NU311 bearing: complete specifications & equivalents
The NU311 is a ISO 15 radial bearing sized 55 x 120 x 29 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 | 55 mm |
| Outer diameter D | 120 mm |
| Width B | 29 mm |
| Seal / shield | open |
| Internal clearance | CN / C3 / C4 available |
| Tolerance class | P0 |
| Cage | steel |
| Operating temperature | -30 to +120 °C |
| Mass (geometry estimate) | 1322 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 NU311 are being backfilled — use the inquiry form for certified values.
Dimensions in inches & NU300-series range
| Bore d | 2.165 in |
|---|---|
| Outer diameter D | 4.724 in |
| Width B | 1.142 in |
| ISO bore code | 11 |
| Radial section height (D−d)/2 | 32.5 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) | 8 of 17 |
| Bore above smallest NU300-series size | 35 mm |
Cross references & equivalents
No verified interchange data yet. Send an inquiry and our team will confirm equivalents manually.
How the NU311 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 NU311 are bore code 11 = 55 mm (code × 5 from 04 upward). The ISO 15 envelope is 120 mm outside diameter and 29 mm width — 2.165 × 4.724 × 1.142 in, with a 32.5 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
- medium electric motor drives
- centrifugal pump and compressor shafts
- machine tool gearboxes and feed drives
- conveyor and hoist gearcase shafts
- wind turbine gearbox intermediate shafts
Known failure patterns
- 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
- roller and raceway smearing when a lightly loaded NU row skids at start-up or below the minimum load
- edge loading and rib spalling from shaft-to-housing misalignment — cylindrical rollers run on line contact and cannot self-align
- roller-end and flange scoring when an NU bearing is forced to locate axially; only NJ/NUP designs carry thrust at the shoulders
Selection pitfalls — read before you substitute
- On this 55 mm-bore NU311, 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 55 mm shaft reaches operating temperature.
- The NU3 medium series at 55 mm bore uses the 120 mm OD and 29 mm width for heavy radial load; it needs stiffer shaft and housing shoulders and runs lower limiting speed than an NU211, so it is a poor choice in a high-rpm gearbox stage sized for the light row.
- Check the shoulder geometry for NU311: a generous fillet on the 55 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 120 mm, 29 mm-wide housing bore.
- Large NU311 rings need real draw-up force: use a sleeve or hydraulic press against the inner ring face only, verify the 55 mm seat temperature before assembly, and never strike the roller cage — a dented cage window on the 29 mm-wide set locks the rollers at the first hot run.
- Read the damage on a removed NU311: uniform roller smearing points to minimum-load skidding, while spalling on one edge of the 120 mm raceway means misalignment or a tilted housing face — the two failures need opposite corrections, so inspect before ordering the replacement fit.
- Apply mounting force to the ring with the interference fit — inner ring for a shaft fit, outer ring for a housing fit; pushing the 55 mm inner ring home through the rollers brinells the 120 mm outer raceway.
- Order clearance for the fit: a rotating interference inner ring on a 55 mm shaft plus thermal growth consumes CN play, so C3 is the normal industrial default and C4 serves hot or vibratory mounts; CN suits loose outer-ring floating arrangements in bored housings.
Frequently asked questions
What are the dimensions of an NU311 cylindrical roller bearing?
An NU311 single-row cylindrical roller bearing per ISO 15 has a 55 mm bore, 120 mm outer diameter and 29 mm width (2.165 × 4.724 × 1.142 in), a 32.5 mm radial section and roughly 1322 g geometry-based estimated mass; catalogue masses vary with cage and E design.
How does the NU311 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 11: 11 × 5 = 55 mm. The 120 mm OD and 29 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 NU311) 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 NU311 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 NU311E) is the reinforced design: larger/optimised rollers raise the basic load rating over the basic designation in the same 55×120×29 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.