FafnirMM9104WIDBSuper-Precision Spindle Angular Contact Ball Bearing Pair – 20 mm ID, 42 mm OD, 12 mm Width, Open, Back-to-Back, ABEC 7/9, 2 Pcs

Product Description

What this part is

Fafnir MM9104WIDB super-precision spindle angular contact ball bearings, supplied as a matched back-to-back pair. Bore: 20 mm. Outside Diameter: 42 mm. Overall Width: 12 mm per bearing, 24 mm for the pair. Number Of Rows: 1 per bearing. Rolling Elements: Ball. Bore Type: Straight. Closure Type: Open. Bearing Material: chrome bearing steel. Series: 9100WI, the Fafnir spindle series that maps to ISO 70xx, so this size is a 7004.

Decoding the designation

Precision Class: P4S, equivalent to ABEC 7/9 — the Fafnir MM prefix carries that class. Mounting Arrangement: DB, back-to-back. Cage Material: laminated phenolic. Cage Type: machined, outer-ring land-riding, from the CR token on the carton. Bore code 9104 maps to ISO 7004, a 20 mm bore. Searchable forms of the number: MM9104WIDB, MM-9104-WI-CR-DB, MM 9104 WI CR DB, MM9104WI, 7004-C.

What these specs mean in practice

A back-to-back pair puts the two contact-angle lines diverging outward, which spreads the effective load centres further apart than the physical bearing width and gives the mount its resistance to overturning moment. That is the arrangement a spindle nose or a screw-support position uses when it has to hold angular stiffness under a radial load applied off the bearing centre. Both directions of thrust are taken, one bearing per direction. P4S means ABEC 7 dimensional tolerances with ABEC 9 running accuracy, which is what holds spindle runout down at speed. The phenolic cage rides on the outer ring land, so it is guided by a stationary surface, and its low mass and self-lubricating character keep cage friction and heat down.

Quantity and configuration

Unit Quantity: 2 Pcs. Two bearings, matched and ground as a back-to-back set. Internal Clearance: N/A. A universally ground super-precision pair is preload-set rather than clearance-set.

Fitting notes

Lubricant: grease or oil. Maximum Operating Temperature: 120 °C, set by the phenolic cage. The high point of radial runout is marked on the rings; aligning those marks between the two bearings and opposing them to the shaft or housing eccentricity minimises assembled runout.

Interchange guidance

Carton designations carried on this part: MM 9104 WI DB/DF/DU, 7004 C P4A DGA and 7004 C. The DF and DU tokens on the same carton are the face-to-face and universal-single forms of the identical bearing, which is why Fafnir prints all three. The 20 x 42 x 12 mm envelope is shared with SKF 7004 CD/P4A, NSK 7004CTRSU, RHP 7004CTSULP4 and FAG B7004-C-T-P4S-UL.