Fafnir2MM210WIDUMSuper-Precision Angular Contact Ball Bearing 50x90x20 mm 15 Degree Duplex Matched Pair Medium Preload ABEC 9

Product Description

What this part is

A super-precision angular contact ball bearing in the Fafnir WI spindle series, supplied as a factory-matched duplex pair. Bore 50 mm, outside diameter 90 mm, width 20 mm per bearing.

Decoding the designation

2MM — 15° contact angle in the Fafnir scheme. A 3MM prefix on the same envelope would be 25°.
210 — no leading 9, so this is the ISO 72 series: the heavier section, corresponding to a 7210 at 50 mm bore and 90 mm outside diameter.
WI — super-precision spindle bearing series.
CR — phenolic cage, riding on the outer ring land.
DUM — duplex universal pair, medium preload.
The MM prefix carries the P4S class: ABEC 7 dimensional tolerances with ABEC 9 running accuracy.

What these specs mean in practice

The 72-series section puts more material and a larger ball complement behind a 50 mm shaft than the 70-series alternative, raising both radial and thrust capacity at the cost of a larger outside diameter and more rotating mass. Medium preload raises axial and radial stiffness over the light grade and holds running accuracy better as load varies, in exchange for higher friction and a higher steady-state temperature. The 15° contact angle keeps this combination toward the speed end of the range despite the heavier section.

Quantity and configuration

2 Pcs — one matched duplex pair. Universal grinding allows the pair to be assembled back-to-back, face-to-face or in tandem.

Fitting notes

The preload is ground into the ring faces and is realised only when both inner rings and both outer rings are clamped solidly against their seats. Alignment marks on the outer ring faces set the intended arrangement.

Interchange guidance

The ISO shorthand for this size and angle is 7210 C. Same-envelope bearings at a 25° contact angle, such as a 3MM210WI, or at light or heavy preload, are related sizes rather than interchanges. A 2MM9110WI is the lighter 70-series section on the same 50 mm bore, not a substitute.