Fafnir2MM9124WIDUMSuper Precision Angular Contact Ball Bearing 120 x 180 x 28 mm, 15° Contact Angle, P4S / ABEC 7 Class, Phenolic Cage, Single Universal Medium Preload

Product Description

What this part is

A super precision angular contact ball bearing in the 7024 envelope — 120 mm bore, 180 mm outside diameter, 28 mm width. This is Fafnir’s spindle bearing range: single row, open, universally ground, and held to the tolerance grade machine tool builders specify for main spindle positions.

Decoding the designation

2MM — Fafnir super precision series with a 15° contact angle. The MM prefix carries P4S tolerance (ABEC 7 running accuracy with tighter ABEC 9 bore and outside diameter control).
9124 — the Fafnir 91xx block maps to the ISO 70xx dimension series, so this is the 7024 envelope: 120 x 180 x 28 mm.
WI — Fafnir super precision angular contact spindle bearing.
CR — phenolic outer ring land riding cage.
SUM — single, universally ground, medium preload.

What these specs mean in practice

The 15° contact angle is the speed-biased choice. A shallow angle keeps the ball from climbing the raceway shoulder, which cuts ball spin, friction and heat, so the bearing runs faster and cooler than the 25° version of the same envelope. What you trade away is thrust capacity and axial stiffness. If the limiting factor on your spindle is rpm — high-speed milling, grinding, precision turning — 15° is right. If it is deflection under heavy cutting load, the 25° bearing (Fafnir 3MM9124WI) is the better tool.

Medium preload partly offsets that. It gives more rigidity than light preload at the cost of some heat generation and speed, so a 15° bearing at medium preload is a deliberate middle setting: keep the low-friction geometry, buy back some stiffness through the clamp.

The phenolic outer land riding cage reinforces the same priorities. At roughly a fifth the density of brass it loads itself far less under centrifugal force, and the laminate holds a small oil reserve that helps the bearing survive a brief lubrication interruption. Outer land riding keeps the cage guided by the outer ring so it stays concentric at speed. In exchange, phenolic will not tolerate sustained high temperature, so lubrication has to be clean and grease fills light.

Quantity and configuration

The SUM suffix denotes a single universally ground bearing, and one bearing is what ships. Universal grinding is what makes a single useful: two or more of these clamp together into a back-to-back (DB) set for maximum moment stiffness and two-direction thrust, a face-to-face (DF) set where housing alignment is imperfect, or a tandem (DT) set to double one-direction thrust — each developing the specified medium preload with no shims and no spacer stack to calculate. Buy one to replace a damaged half of an existing pair, or the matching count to build a complete set.

Fitting notes

Super precision bearings only deliver their accuracy on seats ground to match. Hold shaft journal and housing bore runout and roundness to a few microns and keep the abutment faces square. Mount with controlled heat on the inner ring or a press acting only on the ring being fitted — never drive mounting load through the balls. When building a set, keep the marked faces oriented as intended; reversing one bearing changes the preload. Lubricate with a spindle grease or an oil mist or oil-air system sized for the bore and speed, and resist over-filling — excess grease in a preloaded bearing is a leading cause of early thermal failure.

Interchange guidance

Fafnir super precision is now supplied under the Timken brand and the Fafnir numbers remain in active cross-reference use. Same-envelope 15° equivalents include NSK 7024C, FAG B7024-C-T-P4S, SKF 7024 CD/P4A, RHP 7024-C-T-SUM-P4 and NTN 7024C in the corresponding precision grade. The 7024-C designation carried on this listing is consistent: C denotes 15° in FAG and NSK nomenclature, which matches the Fafnir 2MM prefix. The 25° sibling is 3MM9124WI — same dimensions, different contact angle, and not a substitute inside an existing preloaded set. DUM also appears in the source record; SU and DU are mutually exclusive, so the single reading is published and DUM is carried as a search term only.