Fafnir2MM9128WI SUMSuper-Precision Angular Contact Ball Bearing – 140 mm Straight Bore, 210 mm OD, 33 mm Width, 15° Contact Angle, Open, Phenolic Machined Cage, Medium Preload, Universally Ground

Product Description

What this part is:

Fafnir 2MM9128WI-SUM super-precision angular contact ball bearing, 140 mm bore, 210 mm outside diameter, 33 mm width. Single row, open on both faces, machined phenolic outer-land-riding cage. ISO base number 7028.

Decoding the designation:

2MM sets a 15° contact angle in the Fafnir WI super-precision scheme, and the MM prefix carries P4S precision — ABEC 7 dimensional tolerances with ABEC 9 running accuracy. The leading 9 of 9128 places the bearing in the ISO 70 light dimension series, so the ISO base number is 7028, matching the 7028-C marking. WI is the ground spindle execution, SU a single universally ground bearing, and M medium preload.

What these specs mean in practice:

Basic dynamic capacity is 145 kN and static capacity 125 kN, with a limiting speed of 5,400 rpm. The 15° contact angle favours speed and radial stiffness; a 25° bearing of the same envelope carries more thrust and turns slower. This is a preloaded bearing rather than a clearance-fitted one: the medium preload is built into the ground ring offsets, so the rolling elements are always loaded and the spindle holds position without lost motion.

Quantity and configuration:

One bearing. Universally ground, so it stacks with any other bearing of the same designation back-to-back, face-to-face or in tandem without a matched-set order code.

Fitting notes:

Straight bore on an interference fit to the spindle journal, outer ring clamped square in the housing bore. Where two or more are stacked the marked faces are oriented together per the chosen arrangement, and the medium preload is developed by the ground offsets rather than by shimming. Operating range is –30 to 100 °C on oil or grease.

Interchange guidance:

Same-specification 15° medium-preload equivalents include the SKF 7028 CD/P4A, FAG B7028-C-T-P4S-UM and RHP 7028CTSUMP4. A 3MM9128WI of the same envelope is a 25° bearing and a related size rather than a substitution, and a DUM version of this bearing is a matched duplex pair rather than a single.