Fafnir3MM311WICRDULSuper Precision Angular Contact Ball Bearing Matched Duplex Pair 55x120x29mm 25 Degree ABEC 7 Phenolic Cage Light Preload

Product Description

What this part is

A matched duplex pair of Fafnir 3MM311WI super-precision angular contact ball bearings, universally ground for light preload (DUL). Each ring set measures 55 mm bore x 120 mm OD x 29 mm width, so the pair stacks to 58 mm overall. This is 7311-envelope hardware built for machine tool spindles, grinding heads, high-speed gearboxes and any position that has to take radial load plus thrust with tight running accuracy.

Decoding the designation

3MM is the Fafnir super-precision series at a 25° contact angle. The MM prefix implies ABEC 7/9 (P4S) tolerance class, not plain P4. 311 is the ISO 7311 envelope, 55 x 120 x 29 mm. WI is the Fafnir super-precision spindle bearing design. CR is a phenolic outer-land-riding cage. DUL is duplex, universally ground, light preload.

What these specs mean in practice

A 25° contact angle trades some radial capacity for substantially more thrust capacity and axial stiffness than a 15° bearing of the same size — the usual choice where the spindle carries real cutting thrust. Light preload keeps heat generation and power draw down, favouring higher speeds over maximum rigidity. The phenolic outer-land-riding cage is light, self-lubricating and tolerant of high speed.

Published specifications

Bore: 55 mm. Outside diameter: 120 mm. Width: 29 mm each ring set. Total width mounted as a pair: 58 mm. Contact angle: 25°. Tolerance class: ABEC 7/9 (P4S). Preload: light. Cage: phenolic, outer-land riding. Rows: single row each, mounted as a duplex pair. Dynamic load rating: 88 kN. Static load rating: 60 kN. Limiting speed: 12000 rpm. Mass: 2.80 lbs.

Quantity and configuration

Two bearings, supplied as the factory-matched set. Universally ground rings mean the pair can be arranged back-to-back (DB), face-to-face (DF) or in tandem (DT) and the ground offsets will still deliver the intended light preload.

Interchange guidance

Cross-references at the same 25°, ABEC 7, light-preload specification include 7311 super-precision duplex sets from other makers and the Fafnir/Timken 3MM311WI-CR-DUL and 3MM311WIDUL forms of the same number. Related sizes that are NOT interchangeable: 2MM311WI is the same envelope at 15°, and DUM/DUH variants carry medium or heavy preload. Contact angle and preload must both match the spindle design.