NK21-16-XL bearing: complete specifications & equivalents

Specification table

Source: catalog reference (HR1 2026-09-25) · extracted 2026-09-30

Parameters extracted from the official FAG Rolling Bearings catalogue (HR1 2026-09-25); FAG® is a trademark of Schaeffler Technologies AG & Co. KG, used here for identification only — mpnbase is not an authorized FAG/Schaeffler distributor. Boundary dimensions are ISO 15 interchangeable; current production values may differ, verify at schaeffler.com.

Cross references & equivalents

Interchange levels: full standard interchange · partial verify parameters · verify manual confirmation required. Links resolve to the equivalent model's specification page where available.

How the NK21-16-XL model number is decoded

envelope 21×29×16 mm: 21 mm seat is the hardened and ground shaft journal itself — there is no inner ring, with a thin 4.0 mm radial section packed with small-diameter needle rollers C is 15.2 kN and C0 18.7 kN at the 21 mm seat; the C/C0 ratio of 0.81 is characteristic of a needle bearing — high static rating relative to dynamic in a compact 29 mm outside diameter the fatigue load limit Cur is 3.5 kN, the limiting speed 20,600 r/min and the thermal reference speed 12,600 r/min; for small needle bearings the reference speed can exceed the kinematic limit, so the lower of the two governs the application dynamic capacity per millimetre of bore is 0.72 kN/mm and per millimetre of width 0.95 kN/mm — the reason 16 mm-wide needle bearings routinely replace much larger ball or roller positions mass is 28.1 g, just 0.06 g per mm² of projected 29×16 mm envelope, which keeps unbalanced forces low in fast-actuating linkages the 21 mm bore falls in the small band, the workhorse size for gearbox planetary positions, hydraulic pumps and general cam and linkage duty This bearing accepts axial displacement between shaft and housing because the rollers rotate on the free journal; it locates nothing axially and must be flanked by guide faces With Cur 3.5 kN and a 20,600 r/min limiting speed the unit suits pivots, rocker arms, pumps and compact gear trains running on hardened pins X-life status (-XL) indicates the current Schaeffler premium execution at 21 mm bore; the 18.7 kN C0 rating reflects the improved rolling-contact geometry. Typical applications include hydraulic and pneumatic pump eccentrics, copying and printing machine mechanisms and agricultural machine pivots and linkages, where the 29 mm outside diameter and 15.2 kN dynamic rating match the duty. In hydraulic and pneumatic pump eccentrics the unit fits a compact 21 mm seat, and in copying and printing machine mechanisms the 28.1 g mass keeps unbalanced forces low in fast linkages. Designers also specify it for agricultural machine pivots and linkages, relying on the 16 mm width and the published 18.7 kN static rating. Open ends draw contamination directly onto the needles; specify sealed or shielded positions in dusty environments rather than relying on grease alone. Do not press an RNA/NK assembly in through the outer ring when mounting it on a shaft, and vice versa — rolling forces through the wrong ring bruises the raceways.

Typical equipment & failure modes

Selection pitfalls — read before you substitute

• Open ends draw contamination directly onto the needles; specify sealed or shielded positions in dusty environments rather than relying on grease alone.
• Do not press an RNA/NK assembly in through the outer ring when mounting it on a shaft, and vice versa — rolling forces through the wrong ring bruises the raceways.
• Needle bearings are pure-radial units — any thrust must be taken by separate washers or an adjacent bearing, especially on flangeless RNAO/NAO.