NU308 bearing: complete specifications & equivalents
The NU308 is a ISO 15 radial bearing sized 40 x 90 x 23 mm (bore x OD x width). Verify dimensions against the official catalog before ordering.
Specification table
| Parameter | Value |
|---|---|
| Brand | Generic ISO interchange (unbranded) |
| Bore diameter d | 40 mm |
| Outer diameter D | 90 mm |
| Width B | 23 mm |
| Seal / shield | open |
| Internal clearance | CN / C3 / C4 available |
| Tolerance class | P0 |
| Cage | steel |
| Operating temperature | -30 to +120 °C |
| Mass (geometry estimate) | 599 g |
| Standard | ISO 15 |
| Catalog version | iso15-crb-geometry |
| Last verified | 2026-09-24 |
Source: ISO 15 boundary-dimensions standard · extracted 2026-09-24
Standard-interchange bearing: dimensions per ISO 15; CN clearance, P0 tolerance and a stamped-steel cage are catalogue defaults unless the supplier states otherwise, and mass is a geometry-derived estimate. Brand catalogue load ratings and limiting speeds for NU308 are being backfilled — use the inquiry form for certified values.
Dimensions in inches & NU300-series range
| Bore d | 1.575 in |
|---|---|
| Outer diameter D | 3.543 in |
| Width B | 0.906 in |
| ISO bore code | 08 |
| Radial section height (D−d)/2 | 25 mm |
| Smallest bore in NU300-series | 20 mm |
| Largest outer diameter in NU300-series | 215 mm |
| Width range across NU300-series | 15–47 mm |
| Bore position in NU300-series (smallest first) | 5 of 17 |
| Bore above smallest NU300-series size | 20 mm |
Cross references & equivalents
No verified interchange data yet. Send an inquiry and our team will confirm equivalents manually.
How the NU308 model number is decoded
In the NU300-series (ISO 15 dimension series) the leading NU marks a single-row cylindrical roller: the outer ring has two integral shoulders and the inner ring none, so the inner-ring/roller assembly floats axially both ways and the bearing locates radially only; the last two digits of NU308 are bore code 08 = 40 mm (code × 5 from 04 upward). The ISO 15 envelope is 90 mm outside diameter and 23 mm width — 1.575 × 3.543 × 0.906 in, with a 25.0 mm radial section height. Straight cylindrical rollers give line contact for high radial capacity at low friction, with no self-alignment; catalogue practice for a rotating inner ring is a k5 shaft seat, H7 housing and C3 clearance. NJ adds one inner shoulder (one-direction location), NUP two shoulders (full location); E denotes a reinforced roller complement — verify against the actual load direction.
Typical equipment & failure modes
Common applications
- conveyor and hoist gearcase shafts
- wind turbine gearbox intermediate shafts
- industrial helical and planetary gearboxes
- medium electric motor drives
- centrifugal pump and compressor shafts
Known failure patterns
- cage fatigue and fracture after sustained over-speed with marginal lubrication
- fretting corrosion on an interference inner-ring seat that creeps under a loose fit
- false brinelling indentations at the roller pitch when an idle bearing sits in machine vibration
- inner-ring shoulder damage from cold mounting force transmitted through the roller set
- thermal seizure when an interference fit plus hot running removes all residual C3 clearance
Selection pitfalls — read before you substitute
- On this 40 mm-bore NU308, set C3 as the default for an interference inner-ring fit and recheck play after mounting; a row that turned freely on the bench can be fully clamped once the 40 mm shaft reaches operating temperature.
- The NU3 medium series at 40 mm bore uses the 90 mm OD and 23 mm width for heavy radial load; it needs stiffer shaft and housing shoulders and runs lower limiting speed than an NU208, so it is a poor choice in a high-rpm gearbox stage sized for the light row.
- Check the shoulder geometry for NU308: a generous fillet on the 40 mm shaft looks strong but stops the inner ring seating home; use the catalogue corner-relief radius and a withdrawal groove so the outer ring presses flush in the 90 mm, 23 mm-wide housing bore.
- Large NU308 rings need real draw-up force: use a sleeve or hydraulic press against the inner ring face only, verify the 40 mm seat temperature before assembly, and never strike the roller cage — a dented cage window on the 23 mm-wide set locks the rollers at the first hot run.
- Read the damage on a removed NU308: uniform roller smearing points to minimum-load skidding, while spalling on one edge of the 90 mm raceway means misalignment or a tilted housing face — the two failures need opposite corrections, so inspect before ordering the replacement fit.
- Cylindrical rollers have no self-aligning capacity: line contact across the 23 mm-wide rollers means even a few tenths of a degree of misalignment edge-loads the 90 mm raceway; where shaft deflection is unavoidable, use a spherical roller or a self-aligning ball bearing instead.
- Keep the minimum load above the skid threshold (catalogue rule of thumb roughly 1–2% of the dynamic rating); an unloaded NU308 on an idler or in a cold-start gearbox develops roller smearing, so add a spring preload or downsize the bearing if the duty is mostly empty.
Frequently asked questions
What are the dimensions of an NU308 cylindrical roller bearing?
An NU308 single-row cylindrical roller bearing per ISO 15 has a 40 mm bore, 90 mm outer diameter and 23 mm width (1.575 × 3.543 × 0.906 in), a 25.0 mm radial section and roughly 599 g geometry-based estimated mass; catalogue masses vary with cage and E design.
How does the NU308 bore code work?
The letters give the ring/shoulder design (NU = axially free, NJ = one inner shoulder, NUP = two shoulders) and the last two digits are bore code 08: 08 × 5 = 40 mm. The 90 mm OD and 23 mm width come from the ISO 15 dimension series (NU300), which shares its boundary plan with the 6-series deep-groove envelopes.
What is the difference between NU, NJ, N and NUP designs?
NU (like the NU308) has a shoulderless inner ring and floats axially in both directions; NJ has one integral inner-ring shoulder and locates the shaft in one direction; N has two inner shoulders and a shoulderless outer ring (mirror of NU); NUP adds a second, loose shoulder so the inner ring locates in both directions. Select by the shaft locating scheme, not by load rating — the envelopes are identical.
Can an NU308 take axial load or misalignment?
No thrust in the NU design: any sustained axial force scores the roller ends and ring flanges, so locate the shaft with an NJ/NUP row, a paired angular-contact bearing or a deep-groove row elsewhere. Misalignment is likewise not tolerated — line contact between cylindrical rollers and raceways edges under even small angular errors; use a spherical roller bearing where shaft deflection or housing misalignment is expected.
What does the E suffix mean and which clearance should be ordered?
E (as in NU308E) is the reinforced design: larger/optimised rollers raise the basic load rating over the basic designation in the same 40×90×23 mm envelope. For clearance, C3 is the usual default with an interference fit on a rotating inner ring because fit and thermal growth reduce residual play; use CN mainly for loose floating outer-ring mounts and C4 for hot or vibratory duty.
Reviewed & verified by the MPN Base engineering desk against source catalog iso15-crb-geometry on 2026-09-24. Read our editorial & correction policy.