30308-NTN bearing: complete specifications & equivalents
The 30308-NTN is a ISO 355 radial bearing sized 40 x 90 x 25.25 mm (bore x OD x width). Verify dimensions against the official catalog before ordering.
Specification table
| Parameter | Value |
|---|---|
| Brand | NTN |
| Bore diameter d | 40 mm |
| Outer diameter D | 90 mm |
| Width B | 25.25 mm |
| Material | GCr15 |
| Seal / shield | open |
| Internal clearance | CN |
| Tolerance class | P0 |
| Cage | steel |
| Basic dynamic load rating C | 101 kN |
| Basic static load rating C0 | 102 kN |
| Limiting speed (mechanical) | 4400 rpm |
| Operating temperature | -40 to +120 °C |
| Mass | 765 g |
| Manufacturer designation | NTN 30308 |
| Standard | ISO 355 |
| 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 & 30300-series range
| Bore d | 1.575 in |
|---|---|
| Outer diameter D | 3.543 in |
| Width B | 0.994 in |
| ISO bore code | 08 |
| Radial section height (D−d)/2 | 25 mm |
| Smallest bore in 30300-series | 15 mm |
| Largest outer diameter in 30300-series | 500 mm |
| Width range across 30300-series | 14.25–105 mm |
| Bore position in 30300-series (smallest first) | 7 of 32 |
| Bore above smallest 30300-series size | 25 mm |
Cross references & equivalents
| Equivalent model | Brand | Match level | Note |
|---|---|---|---|
| 30308 | SKF | full Dimensions and internal design equivalent — standard interchange. | Identical ISO 15 envelope, seal and ratings band — standard interchange; re-verify precision class and clearance per supplier. |
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 30308-NTN model number is decoded
For a 40 mm shaft with a thrust component, NTN lists 30308, a metric 4T tapered roller bearing of 101 kN C and 102 kN C0 inside the 90 mm OD, 25.25 mm-wide assembled set. The last two digits 08 are the ISO bore code, ×5 = 40 mm (codes 00–03 use the 10/12/15/17 exception table) under the ISO 355 numbering; the 25.25 mm value printed here is T, the width of the fully assembled cone-plus-cup set, not the inner-ring width alone. The pressed-steel cage and surface-finished roller ends are standard 4T execution; ULTAGE-marked variants use upgraded material and internal geometry aimed at longer rating life with the same ISO 355 envelope. Grease relubrication at 4,400 r/min limiting speed targets the rib contact; if the drive needs 5,900 r/min it must be arranged for oil lubrication with a defined flow to the large roller ends. After mounting check the contact pattern with marking blue: a pattern biased to the large or small roller end means the 40 mm pair is misadjusted and the 25.25 mm T set will edge-load in service.
Typical equipment & failure modes
Common applications
- industrial gearbox and speed-reducer shafts
- bevel and helical pinion drives with high thrust component
- pump and compressor drive shafts carrying combined load
- machine-tool gearbox and headstock shafts
Known failure patterns
- cup creep and housing fretting when the specified outer-ring press fit is not achieved
- preload loss from racewear when one bearing of the paired set is replaced without re-adjusting end play
- rib scoring when axial load is carried beyond the cone rib capacity of the adjusted pair
- cage damage from a mounting blow delivered through the roller complement instead of the ring face
- water-induced pitting when the open bearing is exposed to washdown without external seals
Selection pitfalls — read before you substitute
- Hub and pinion bearings of this 40 mm bore class are commonly adjusted in pairs; verify end play against the axle drawing and re-torque after bedding-in rather than trusting the initial preload on 30308.
- The steeper contact angle of the large-OD 30300 family with the steeper contact angle raises axial capacity but lowers the speed ceiling against a 302/322 bearing at 40 mm bore; do not swap to the lighter family without re-checking the 4,400 r/min grease limit and the thrust load split.
- On slash-size and T-code bearings confirm the full code on both cone and cup before fitting: the 40 mm bore alone does not identify the cup, and a mismatched cone/cup set has no catalogue rating.
- Open 30308 units have no seals: a wheel-hub or gearbox application needs external sealing or the 4,400 r/min grease figure is meaningless once water reaches the cone rib.
- Replacing one half of an adjusted pair without resetting end play negates the 25.25 mm T dimension logic: re-adjust the set and re-verify the contact pattern rather than reusing the old spacer stack.
- The 101/102 kN ratings here are NTN Ball and Roller Bearings CAT.No.2203E figures for 30308 (ISO 355 2FB); internal geometry legitimately differs between manufacturers, so re-verify ratings, adjustment and suffix before substituting another make on a rated drive.
Frequently asked questions
What are the dimensions and catalogue ratings of NTN 30308?
30308 is a metric 4T tapered roller bearing to ISO 355: 40 mm bore, 90 mm outside diameter and 25.25 mm assembled width T (1.575 × 3.543 × 0.994 in). From NTN Ball and Roller Bearings CAT.No.2203E: C 101 kN, C0 102 kN (2203E prints no Pu for this size), grease limiting speed 4,400 r/min, oil speed 5,900 r/min, mass about 765 g. It is an open, separable cone/cup bearing with a pressed-steel cage.
What does the 4T prefix mean on 30308?
4T is NTN's tapered roller bearing brand mark, stamped on the cone and cup in the factory. It identifies the NTN 4T programme and is not a dimension, so the catalogue and this page list the bare ISO 355 number (30308); the lower-case 4t- factory spelling of the code is retained among the search aliases.
How does the 30308 designation encode the 40 mm bore and what is 2FB?
last two digits 08 are the ISO bore code, ×5 = 40 mm (codes 00–03 use the 10/12/15/17 exception table). The 90 mm OD and 25.25 mm assembled T follow the ISO 355 dimension-series plan. 2FB is the maker-neutral ISO 355 abbreviation for this dimension series (the same code other manufacturers print); 4T is NTN's own brand mark.
Why does 30308 list two speeds, 4,400 and 5,900 r/min?
The lower figure, 4,400 r/min, is the grease-lubricated limiting speed — the mechanical ceiling for the cage, rollers and cone-rib contact. With oil lubrication NTN rates 30308 at 5,900 r/min because oil removes rib-contact heat better. Real limits still depend on preload, oil flow and cooling.
What does the suffix on 30308 mean?
It is the basic open 4T execution — steel cage, adjusted mounting, no seal or feature suffix. NTN uses U for the uprated design, UTG for ULTAGE large-bearing specs, X for the ISO boundary section, and C/CA/D/A for contact-angle or internal-design variants; all are separate order numbers.
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.