TimkenBall Screw Support Bearing, 40 x 72 x 15 mm, 60 Degree Contact Angle, Heavy Preload, Phenolic Outer-Land-Riding Cage, Universally Ground Matched Set

Product Description

What this part is

Timken Fafnir MM40BS72, a ball screw support bearing for the end support of a precision ball screw. Brand: Timken (Fafnir). Bore: 40 mm. Outside Diameter: 72 mm. Overall Width: 15 mm per bearing. Contact Angle: 60°. Preload: Heavy. Cage Material: Phenolic. Cage Type: CR, outer-land-riding. Mounting Arrangement: Universally ground, preloaded matched set. Number Of Rows: 1 row per bearing. Rolling Elements: Steel balls. Bearing Material: Bearing steel. Closure Type: Open. Bore Type: Straight. Lubricant: Oil or grease. Series: MM..BS Fafnir and Timken ball screw support. Base Number: MM40BS72. Design Units: Metric. Item Weight: 0.85 lb. Harmonization Code: 8482.10.50. The 60° contact angle makes this a thrust bearing first, optimised for axial load and axial stiffness rather than speed.

Published ratings and mounting data

Dynamic Radial Load Rating C1 at 1 million revolutions: 8,160 lbf, 36,300 N. C1 ISO Dynamic Radial Rating per row: 7,360 lbf, 32,700 N. Static Radial Load Rating C0: 10,200 lbf, 45,400 N. Limiting Speed: 2,900 rpm on grease, 5,800 rpm on oil. Maximum Shaft Fillet Radius: 0.8 mm. Shaft Backing Shoulder Diameter: 54.483 mm. Housing Backing Shoulder Diameter: 55.611 mm. Outer Ring To-Clear Radius: 0.8 mm. Ball Type: steel. These are Timken’s published catalogue figures for MM40BS72DUH.

Decoding the designation

MM: Timken and Fafnir super precision metric prefix, indicating the P4S grade. 40: 40 mm bore. BS: ball screw support bearing, the code that identifies the product type and the reason for the 60° contact angle. 72: 72 mm outside diameter. DU: duplex universally ground, a matched set of two. QU: quadruplex universally ground, a matched set of four. H: heavy preload. Searchable variants: MM40BS72DUH, MM-40-BS-72-DUH, MM 40 BS 72 DUH, MM40BS72QUH, MM-40-BS-72-QUH, MM40BS72, 40BS72.

What these specs mean in practice

The 60° contact angle is the defining specification. At 60° the ball load line runs steeply, close to axial, so almost all of the capacity is thrust capacity. That is what a ball screw needs: the screw generates a large axial force and essentially no radial load, and the end support reacts that force with as little axial deflection as possible, because deflection at the support shows up directly as positioning error at the tool.

The trade is speed. A steep contact angle means high ball spin, large gyroscopic moments and heavy cage loading, and the published limits reflect that: 2,900 rpm on grease against the 10,000 to 20,000 rpm typical of a 15° spindle bearing in a comparable size. A ball screw turns slowly compared with a spindle, so this is the right trade for the application.

The static rating sitting well above the dynamic rating is characteristic of a 60° bearing. Static axial capacity is very large relative to the rotating fatigue rating, because the part spends its life reacting large forces at low speed rather than accumulating revolutions.

Heavy preload is normal on a ball screw support set. Preload removes axial backlash from the screw’s end bearing, and backlash there translates directly into lost positioning accuracy. The cost is friction and heat, which is acceptable at 2,900 rpm and is the reason the phenolic outer-land-riding cage is fitted: it is light and mildly self-lubricating, so it adds far less heat than a steel cage. Phenolic is attacked by some solvents.

Fitting notes

Ball screw support bearings mount in a stack and are clamped by a locknut against a shoulder, and the preload they were ground to deliver develops from that clamping. The whole set goes in together, in the ground orientation, with clean and square faces; a burr, a dirt film or a shoulder that is not square translates directly into wrong preload, which on a ball screw means lost positioning accuracy. Bearings stay with their own set. The 54.483 mm shaft and 55.611 mm housing backing shoulder diameters apply, with 0.8 mm fillet clearance. Locknut torque follows the machine builder’s figure.

Interchange guidance

Closest 60° ball screw support bearings: NSK 40TAC72B and SKF BSD 4072 C, both 40 x 72 mm; preload and cage are not verified for either. FAG 7602040-TVP is a 60° ball screw support bearing with a 68 mm outside diameter. An ordinary 72-series angular contact bearing in the same size is a spindle bearing with a shallow contact angle and is a related size. Fafnir bearings are supplied under the Timken brand, so Fafnir and Timken designations of this form are the same product line.