NSK 6811 DDU Kugellager (55x72x9)

At 55x72x9 mm and a unit weight of 0.088 kg, the installation envelope is fixed; further technical details are held in the specification table. This variant features a contact seal. What this version gains in protection it gives up in friction — the lip produces a torque that is present independently of the load and shows up in heating and in the permissible speed.

The contact seal consists of an elastomeric sealing body with a lip that bears against a shoulder or in a groove of the inner ring, guided there with a defined contact pressure. The difference from a non-contacting version is fundamental: a restriction slows entry, a contact prevents it — which is why this version keeps not only coarse particles but also splash water, spray mist and fine dust away from the rolling chamber, and retains the grease charge even where the position stands vertically or centrifugal forces act on it.

This effect is bought with a frictional torque that depends not on the load but on the lip geometry and its contact pressure: it is present even at no load, stands out clearly at breakaway, and generates a heat source of its own directly at the inner ring, which is why the permissible speed of this version lies below that of an open or non-contacting variant.

The grease charge is enclosed and dimensioned for the service life, so relubrication is not foreseen; the design of the position therefore has to be oriented to the grease service life at the actual operating temperature. As a wearing part the lip is subject to an ageing that depends on circumferential speed, temperature and medium, so the barrier effect declines over operating time; in applications with long standstill periods and changing humidity this course has to be taken into account.

Normal radial clearance is matched to one interference-fitted ring and a moderate temperature difference between the rings, whereby the heat introduced by the seal has to be attributed to the inner ring. Through-hardened standard bearing steel sets the frame in which the ratings listed in the catalogue apply; the temperature limit of the sealing material is independent of it and is to be taken from the designation.