NU304 bearing: complete specifications & equivalents
The NU304 is a ISO 15 radial bearing sized 20 x 52 x 15 mm (bore x OD x width). Verify dimensions against the official catalog before ordering.
Specification table
| Parameter | Value |
|---|---|
| Brand | SKF |
| Bore diameter d | 20 mm |
| Outer diameter D | 52 mm |
| Width B | 15 mm |
| Material | GCr15 |
| Seal / shield | open |
| Internal clearance | CN |
| Tolerance class | P0 |
| Cage | polymer (PA66 glass-fibre reinforced) |
| Basic dynamic load rating C | 35.5 kN |
| Basic static load rating C0 | 26 kN |
| Fatigue load limit Pu | 3.25 kN |
| Reference speed (thermal) | 15000 rpm |
| Limiting speed (mechanical) | 18000 rpm |
| Operating temperature | -40 to +120 °C |
| Mass | 140 g |
| Manufacturer designation | NU 304 ECP |
| Standard | ISO 15 |
| Catalog version | SKF PUB BU/P1 17000/1 EN (Oct 2018), ch. 6.1 single-row cylindrical roller bearings |
| Last verified | 2026-09-26 |
Source: catalog reference (SKF PUB BU/P1 17000/1 EN (Oct 2018), ch. 6.1 single-row cylindrical roller bearings) · extracted 2026-09-26
Parameters extracted from the official SKF Rolling Bearings catalogue (SKF PUB BU/P1 17000/1 EN (Oct 2018), ch. 6.1 single-row cylindrical roller bearings); 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 & NU300-series range
| Bore d | 0.787 in |
|---|---|
| Outer diameter D | 2.047 in |
| Width B | 0.591 in |
| ISO bore code | 04 |
| Radial section height (D−d)/2 | 16 mm |
| Smallest bore in NU300-series | 17 mm |
| Largest outer diameter in NU300-series | 620 mm |
| Width range across NU300-series | 14–109 mm |
| Bore position in NU300-series (smallest first) | 2 of 34 |
| Bore above smallest NU300-series size | 3 mm |
Cross references & equivalents
| Equivalent model | Brand | Match level | Note |
|---|---|---|---|
| 6304 | SKF | verify Commonly quoted cross reference; confirm against the target catalog before ordering. | Identical envelope but a different bearing type — confirm load and speed suitability before substitution. |
| 6304-2RS | SKF | verify Commonly quoted cross reference; confirm against the target catalog before ordering. | Identical envelope but a different bearing type — confirm load and speed suitability before substitution. |
| 6304-RSR | SKF | verify Commonly quoted cross reference; confirm against the target catalog before ordering. | Identical envelope but a different bearing type — confirm load and speed suitability before substitution. |
| 6304-ZZ | SKF | verify Commonly quoted cross reference; confirm against the target catalog before ordering. | Identical envelope but a different bearing type — confirm load and speed suitability before substitution. |
| N304-ECP | SKF | full Dimensions and internal design equivalent — standard interchange. | Same ISO 15 boundary envelope and equivalent catalogue ratings; SKF N 304 ECP is a different execution with its own cage/flange configuration. |
| NJ304-ECP | SKF | full Dimensions and internal design equivalent — standard interchange. | Same ISO 15 boundary envelope and equivalent catalogue ratings; SKF NJ 304 ECP is a different execution with its own cage/flange configuration. |
| NUP304-ECP | SKF | full Dimensions and internal design equivalent — standard interchange. | Same ISO 15 boundary envelope and equivalent catalogue ratings; SKF NUP 304 ECP is a different execution with its own cage/flange configuration. |
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 NU304 model number is decoded
NU304 is a single-row cylindrical roller bearing in the NU300 family: line-contact rollers between ground inner and outer raceways, separated by a glass-fibre reinforced polyamide (PA66) cage, listed by SKF in SKF catalogue PUB BU/P1 17000. On the SKF nameplate, NU304 follows the ISO 15 convention: NU for the separable-inner-ring type, 3 for the standard, final digits 04 for the 20 mm bore. The SKF catalogue adds the ECP suffix for the standard glass-fibre reinforced polyamide cage; ECJ (pressed steel) and ECM (machined brass) are alternative executions with different speed and temperature ratings. The 16.0 mm radial section height ((52−20)/2) and 140 g catalogue mass frame a 20 mm bore bearing rated at 35.5 kN C — roughly 1.77 kN of dynamic capacity per millimetre of bore before application factors. When press-fitting the inner ring, support the ring face squarely; cocking the ring over a keyway or burr brinells the raceway and shows as vibration within the first running hours.
