NTN 6903 JRXZZ/5K Kugellager (17x30x7)

The principal dimensions are 17x30x7 mm at a weight of 0.018 kg; the further technical details are listed in the specification table. A gap is a barrier against coarse particles, not against liquid: splash water, vapour or fine dust are to be kept out by a contact version or by the design itself. The shield is a sheet-metal part fixed in the outer ring that reaches close to a shoulder of the inner ring without touching it.

The whole character of the version follows from this geometry: the remaining gap holds back coarse contamination and keeps the grease charge in the rolling chamber, yet generates neither an additional frictional torque nor a heat source, so that breakaway torque, no-load losses and permissible speed stay close to those of an open version.

The price lies in the selectivity: a gap lets water, vapour and fine dust through, and where the position is exposed to such media, either a contact version or an upstream seal in the design is to be provided. Since the shields are fixed in the outer ring, the grease charge is part of the delivered condition and is dimensioned for the service life; the life of that charge — rather than material fatigue — is frequently the limiting quantity at moderate load, and it depends directly on the operating temperature and the speed of the position.

Normal radial clearance is matched to one interference-fitted ring and a moderate temperature difference between inner and outer ring; because the shields do not touch the rings, they exert no influence on the clearance themselves. During mounting it has to be observed that the fitting force is introduced through the ring carrying the interference fit: a force routed through the outer ring, or a heavily compressed housing bore, can deform the shield in its seat and close the gap.

Through-hardened standard bearing steel sets the temperature and load frame within which the ratings listed in the catalogue apply.