Fafnir2MM202WIDUMSuper Precision Angular Contact Ball Bearing Matched Duplex Pair, 15 x 35 x 11 mm, 15 Degree Contact Angle, Medium Preload, Universally Ground

Product Description

What this part is

A factory-matched duplex pair of super precision angular contact ball bearings from the Fafnir WI machine-tool spindle series, 15 mm bore, 35 mm outside diameter, 11 mm width per bearing, single row each. The rings are ground to super precision geometry and matched to one another so the pair develops a defined medium preload when it is clamped up. This is a spindle bearing set, not a general-purpose bearing.

Decoding the designation

2MM – in the Timken/Fafnir WI scheme, a 15° nominal contact angle. A 3MM prefix on the same envelope would be 25°.
202 – the size code corresponding to the ISO 7202 envelope, 15 x 35 x 11 mm.
WI – the super precision machine-tool spindle series.
DUM – duplex universal, medium preload. DUL would be light preload and DUH heavy.
UM on the ring – each individual bearing of a DUM set is stamped 2MM202WIUM. The set-level DUM appears on the carton, the per-ring UM appears on the outer ring.
T BORE and T O/D stamped on the outer ring face – the matched bore and outside-diameter tolerance grading applied to the set.

Correcting the source data

Two things in the previous listing were wrong and are corrected here. First, country of origin: the earlier text said made in the United States. The Fafnir carton in the photographs is printed MADE IN ENGLAND, and the outer rings are stamped 07J U.K. This was Fafnir’s British production under The Torrington Company, and the box includes a period Torrington leaflet reading “Part of worldwide Ingersoll-Rand”. Second, the earlier text said “unit quantity of 1” while the title said two pieces, which reads as a contradiction. It is not one: one unit is one matched pair, and a matched pair is two physical bearings. Both statements were describing the same lot from different ends. The reconciliation is set out below. The earlier text also described the set as suited to general industrial automation and motion control; that framing is dropped, because a 15° ABEC-class matched spindle pair is a machine-tool spindle component.

Quantity and configuration

One lot equals one matched pair equals two physical bearings. The stock quantity of 1 refers to one saleable pair, and that is correct rather than a data error. The photographs confirm it: two individually wrapped bearings, both stamped 2MM202WIUM, both from LOT # 29448, alongside one carton labelled 2MM202WI DUM. The two bearings must stay together — the preload exists in the relationship between their ground faces, and splitting the set across two machines throws that away.

Why 15 degrees and medium preload

Contact angle sets the balance between speed and axial stiffness. A 15° angle keeps more of the load line radial, which reduces the centrifugal and gyroscopic work the ball set does, so the bearing runs faster and cooler than a 25° equivalent of the same size. What you give up is axial rigidity under heavy thrust. On a 15 mm bore spindle — a small grinder, an instrument spindle, a high-frequency head — 15° is almost always the right choice.

Preload class then sets how hard the pair is squeezed together. Medium preload sits between light and heavy: stiffer and more accurate under cutting load than light preload, but running warmer and with a lower speed ceiling than light. The critical rule is that preload class must match across a pair. Two bearings of different classes clamped together do not average out; the stiffer one takes a disproportionate share of the load and fails early, and the spindle’s axial position becomes unpredictable. That is why matched sets are sold as sets and why the DUM marking matters more than the base number.

What universally ground rings buy you

Because the faces are relieved for universal pairing, the fitter chooses the arrangement rather than the part number. Back-to-back gives the widest effective load-line spread and the greatest moment stiffness, which is what a spindle nose wants. Face-to-face tolerates a little shaft-to-housing misalignment. Tandem doubles axial capacity in one direction. Any of these arrives at the same intended preload with the same two parts.

Published ratings and mounting data

Timken does not publish dynamic load rating, static load rating or limiting speed for the legacy Fafnir 2MM202WI-DUM designation in any current or reachable source; the data for this line survives only in out-of-print paper catalogues. No figure is estimated and none is borrowed from a sibling size or another brand, because contact angle, preload class and cage design all move the ratings materially and a borrowed number would be quietly wrong. The boundary dimensions above are ISO-standardised and reliable. A one-piece land-riding cage is visible in the photographs; it is dark rather than the brown seen on CR-suffixed phenolic parts, and no cage-material code appears in the designation, so the material is left undocumented rather than guessed.

Fitting notes

Each outer ring is marked THRUST on one face; that face must face the thrust load, and in a back-to-back pair the two thrust faces point outward. Fitted backwards, load lands on the low shoulder of the outer ring and the bearing fails quickly. Keep both bearings in their original wrappers until the moment of assembly, handle them by the outer ring, clamp against square shoulders and a square nut or spacer face, mount by ring temperature or a proper fixture rather than driving through the ball set, and lubricate to your spindle specification before running.

Interchange guidance

Equivalent designation forms include 2MM202WI-DUM, 2MM202WIDUM, 2MM202WI DUM, 2MM-202-WI-DUM, 2-MM-202-WI-DUM, 2MM202WIUM and 2MM202WI. The ISO base number is 7202 and the paired 15° form is written 7202-C-DUM or 7202CDUM. The obsolescence trail is Fafnir to Torrington to Timken, and Timken still lists the successor 2MM202WI family. Same-envelope equivalents include NSK 7202CTYDUMP4, NTN 7202CGD2GNP4, FAG/Schaeffler B7202-C-T-P4S-UM and SKF 7202 CD/P4A, but preload class and cage material differ between brands, so match the full suffix rather than the base number alone.