Typical equipment & failure modes
Common applications
- pump and compressor crankshafts
- machine-tool spindle support (radial set)
- agricultural machinery power-take-off shafts
- automotive alternator and starter shafts
Known failure patterns
- inner-ring raceway spalling from overload or contamination in the line-contact hertz zone
- outer-ring raceway fatigue after extended service at loads above the catalogue L10 rating
- cage pocket wear from roller skidding at insufficient radial load or during start-up before oil film build-up
- roller-end scoring from inadequate oil flow in the roller-raceway contact
- brinelling from static overload or impact during handling and mounting
Selection pitfalls — read before you substitute
- At a 20 mm bore the thin roller and raceway are sensitive to misalignment; keep shaft deflection within the 3–4 minute alignment limit and check seat runout (target Ra ≤ 0.8 µm) — a tilted NU304 develops edge loading and early spalling.
- The NU300 series at 20 mm bore (52×15 mm) is the standard-section cylindrical roller envelope; stepping to a wide-section 22/23 variant changes OD, width and heat generation — it is not a drop-in swap when the housing and speed are fixed.
- Catalogue reference speed and limiting speed are different limits: the 15,000 r/min reference is a thermal criterion under load, while the 18,000 r/min limiting value is the mechanical ceiling — observe both.
- Catalogue reference speed and limiting speed are different limits: the 15,000 r/min reference is a thermal criterion under load, while the 18,000 r/min limiting value is the mechanical ceiling — observe both.
- The 35.5/26 kN ratings are SKF catalogue 17000 values for the ECP polymer-cage execution; SKF can revise internal geometry without changing the order number, so keep the catalogue version (PUB BU/P1 17000/1 EN, 2018) with the specification and re-verify for critical drives.
- SKF lists several executions of NU304 (ECP polymer cage, ECJ pressed steel, ECM machined brass); they are not interchangeable order numbers — quote the full NU 304 ECP designation when purchasing and match the execution to the speed and temperature requirements.
Frequently asked questions
What are the dimensions and catalogue data of NU304?
The NU 304 ECP is a single-row cylindrical roller bearing with 20 mm bore, 52 mm outer diameter and 15 mm width (0.787 × 2.047 × 0.591 in) per ISO 15. From SKF catalogue PUB BU/P1 17000: basic dynamic load rating C 35.5 kN, C0 26 kN, fatigue load limit Pu 3.25 kN, reference speed 15,000 r/min, limiting speed 18,000 r/min, mass about 140 g. It is an open, separable bearing with a glass-fibre reinforced polyamide cage.
How does the NU304 bore code work?
The last two digits of NU304 are bore code 04, so 04 × 5 = 20 mm bore. The 52 mm OD and 15 mm width come from the ISO 15 dimension-series plan, shared with other cylindrical bearings of the same code.
What does the ECP suffix mean on NU304?
ECP stands for a glass-fibre reinforced polyamide cage (PA66), the SKF standard for this size. Alternative executions include ECJ (pressed steel cage) and ECM (machined brass cage), which have different speed and temperature ratings and are separate order numbers.
Can NU304 take axial load?
No — the NU design has no ribs on the outer ring and cannot support axial load in either direction. It is intended for pure radial load with axial location provided by adjacent components. If axial location is required, consider NJ (one-direction) or NUP (two-direction) variants, or pair the NU bearing with a thrust or angular-contact bearing.
How does NU304 compare with a ball bearing of the same bore?
A cylindrical roller bearing has line contact and much higher radial capacity than a ball bearing of the same bore, but it cannot take axial load in the NU design and is sensitive to misalignment (≈ 3–4 minutes of arc). Speed limits are generally lower than deep-groove ball bearings. A ball bearing cannot be dropped into a pure radial cylindrical-roller position without re-checking life and deflection, and vice versa.
Reviewed & verified by the MPN Base engineering desk against source catalog SKF PUB BU/P1 17000/1 EN (Oct 2018), ch. 6.1 single-row cylindrical roller bearings on 2026-09-26. Read our editorial & correction policy.