FafnirMM92EXInch-Series Super Precision Ball Bearing Pair 1-3/4 x 3-7/16 x 13/16 in (44.45 x 87.31 x 20.64 mm), ABEC 7 / P4 Class, Phenolic Outer-Land Cage, 2 Pieces

Product Description

What this part is

A pair of inch-series super precision ball bearings from Fafnir’s MM range — 1-3/4 in bore, 3-7/16 in outside diameter, 13/16 in width per bearing (44.45 x 87.31 x 20.64 mm). This is the precision inch family that turns up in older American-built spindles, where the shaft was cut to an inch dimension and no metric bearing will fit.

Decoding the designation

MM — Fafnir super precision series. The MM prefix carries ABEC 7 / ISO P4 tolerances as standard.
92 — size code within the inch MM range, corresponding to the 1-3/4 in bore.
EX — Fafnir execution suffix, carried here as a searchable reference rather than published as a specification.
CR — phenolic outer ring land riding cage.

Correcting the source data

Earlier copy described this as an angular contact ball bearing. That needs a caveat, because Fafnir drew a clear distinction inside the MM family: the MM-WI designations (2MM…WI and 3MM…WI) are the angular contact spindle bearings, identified by a low shoulder on the outer ring and a stated contact angle in the prefix. A plain MM number without the WI element is the single-row radial super precision bearing — deep groove raceway geometry held to ABEC 7 tolerances. There is no WI in MM92EX and no contact angle code, so no contact angle is published here.

The practical consequence is a good one. Because the raceway is a radial deep groove form rather than an inclined contact, this bearing carries thrust in either direction on its own and does not have to be opposed and preloaded against a second bearing to work. Fafnir positioned exactly this range for precision applications where a duplex angular contact set is not a definite requirement — woodworking spindles, aircraft accessory units, and machine tool positions that need accuracy and speed more than they need one-direction thrust stiffness.

What these specs mean in practice

ABEC 7 running accuracy means the bearing itself will hold runout to a few microns. Whether the spindle does depends entirely on the seats it is mounted in. The value of a super precision bearing in a radial deep groove form is that you get that accuracy and the high speed capability of a low-friction geometry, with the convenience of a bearing that locates the shaft in both directions by itself.

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 pushed 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

This listing is a set of 2 pieces — two bearings, open on both sides, with no seals or shields. Used together at the two ends of a spindle, or stacked at one end for extra capacity, they give a matched pair of the same precision grade from the same source, which is worth having when both positions in a spindle need the same accuracy.

Fitting notes

Hold shaft journal and housing bore runout and roundness to a few microns and keep abutment faces square, or the bearing’s accuracy is thrown away before it turns. 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, which on a 13/16 in wide bearing is easy to do by accident and leaves damage you cannot see. Because the bore is an inch dimension, check the shaft with inch gauging rather than assuming a nominal metric size; a 44.45 mm shaft and a 45 mm shaft are not the same fit. Lubricate with a spindle grease or oil mist rated for the intended 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. Inch-series super precision bearings have no exact metric equivalent: the nearest metric envelopes to 44.45 x 87.31 x 20.64 mm are the 45 mm bore 7209 and 6209 sizes, which are close but not the same, and substituting one will change both the shaft fit and the housing fit. If you are replacing this bearing in an existing inch spindle, match the Fafnir number. If you are designing new, move the whole position to a metric bearing rather than mixing. The angular contact members of the same family are 2MM…WI (15°) and 3MM…WI (25°) — a different bearing type, carried here as searchable references because buyers commonly search the family that way.