Fafnir2MM210WICRDUMSuper Precision Angular Contact Ball Bearing 50 x 90 x 20 mm, 15° Contact Angle, ABEC 7/9, Phenolic Cage, Medium Preload

Product Description

What this part is

One Fafnir (now Timken) 2MM210WICR super precision angular contact ball bearing, 50 mm bore × 90 mm outside diameter × 20 mm wide, with a phenolic cage and a 15° contact angle. It is from the 200 light series — the ISO 7210 envelope — ground to ABEC 7/9 and universally flush ground so it butts up against a second bearing to form a back-to-back, face-to-face or tandem set. Made in USA. This is a machine tool spindle bearing, not a general-purpose or automotive part.

Decoding the designation

2 — Timken code for a 15° contact angle. ISO writes this as C.
MM — super precision, between ABEC 7 (ISO P4) and ABEC 9 (ISO P2). The ISO equivalent code is P4S.
210 — 200 light series, bore code 10 (10 × 5 = 50 mm).
WI — angular contact with a low shoulder on the outer ring only.
CR — phenolic (composition) outer-land-riding cage, Timken’s standard for this family. ISO writes the same cage as T.
DUM — universally flush ground duplex bearings at medium preload. DU means the part was made and sold as a matched pair; M is the medium preload step between L (light) and H (heavy).

Correcting the source data

The source title carried two contradictory preload designations at once — DUM in the part number and DUL in the hyphenated variant. These are different products: at this size the published preload is 220 N for DUL and 560 N for DUM, a factor of two and a half apart, with real consequences for stiffness, heat and speed. The bearing itself settles it. The ring is etched 2MM210WI CR DUM, and the bag label reads 2MM210WICRDUM. This is a DUM, medium preload. The DUL reference has been removed from the title and retained only as a cross-reference so anyone who searched the old wording still lands here.

Quantity and configuration — read this before you buy:
The DUM suffix describes a matched pair, but only one bearing is physically here. Duplex sets get split over the years and the surviving half keeps the pair’s marking. What you are buying is a single bearing whose ring says DUM — not two bearings, and not a complete matched set. Practically, that means one of three things: use it as a single-bearing position; pair it with the other half of its original set if you have it; or buy a fresh matched pair if your spindle needs a preloaded duplex mount, because two arbitrary DUM singles from different lots will not give you the designed preload. The quantity marked on the carton is not the quantity in the box, and we would rather say so than have you find out on the bench.

What these specs mean in practice

15° is the high-speed, lower-thrust option. It gives the best speed capability and the least axial stiffness of the standard angles; the 25° version (Timken 3MM, ISO E) trades speed for thrust capacity. Medium preload sits in the middle of the trade-off that governs every preloaded spindle: more preload buys stiffness and running accuracy under load, and costs you speed, friction and heat. At this size the published back-to-back speed drops from 12,500 rpm on grease at light preload to 9,600 rpm at medium and 6,400 rpm at heavy, while axial stiffness climbs from 64.19 to 95.67 to 139.57 N/µm. The phenolic cage is the reason this bearing tolerates high speed at all: it is light, it runs quiet, it holds oil in the resin, and it is far more forgiving of momentary lubrication starvation than a steel cage — at the cost of a ceiling near 93 °C (200 °F), above which machined bronze is the usual substitute.

Published ratings and mounting data

From the Timken Fafnir Super Precision Bearings for Machine Tool Applications catalogue (order 5918), 15° steel-ball columns:

Basic dynamic radial load rating C — 47,400 N (10,700 lbf)
Basic static radial load rating C0 — 31,700 N (7,130 lbf)
Speed rating (Ng) — 16,000 rpm, for a single grease-lubricated spring-preloaded bearing
Ball complement — 14 steel balls, 12.70 mm (1/2 in) diameter
Bearing weight — 0.457 kg (1.01 lb)
Preload, matched pair — DUM 560 N (125 lbf); for comparison DUX 130 N and DUL 220 N
Back-to-back speed at medium preload — 9,600 rpm grease, 16,300 rpm oil
Stiffness at medium preload — 95.67 N/µm axial, 496.89 N/µm radial
Maximum shaft fillet radius — 1.0 mm
Suggested shaft shoulder diameter — 57.9 / 57.4 mm; suggested housing shoulder diameter — 83.3 / 82.8 mm
Grease fill — 2.15 g (10 %) to 3.20 g (15 %) at high speed; 4.80 g (25 %) to 7.60 g (40 %) at low to moderate speed

Note that the preload and paired-speed figures above are published for a pair. A single bearing will not develop them on its own.

Fitting notes

The outer ring is etched THRUST on one face. The WI construction has a low shoulder on the outer ring only, so the bearing carries thrust in one direction and the marked face must go toward the load — fitted backwards it will not carry the thrust it was bought for. The ring also carries 01P USA LOT #15707, which is a factory lot reference, not a part number. The bearing is sealed in its original factory poly bag; there is no carton. Leave it wrapped until you are ready to mount, and mount clean. Observe the shoulder diameters and the 1.0 mm maximum fillet radius above.

Interchange guidance

The 50 × 90 × 20 mm, 15°, phenolic-cage, ABEC 7/9 combination is ISO 7210C-T-P4S, and Timken’s own catalogue prints the pairing as 2MM210WICR / 7210C-T-P4/P4S. Same-envelope super precision bearings from other makers include NSK 7210CTRDUMP4, FAG B7210C.T.P4S.UM, SKF 7210 CD/P4A and NTN 7210CT1GD2GLP4 — dimensionally and functionally comparable, though preload class and cage must be matched to your application and none of these is a Timken-published interchange. Fafnir is a retired brand name: Fafnir super precision became Timken super precision, so the current-production part is ordered as a Timken 2MM210WICRDUM — and if you need a pair, order it as a pair.