FafnirMM209WICRDUXSuper Precision Angular Contact Ball Bearing Duplex Pair, 45x85x19 mm Each, Phenolic Cage, ABEC 7, Universally Ground — 2 Pcs

Product Description

What this part is

A matched duplex set of two Fafnir MM209WI super-precision angular contact ball bearings, 45 mm bore x 85 mm outside diameter x 19 mm wide each — the 7209 / 209 envelope. The outer rings are counterbored on one side so the ball complement can be loaded and so the bearing develops a contact angle: each bearing therefore takes radial load plus thrust in one direction, and the pair working together handles thrust reversal.

Decoding the designation

MM – the doubled M is Fafnir’s super-precision grade, ABEC 7. This is the letter that catches people out: a single M is ABEC 3, MM is ABEC 7, and M with an FS6xxxx suffix is ABEC 5.
209 – 200 series, bore code 09, so 45 mm bore.
WI – maximum-capacity (filling-slot) type with the outer ring counterbored, which is what makes the bearing angular contact rather than deep groove.
CR – composition retainer: a phenolic laminated-fabric cage. Fafnir stamps CR on the carton rather than the ring, so the rings may read only MM209WI.
DUX – duplex, universally ground. DU means the faces are ground so the two rings can be clamped together and arrive at a repeatable preload. The X in place of Fafnir’s usual L, M or H preload letters marks a special (non-catalogue) preload rather than one of the three standard classes, so it is carried here as a searchable reference rather than published as a preload figure.

What these specs mean in practice

Universal grinding is the whole point of a duplex set. Because the ring faces are ground to a controlled offset, the two bearings can be clamped face-to-face, back-to-back or in tandem and will develop a predictable preload without shims or measurement at the bench. That preload is what removes axial play and holds spindle running accuracy under thrust reversal — and it only exists as long as the pair stays together and is fully clamped.

ABEC 7 controls bore, outside diameter, width and, critically, radial and axial runout, so the rings can be fitted in any rotational orientation without chasing a high spot. The phenolic cage is the high-speed complement to that: light, self-lubricating, quiet, and soft enough that a ball skid polishes the pocket instead of galling. Its limits are heat above roughly 120 °C and attack by some synthetic lubricants and solvents.

Published dimensions

Bore (d): 45 mm
Outside diameter (D): 85 mm
Width (B): 19 mm per bearing, 38 mm for the clamped pair
Rows: 1 per bearing, 2 in the mounted set
Cage: phenolic / laminated fabric composition (CR)
Precision: ABEC 7
Closure: open, no seals or shields
Outer ring: counterbored one side

Quantity and configuration

Two bearings ship together as one matched duplex set. Do not split the set and do not mix one of these rings with a bearing from another set — the preload comes from the ground offset between these two specific rings and is only correct as supplied.

Fitting notes

Mount to the arrangement the machine drawing calls for and clamp the rings fully; a partially clamped duplex pair never develops its designed preload and simply loses stiffness. Orient each bearing so the counterbored (low shoulder) side faces away from its thrust load — fitted backwards the balls climb the raceway shoulder and the bearing fails fast. Press or heat, never hammer an ABEC 7 bearing, and never drive force through a phenolic cage. Grease sparingly, roughly a quarter to a third fill, or run the oil-air system the spindle was designed around.

Interchange guidance

Fafnir is now sold by Timken, so Timken MM209WI and Fafnir MM209WI are the same part. Fafnir does not publish a contact angle for the plain WI series — in Timken/Fafnir numbering it is the numeric prefix that names the angle, with 2MM = 15° and 3MM = 25° — so no angle is claimed here. Same-envelope 45 x 85 x 19 mm super-precision duplex sets, with the differences that matter named: NSK 7209CTYNDULP4 (C is NSK’s 15° code) and 7209A5TYNDULP4 (A5 is 25°), SKF 7209 ACD/P4ADGA (AC is SKF’s 25° code), FAG B7209-E-T-P4S-UL (E is FAG’s 25° code), NTN 7209CT1GD2GLP4 (C is 15°), RHP 7209ETDULP4 (RHP E is 25°). All six sit in this envelope but each names a specific contact angle, so use them as dimensional cross-references and match the angle your spindle was designed around. The 7209-B reading that has travelled with this record is FAG’s 40° code and is carried as a searchable reference only.