Fafnir3MM224WI3SULSuper Precision Angular Contact Ball Bearing 120 x 215 x 40 mm, 25° Contact Angle, P4S / ABEC 7 Class, Heavy 72xx Section, Single Universal Light Preload

Product Description

What this part is

A super precision angular contact ball bearing in the 7224 envelope — 120 mm bore, 215 mm outside diameter, 40 mm width. This is Fafnir’s spindle bearing range in a heavy section: single row, open, universally ground, and held to the tolerance grade machine tool builders specify for main spindle and precision rotary 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).
224 — the Fafnir 2xx block maps to the ISO 72xx dimension series, so this is the 7224 envelope: 120 x 215 x 40 mm.
WI — Fafnir super precision angular contact spindle bearing.
SUL — single, universally ground, light preload.

What these specs mean in practice

The 7224 is a deliberately heavy choice for a 120 mm shaft. Compare it with the 7024 in the same bore: 215 mm outside diameter against 180, 40 mm wide against 28. All of that extra section goes into a larger ball complement and thicker rings, which buys load capacity and stiffness at the cost of mass and limiting speed. You specify a 72xx over a 70xx when the spindle or table has to hold position under heavy cutting or heavy overhung load, not when you are chasing rpm.

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. Light preload then pulls back the other way — minimum internal friction, minimum heat generation, the highest permissible speed of the three preload classes, at the cost of some stiffness. A heavy section at light preload is a coherent combination: get the capacity from the geometry rather than from clamping the bearing up tight.

Quantity and configuration

The SUL 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 light 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. At this size and mass, thermal mounting is the practical method — heat the inner ring evenly in an induction heater or oil bath to a controlled temperature and slide it onto the journal. Never press through the balls and never use a torch. When building a set, keep the marked faces oriented as intended; reversing one bearing changes the preload and can turn a light-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 keep the fill light.

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 RHP 7224-E-T-SUL-P4 (RHP E is 25°, so this is a true contact angle match), SKF 7224 ACD/P4A (SKF AC is 25°, also a true match), NSK 7224A5, FAG B7224-E-T-P4S and NTN 7224 in the corresponding precision grade. The 7224A reference also carried on this listing shares the 120 x 215 x 40 mm envelope; a plain A suffix in NSK nomenclature indicates 30°, so it is kept as a dimensional cross-reference rather than a contact angle claim. The 15° sibling is 2MM224WI — a different bearing, not a substitute inside an existing set. The -3 element in the source designation is an order or customer specific suffix and is carried here as a searchable reference rather than published as a specification.