NSK 6308 NRC3 Kugellager (40x90x23)

The principal dimensions are 40x90x23 mm at a weight of 0.676 kg; the further technical details are listed in the specification table. This variant features increased radial clearance (C3). Increased clearance (C3) is not a loose specification but an allowance: the interference fit and the temperature difference between the rings consume part of the clearance in service, and only what remains governs running behaviour.

Two losses lie between the clearance as delivered and the clearance in service, and they add up. The interference fit on the shaft expands the inner ring and takes a first share; the sustained temperature difference between the warmer inner ring and the cooler outer ring, coupled to the housing, takes a second. Increased clearance (C3) compensates for exactly these two shares so that a residual clearance within the intended range establishes itself once the arrangement has warmed up — it is a calculated quantity of the bearing position, not a property that would be selected on its own.

The residual clearance acts directly on load distribution: the wider it is, the smaller the circumferential sector in which balls carry and the higher the load on the individual ball; the tighter it is, the sooner the rolling contact is preloaded and generates additional heat, which closes the clearance further. Because this bearing is supplied in the open form, the lubricant carries the frictional heat directly out of the rolling chamber; oil and circulating lubrication are possible, and the temperature difference between the rings therefore develops differently than in a grease-filled, closed version.

Through-hardened standard bearing steel continues to define the temperature and load frame within which the ratings listed in the catalogue apply. At low radial load the wider clearance has to be taken into account: the unloaded part of the ball set is guided only by the cage and, without sufficient minimum load, it can slide instead of rolling.

Whether increased clearance (C3) is the right choice for a specific position therefore follows from the fit, the temperature gradient and the actual load taken together — not from any one of the three on its own.