Fafnir3MM9115WIDUMSuper Precision Angular Contact Ball Bearing 75x115x20 mm, 25° Contact Angle, P4S, Phenolic Cage, Duplex Universally Ground, Medium Preload

Product Description

What this part is

A super-precision angular contact ball bearing in the 7015 envelope — 75 mm bore, 115 mm outside diameter, 20 mm width — built to Fafnir’s MM super-precision spindle-bearing standard. Open (no shields or seals), 25° contact angle, phenolic cage, and ground so that it can be stacked into a duplex set with a defined medium preload. Fafnir super-precision production passed to The Timken Company, so this designation is the direct ancestor of Timken’s current 2MM/3MM world numbering.

Decoding the designation

3MM — Fafnir super-precision prefix carrying a 25° contact angle. (The companion 2MM prefix is 15°; the two are not interchangeable.)
MM prefix — implies precision class P4S (ABEC 7/9 tolerances), not plain P4. A cross-reference quoting P4 for an MM number is reading the class down.
9115 — the 7015 dimension series, 75 x 115 x 20 mm.
CR — phenolic, outer-land-riding cage.
DUM — DU = duplex universally ground, M = medium preload. DU and SU (single universally ground) are mutually exclusive, so any cross-reference asserting both is wrong.
WI — carton and ring marking carried on this part; Fafnir published no public decode for it, so it is retained as a search term rather than translated into a spec.

What these specs mean in practice

The 25° contact angle is the stiffer of the two common spindle choices. It carries substantially more thrust per unit of radial load than a 15° bearing and holds axial position better under interrupted cutting, at the cost of a lower speed ceiling and more heat generation at a given rpm. If the spindle you are rebuilding was originally supplied with 15° bearings, fitting 25° will change its stiffness and thermal behaviour — check the machine builder’s spec before substituting.

The phenolic outer-land-riding cage is the reason these run quietly and tolerate marginal lubrication: phenolic is self-lubricating, absorbs oil, and is far gentler on the balls than a pressed-steel cage during momentary oil starvation. It is also the limiting component on temperature — phenolic cages are not for high-temperature service.

Medium preload sits between light (highest speed, lowest stiffness) and heavy (highest stiffness, most heat). It is the usual default for general machine-tool spindles.

Published ratings and mounting data

No dynamic or static load rating, and no limiting speed, is published for this legacy Fafnir designation by a source we will stand behind. We have deliberately left those cells blank rather than borrow figures from an adjacent designation — ratings for 2MM9115 (15°) or for other members of the 9115 family are not this bearing’s ratings, and substituting them would be misleading. Contact Timken with the full designation if a certified rating is required for a calculation.

Quantity and configuration

A DUM designation describes a matched pair — two bearings ground as a set. Duplex sets in surplus are frequently split, so the number of physical rings on the shelf may not be a whole number of pairs. We are therefore making no claim about how many matched pairs are available. Ask before ordering if you need a complete, factory-matched set rather than individual universally ground rings, and we will count and confirm.

Fitting notes

Universally ground rings can be mounted back-to-back (DB), face-to-face (DF) or in tandem (DT); back-to-back is standard for spindle work because it gives the widest effective bearing spread and the best resistance to moment loads. Preload is built into the ground ring faces, so do not shim or grind the faces — doing so destroys the matched preload. Mount with the marked faces together per the ring etching, and clamp both rings fully; a partially clamped duplex set loses its preload entirely.

Interchange guidance

The ISO base number is 7015. Same-envelope 25° super-precision equivalents from other makers exist, but contact-angle suffix letters mean different things in different schemes — NSK reads A5 as 25° and A as 30°, SKF reads AC as 25°, and Timken reads E as 25° and C as 15°. Always confirm which scheme a candidate part number is written in before treating it as a substitute.