NNU4921K bearing: complete specifications & equivalents
The NNU4921K is a ISO 15 radial bearing sized 105 x 145 x 40 mm (bore x OD x width). Verify dimensions against the official catalog before ordering.
Specification table
| Parameter | Value |
|---|---|
| Brand | NTN |
| Bore diameter d | 105 mm |
| Outer diameter D | 145 mm |
| Width B | 40 mm |
| Material | GCr15 |
| Seal / shield | open |
| Internal clearance | CN |
| Tolerance class | P0 |
| Cage | steel |
| Basic dynamic load rating C | 147 kN |
| Basic static load rating C0 | 268 kN |
| Fatigue load limit Pu | 30.5 kN |
| Limiting speed (mechanical) | 4100 rpm |
| Operating temperature | -40 to +120 °C |
| Mass | 1820 g |
| Manufacturer designation | NTN NNU4921K |
| Standard | ISO 15 |
| Catalog version | NTN Ball and Roller Bearings CAT.No.2203E |
| Last verified | 2026-10-01 |
Source: catalog reference (NTN Ball and Roller Bearings CAT.No.2203E) · extracted 2026-10-01
Parameters extracted from the official NTN Ball and Roller Bearings catalogue (NTN Ball and Roller Bearings CAT.No.2203E); NTN® is a trademark of NTN Corporation, used here for identification only — mpnbase is not an authorized NTN distributor. Boundary dimensions are ISO 15 interchangeable; current production values may differ, verify at ntnglobal.com.
Dimensions in inches & NNU4900-series range
| Bore d | 4.134 in |
|---|---|
| Outer diameter D | 5.709 in |
| Width B | 1.575 in |
| ISO bore code | — |
| Radial section height (D−d)/2 | 20 mm |
| Smallest bore in NNU4900-series | 100 mm |
| Largest outer diameter in NNU4900-series | 650 mm |
| Width range across NNU4900-series | 40–170 mm |
| Bore position in NNU4900-series (smallest first) | 2 of 26 |
| Bore above smallest NNU4900-series size | 5 mm |
Cross references & equivalents
| Equivalent model | Brand | Match level | Note |
|---|---|---|---|
| NNU4921K | ISO 15 | full Dimensions and internal design equivalent — standard interchange. | ISO 15 boundary-dimension envelope; brand-specific OEM equivalents (NSK/FAG/NTN) pending multi-source data. |
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 NNU4921K model number is decoded
The NTN order number NNU4921K denotes a 105×145×40 mm (4.134 × 5.709 × 1.575 in) cylindrical roller bearing in the NNU4900 family, a machined or stamped steel cage per the precision execution. The leading NNU defines the rib arrangement (double row, ribbed outer ring, separable inner); the dimension-series digits and the two-digit bore code resolve to 105 mm, with 145 mm OD and 40 mm width from the ISO 15 general plan. NTN ships these bearings open with normal (CN) radial clearance and P0 tolerance as standard; precision classes and C3/C4 clearance are separate order suffixes, not part of NNU4921K. The 1.4 kN-per-mm-bore figure (147 kN over 105 mm) lets a NNU4921K interchange candidate be screened quickly against the same-envelope ball or roller alternatives. Housing and shaft fillets must clear the ring chamfers of NNU4921K; an oversized radius locks the ring on its face and tilts the 40 mm-wide seat.
Typical equipment & failure modes
Common applications
- high-speed turbine and turbo-blower rotor supports
- coordinate-measuring and semiconductor-equipment drives
- machine-tool main spindles requiring high radial stiffness
- precision gearbox and indexing-drive shafts
Known failure patterns
- inner-ring raceway spalling from overload in the line-contact Hertzian zone
- cage distortion after a mounting blow carried through the roller set
- grease starvation and cage drag when relubrication intervals are extended past the grease life
- rolling-contact fatigue after the catalogue L10 life is exceeded
- rolling-contact fatigue after the catalogue L10 life is exceeded
Selection pitfalls — read before you substitute
- Induction-heat the 105 mm inner ring (max 120 °C) and mount it before fitting the outer ring; the ring mass makes square guidance essential — driving through the rollers brinells the 145 mm raceway.
- The 20.0 mm section at 105 mm bore leaves a narrow oil path; with the standard cage the relubrication groove and oil-level design must keep grease from churning in the 40 mm-wide cavity.
- Minimum radial load is frequently missed in oversized motor and fan selections: an underloaded NNU4921K skids at 4,100 r/min and fails by smearing long before L10 fatigue.
- The K tapered bore of NNU4921K is driven up to a measured clearance: skipping the drive-up procedure in favour of a normal press fit leaves the precision spindle set either preloaded hot or loose under load.
- Never press either ring through the roller complement when fitting: mounting force goes through the ring faces, or the 145 mm race brinells and releases as vibration within hours.
- The 147/268 kN ratings, 30.5 kN Pu and 1820 g mass are NTN Ball and Roller Bearings CAT.No.2203E values for NNU4921K (standard execution); other makes sharing the ISO 15 envelope revise internal geometry without changing the order number, so keep the 2203E reference with the specification for critical drives.
Frequently asked questions
What are the dimensions and catalogue data of NNU4921K?
NNU4921K is an NTN NNU double-row precision cylindrical roller bearing to ISO 15: 105 mm bore, 145 mm OD and 40 mm width (4.134 × 5.709 × 1.575 in). From NTN Ball and Roller Bearings CAT.No.2203E: C 147 kN, C0 268 kN, Pu 30.5 kN, grease limiting speed 4,100 r/min, oil 4,800 r/min, mass about 1820 g. It is open, with a machined or stamped steel cage per the precision execution.
What is the difference between NU, NJ, NUP, N and NF cylindrical roller bearings?
NN and NNU are double-row precision designs rather than single-row rib variants: NN has a ribbed inner ring and separable outer ring, NNU the reverse; both give high radial stiffness for spindles and are commonly ordered with P5/P4 precision and a K 1:12 tapered bore for preload setting. NNU4921K is the NNU execution.
What do the EA, E and ＊ suffixes mean on NTN cylindrical roller bearings?
EA is NTN's high-capacity design with a pressed sheet-steel cage and optimised roller complement in the same ISO 15 envelope; E marks the high-capacity design at the larger bores; the ＊ symbol printed before the code in the NTN catalogue marks the ULTAGE new-generation specification and is not typed as part of the order number. NNU4921K is supplied in the standard open execution shown in the table.
What does the K tapered bore mean on a precision NNU bearing like NNU4921K?
The K suffix denotes a 1:12 tapered bore. NNU4921K mounts on a tapered shaft seat or adapter sleeve and is driven up to a measured residual clearance or preload — this is how precision spindle bearings achieve stiffness without locknut guesswork; the cylindrical-bore NNU4921 execution of the same envelope mounts on a plain seat.
What do Fw and Ew mean for NNU4921K?
Fw is the inner sub-assembly raceway/roller-set envelope diameter and Ew the outer-ring raceway diameter printed for geometric checks: Fw 118 mm. The nominal bore/OD remain 105/145 mm; Fw/Ew describe the rolling-contact path, and speeds still run to 4,100 r/min grease / 4,800 r/min oil.
Reviewed & verified by the MPN Base engineering desk against source catalog NTN Ball and Roller Bearings CAT.No.2203E on 2026-10-01. Read our editorial & correction policy.