NSKSuper Precision Angular Contact Ball Bearing Matched Pair 140 x 210 x 33 mm, 30° Contact Angle, P4 / ABEC 7 Class, Phenolic Cage, Back-to-Back Duplex Arrangement, 2 Pieces

Product Description

What this part is

A matched pair of super precision angular contact ball bearings in the 7028 envelope — 140 mm bore, 210 mm outside diameter, 33 mm width per bearing. Spindle grade, ground as a set for back-to-back mounting, and held to the tolerance class machine tool builders specify for main spindle positions.

Decoding the designation

7028: ISO boundary dimensions in the thin 70xx dimension series, 140 x 210 x 33 mm. A: 30° contact angle in NSK nomenclature. A bare A is 30°, A5 is 25° and C is 15°, so 7028A and 7028A5 are different bearings. T: phenolic textile laminate cage. DB: back-to-back duplex arrangement, ground and supplied to be mounted this way round. P4: super precision tolerance class, equivalent to ABEC 7. C7: NSK preload and execution code, carried here as a searchable reference.

What these specs mean in practice

A 30° contact angle is the high-thrust end of the spindle bearing range. Load passes through the bearing along a line inclined 30° to the radial plane, which gives it substantially more axial capacity and much greater axial stiffness than a 25° or 15° bearing of the same envelope. The trade is speed: a steeper angle makes the ball climb further up the raceway shoulder, which increases ball spin, friction and heat generation. A 30° angle suits a spindle that has to hold position under heavy axial load — heavy drilling, boring, plunge milling, a vertical spindle carrying weight.

Mounted DB, the two bearings’ load lines diverge outwards, which gives the widest effective bearing spread and therefore the greatest resistance to tipping moment, exactly what suits the front of a spindle carrying an overhung tool. It also takes thrust in both directions and locates the shaft axially with no play.

The phenolic cage is the speed-friendly counterweight to the steep contact angle. At roughly a fifth the density of brass it loads itself far less under centrifugal force, and the laminate holds a small oil reserve that helps the bearing survive a brief lubrication interruption.

Quantity and configuration

This listing is a set of 2 pieces — a matched pair supplied for back-to-back (DB) mounting. The pair is ground so that clamping it between the shaft shoulder and the locknut produces the intended preload with no shims.

Interchange guidance

Same-envelope 30° equivalents include SKF 7028 ACD/P4ADBA in the corresponding grade and NTN 7028 in a 30° execution. FAG’s 7028-B series carries a B suffix denoting 40°, a steeper angle again, so it is not a direct swap. The 7028A designation carried on this listing is the correct short form of this bearing. Related sizes in the same 140 x 210 x 33 mm envelope at other angles: FAG B7028-C-T-P4S-UL, Fafnir 2MM9128WI and NSK 7028C are the 15° version, while SKF 7028 ACD, FAG B7028-E and Fafnir 3MM9128WI are the 25° version. Those are carried as dimensional cross-references rather than substitutions inside a 30° preloaded set.