Fafnir3MM9124WISUMSuper Precision Angular Contact Ball Bearing 120 x 180 x 28 mm, 25° 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

3MM — Fafnir super precision series with a 25° 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 70xx series is the thin-section spindle family: a comparatively slim ring for a 120 mm bore, which keeps mass and outside diameter down and lets the bearing run faster than a heavier 72xx section would. It is the standard choice for machine tool main spindles where the shaft diameter is set by torsional stiffness rather than by bearing load.

A 25° contact angle sits in the middle of the spindle range. Against a 15° bearing it takes substantially more thrust and gives a stiffer axial response; against a 30° or 40° bearing it runs faster and generates less heat. Medium preload follows the same logic — more rigidity under cutting load than light preload, more heat generated at speed. That pairing suits a spindle doing heavier work at moderate rpm rather than one chasing maximum surface speed.

The phenolic outer land riding cage is the speed-friendly choice. 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 instead of being shoved around by the balls. 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 and can turn a medium-preload stack into a clamped one. 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 25° equivalents include NSK 7024A5, FAG B7024-E-T-P4S, SKF 7024 ACD/P4A, RHP 7024-E-T-SUM-P4 and NTN 7024 in the corresponding precision grade. The 7024-A reference also carried on this listing shares the 120 x 180 x 28 mm envelope; note that a plain A suffix in NSK nomenclature indicates a 30° contact angle, so it is kept here as a dimensional cross-reference rather than a contact angle claim. The 15° sibling is 2MM9124WI — a different bearing, 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.