NN4921K bearing: complete specifications & equivalents
The NN4921K 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 | 1730 g |
| Manufacturer designation | NTN NN4921K |
| 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 & NN4900-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 NN4900-series | 100 mm |
| Largest outer diameter in NN4900-series | 440 mm |
| Width range across NN4900-series | 40–118 mm |
| Bore position in NN4900-series (smallest first) | 2 of 18 |
| Bore above smallest NN4900-series size | 5 mm |
Cross references & equivalents
| Equivalent model | Brand | Match level | Note |
|---|---|---|---|
| NN4921K | 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 NN4921K model number is decoded
For pure radial duty on a 105 mm shaft NTN supplies NN4921K: two integral ribs on the outer ring and a ribless, separable inner ring; the open execution is rated 4,100 r/min on grease and 4,800 r/min on oil. Digit by digit, NN4921K reads as NN (double row, ribbed inner ring, separable outer), ISO 15 series, and bore 105 mm; trailing letters such as EA or E mark the cage/capacity execution and are part of the order number. The 20.0 mm radial section ((145−105)/2) frames the roller complement; capacity per millimetre of bore is about 1.4 kN/mm before application factors. Mass 1730 g for the 105×145×40 mm NN4921K execution covers the pressed-steel cage and full roller complement; substitute on mass plus rating, not envelope alone. On NN arrangements confirm which ring is separable before heating: forcing the ribbed ring off over the roller complement damages the cage and rib faces.
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
- roller smearing (microwelding) at start-up before the oil film builds under insufficient radial load
- pressed-steel cage pocket wear when rollers skid at light load or too-thin lubricant
- grease starvation and cage drag when relubrication intervals are extended past the grease life
- false brinelling on standby machines exposed to external vibration
- fretting corrosion on a loose rotating shaft seat
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.
- Minimum radial load is frequently missed in oversized motor and fan selections: an underloaded NN4921K skids at 4,100 r/min and fails by smearing long before L10 fatigue.
- 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 NN rib arrangement is not interchangeable with another prefix on the same 105 mm seat: double row, ribbed inner ring, separable outer. Swapping NU for NJ changes axial location behaviour even though the 145×40 mm envelope matches.
- 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.
- The 147/268 kN ratings, 30.5 kN Pu and 1730 g mass are NTN Ball and Roller Bearings CAT.No.2203E values for NN4921K (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 NN4921K?
NN4921K is an NTN NN 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 1730 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. NN4921K is the NN 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. NN4921K is supplied in the standard open execution shown in the table.
What does the K tapered bore mean on a precision NN bearing like NN4921K?
The K suffix denotes a 1:12 tapered bore. NN4921K 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 NN4921 execution of the same envelope mounts on a plain seat.
What do Fw and Ew mean for NN4921K?
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.