NU vs NJ vs NUP cylindrical roller bearings: which flange locates the shaft
Cylindrical roller bearings exist to carry heavy radial loads on shafts that must also move axially — thermal expansion has to go somewhere. The N-letter prefix is a flange map, and the flange map alone decides whether the bearing locates, half-locates, or floats the shaft.
The flange map behind the letters
A cylindrical roller assembly has an inner ring, an outer ring, the roller-and-cage set, and integral flanges that stop the rollers running off the ring axially. Whichever ring carries the flanges becomes the axially fixed member.
| Design | Inner ring flanges | Outer ring flanges | Axial behaviour |
|---|---|---|---|
| N | 2 | 0 | outer ring free axially (inner fixed to shaft) |
| NU | 0 | 2 | free: shaft/inner ring displaces both directions |
| NJ | 1 | 2 | one-direction locating: fixed against the flange, free the other way |
| NUP | 1 integral + 1 loose ring | 2 | locating both directions (fixed end) |
| NJ + L ring | 1 + separate angle ring | 2 | locating both directions, assembled on site |
All data in the database currently carries the NU form — NU205 (25×52×15 mm), NU210, NU308, the wide NU2205 and NU2310 — because the free end is where substitution questions actually arise in existing machines.
Series and envelope: the letter does not change the size
The dimension series after the prefix works like the ball-bearing numbering system:
- NU2xx — light series (NU205 = 25×52×15 mm)
- NU3xx — medium series, higher C in a bigger envelope (NU308 = 40×90×23 mm)
- NU22xx — light-wide series for more rollers (NU2205 = 25×52×18 mm)
- NU23xx — medium-wide, the workhorse of heavy gearboxes (NU2310 = 50×110×40 mm)
NU2310, NJ2310 and NUP2310 share the 50×110×40 envelope and the same basic load rating; an E suffix (e.g. NU2310 E C3) denotes a reinforced roller set with a higher dynamic rating in the same envelope. Because the bore and outer section are identical across the flange variants, envelopes like these appear in the dimensions finder alongside spherical and tapered rollers of similar size.
The locating/free arrangement (the reason this matters)
Every shaft between two bearing positions grows as it warms. Fix both ends axially and the growth has nowhere to go — the bearings preload each other. The standard solution:
- Fixed (locating) end — one NUP, an NJ + L ring, or a locating ball/angular-contact unit; takes residual axial forces and sets shaft position.
- Free end — one NU (or N); the inner ring walks slightly on the outer ring raceways as the shaft expands, producing zero axial reaction.
- Heavy shafts, long centres — both ends may use NU/NJ pairs with the location handled by a dedicated pair of angular-contact bearings elsewhere, as in the paired-arrangement guide.
Substitution rules from the field
- NU → NJ/NUP is not a drop-in: you are changing the machine’s thermal-expansion path. If the free end suddenly locates, the fixed end may fight it and both bearings run hot.
- NJ + L vs NUP: equivalent locating function; the loose flange ring is an interchangeable part brand-to-brand in principle, but thickness tolerances mean the L ring should come from the bearing’s manufacturer where possible.
- Clearance choice leans one group looser on cylindrical rollers in many catalogue recommendations because interference fits are common and axial sliding must stay free — see C2/CN/C3/C4 clearance.
- Lubrication access: flange-less inner rings (NU) make oil or grease distribution to the roller set easier, which matters in high-speed gearboxes; check the cage suffix (M brass vs polymer) against the oil temperature.
Verification path for an actual replacement
- Read the prefix and any L ring on the removed bearing — do not assume NU from a partial stamp.
- Confirm envelope and rating on the model page: NU205, NU2205, NU2310.
- Keep the same flange design unless the locating arrangement is being redesigned on paper.
- Check brand-naming pages for suffix spelling — SKF / NSK — and use the cross reference tool for graded equivalents.
- Revisit designation fundamentals any time the removed stamp is ambiguous: how to read bearing numbers.
Frequently asked questions
What is the difference between NU and NJ cylindrical roller bearings?
An NU bearing has two integral flanges on the outer ring and none on the inner ring, so the inner ring (and shaft) can move axially in both directions — it is a free bearing. An NJ bearing has two outer flanges plus one inner flange, locating the shaft in one axial direction while allowing displacement the other way.
Can an NJ bearing be turned into a locating bearing?
Yes, with a separate L-type angle ring fitted against the flange-less side of the inner ring, an NJ plus L ring behaves as a locating arrangement. A factory NUP bearing provides the same function with a loose second flange ring included, so the choice is inventory and assembly preference rather than capacity.
Why are cylindrical roller bearings paired with a ball bearing on the same shaft?
A single cylindrical roller cannot locate the shaft axially — that is the point of its line-contact, displacement-friendly design. The common arrangement is one locating bearing (NUP, NJ+L or a paired angular-contact/deep-groove bearing) that fixes axial position, plus one free NU that absorbs shaft thermal expansion without preloading the system.
Do NU and NUP bearings in the same series have the same dimensions and load rating?
Yes — within a series (for example NU2310 and NUP2310) the d/D/B envelope and basic dynamic/static ratings are the same; the letter changes only the flange arrangement and axial-location capability. Always confirm on the exact model page because E-reinforced and cage variants exist within each series.