Fafnir3MM9122WIDUMSuper Precision Angular Contact Ball Bearing 110 x 170 x 28 mm, 25° Contact Angle, ABEC 7, Phenolic Cage, Duplex Medium Preload, Matched Pair

Product Description

What this part is

A matched pair of super precision angular contact ball bearings in the 7022 envelope — 110 mm bore, 170 mm outside diameter, 28 mm width per bearing. Spindle grade, universally ground, with a phenolic cage and medium preload built in at the factory.

Decoding the designation

3MM — Fafnir super precision series, 25° contact angle. The MM prefix carries P4S tolerance (ABEC 7 running accuracy with tighter ABEC 9 bore and outside diameter control).
9122 — the Fafnir 91xx block maps to the ISO 70xx dimension series, so this is the 7022 envelope: 110 x 170 x 28 mm.
WI — Fafnir super precision angular contact spindle bearing.
CR — phenolic (cotton-reinforced laminate) cage, outer ring land riding.
DUM — duplex, universally ground, medium preload.

What these specs mean in practice

The phenolic cage is the reason this bearing goes fast. Phenolic is roughly a fifth the density of bronze, so the centrifugal load on the cage at high rpm is far lower, and the material is slightly porous so it holds oil and behaves well through a marginal lubrication event. The trade-off is temperature: phenolic does not like sustained high heat, so it wants clean lubrication and a spindle that is not running hot. Outer ring land riding keeps the cage guided and concentric rather than letting it wander on the balls.

A 25° contact angle with medium preload is the setup for a spindle doing real cutting work at moderate to high speed. Medium preload buys axial and moment stiffness over light preload — less deflection under a side load, better surface finish, better tool life — at the cost of more internal friction and more heat. If your spindle is speed-limited rather than stiffness-limited, the light preload version (DUL) is the better choice.

Quantity and configuration

This listing is a set of 2 pieces — a duplex pair. Because they are universally ground, they can be mounted back to back (DB) for maximum moment stiffness and two-direction thrust, face to face (DF) where housing alignment is imperfect, or in tandem (DT) to double one-direction thrust capacity. The medium preload develops as the pair is clamped between the shaft shoulder and the locknut. Do not insert shims or a spacer unless you are deliberately changing the preload.

Fitting notes

Grind the shaft and housing seats to a precision that matches the bearing — a few microns of runout and roundness — and make sure the abutment faces are square, or the bearing’s accuracy is thrown away before it turns. Keep the pair oriented as marked; the ground faces are matched and flipping one bearing changes the preload. Mount with controlled heat or a proper press on the correct ring only, never through the balls. With a phenolic cage, lubrication discipline matters more than usual: use a clean spindle grease or an oil mist or oil-air system, keep the grease fill light, and avoid running the spindle hot.

Interchange guidance

Fafnir super precision is now supplied under the Timken brand. Same-envelope 25° equivalents include NSK 7022A5, FAG B7022-E-T-P4S, SKF 7022 ACD/P4A and NTN 7022 in the corresponding precision grade. The 7022C reference carried on this listing shares the 110 x 170 x 28 mm envelope; note that C denotes a 15° contact angle in FAG, NSK and SKF nomenclature, so it is kept here as a dimensional cross-reference rather than a contact angle claim — this bearing is 25°. The true 15° Fafnir sibling is 2MM9122WI. Never mix contact angles or cage types inside one preloaded stack.