RHP7211ETSUMP4Super Precision Angular Contact Ball Bearing 55x100x21 mm, 25° Contact Angle, Phenolic Cage, ABEC 7, Medium Preload, Single Universally Ground, Medium 72-Series Section

Product Description

What this part is

A single-row super precision angular contact ball bearing in the ISO 72-series medium section — 55 mm bore, 100 mm outside diameter, 21 mm width. It is a machine tool spindle bearing with a 25° contact angle, a machined phenolic cage, ABEC 7 running accuracy, and faces universally ground for medium preload.

Decoding the designation

72 – ISO 72-series medium section angular contact ball bearing (heavier than the 70-series light section on the same bore)
11 – bore code, 55 mm bore
E – 25° nominal contact angle
T – machined phenolic cage
SUM – single, universally ground, medium preload
P4 – ISO tolerance class 4, equivalent to ABEC 7

Correcting the source data

The previous listing text described this bearing as both SUM and DUM. Those are mutually exclusive: SU means single universally ground, DU means duplex universally ground. The part number stamped on the part, 7211ET SUMP4, is the governing designation, so this is published as a single universally ground bearing at medium preload. The DUM forms that appeared in the old copy, including the Fafnir-style number 3MM211WI CR DUM and the designation B7211X3TADUMEP7, are retained only as search terms and cross-references. They are not specifications of this item and should not be read as evidence that a pair is supplied.

The old copy also claimed the bearing was manufactured in the United Kingdom. There is no carton or ring marking we have verified to support an origin claim, so no origin is claimed here. Descriptions of this bearing as suitable for “DIY” use have also been removed — this is a super precision spindle bearing with fitted-tolerance seats, not a general-purpose part.

What these specs mean in practice

The 25° contact angle is the axial-capacity choice. Steepening the angle from 15° to 25° transfers more of the ball load into the thrust direction, so the bearing carries more axial load and is axially stiffer — which is what you want on a spindle that takes real cutting thrust, or on any shaft where axial deflection sets the accuracy. What you give up is speed: a steeper angle increases ball centrifugal and gyroscopic effects and cage loading, so the permissible speed and the heat budget are tighter than a 15° C bearing in the same envelope.

The 72-series medium section backs that up with more material and larger balls than a 70-series bearing of the same bore, which is why this is the choice where load capacity, not minimum envelope, is the constraint.

Medium preload is the middle of the three grades. More preload means higher stiffness and better ball guidance under varying load, at the cost of more friction heat and lower permissible speed. It is the usual selection for a general-purpose or roughing spindle; a high-rpm finishing spindle is normally specified light instead.

The phenolic cage is light, quiet and holds lubricant in its own porosity, which is why phenolic-caged bearings survive marginal lubrication far better than steel or brass. The trade is a ceiling on operating temperature, so phenolic does not belong in a hot, uncooled housing.

Quantity and configuration

This is a single bearing, and the SU in the designation says so. Because it is universally ground, it can be used on its own or combined with another bearing of the identical designation to form a back-to-back, face-to-face or tandem set that comes up to the specified medium preload with no shims and no grinding. If you need a pair, order two of the same designation.

Fitting notes

Even used singly, a universally ground bearing has a defined high face and low face, and if you are building a set the ring markings determine the orientation. Choose the arrangement for the job: back-to-back for stiffness and moment capacity, face-to-face for tolerance of shaft-to-housing misalignment, tandem to share one-directional thrust between two bearings.

Precision practice throughout. Shaft and housing seats clean, round and to the tolerance the P4 class assumes; even, symmetrical clamping so the rings are not distorted; controlled quantity of clean lubricant, because over-filling raises running temperature and shifts the preload you specified. Never hammer, and never drive mounting force through the balls.

Published ratings and mounting data

Basic dynamic load rating, basic static load rating and limiting speed are left blank. NSK’s super precision bearing tables were not retrievable at the time of writing, and the ratings for a 25° E bearing differ from the 15° C bearing in the same 55x100x21 envelope, so borrowing a sibling’s figures would be misleading rather than helpful. Take them from the current NSK super precision catalogue for your duty calculation.

Interchange guidance

RHP is now NSK, so this is cross-shopped against NSK 7211CTRSUMP4-family numbering in the 25° variant, and against the Timken/Fafnir world number 3MM211WI CR, where the 3MM prefix denotes a 25° contact angle and CR a phenolic outer-land-riding cage — the same specification intent in a different scheme. Note that Fafnir MM-prefixed numbers normally carry P4S tolerance, which is a tighter class than P4. Same-envelope equivalents elsewhere: SKF 7211ACD/P4A (SKF AC = 25°), FAG B7211E.T.P4S, NTN 7211CGD2GLP4 in its 25° form. Before substituting, match envelope, contact angle, tolerance class and preload grade — and do not swap a 25° E bearing for a 15° C one, because spindle stiffness and speed capability both change.