Fafnir3MM9122WICRDUMSuper-Precision Angular Contact Ball Bearing – 110 mm ID, 170 mm OD, 28 mm Width, 25° Contact Angle, Universally Matched Duplex Pair, Medium Preload, 2 Pc

Product Description

What this part is

A Timken Fafnir super-precision angular contact ball bearing in the 9100WI machine-tool spindle series, supplied as a universally matched duplex pair. Bore 110 mm, outside diameter 170 mm, width 28 mm per bearing, giving 56 mm of total stack width. The bearing is open on both faces, single row, with a phenolic outer-land-riding cage and a 25° contact angle.

Decoding the designation

In the Fafnir super-precision scheme the leading 3MM sets both the contact angle and the class: 3 indicates a 25° contact angle, and the MM prefix denotes P4S tolerances, equivalent to ABEC 7 dimensional accuracy with ABEC 9 running accuracy. 9122 is the size code, and the leading 9 marks the ISO 70 light series, so 9122 corresponds to an ISO 7022 envelope of 110 x 170 x 28 mm. WI identifies the machine-tool spindle bearing line. CR is a phenolic cage that rides on the outer ring land. DUM is a universally matched duplex pair set to medium preload.

What these specs mean in practice

A 25° contact angle carries substantially more thrust load than the 15° equivalent and gives greater axial stiffness, at the cost of a lower limiting speed. The phenolic outer-land-riding cage is light, self-lubricating and tolerant of oil-mist and oil-air lubrication, which is why it dominates grinding and high-speed milling spindles. Medium preload sits between the light preload chosen for maximum speed and the heavy preload chosen for maximum stiffness, and is the usual selection for general machine-tool spindle duty. Because the pair arrives preload-set at the factory, radial internal clearance is not a specification of this bearing.

Quantity and configuration

Two bearings, supplied as one universally matched duplex set. Universal grinding means the two rings can be assembled back-to-back for maximum moment stiffness, face-to-face for greater tolerance of shaft misalignment, or in tandem for thrust in one direction only, and the medium preload is developed in any of those arrangements without shims or spacers.

Fitting notes

Rings are ground with matched offsets, so the pair is installed together and in the orientation the arrangement calls for. Shaft and housing seats to P4S practice, and clamping is applied evenly across the full ring face so the ground offset is closed uniformly. Phenolic cages have an upper service temperature well below the steel, which sets the practical temperature ceiling for the assembly.

Interchange guidance

The ISO base number is 7022, and Timken’s own machine-tool spindle interchange lists FAG B7022-E-T-P4S-UM, SKF 7022 CDGB/P4A, NSK 7022A5 series, RHP 7022ET, GMN S6022, and SNFA EX110 7CE3 as the equivalent super-precision builds at matching angle and class. The 2MM9122WI variant shares the same 110 x 170 x 28 mm envelope but carries a 15° contact angle, which makes it a related size rather than a substitution. Light and heavy preload builds of the same number, DUL and DUH, are likewise different parts.