Fafnir2MM9121WIDUMSuper-Precision Angular Contact Ball Bearing – 105 mm ID, 160 mm OD, 26 mm Width, Open, 15° Contact Angle, Machined Phenolic Cage, Medium Preload, Duplex Pair

Product Description

What this part is

Fafnir 2MM9121WI-CR-DUM super-precision angular contact ball bearing, 105 mm bore, 160 mm outside diameter, 26 mm width, in the 9100WI spindle series. Steel rings and balls in a machined phenolic cage guided on the outer ring land, open on both faces, supplied as a universally ground duplex pair set to medium preload.

Decoding the designation

2MM is the Fafnir super-precision prefix at a 15° contact angle, built to P4S tolerances. In the 9100WI series the last two digits are the bore code, so 21 is 105 mm and the envelope corresponds to an ISO 7021, which is how the carton also marks it. CR is the phenolic cage riding on the outer ring land. DUM breaks down as D for duplex, U for universally ground and M for medium preload, which is why two bearings make one set.

What these specs mean in practice

The 15° contact angle favours speed and radial stiffness; a 25° bearing in the same envelope trades speed for thrust capacity. Medium preload sits between the low-heat, high-speed setting and the maximum-stiffness setting, and is the usual choice where a spindle needs both position control under load and a useful speed range. The machined phenolic outer-land-guided cage is light, damps well and holds oil at the ball track. Dynamic radial rating is 75.5 kN (16,973 lbf) and static radial rating 76.5 kN (17,198 lbf) per bearing, with a limiting speed of 9,000 rpm. P4S tolerances hold bore, outside diameter, width and runout to machine tool spindle grade.

Quantity and configuration

Two bearings, one duplex pair, 52 mm across the pair. Universal grinding lets them be mounted back to back, face to face or in tandem and reach the ground medium preload when clamped solid. Sale unit is 2 Pcs.

Installation and setup notes

Preload is ground into the ring faces, so the pair clamps against square shoulders with no shims between the rings. Ring faces carry the match marking and the stack orientation follows the chosen arrangement. Oil lubrication suits the phenolic cage and the speed range. This bearing has no relubrication feature.

Interchange guidance

SKF 7021 CD/P4ADGB shares the 105 x 160 x 26 mm envelope, the 15° angle and medium-preload duplex grinding, as do NSK 7021CTYNDUMP4 and FAG B7021-C-T-P4S-DUM. The 3MM9121WI version of this number is a 25° bearing in the same envelope, a related size rather than a substitution, and a 71921 is a different envelope at 105 x 145 x 20 mm.