NTN 6311 ZZC4/5K Kugellager (55x120x29)

The principal dimensions are 55x120x29 mm at a weight of 1.38 kg; the further technical details are listed in the specification table. This variant features greatly increased radial clearance (C4). If the housing bore is made tight, the compression of the outer ring carries right through to the seat of the shield: the gap to the inner ring is a function of how that ring deforms.

In this combination the clearance class and the design belong together more closely than it first appears. Strongly increased clearance (C4) is selected when not only the inner ring is expanded but the outer ring is also compressed in a tight housing bore, or when a high temperature gradient is added; both shares reach the rolling chamber at the same time and leave too little of the normal clearance.

At the same time the outer ring is the component in whose groove the shields are fixed. An interference fit large enough to raise a clearance class deforms that ring measurably, and the deformation also reaches the shield seat: the gap to the inner ring can change unevenly in the process, in an unfavourable case as far as contact. Where the housing fit lies in this range, free running is to be checked after assembly and the fitting force introduced solely through the ring that carries the interference fit.

If the shield is undamaged, its contribution remains unchanged: a non-contacting gap that keeps the grease charge in the bearing and coarse contamination out, without contributing a frictional torque or a heat source, and which does not hold back liquid or fine dust. At low radial load the wide clearance has to be taken into account, because the load zone becomes narrow and the load-free part of the ball set can slide without sufficient minimum load.

The grease charge is enclosed and dimensioned for the service life; relubrication is not foreseen. Through-hardened standard bearing steel sets the frame for hardness and dimensional accuracy.