NSK 6008 DDUC4 Kugellager (40x68x15)

The principal dimensions are 40x68x15 mm at a weight of 0.184 kg; the further technical details are listed in the specification table. This variant features a contact seal and greatly increased radial clearance (C4). Strongly increased clearance (C4) points to tight seats or a high temperature gradient; at a position sealed by contact on both sides, both are first to be held against the temperature limit of the sealing material.

The pairing of a contact seal on both sides with strongly increased clearance (C4) is one of the more demanding combinations in this series, because both features act at the same place. The wide clearance class is selected when tight seats on both rings or a sustained temperature gradient consume a large part of the mounted clearance. The remaining residual clearance permits a greater mutual displacement of inner and outer ring, and it is precisely this movement that is transmitted to the seal: the sealing body fixed in the outer ring has to keep its lip against the shoulder or in the groove of the inner ring under all operating states, so that the design of the seal and the choice of clearance class are not independent of one another.

The second point is thermal: a closed version carries its heat away through the rings and not with a flow of lubricant, so the temperature that prompted the clearance class remains effective inside the bearing — it is to be held against the temperature limit of the sealing material and against the service life of the enclosed grease charge, which cannot be renewed.

Both figures are to be taken from the designation; where the operating temperature lies above them, an open or relubricatable solution is the applicable answer rather than a still wider clearance class. The seal continues to bring a load-independent frictional torque that limits the permissible speed and forms a heat source of its own at the inner ring.

At low radial load it must additionally be checked whether the minimum load is reached, because the load zone turns out narrow at this clearance class. Through-hardened standard bearing steel sets the frame for hardness and dimensional accuracy of the rings.