Fafnir3MM305WICRDUMSuper Precision Angular Contact Ball Bearing 25 x 62 x 17 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 7305 envelope — 25 mm bore, 62 mm outside diameter, 17 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 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).
305 — the Fafnir 3xx block maps to the ISO 73xx dimension series, so this is the 7305 envelope: 25 x 62 x 17 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 7305 is a heavy-section bearing for its bore — the 73xx series puts a much thicker ring and a larger ball complement around a 25 mm shaft than the 72xx or 70xx series would. That is the whole point of choosing it: stiffness and load capacity on a small shaft, where the shaft diameter is fixed by the machine and the loads are not.

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 reasoning — more rigidity under cutting load than light preload, more heat at speed. Medium is the right pick where the spindle is doing real work at moderate rpm rather than 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 instead of being shoved around by the balls. The trade-off is that 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 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 7305A5, FAG B7305-E-T-P4S, SKF 7305 ACD/P4A, RHP 7305-E-T-SUM-P4 and NTN 7305 in the corresponding precision grade. The 15° sibling of this bearing is 2MM305WI — a different bearing, and not a substitute inside an existing preloaded set. The DUM element that also appears in the source record is the duplex reading of the same preload class; SU and DU are mutually exclusive, so the single reading is published here and DUM is carried as a search term only.