Fafnir2MM9108WIDUMSuper Precision Angular Contact Ball Bearing – 40 mm ID, 68 mm OD, 15 mm Width, Open, 15°, Medium Preload Duplex Pair

Product Description

What this part is

A Timken Fafnir 2MM9108WI-DUM super-precision angular contact ball bearing, 40 mm bore by 68 mm outside diameter by 15 mm width, supplied as a factory-matched duplex pair. The ISO equivalent designation is 7008C. Rolling elements are steel balls in a machined phenolic cage, and both faces are open.

Decoding the designation

In the Fafnir super-precision scheme the leading 2MM denotes a 15° contact angle, and the MM prefix itself carries the P4S accuracy class — ABEC 7 geometry with ABEC 9 running accuracy. The 9 that opens 9108 places the bearing in the ISO 70 light series, so 9108 corresponds to a 7008 envelope. WI is the machined phenolic cage execution. DUM is a duplex pair set to medium preload.

What these specs mean in practice

A 15° contact angle favours speed over thrust. It carries less axial load than a 25° or 40° bearing of the same envelope but runs cooler and faster, which is why this angle dominates machine tool spindle positions. The machined phenolic cage is light and self-lubricating, supporting the 39,800 rpm limiting speed under oil lubrication. Medium preload sits between the light setting used for maximum speed and the heavy setting used for maximum stiffness, giving a balance of spindle rigidity and thermal headroom.

Quantity and configuration

Two pieces, a universally matched duplex pair supplied together as one set. Because the rings are ground as a matched set, the preload is established by the factory when the two bearings are clamped face to face or back to back.

Fitting notes

Preload is set by the ground ring faces, so the pair is clamped solid against the shaft and housing shoulders with no shims. Oil lubrication suits the published limiting speed; grease lowers it. Maximum operating temperature is 100 °C.

Interchange guidance

Same-specification equivalents at 15° and medium preload are the SKF 7008 CD/P4ADGB and the NSK 7008CTRDUMP4Y. A 7008 in a different contact angle, preload class or cage execution shares the envelope but is a related size rather than an interchange, because the angle and preload set the load direction and stiffness.