NK21-20-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 NK21-20-XL model number is decoded

envelope 21×29×20 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 19.3 kN and C0 25.5 kN at the 21 mm seat; the C/C0 ratio of 0.76 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 4.5 kN, the limiting speed 20,600 r/min and the thermal reference speed 12,300 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 0.92 kN/mm and per millimetre of width 0.97 kN/mm — the reason 20 mm-wide needle bearings routinely replace much larger ball or roller positions mass is 35.2 g, just 0.06 g per mm² of projected 29×20 mm envelope, which keeps unbalanced forces low in fast-actuating linkages at 21 mm bore this size balances the 29 mm outside diameter against machineability of the adjacent parts — the most frequently specified band in the HR1 range NK21/20-XL has no inner ring: the needle rollers run directly on the shaft journal, so the seat must be hardened (58–64 HRC) and ground to bearing raceway quality 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 The -XL suffix is the Schaeffler X-life mark: interchangeable envelope with the basic bearing, but higher load capacity and lower running temperature at the 20,600 r/min limit. Typical applications include textile machine heald and cam shafts, agricultural machine pivots and linkages and connecting-rod small ends, where the 29 mm outside diameter and 19.3 kN dynamic rating match the duty. In textile machine heald and cam shafts the unit fits a compact 21 mm seat, and in agricultural machine pivots and linkages the 35.2 g mass keeps unbalanced forces low in fast linkages. Designers also specify it for connecting-rod small ends, relying on the 20 mm width and the published 25.5 kN static rating. Needle bearings are pure-radial units — any thrust must be taken by separate washers or an adjacent bearing, especially on flangeless RNAO/NAO. Without an inner ring the shaft is the raceway: 58–64 HRC and a ground, polished journal are mandatory, not recommendations.

Typical equipment & failure modes

Selection pitfalls — read before you substitute

• Needle bearings are pure-radial units — any thrust must be taken by separate washers or an adjacent bearing, especially on flangeless RNAO/NAO.
• Without an inner ring the shaft is the raceway: 58–64 HRC and a ground, polished journal are mandatory, not recommendations.
• 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.