FafnirMM60BS120QUHBall Screw Support Bearing Duplex Pair – Straight Bore, 60 mm ID, 120 mm OD, 20 mm Width Each, 40 mm Stacked, Open, 60 Degree Contact Angle, Heavy Preload, 2 Pc

Product Description

What this part is

A Timken Fafnir MM60BS120 ball screw support bearing set, supplied as two pieces functioning as a DUH duplex pair. Each bearing is 60 mm bore by 120 mm outside diameter by 20 mm wide, giving 40 mm across the mounted pair. Contact angle is 60°, both faces are open, and the preload is heavy. This is a thrust-biased precision bearing built for the fixed end of a ball screw.

Decoding the designation

MM is the Fafnir super precision prefix and carries the P4S tolerance grade. BS identifies the ball screw support series, which fixes the contact angle at 60°. The two numbers either side of BS are the bore and the outside diameter in millimetres, so MM60BS120 is 60 mm on 120 mm. The trailing letters give the set size and preload: DUH is a duplex pair at heavy preload, QUH a quad set of four at heavy preload. Both codes appear on this item, and the reason is set out under quantity and configuration.

What these specs mean in practice

A 60° contact angle puts almost the whole load path along the shaft axis, so the bearing converts a ball screw’s thrust into very high static capacity — 76 kN static against 29 kN dynamic per bearing, which is the inverse of the usual relationship and is exactly what a screw end needs. The trade is speed and radial capability: the limiting speed is 3000 rpm and radial load should be carried elsewhere.

Timken holds the MM-BS family to ABEC 9 / ISO P2 axial running accuracy, tighter than the ABEC 7 bore and outside diameter tolerance implied by the MM prefix. Axial run-out is the figure that matters on a screw end, because every micron of it appears directly as positioning error at the tool. Heavy preload removes axial play entirely and raises the stiffness of the screw mounting, so the machine holds position under reversing cutting load.

Quantity and configuration

Two pieces. The manufacturer’s carton is marked MM60BS120-QUH, a factory quad set of four bearings at heavy preload. That set has been broken down and two of the four bearings remain. Since every bearing in a universal heavy-preload stack is ground to the same faces, the two pieces on offer stack as a DUH duplex pair — mounted back-to-back at the correct heavy preload with no shimming. Sale unit is 2 Pc. Mass is 1.80 lbs for the pair. Temperature range is –30 °C to 120 °C. Supplied without a snap ring.

Installation and setup notes

The pair mounts back-to-back at the fixed end of the screw with the ground match faces together and is clamped solid against square, ground shoulders. Preload is set by the factory grind, so no shimming or measured axial setting applies. The free end of the screw carries a separate radial bearing so that thermal growth is taken up there rather than loading this pair.

Interchange guidance

Direct equivalents at the same 60° angle and duplex configuration are the NSK 60TAC120B and the SKF BSD 60120 C. Related sizes within the same envelope differ by set size and preload: MM60BS120-TUH is the triplex heavy-preload set of three, MM60BS120-QUH the quad set of four, and MM60BS120-TUM and -TUL the triplex medium and light preload versions. MM60BS120-TBH is the same bearing in a back-to-back designation.