TimkenMM40BS72DUHBall Screw Support Bearing, 40 x 72 x 15 mm, 60° Contact Angle, Heavy Preload, Phenolic Outer-Land-Riding Cage, Universally Ground Matched Set

Product Description

What this part is

A ball screw support bearing, 40 mm bore by 72 mm outside diameter by 15 mm wide per bearing, with a 60° contact angle and a phenolic outer-land-riding cage. It is supplied as a preloaded, universally ground matched set for the end support of a precision ball screw.

This is worth stating clearly because the original listing called it simply an “angular bearing”. A 60° ball screw support bearing is a specialised part and it is not a substitute for — nor substitutable by — an ordinary 15° or 25° spindle angular contact bearing in a similar envelope. The whole design is optimised for axial load and axial stiffness rather than speed.

Decoding the designation

MM — Timken/Fafnir super precision metric prefix. It implies the P4S precision grade rather than P4.
40 — 40 mm bore.
BS — ball screw support bearing. This is the code that identifies the product type, and it is 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. These are mutually exclusive; see the configuration section.
H — heavy preload.

Quantity and configuration

The record carries two conflicting arrangement codes and both are genuine markings, so both are published rather than one being quietly discarded. MM40BS72DUH describes a duplex matched set of two bearings. MM-40-BS-72-QUH describes a quadruplex matched set of four. A set cannot be both two and four bearings.

The photographs could not be examined to settle it, so no piece count is claimed on this page. Timken publishes the same load ratings and limiting speeds against this part number, so the ratings below hold either way, but the number of bearings you receive and the assembled stack height obviously do not. If the count matters — and on a ball screw end support it almost always does, because the preload and the axial stiffness of the mounting depend on how many rows are stacked — ask before ordering. The carton marking and the physical count are checked and confirmed in writing before dispatch.

What these specs mean in practice

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

What you trade for it 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 and completely the wrong one for a spindle.

The static rating being much higher than the dynamic rating is also characteristic and not a data error. On a 60° bearing the 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 is what removes axial backlash from the screw’s end bearing, and backlash there translates directly into lost positioning accuracy and possible axis instability. The cost is friction and heat — acceptable at 2,900 rpm, and 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. Never clean these in solvents that attack phenolic, and never run them dry.

Published ratings and mounting data

Contact angle: 60°
Preload: heavy
Bore 40 mm, outside diameter 72 mm, width 15 mm per bearing
Dynamic radial load rating C1, 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, grease: 2,900 rpm
Limiting speed, oil: 5,800 rpm
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
Cage: phenolic, outer-land-riding
Ball type: steel

These are Timken’s own published catalogue figures for MM40BS72DUH. Timken does not publish a precision class or a maximum operating temperature on its catalogue page for this part, so neither is stated as a specification here.

Correcting the source data

The original listing quoted a dynamic load rating of 29.5 kN and a static rating of 65.5 kN. Neither figure matches Timken’s own published data, which gives 36,300 N dynamic and 45,400 N static. The published figures above are Timken’s and supersede the previous ones.

The listing also stated a precision rating of “ABEC 7 / P4” and a maximum temperature of 120 °C. Timken publishes neither of those against this part number, so both have been removed rather than repeated. Note in particular that the MM prefix implies P4S, not P4, so the P4 in the original was wrong on its own terms. The bearing was additionally described as having an “open seal”, which is a contradiction: it is open, meaning it has no seal and no shield at all.

Fitting notes

Ball screw support bearings are almost always mounted in a stack and clamped by a locknut against a shoulder, and the preload they were ground to deliver is developed by that clamping. The consequence is that the whole set must go 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 rather than merely a shorter bearing life. Do not mix bearings from different sets. Respect the 54.483 mm shaft and 55.611 mm housing backing shoulder diameters and leave the 0.8 mm fillet clearance. Tighten the locknut to the machine builder’s figure; guessing at it defeats the point of buying a preloaded set.

Interchange guidance

Timken does not publish a world part number for this designation. A valid substitute must match the 60° contact angle, the heavy preload, the phenolic cage and the number of rows in the set, as well as the 40 x 72 x 15 mm envelope. Be careful with same-envelope bearings from other makers: FAG’s 7602-series TVP bearings are also 60° ball screw support bearings, whereas an ordinary 72-series angular contact bearing in the same size is a spindle bearing with a shallow contact angle and is not a substitute. Fafnir bearings are now supplied under the Timken brand, so Fafnir and Timken designations of this form are the same product line.