NSK 6904 VV Kugellager (20x37x9)

This NSK bearing measures 20x37x9 mm and weighs 0.04 kg; the specification table carries the supplementary technical details. What does not bear does not wear — the barrier effect of this version stays largely unchanged over the operating period, whereas a bearing lip weakens with the operating hours. The non-contact seal consists of an elastomeric sealing body on a sheet-metal insert, fixed in the outer ring, whose contour follows the shoulder or groove of the inner ring down to a narrow distance without touching it.

The difference from a shield lies less in the principle than in what can be realised: an elastomeric body can be moulded and guided more closely than a sheet-metal part, so the remaining passage cross-section comes out smaller and the grease charge is better retained in the rolling chamber. The difference from a contact version, by contrast, is fundamental: without contact pressure no load-independent frictional torque arises, no additional heat source at the inner ring and no wear point, which is why the permissible speed stays close to that of the open form and the barrier effect does not decline over the operating period through lip wear.

The limit of this version follows from the same geometry: a gap, however closely it may be guided, does not hold back liquid permanently, so for flushed or continuously wetted positions a contact version or an upstream seal in the design is to be provided. The grease charge is included in the delivered condition and dimensioned for the service life; its ageing at the actual operating temperature is frequently the limiting quantity at moderate load.

Normal radial clearance is matched to one interference-fitted ring and a moderate temperature difference between the rings, whereby the seal itself makes no contribution to the warming and therefore does not enter the consideration of the residual clearance. Through-hardened standard bearing steel sets the temperature and load frame of the rings; the temperature limit of the sealing material is to be taken from the designation.