NACHI 6904-2NKECM Kugellager (20x37x9)

At 20x37x9 mm and a unit weight of 0.04 kg, the installation envelope is fixed; further technical details are held in the specification table. This version brings together two features that both aim at uniformity: a sealing without contact and a clearance with reduced scatter. In this version the two features complement one another in a way that shows up in the frictional torque.

The non-contact seal follows the contour of the inner ring with an elastomeric sealing body down to a narrow distance without touching it; this removes the contribution that dominates breakaway and no-load torque in a contact version — and removes its scatter with it, since a lip is subject to manufacturing tolerances of its contact pressure and to a wear that changes over the operating period.

What then remains are the grease charge and the load distribution in the rolling contact, and the load distribution is a direct consequence of the residual clearance: it determines how many balls carry at the same time and how the outer ring settles relative to the inner ring under load. Normal radial clearance with narrowed tolerances reduces the scatter of that quantity within the normal class without shifting the range, so that no-load torque, radial stiffness and vibration behaviour lie closer together across a production series.

Applications in which this counts check those values on the finished product and can no longer adjust them after joining. The choice of class is not replaced by it: interference fit and temperature gradient still consume the same share as under the undivided normal clearance, and where that share is large, a wider class remains the applicable answer.

The gap of the seal does not permanently hold back liquid and very fine dust; the grease charge is enclosed and dimensioned for the service life. Through-hardened standard bearing steel supplies the hardness and dimensional accuracy on which a narrow distribution can rest.