NU, NJ, N, NUP: cylindrical roller types
An NU 205 and an N 205 are both 25 × 52 × 15 mm. They will fit the same shaft and housing. And they are not interchangeable — swapping one for the other can let a shaft walk out of position or over-constrain it until the roller ends wear. The reason is the letter, and it is worth understanding because cylindrical roller bearings are the type where "same size, different job" bites hardest.
Ribs are the whole story
In a cylindrical roller bearing the rollers run between guide ribs (also called flanges). The type letter says which ring carries those ribs, and that decides whether — and in which direction — the bearing can hold the shaft in place along its axis. The radial load capacity is similar across the types; the axial behaviour is completely different.
The five common types
| Type | Ribs | Axial function |
|---|---|---|
| NU | On the outer ring only; inner ring plain | None — shaft floats both ways (non-locating / free) |
| N | On the inner ring only; outer ring plain | None — floats at the outer seat (non-locating) |
| NJ | Two on the outer ring plus one on the inner ring | Locates the shaft in one direction |
| NUP | NJ plus a loose flanged washer (HJ) | Locates in both directions |
| NF | The N counterpart with one outer-ring rib | Locates in one direction |
Why they cannot be swapped
In a typical shaft with two bearings, one is the locating bearing (it fixes the shaft's axial position and takes thrust) and the other is the non-locating bearing (it lets the shaft grow and shrink with temperature). Cylindrical roller bearings are chosen for these roles by type letter:
- Replace a locating NJ with a free NU and the shaft has nothing holding it in place in that direction — it can walk under axial load.
- Replace a free NU with a locating NUP on a shaft that already has a locating bearing elsewhere, and you have now fixed the shaft at both ends. It can no longer expand with heat, so it is axially over-constrained — the result is edge loading and roller-end wear.
Same dimensions, same radial rating, opposite mechanical job. The type letter is not a variant to be matched loosely — it must be identical.
Separable rings
Cylindrical roller bearings are also separable: the inner ring, and the outer ring with its roller-and-cage assembly, come apart. This is handy for mounting on tight fits, but it has a cross-reference consequence — the two rings are matched at manufacture. Do not mix an inner ring from one brand or part number with an outer ring from another. The raceway finish and matching tolerances are not guaranteed to agree.
The "E" suffix is not cosmetic either
A suffix like NU 205 E denotes a reinforced internal design — larger rollers, more of them, higher load capacity — and it is not identical to a non-E ring set of the same number. If you are replacing one ring of a pair, keep both rings the same E/non-E design.
How to replace one correctly
- Read the full type code, not just the size — NU vs NJ vs N vs NUP vs NF.
- Match the E / design suffix.
- Match the cage (steel, brass, polyamide) and the clearance (CN/C3/C4 — roller bearings are clearance-sensitive).
- Keep inner and outer rings from the same brand and part number.
Do that and a cross-brand swap of the same code is sound. Match only the number, and you may have bought a bearing that fits and does the wrong thing.
General reference only. Confirm the locating/non-locating arrangement of your shaft system and the correct type with the equipment or bearing manufacturer.