Fafnir3MM9140WISuper Precision Angular Contact Ball Bearing 200 x 310 x 51 mm, 25° Contact Angle, ABEC 7, Universally Ground Single Bearing

Product Description

What this part is

A single super precision angular contact ball bearing in the 7040 envelope — 200 mm bore, 310 mm outside diameter, 51 mm width. This is the large end of the spindle bearing range, the size found in horizontal boring mills, large vertical turning centres and heavy rotary and indexing tables.

Decoding the designation

3MM — Fafnir super precision series, 25° contact angle. The MM prefix carries P4S tolerance (ABEC 7 running accuracy with tighter ABEC 9 bore and outside diameter control).
9140 — the Fafnir 91xx block maps to the ISO 70xx dimension series, so this is the 7040 envelope: 200 x 310 x 51 mm.
WI — Fafnir super precision angular contact spindle bearing.

No SU or DU element appears in this designation, so no preload class is called out on the order code and the bearing is supplied as a single piece.

What these specs mean in practice

At 200 mm bore, the reason to reach for a 25° angular contact bearing rather than a deep groove ball or a cylindrical roller is stiffness under combined load. The 25° angle takes meaningful thrust as well as radial load and resists the tipping moment a long boring bar or a heavy faceplate puts into a spindle. Against a 15° bearing it gives more axial stiffness and thrust capacity; against a 30° or 40° bearing it runs faster and generates less heat, which matters a great deal at this diameter because surface speed at the ball track climbs quickly with bore size.

P4S tolerance means the bore, outside diameter and running accuracy are all held tightly enough that the spindle’s own geometry, not the bearing, becomes the limit on achievable runout. That is only true if the seats are ground to match — see the fitting notes.

Quantity and configuration

This listing is for a single bearing. Because the rings are universally ground, a single WI bearing can be used on its own or combined with a second matched bearing of the same designation to form a back-to-back (DB), face-to-face (DF) or tandem (DT) set, or with two more for a triplex arrangement. If you are rebuilding a spindle that ran a pair or a triplex, order the matching count — mixing a bearing from a different production batch into an existing preloaded stack will not give a predictable preload.

Fitting notes

At this size, thermal mounting is the practical method: heat the inner ring evenly in an induction heater or an oil bath to a controlled temperature and slide it onto the journal. Never press through the balls, and never use a torch. Shaft and housing seats need to be ground with runout and roundness held to a few microns, and the abutment faces need to be square, or the bearing will run with a built-in wobble no matter how good it is. Use a spindle-rated grease or an oil mist or oil-air system sized for the bore, and keep the fill light; a large preloaded bearing packed with grease will overheat before it ever reaches rated speed.

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 7040A5, FAG B7040-E-T-P4S, SKF 7040 ACD/P4A and NTN 7040 in the corresponding precision grade. The 7040A reference carried on this listing shares the 200 x 310 x 51 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 2MM9140WI — a different bearing, not a substitute inside an existing set.