Fafnir3MM212WIDUMSuper-Precision Angular Contact Ball Bearing 60 x 110 x 22 mm, 25°, Duplex Medium Preload, Phenolic Cage

Product Description

What this part is

A Fafnir 3MM212WI-CR-DUM super-precision angular contact ball bearing, 60 mm bore by 110 mm outside diameter by 22 mm wide, supplied as a universally ground duplex pair set to medium preload. Fafnir is Timken’s super-precision spindle bearing line, and the WI family is its standard single-row angular contact spindle bearing. The ISO equivalent envelope is the 7212 size.

Decoding the designation

The leading 3 in 3MM gives a 25° contact angle. The MM prefix itself denotes P4S precision, which is ABEC 7 on dimensional tolerances with ABEC 9 running accuracy. With no 9 ahead of the size block the bearing sits in the ISO 72 dimension series, and the bore code 212 gives 60 mm. WI identifies the super-precision angular contact family. CR is a phenolic cage riding on the outer ring land. DUM is a duplex universally ground pair at medium preload.

What these specs mean in practice

A 25° contact angle carries more thrust and less speed than a 15° bearing of the same size, which makes this the choice where a spindle sees heavy axial cutting load rather than top rpm. The outer-land-riding phenolic cage is light, self-lubricating and quiet, and it is the reason super-precision spindle bearings run at the speeds they do; it also sets a practical temperature ceiling well below that of a brass cage. Medium preload sits between the light preload used for maximum speed and the heavy preload used for maximum stiffness.

Quantity and configuration

2 Pcs, one matched duplex pair. Universally ground rings allow the pair to be mounted back-to-back, face-to-face or in tandem, with the preload established by the ground offsets rather than by shims.

Fitting notes

Duplex pairs are mounted with the marked faces together in the arrangement chosen, and the rings are clamped fully against their shoulders so the ground offset sets the preload. Shaft and housing fits for a P4S spindle bearing are held far tighter than for a general-purpose bearing, and the bearing is fitted clean and lubricated to its final grease or oil specification before clamping.

Interchange guidance

Direct interchange requires the same 25° angle, the same P4S class and the same duplex medium preload: an NSK 7212A5 or an SKF 7212 ACD at P4A in a DUM-equivalent pair is the same specification. A 7212 in the 15° C or 40° B angle is a related size in the same envelope, not a substitution, because the load split between radial and axial changes with the angle. The Fafnir 2MM212WI is the 15° version of this same size.