference between inner and outer ring in service expands the inner ring more than the outer, closing it further.
This is why electric motors and gear units so often specify C3. By the time the machine is hot and running, a C3 bearing has arrived at roughly normal residual clearance.
Too Little And Too Much
Too little residual clearance preloads the bearing, raises temperature, and can run away as heat closes the clearance further. Too much lets the load zone shrink, which concentrates load on fewer rolling elements and increases noise and vibration.
Practical Rule
Follow the machine builder's specification where one exists. Where none does, use CN for a normal fit at moderate temperature, and step up to C3 where the inner ring fit is tight, where the shaft runs hot, or where the outer ring is in a light alloy housing.
