NSK7904A5TRDULP4YSuper Precision Angular Contact Ball Bearing Matched Set 20x37x9 mm, 25 Degree Contact Angle, ABEC 7, Duplex Light Preload, Phenolic Cage, Extra Thin 79 Series

Product Description

What this part is

A matched pair of super precision angular contact ball bearings in the extra thin ISO 79 series, 20 mm bore, 37 mm outside diameter, 9 mm width per bearing. The 79 series is the slimmest of the angular contact families, chosen when the radial space between shaft and housing is the binding constraint.

Decoding the designation

7904 — ISO 79 extra thin series, 20 x 37 x 9 mm. Also written 71904, which is the same envelope; both forms are kept in the title so the part is findable either way.
A5 — NSK contact angle code for 25°. NSK’s A is 30° and C is 15°.
TR — phenolic cage.
DU — Duplex, Universally ground.
L — light preload.
P4 — ABEC 7 tolerance class. The trailing Y is a factory special or customer specific variant whose meaning NSK does not publish; it is retained as part of the number and recorded as undecoded rather than dropped.

Why the thin section and the phenolic cage matter:
A 9 mm wide, 8.5 mm wall bearing has less material to carry load than a 70 series bearing on the same shaft, so the 79 series trades capacity for envelope. What you get back is speed and low mass: less rotating inertia, less heat, and the ability to fit a preloaded pair where a wider bearing would not go. The phenolic cage reinforces that: it is a fraction of the weight of a steel cage, it is porous enough to retain oil in its own structure and so survives marginal lubrication, and it is the cage material specified on genuinely high speed spindle bearings. Brown or bronze coloured cage material in the photographs is the phenolic, not a coating.

What these specs mean in practice

25° with light preload is the standard compact spindle recipe: enough axial stiffness to hold position, low enough friction that the pair does not overheat in a small, poorly cooled housing. Because the bearings are universally ground, preload comes from ground ring offsets when both rings are clamped, not from a spacer you have to size.

Quantity and configuration

Supplied as a matched DUL pair, and the pair must stay together — the light preload is a property of the two rings ground to each other, not of either bearing alone. Note that DU and SU are mutually exclusive designations and this is DU, a duplex. Standing stock on this SKU is held in a way that does not distinguish pairs from individual pieces, so no total piece count is published here.

Fitting notes

Thin section rings are easy to distort. Press on the correct ring only, never through the balls, and check that housing bore and shaft roundness are within tolerance before assembly — a 37 mm outer ring in an out of round bore will show up as runout that no amount of bearing quality can fix. Clamp both rings fully or the ground offset never closes.

Published ratings and mounting data

Load ratings and limiting speed are not published by NSK for this exact suffix combination in a verifiable source and are left blank; set ratings are not double the single bearing figures.

Interchange guidance

Same envelope 25° extra thin alternatives are found as 71904ACD/P4A (SKF, AC = 25°) and B71904E-T-P4S (FAG). RHP’s form of this angle would carry the E code. Confirm preload class and cage material, since a polyamide caged equivalent is not a substitute in a high speed application.