What the type offers
A cylindrical roller bearing uses straight rollers that touch the raceways along a line. That gives much higher radial capacity than a ball bearing of the same size, plus good stiffness and the ability to run at fairly high speed. Because the rings are separable in most designs, the bearing is also easier to mount with heavy interference fits: the inner ring can be pressed on the shaft and the outer ring in the housing, then brought together.
The price of that line contact is that the bearing carries little or no axial load, depending on the type, and is sensitive to misalignment. Where the shaft deflects or the housing bores are not aligned, the roller ends load unevenly and life drops. If misalignment is expected, see the spherical roller or self-aligning ball options instead.
Reading NU, NJ, NUP and N
The letters describe the flanges, called ribs, that guide the rollers. A rib on the inner ring or outer ring also provides axial location in one direction. Four types cover most orders.
| Type | Ribs | Axial behaviour | Typical position |
|---|---|---|---|
| NU | Two on outer ring, none on inner | Free axial movement of the shaft relative to the housing | Floating position |
| N | Two on inner ring, none on outer | Free axial movement | Floating position |
| NJ | Two on outer ring, one on inner | Locates the shaft in one direction | Semi-fixed position |
| NUP | Two on outer ring, one on inner plus a loose rib ring | Locates the shaft in both directions | Fixed position |
NU is by far the most commonly ordered because it gives the housing and shaft freedom to expand with heat. A common shaft design uses one NUP or a different locating bearing at one end and an NU at the other, so the NU takes radial load only and the locating bearing takes the axial position. An NU can also be turned into a one-direction locating bearing with an HJ angle ring, which sits against the inner ring. Axial load capacity, where the type allows any, is limited and comes from the supplier datasheet; do not use these bearings for substantial thrust.
Series and nominal sizes
The same dimension series logic as in ball bearings applies: the NU2 series is the light one and NU3 the medium. Nominal ISO boundary dimensions for a few common designations are below; confirm against the drawing and datasheet.
| Designation | Bore d | OD D | Width B |
|---|---|---|---|
| NU204 | 20 | 47 | 14 |
| NU205 | 25 | 52 | 15 |
| NU206 | 30 | 62 | 16 |
| NU304 | 20 | 52 | 15 |
Boundary dimensions of NU, NJ, NUP and N bearings in the same series are typically the same, so a designation can change type without changing the fit; the difference is in the ribs. For bore codes, multiply the last two digits by five for codes of 04 and above. The size chart lists more sizes.
Cage, clearance and class
Cage choice follows speed and shock. Pressed steel cages are standard for many sizes. Machined brass cages are common in heavy or high-speed work and in larger sizes; glass-fibre nylon cages are used in some smaller sizes where quiet running matters. Internal clearance is normal by default; C3 is typical for electric motors and for fits where the inner ring warms. Because the rings are separable, the radial clearance of a cylindrical roller bearing is measured differently from that of a ball bearing, so ask the plant to state the clearance group rather than a figure in microns. See C3 clearance explained.
Tolerance class is normal for most uses; tighter ISO 492 classes appear on machine-tool and precision axes. Running accuracy depends on roller grading, so ask whether the plant sorts rollers into diameter groups within one bearing. Load ratings and limiting speeds depend on the size and cage; take them from the datasheet.
Where the types are used
Cylindrical roller bearings appear in electric motors and generators of larger frame sizes, in gearboxes and gear pumps, in machine-tool headstocks, in rolling-mill and paper-machine supports, and in many industrial fans. They are less common in agricultural wheel hubs, where the tapered type dominates; the choice between the two is set out in tapered vs cylindrical roller bearings. For heavy axial plus radial load, a tapered or angular contact arrangement is usually better.
Mounting and fits
Separable rings make mounting flexible but also create handling risks. When the inner ring and roller assembly are slid into the outer ring, they must go in square and slowly; forcing them in at an angle scores the raceway and rollers and leaves damage that is invisible until the bearing runs rough. Mark matched parts, because rollers are graded to the outer ring in some designs, and never mix parts between bearings. With rotating inner rings, an interference fit on the shaft is normal, and the outer ring is a lighter fit in the housing; the common fit classes are discussed in the shaft and housing fits note. Keep shaft and housing shoulders square, because the straight rollers are unforgiving of tilt. After mounting, turn the shaft by hand to check for smooth rotation before connecting the drive.
Avoiding wrong-type shipments
A frequent buying mistake is to order by the number alone and receive a bearing of a different rib type. The bore, outside diameter and width are identical across NU, NJ and NUP, so a wrong type fits physically but locates the shaft differently, and the error may only show up after assembly. State the full designation, the position of the bearing on the shaft (floating or locating), and whether an HJ ring is needed. Ask for loose rings and rib rings to be packed and labelled with the bearing, and confirm that the inner ring and the roller-cage assembly are from the same batch.
- Full designation with letter prefix, series and bore code.
- Position on the shaft: floating, one-direction or two-direction location.
- Cage type, clearance group, tolerance class.
- Application: radial load, speed, temperature, misalignment.
- Quantity and packing, including loose rings and angle rings.
Typical reference MOQs for standard designations are about 100 to 500 pieces and are confirmed in writing for each program (see the MOQ page). Send the list through the quote form.