Fafnir3MM307WIDUHSuper Precision Spindle Angular Contact Ball Bearing 35x80x21 mm, 25 Degree Contact Angle, Phenolic Outer-Land-Riding Cage, Duplex Universal Heavy Preload, ISO P4S

Product Description

What this part is

A Fafnir super-precision spindle angular contact ball bearing, 35 mm bore × 80 mm outside diameter × 21 mm width, 25° contact angle, ground to duplex universal heavy preload. Mounted as a pair the set occupies 42 mm of shaft length. This is the heavy-section 73-series, so it carries substantially more load than a 70- or 72-series bearing of the same bore.

Decoding the designation

3MM — super-precision spindle bearing at the high contact angle, 25°. Timken’s catalogue states plainly that 3 is the Timken designation and E the ISO designation for 25°; 2MM is the 15° series.
307 — 73-series bore code: 35 mm bore in the 80 mm outside diameter envelope.
WI — angular contact geometry with full shoulders both sides of the inner ring and a low shoulder on the outer ring; carries thrust one direction, non-separable.
CR — composition cage, meaning phenolic, riding on the outer-ring land, rated for service to 93 °C.
DUH — flush-ground duplex pair at heavy preload. The series runs DUX extra-light, DUL light, DUM medium, DUH heavy.

Why heavy preload changes the buying decision

DUH is the specification most often bought by mistake. Heavy preload gives the greatest axial and radial stiffness of the four classes, and the tightest positional accuracy under varying load — which is what a rigid boring or heavy milling spindle needs. What it gives up is speed and thermal margin. The published mounted-pair speed for this bearing at heavy preload is roughly 40 per cent of the single-bearing figure, and the bearing generates considerably more heat, so grease life is shorter and the lubrication system has more work to do.

Practically: if the machine’s original specification called for DUL or DUM, fitting DUH will make the spindle stiffer and hotter, and may take it outside its designed thermal envelope. Preload class is ground in and cannot be adjusted at assembly, so this is a purchase-time decision, not a fitting-time one.

Precision class

The MM prefix denotes ISO P4S: running accuracy and performance to ABEC 9 (ISO P2) levels, other features to ABEC 7 (ISO P4). A world number rendering this as plain P4 understates the running accuracy by one class.

Published ratings and mounting data

Dynamic load rating Cr — 44,500 N (10,000 lbf)
Static load rating Cor — 24,700 N (5,600 lbf)
Ball complement — 10 balls of 14.29 mm diameter
Preload force, DUH — 1,560 N
Speed, single bearing (Ng) — 17,300 rpm
Speed, DUH pair mounted back-to-back — 6,930 rpm grease, 11,790 rpm oil
Assembled pair width — 42 mm

The single-bearing and mounted-pair speed conventions differ substantially on this part, which is why both are given and labelled.

Quantity and configuration

DUH describes a matched pair of bearings ground as a set. Surplus duplex stock is regularly split, and no pair-count claim is published here. Where more than one piece is held, only bearings from a common original set will produce the specified 1,560 N preload when clamped together — two unrelated halves will not.

Fitting notes

Keep the set together, align the etched face marks as ground, and observe the thrust direction identified by the low outer shoulder. Remove the shipping preservative and charge with the specified spindle lubricant. Given heavy preload, confirm the lubrication and cooling arrangement can handle the extra heat before commissioning.

Interchange guidance

ISO world number: 7307E-T-P4S-DUH. Schaeffler format: B7307-E-T-P4S-DUH. The 15° sibling 2MM307WI CR DUH shares the envelope but not the contact angle or the ratings, so it is a search term rather than a substitute. Fafnir spindle bearings are now branded Timken.