NSK 6008 DDUC3 Kugellager (40x68x15)

For this NSK bearing, 40x68x15 mm and a weight of 0.192 kg apply; the specification table holds the further technical particulars. This variant features a contact seal and increased radial clearance (C3). Increased clearance (C3) does not make a sealed version faster; it prevents the rolling contact from being preloaded when interference fit, seal friction and ambient heat come together.

In a version sealed by contact on both sides an elastomeric lip bears against the inner ring and keeps moisture and fine dust away from the rolling chamber. The price for this is a frictional torque that is present independently of the load — and that torque becomes visible not only as lost power but first of all as heat at the place where it arises: at the contact face between lip and inner ring.

As a result the temperature difference that closes the clearance arises partly inside the bearing itself and not exclusively through an external heat source. To this share is added the interference fit on the shaft, which expands the inner ring, and in many applications the ambient heat of the drive as well. Increased clearance (C3) holds an allowance ready for this sum so that a residual clearance establishes itself in the warm running state at which the rolling contact does not run under constraint; if it turns out too small, the frictional torque rises further and the additional heat reduces the remaining clearance further still.

The grease charge is enclosed by the seals and dimensioned for the service life, which is why its ageing at the elevated operating temperature has to be assessed as well — it, rather than material fatigue, frequently limits the life at moderate load. In this version the permissible speed follows from the seal and not from the clearance class: a wider clearance does not raise the limit that the contact version sets.

Through-hardened standard bearing steel determines the temperature and load frame of the rings; the temperature limit of the sealing material is to be taken separately from the designation.