NSKNN3008MBKRE44CC1P4Double Row Cylindrical Roller Bearing 40x68x21 mm, 1:12 Tapered Bore, Brass Cage, ABEC 7 Precision

Product Description

What this part is

A double-row cylindrical roller bearing in the 40 mm bore × 68 mm outside diameter × 21 mm width envelope, the ISO NN3008 size. Two rows of cylindrical rollers, machined brass cage, with a 1:12 tapered bore and super-precision tolerances.

Decoding the designation

The NN prefix marks a double-row cylindrical roller bearing in which the inner ring carries the guiding ribs. The 30 gives the dimension series and the bore code 08 gives a 40 mm bore, since ISO bore codes above 04 are multiplied by five. MB is the machined solid brass cage, K is the 1:12 tapered bore, and P4 is the precision class equivalent to ABEC 7.

Bore form and mounting

The 1:12 tapered bore of an NN cylindrical roller bearing seats directly on a matching tapered spindle journal. No adapter sleeve applies. This is the reverse of a tapered-bore spherical roller bearing, where a K bore is mounted on a cylindrical shaft by way of a separate H-series adapter sleeve. Radial clearance is set on assembly by how far the inner ring is driven up the taper, which is what makes the tapered bore the preferred arrangement on a precision spindle: clearance becomes an adjustable quantity rather than a fixed one.

What these specs mean in practice

Two rows of cylindrical rollers give very high radial capacity and high radial stiffness in a short axial length, which is why the NN type is the standard non-locating radial bearing at the working end of a machine-tool spindle. Line contact between roller and raceway spreads load along a length rather than at a point, so radial capacity is far above a ball bearing in the same envelope. The design carries no axial load and provides no axial location; a separate thrust or angular contact bearing sets the shaft position.

The machined brass cage tolerates higher temperature and more marginal lubrication than a pressed-steel cage, and suits high speed and heavy load together. P4 holds bore, outside diameter, width and running accuracy to single-digit micron limits, which is what allows the adjustable clearance to be set meaningfully.

Quantity and configuration

1 Pc. One complete bearing, inner ring, outer ring, both roller rows and cage. Supplied without a housing, locknut, withdrawal sleeve or spacers.

Fitting notes

The inner ring is drawn up the tapered journal against a locknut, and clearance is measured as it advances rather than being taken as given. Cylindrical roller bearings of this class are sensitive to housing roundness, since an out-of-round bore concentrates load on part of the roller length. Seats at this class are ground and measured, and cleanliness governs the result.

Interchange guidance

The same-spec equivalents are SKF NN3008KTN9/SP in its precision tapered-bore form, FAG NN3008-AS-K-M-SP and NTN NN3008K. A straight-bore NN3008 without the K is a related size rather than a substitution, since it cannot be clearance-adjusted on the journal. An NNU3008, with the ribs on the outer ring, is a different design in the same envelope.