NKI40-20-TN bearing: complete specifications & equivalents
The NKI40-20-TN is a DIN 616 radial bearing sized 40 x 55 x 20 mm (bore x OD x width). Verify dimensions against the official catalog before ordering.
Specification table
| Parameter | Value |
|---|---|
| Brand | SKF |
| Bore diameter d | 40 mm |
| Outer diameter D | 55 mm |
| Width B | 20 mm |
| Material | GCr15 |
| Seal / shield | open |
| Internal clearance | CN |
| Tolerance class | P0 |
| Cage | polymer (PA66 glass-fibre reinforced) |
| Basic dynamic load rating C | 31.4 kN |
| Basic static load rating C0 | 65.5 kN |
| Fatigue load limit Pu | 8.3 kN |
| Reference speed (thermal) | 9000 rpm |
| Limiting speed (mechanical) | 10000 rpm |
| Operating temperature | -40 to +120 °C |
| Mass | 140 g |
| Manufacturer designation | NKI 40/20 TN |
| Standard | DIN 616 |
| Catalog version | SKF PUB BU/P1 17000/1 EN (Oct 2018), ch. 7.4 machined rings with flanges, with an inner ring |
| Last verified | 2026-09-26 |
Source: catalog reference (SKF PUB BU/P1 17000/1 EN (Oct 2018), ch. 7.4 machined rings with flanges, with an inner ring) · extracted 2026-09-26
Parameters extracted from the official SKF Rolling Bearings catalogue (SKF PUB BU/P1 17000/1 EN (Oct 2018), ch. 7.4 machined rings with flanges, with an inner ring); SKF® and SKF Explorer® are trademarks of AB SKF, used here for identification only — mpnbase is not an authorized SKF distributor. Boundary dimensions are ISO 15 interchangeable; current production values may differ, verify at skf.com.
Dimensions in inches & NKI-series range
| Bore d | 1.575 in |
|---|---|
| Outer diameter D | 2.165 in |
| Width B | 0.787 in |
| ISO bore code | — |
| Radial section height (D−d)/2 | 7.5 mm |
| Smallest bore in NKI-series | 5 mm |
| Largest outer diameter in NKI-series | 130 mm |
| Width range across NKI-series | 12–40 mm |
| Bore position in NKI-series (smallest first) | 17 of 30 |
| Bore above smallest NKI-series size | 35 mm |
Cross references & equivalents
| Equivalent model | Brand | Match level | Note |
|---|---|---|---|
| RPNA40-55 | SKF | partial Main dimensions match; tolerances, seals or internal clearance differ. | Same 40×55×20 mm envelope, but catalogue C/C0 differ between the executions (31.4/65.5 vs 26.4/51 kN) — boundary dimensions match, not a load-rating equivalent. |
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 NKI40-20-TN model number is decoded
In SKF catalogue PUB BU/P1 17000, NKI 40/20 TN denotes a complete needle roller bearing for a 40 mm shaft — machined inner and outer rings, integral flanges on the outer ring retaining the roller set, 55 mm OD and 20 mm width. Reading the designation: the letters name the series with inner ring, the numbers the boundary plan — 40 mm bore (d), 20 mm width; the 55 mm OD follows the dimension series, and the inner ring is a separate ground component. The 2RS executions of the 49 series carry contact seals on both faces and a wider outer ring — their B and s dimensions differ from the open execution, so housings must be re-checked before substituting. The 140 g catalogue mass and 7.5 mm radial section ((55−40)/2) frame a rating of about 0.78 kN dynamic per millimetre of bore — the needle construction's density advantage over ball bearings of the same envelope. Because the inner ring is separable, mount it first against its shoulder, then slide the outer ring and roller set over it — the order avoids forcing rollers across the shaft threads or burrs.
Typical equipment & failure modes
Common applications
- conveyor roller ends and return rollers
- agricultural implement pivots and cutter heads
- electric motor armature shafts with ground seats
- printing and paper-handling rollers
Known failure patterns
- needle edge loading and raceway denting when the shaft deflects beyond the compensation range
- polymer cage deformation when the local temperature exceeds the 120 °C sustained limit
- abrasive paste formation from dust ingressing past open ends without effective housing seals
- brinelling of raceways from impact or pressing force transmitted through the rolling elements
- inner-ring bore fretting on a loose shaft seat, migrating hard particles into the rolling contact
Selection pitfalls — read before you substitute
- At 40 mm bore, verify shaft deflection against the needle length: these rollers tolerate little edge loading, and a simply supported span between 40 mm seats can bend past the compensation range long before the load rating is exhausted.
- Contamination, not fatigue, kills most small needle bearings: with raceways this narrow, one hard particle dents a full-width line — seal the housing even when the bearing execution itself is open.
- Do not interchange envelope-identical needle families blindly: NKI 40/20 TN shares its 40×55×20 mm outline with inner-ring and sealed siblings whose ratings differ — the family letters are part of the specification, not decoration.
- The 31.4/65.5 kN ratings and 140 g mass are SKF catalogue values transcribed from ch. 7.4 of PUB BU/P1 17000 (PDF table page 642); combined-type axial ratings appear in the text above because the structured record carries radial values only.
Frequently asked questions
Is the NKI 40/20 TN sealed?
No — the open execution relies on housing seals; 2RS executions of the 49 series add contact seals with a wider outer ring as a separate order number.
Which dimensions does the NKI 40/20 TN have?
A NKI 40/20 TN measures 40 mm bore/Fw × 55 mm outside diameter × 20 mm width (1.575 × 2.165 × 0.787 in). From SKF catalogue PUB BU/P1 17000: radial dynamic rating C 31.4 kN, static C0 65.5 kN, fatigue limit Pu 8.3 kN, limiting speed 10,000 r/min, reference speed 9,000 r/min, mass about 140 g.
What does NKI 40/20 TN mean in SKF part numbering?
The letters mark a machined-ring needle roller bearing with inner ring: 40 mm bore, 20 mm width, 55 mm OD per the series plan. The hardened inner ring is the raceway, so an ordinary k5 shaft seat suffices.
Does the NKI 40/20 TN need a hardened and ground shaft?
Yes — that is the point of this family. The NKI 40/20 TN carries its own hardened, ground inner ring on a 40 mm seat, so the shaft needs only a normal k5/j6 finish. The inner ring is separable and replaceable.
Reviewed & verified by the MPN Base engineering desk against source catalog SKF PUB BU/P1 17000/1 EN (Oct 2018), ch. 7.4 machined rings with flanges, with an inner ring on 2026-09-26. Read our editorial & correction policy.