Fafnir3MM9124WICRDULSuper-Precision Angular Contact Ball Bearing Duplex Pair, 120 mm Bore, 180 mm OD, 28 mm Width, 25 Degree Contact Angle, Light Preload, ABEC 7 / P4S

Product Description

What this part is

A matched pair of Fafnir super-precision single-row angular contact ball bearings, 120 mm bore, 180 mm outside diameter, 28 mm width each. Open on both faces, machined phenolic cage, chrome steel rings and balls. Fafnir is Timken’s super-precision line, so the pair is marked and catalogued under both names.

Decoding the designation

The leading 3 of 3MM sets a 25° contact angle. MM carries P4S running accuracy, equivalent to ABEC 7 with ABEC 9 tolerances on the critical features. The 9 that opens 9124 places the bearing in the ISO 70 light dimension series and 24 is the bore code for 120 mm, which is why the envelope matches a 7024. CR is the outer-land-riding phenolic cage. DUL is a duplex set, universally ground, at light preload.

What these specs mean in practice

A 25° contact angle carries more thrust and less speed than a 15° bearing in the same envelope, which suits spindle positions where axial cutting load dominates. Light preload gives lower running torque and less heat than medium or heavy preload, at the cost of some axial stiffness. Universal grinding means the two rings can be set back-to-back, face-to-face or in tandem and the preload comes out at the designed value without shims. The outer-land-riding phenolic cage is light and quiet at speed, and it sets a working ceiling near 120 °C.

Quantity and configuration

2 Pcs, supplied as one matched duplex pair. Open both faces, without snap ring.

Fitting notes

Back-to-back mounting of the pair gives the widest effective bearing spread and the greatest resistance to moment loading; face-to-face gives a narrower spread and tolerates a little more shaft misalignment; tandem stacks both rows against thrust in one direction. Ring faces carry the arrangement marks that indicate which faces meet. Mounting is by cold press or induction heat on the inner rings, with no load path through the balls, and the rings are kept as a set.

Interchange guidance

A same-spec equivalent duplex pair is the SKF 7024 ACD/P4ADGA. A 2MM9124WI is the same envelope at a 15° contact angle, and a DUM or DUH suffix is the same pair at medium or heavy preload; those are related sizes rather than substitutions, because contact angle and preload change the stiffness and speed behaviour of the mounting. A single SU bearing does not meet a duplex set requirement.