NACHI 6909 ZZECM Kugellager (45x68x12)

The mounting dimensions are 45x68x12 mm and the weight is 0.14 kg; technical details beyond these are shown in the specification table. With no rubbing sealing lip, the running noise of this version is governed by the residual clearance and by the lubricant — and a narrowed tolerance turns that into a result comparable from piece to piece.

Normal radial clearance with narrowed tolerances does not shift the range in which the clearance lies but reduces the permitted scatter within that range. For the combination with shields on both sides this makes a fitting pair, because both features aim at the same property: predictable running and noise behaviour. Since the shields do not touch the rings, the contribution of a sealing lip to frictional torque and noise falls away entirely, so that the remaining influencing quantities are the residual clearance, the grease charge and the surface quality of the raceways.

Of these the residual clearance is the one open to design influence: a wider clearance lets the outer ring move further relative to the inner ring under changing load and stands out in vibration measurements and in radiated noise, while a tighter clearance raises radial stiffness but works towards preload once the interference fit and the warming have taken their share.

The narrowed tolerance reduces the band within which this result establishes itself across a production series and makes a noise or vibration acceptance reproducible; it does not, however, replace the correct choice of class, since fit and temperature gradient still consume the same share as under the undivided normal clearance. The grease charge is enclosed by the shields and dimensioned for the service life; in noise-critical applications its quantity and consistency belong to the specification just as the clearance class does.

Through-hardened standard bearing steel supplies the hardness and dimensional accuracy on which a narrow distribution can rest at all.