Fafnir3MM9312WICRDUMThin-Section Super Precision Angular Contact Ball Bearing, Duplex Pair, 60 x 85 x 13 mm, 25 Degree Contact Angle, Medium Preload, Phenolic Outer-Land-Riding Cage, ABEC 7/9

Product Description

What this part is

Fafnir 3MM9312WI-DUM, a Timken super precision angular contact ball bearing for machine tool spindles, supplied as a universally ground duplex pair at medium preload. Brand: Fafnir (Timken). Bore: 60 mm. Outside Diameter: 85 mm. Overall Width: 13 mm per bearing. Total Width: 26 mm as a duplex pair. Contact Angle: 25°. Preload: Medium. Precision Class: ISO P4S, ABEC 7/9. Arrangement: DU, duplex universally ground. Number Of Rows: 2, one row per bearing. Cage Material: Phenolic, Timken cage designation MBR. Cage Type: CR, outer-land-riding. Rolling Elements: Steel balls. Bearing Material: Chrome steel. Closure Type: Open. Bore Type: Straight. Lubricant: Oil. Running Accuracy: High precision. Series: 9300WI, 3MM super precision metric, the ISO 19-series and 71900 thin-section envelope. Base Number: 3MM9312WI. Design Units: Metric. Item Weight: 1.1 lb. Harmonization Code: 8482.10.50. Timken’s published world part number is 71912E-T-P4S-DUM.

Published ratings and mounting data

Dynamic Radial Load Rating C1 at 1 million revolutions: 4,820 lbf, 21,400 N. C1 ISO Dynamic Radial Rating per row: 4,380 lbf, 19,500 N. Static Radial Load Rating C0: 4,270 lbf, 19,000 N. Limiting Speed: 15,200 rpm on grease, 27,600 rpm on oil. Maximum Shaft Fillet Radius: 1 mm. Shaft Backing Shoulder Diameter: 65.3 mm. Housing Backing Shoulder Diameter: 80.2 mm. Outer Ring To-Clear Radius: 1 mm. Operating Temperature Range: -40 °C to 100 °C. These are Timken’s published catalogue figures for 3MM9312WI DUM and belong to the 25° 3MM variant; the 15° 2MM9312WI carries different ratings. On a thin-section bearing the static and dynamic ratings sit close together, which is normal for this envelope.

Decoding the designation

3MM: Fafnir and Timken super precision metric series, 25° contact angle. 9312: 9300WI dimension series, 60 mm bore, the thin-section family equivalent to the ISO 19-series or 71900 envelope. WI: the standard Fafnir super precision spindle angular contact ball bearing series. CR: phenolic outer-land-riding cage, Timken designation MBR. DU: duplex universally ground, a matched pair that mounts in any orientation without shims. M: medium preload. The MM prefix carries ISO P4S, ABEC 7/9. Searchable variants: 3MM9312WI-DUM, 3MM9312WIDUM, 3MM-9312-WI-CR-DUM, 3MM9312WICRDUM, 3MM 9312 WI CR DUM, 3MM9312WI, 71912E-T-P4S-DUM, 71912E, 71912, 7912A, 71912-A5.

What these specs mean in practice

The thin section is the defining feature of the 9300WI and 19-series envelope. At 60 mm bore the outside diameter is only 85 mm and the width only 13 mm, so the bearing fits where a 200-series bearing never would. That compactness costs capacity: the smaller balls and shallower cross-section carry less load than a heavier series on the same shaft. Thin-section super precision bearings suit a space-constrained design where stiffness per millimetre matters more than absolute rating.

The 25° contact angle is the rigidity-biased option. Against the 15° version of the same envelope, the steeper angle carries more thrust and deflects less axially under load, which suits a spindle taking real cutting thrust or a screw-driven axis. It costs speed: a steeper angle increases ball spin and cage loading, so limiting speed and heat generation at a given rpm are both worse than the 15° part. 25° buys stiffness; 15° buys rpm.

Medium preload sits between the extremes: more axial stiffness and better load sharing between the two rows than light preload, with less friction, heat and grease consumption than heavy. Preload class is part of the spindle design; a spindle designed around DUM loses its designed stiffness with a DUL set and runs hotter with a DUH set.

The phenolic outer-land-riding cage is what keeps a 25° bearing viable at speed. Phenolic is light and mildly self-lubricating, generating far less heat than a steel cage, and riding on the outer ring land lets centrifugal force stabilise the cage instead of loading it. Phenolic is attacked by some solvents and depends on the oil film.

Quantity and configuration

Unit Quantity: 1 matched duplex pair, two bearings. DUM designates a duplex pair, so the sale unit is two bearings ground and matched to each other as a set, supplied and priced as one set.

Fitting notes

Universally ground rings allow the pair to be mounted back-to-back, face-to-face or in tandem, assembled in the orientation the machine calls for and clamped square. Preload develops from clamping the ground faces solidly together, so a burr, a dirt film or an out-of-square abutment translates directly into wrong preload and lost running accuracy, and a thin-section bearing with 13 mm of width has very little section to absorb a distorted mounting. The 65.3 mm shaft and 80.2 mm housing backing shoulder diameters apply, with 1 mm fillet clearance. Handling is by the outer ring, in the original packaging until assembly, with a progressive run-in before full speed.

Interchange guidance

Timken’s published world equivalent is 71912E-T-P4S-DUM, where E denotes the 25° contact angle, T the phenolic cage and P4S the precision grade. The FAG-scheme form B71912E-T-P4S-DUM describes the same 25°, phenolic-caged, P4S, medium-preload duplex execution in the same envelope. Same-envelope related sizes: SKF 71912 ACD/P4ADGB, NSK 71912A5TRDUMP4, and the 15° 2MM9312WI or 71912C. A matching substitute carries the contact angle, precision grade, preload class, cage material and arrangement together alongside the 60 x 85 x 13 mm envelope. Fafnir super precision bearings are supplied under the Timken brand, so Fafnir and Timken designations of this form are the same product line.