Fafnir3MM204WICRDULSuper Precision Spindle Angular Contact Ball Bearing 20x47x14 mm, 25 Degree Contact Angle, Phenolic Outer-Land-Riding Cage, Duplex Universal Light Preload, ISO P4S

Product Description

What this part is

A Fafnir super-precision spindle angular contact ball bearing, 20 mm bore × 47 mm outside diameter × 14 mm width, 25° contact angle, ground to duplex universal light preload. Mounted as a back-to-back pair the set occupies 28 mm of shaft length. Fafnir is now part of Timken, and the MM-prefix spindle range remains in current production.

Decoding the designation

3MM — super-precision spindle bearing at the high contact angle, 25°. Timken’s own catalogue footnote states that 3 is the Timken designation and E is the ISO designation for 25°, and 2MM is the corresponding 15° series. The MM prefix also carries the precision spec, below.
204 — 72-series bore code, 20 mm bore in the 47 mm outside diameter envelope.
WI — Timken’s angular contact geometry code: full shoulders both sides of the inner ring and a low shoulder on the outer ring, so the bearing carries thrust in one direction and is non-separable.
CR — composition cage, that is phenolic, riding on the outer-ring land. Standard service to 93 °C.
DUL — flush-ground duplex pair, light preload. DUM is medium, DUH heavy, DUX extra-light; SUL is the single flush-ground equivalent.

Precision class — what MM actually buys you

The MM prefix denotes ISO P4S, which is not the same as P4. Timken’s specification is that running accuracy and performance meet ABEC 9 (ISO P2) levels while other features conform to ABEC 7 (ISO P4) requirements. So a world number that renders this bearing as plain P4 is understating it — the running accuracy is a class higher than the dimensional tolerance. That matters when the bearing is being selected for spindle runout rather than fit.

Published ratings and mounting data

Dynamic load rating Cr — 15,500 N (3,490 lbf)
Static load rating Cor — 7,700 N (1,730 lbf)
Ball complement — 10 balls of 7.94 mm diameter
Preload force, DUL — 130 N
Speed, single bearing (Ng) — 32,100 rpm
Speed, DUL pair mounted back-to-back — 25,740 rpm grease, 43,740 rpm oil
Assembled pair width — 28 mm

Three different speed conventions circulate for these bearings — a single-bearing Ng figure, a mounted-pair figure by preload class, and a generic catalogue limiting speed — and they disagree with each other. The figures above are labelled by convention so they can be compared like for like.

What these specs mean in practice

25° with light preload is the middle-ground spindle specification: meaningfully more thrust capacity and axial stiffness than a 15° bearing, but preloaded lightly so it still turns fast and runs cool. It suits milling and machining-centre spindles that see real cutting thrust but also need speed.

The phenolic cage is a genuine selling specification rather than a detail. Riding on the outer-ring land, it damps ball-passage excitation, tolerates marginal lubrication far better than steel or brass, and is what makes very high dN values practical. Its limit is temperature — phenolic is a 93 °C part, so it is the wrong cage for a hot-running or high-ambient position.

Fitting notes

Flush-ground bearings must be kept as a set and stacked with their etched face marks aligned; the light preload is produced by clamping the ground faces together and cannot be adjusted afterwards. Observe thrust direction — the low outer shoulder identifies the unloaded side. Ship preservative must be removed and the correct spindle lubricant applied before running.

Interchange guidance

The ISO world number for this bearing is 7204E-T-P4S-DUL: E for 25°, T for the phenolic machined cage, P4S for the Fafnir MM precision spec. Schaeffler format is B7204-E-T-P4S-DUL. The 15° sibling in the same envelope is 2MM204WI CR DUL and is not interchangeable — different contact angle, different ratings. Fafnir spindle bearings are now sold under the Timken brand.