SKF 6308-2Z/VA208 Kugellager (40x90x23)

The dynamic load rating of 40.5 kN characterises this bearing. The static load rating is 24 kN. This variant features extra-large radial clearance (C5). Before the operating temperature is reached the full clearance is present — on a cold start at a position with very strongly increased clearance (C5) the load zone is at its narrowest and the grease at its stiffest.

Choosing very strongly increased clearance (C5) says that a large part of the mounted clearance is consumed in service at this position — through tight seats on both rings, but above all through a sustained temperature difference between a warm inner ring and a cooler outer ring. In combination with shields on both sides this also names the decisive item to be checked: the grease charge is enclosed, dimensioned for the service life and cannot be renewed, so its temperature resistance and its service life at the actual operating temperature become the real design limit.

That check is to be made against the data for the charge and not inferred from the clearance class; where the temperature lies above it, a relubricatable or an open version is to be preferred even if the clearance class fits. The second point concerns start-up: as long as the position is cold the full clearance is present, the load zone covers only a narrow sector of the ball set, and the load-free part is guided by the cage alone — at low radial load it can slide before a load-carrying lubricant film has built up.

A stiff, cold grease reinforces this condition, which is why start-up behaviour and minimum load are to be assessed together for this combination. The shields themselves do not touch the rings, contribute no frictional torque and exert no influence on the clearance; they keep the charge in the rolling chamber and coarse contamination away, but are no barrier against liquid or fine dust.

Through-hardened standard bearing steel sets the frame for hardness and dimensional accuracy; whether the position may be operated above the usual temperature range is to be taken from the designation. Metal shields on both sides — Dimensional/form-fit equivalent — verify suffix-level details for critical applications. Matching suffixes are also listed by FAG (2Z), NSK (ZZ), NTN (ZZ) and NACHI (ZZE).