NSK 6307 ZZNRC3 Kugellager (35x80x21)

This NSK bearing measures 35x80x21 mm and weighs 0.46 kg; the specification table carries the supplementary technical details. This variant features increased radial clearance (C3). A closed rolling chamber does not carry its frictional heat away with the lubricant but through the rings — the operating temperature therefore establishes itself differently than at a position with lubricant flow, and increased clearance (C3) accounts for that.

The combination of shields on both sides with increased clearance (C3) describes a position that is closed and nevertheless runs warm. Because no lubricant is routed through the rolling chamber, the frictional heat leaves the bearing through the inner and outer ring, and the ring with the better thermal connection — as a rule the outer ring in the housing — stays cooler than the inner ring on the shaft.

It is precisely this difference that consumes clearance: the warmer inner ring expands further and, together with the share the interference fit has already taken, the remaining clearance can fall below the intended range. Increased clearance (C3) holds an allowance ready for that, so that a residual clearance establishes itself in the warm running state at which the rolling contact is not preloaded.

The second effect of the same temperature concerns the lubricant: the grease charge is enclosed by the shields and dimensioned for the service life, so that its ageing at elevated temperature — rather than material fatigue — frequently becomes the limiting quantity. The operating temperature of the position is therefore to be checked against the data for the grease charge; where it lies above, a version with a different filling or a relubricatable solution is to be selected rather than adjusting the clearance class alone.

Since the shields do not touch the rings, they contribute neither a frictional torque nor a heat source of their own; the permissible speed follows from the cage and the lubricant. Through-hardened standard bearing steel sets the frame for hardness and dimensional accuracy within which this interplay holds.