Fafnir3MM9105WIDUMSuper-Precision Angular Contact Ball Bearing 25 x 47 x 12 mm, 25 Degree Contact Angle, P4S, Phenolic Cage, Universally Ground Duplex Pair, Medium Preload, 2 Pcs

Product Description

What this part is

A matched duplex pair of Fafnir (now Timken) super-precision angular contact ball bearings in the 7005 envelope — 25 mm bore, 47 mm outside diameter, 12 mm width per bearing. Two pieces ship as one universally ground set with medium preload built in, for machine-tool spindles, grinding heads and high-speed rotary assemblies where axial stiffness matters.

Decoding the designation

3MM — Fafnir super-precision angular contact series, 25° contact angle; the MM prefix carries P4S tolerances (ABEC 7/9).
9105 — the 7005 size, 25 x 47 x 12 mm.
WI — Fafnir super-precision angular contact design code.
CR — phenolic outer-land-riding cage.
DU — duplex, universally ground.
M — medium preload.

Correcting the source data

Earlier copy on this listing described a machined cage and quoted load ratings and a 56,000 rpm limiting speed. The CR code means a phenolic cage, not a machined metal one, and those ratings did not hold up against the manufacturer’s own data, so they have been withdrawn rather than repeated. Size, contact angle, tolerance class and preload grade below are read straight from the designation.

What these specs mean in practice

Medium preload is the middle setting of three, and it is the practical difference between this bearing and the otherwise identical DUL. More preload means a stiffer spindle — less axial and radial deflection under cutting load, better surface finish, tighter tool position — paid for in friction, heat and a lower usable speed. Light preload spins faster and cooler; heavy preload is stiffer still but generates real heat. The 25° contact angle carries substantially more thrust than a 15° bearing in the same envelope while conceding some speed capability. Universal grinding means clamping the pair together sets the specified preload on its own, with nothing to shim. The phenolic cage is light, quiet and self-lubricating, but dislikes aggressive solvents and sustained heat.

Quantity and configuration

2 Pcs — one universally ground duplex pair, both bearings shipping together as a set.

Fitting notes

Keep the pair together; swapping in a bearing from another set throws away the ground preload. Mount back-to-back for maximum moment stiffness, or face-to-face where shaft and housing alignment is less certain. A medium-preload set is less forgiving of a tight interference fit than a light one, because the fit adds preload on top of what is already ground in — check shaft and housing sizes against the spindle drawing before pressing. Seats must be ground, clean and burr-free, or the P4S tolerance is wasted. Lubricate sparingly with a spindle grease or run oil-air.

Interchange guidance

Fafnir super-precision trades under the Timken name today; the number is unchanged. Genuine 25° medium-preload cross-references in this envelope include SKF 7005 ACD/P4ADGB, FAG B7005-E-T-P4S-UM, NSK 7005A5 in a DUM set and RHP 7005E to medium preload. Related sizes share the same 25 x 47 x 12 mm envelope with a different contact angle — 2MM9105WI is the 15° build, a different part rather than a substitute. Preload grade is not interchangeable either: DUL (light) and DUH (heavy) also exist in this size and change spindle stiffness and speed.