NTN 6309 C4 Kugellager (45x100x25)

At 45x100x25 mm and a unit weight of 0.84 kg, the installation envelope is fixed; further technical details are held in the specification table. This variant features greatly increased radial clearance (C4). On an open bearing the clearance can be re-measured after mounting — with strongly increased clearance (C4) it is precisely this residual value that carries the information, not the figure as delivered.

Strongly increased clearance (C4) answers positions at which the clearance is consumed from two sides at once. If not only the shaft but also the housing bore carries an interference fit, the inner ring is expanded and the outer ring is compressed; both shares occur simultaneously and add up to a loss that normal radial clearance no longer covers.

A permanently high temperature gradient between the rings has the same effect, as it arises at positions with an internal heat source. The price of this specification becomes visible in the mounted state: the wider the remaining clearance, the smaller the circumferential sector in which balls take up load at all, the higher the load on the carrying ball, and the more pronounced the movement of the outer ring relative to the inner ring under a change of load direction.

At low radial load there is the further point that the load-free part of the ball set is carried along by the cage alone and, without sufficient minimum load, can slide in the rolling contact, which stresses the lubricant film. The wide clearance also permits a small misalignment between the rings; the contact ellipse then migrates towards the groove shoulder, so this mobility is to be treated as a manufacturing tolerance rather than as a design principle.

The open form contributes twice in this combination: it allows oil or circulating lubrication, which removes the frictional heat and thereby limits the temperature gradient itself, and it makes the mounted clearance accessible to measurement. Through-hardened standard bearing steel sets the frame for hardness and dimensional accuracy; whether the position may be run permanently above the usual temperature range is to be taken from the designation.