Fafnir3MM220WIDULSuper Precision Angular Contact Ball Bearing 100 x 180 x 34 mm, 25° Contact Angle, ABEC 7, Duplex Light Preload, Matched Pair

Product Description

What this part is

A matched pair of super precision angular contact ball bearings in the 7220 envelope — 100 mm bore, 180 mm outside diameter, 34 mm width per bearing. Spindle grade, universally ground, supplied as a factory-matched duplex set with light preload built in.

Decoding the designation

3MM — Fafnir super precision series, 25° contact angle. The MM prefix carries P4S tolerance (ABEC 7 running accuracy with tighter ABEC 9 bore and outside diameter control).
220 — 7220 boundary dimensions, 100 x 180 x 34 mm.
WI — Fafnir super precision angular contact spindle bearing.
DUL — duplex, universally ground, light preload.

What these specs mean in practice

The 25° contact angle is the middle setting in the spindle range. Compared with a 15° bearing it carries considerably more thrust and gives a stiffer axial response; compared with a 30° or 40° bearing it runs faster and cooler. Light preload is the high-speed choice: less internal friction, less heat, higher permissible rpm, at the cost of some stiffness against cutting load. That pairing — 25° with light preload — is what most general purpose machine tool spindles run.

Universal grinding is the feature that makes these usable as a set. The ring faces are ground so that clamping any two of them together produces a known, repeatable preload with no shims and no spacer stack to calculate.

Quantity and configuration

This listing is a set of 2 pieces — a duplex pair. Mount back to back (DB) for maximum moment stiffness and the ability to take thrust in both directions, face to face (DF) where the housing bores may be slightly misaligned, or in tandem (DT) to double thrust capacity in one direction. The light preload develops when the pair is clamped between the shaft shoulder and the locknut; adding a spacer changes the preload and defeats the matching.

Fitting notes

Shaft and housing seats must be ground to a precision that matches the bearing — a few microns of runout — or the bearing’s accuracy is wasted. Keep the pair oriented as marked, mount with controlled heat or a proper press on the correct ring, and never let mounting force pass through the balls. Use a spindle grease or oil mist rated for the intended speed and resist over-greasing; excess lubricant in a preloaded pair is a leading cause of early thermal failure.

Interchange guidance

Fafnir super precision is now supplied under the Timken brand. Same-envelope 25° equivalents include NSK 7220A5, FAG B7220-E-T-P4S, SKF 7220 ACD/P4A and NTN 7220 in the matching precision grade. The 7220A designation also carried on this listing is a same-envelope reference — note that in NSK nomenclature a plain A suffix denotes a 30° contact angle rather than 25°, so it is retained here as a dimensional cross-reference rather than a contact angle claim. The 15° sibling of this bearing is 2MM220WI; do not mix a 2MM and a 3MM bearing in one pair.