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 — Fafnir super-precision series, 25° contact angleThe MM prefix implies ABEC 7/9 (P4S) tolerance class, not plain P4311 — ISO 7311 envelope, 55 x 120 x 29 mmWI — Fafnir super-precision spindle bearing designCR — phenolic outer-land-riding cageDUL — 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, but it must be handled dry and kept away from solvents and steam.

Published specifications

Bore: 55 mmOutside diameter: 120 mmWidth: 29 mm each ring setTotal width mounted as a pair: 58 mmContact angle: 25°Tolerance class: ABEC 7/9 (P4S)Preload: lightCage: phenolic, outer-land ridingRows: single row each, mounted as a duplex pair

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. Do not split the pair across two machines or mix one of these with a bearing from another set — the preload comes from the matched grind, not from a spacer.

Fitting notes

Handle by the outer ring, keep the set together and mount clean. Duplex pairs are direction-sensitive: mark the faces before removal so the original orientation is reproduced. Housing and shaft seats should be ground to a matching precision class, otherwise the bearing’s own accuracy is wasted. Grease sparingly or use an oil-mist/oil-air system suited to the speed.

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.