NNU4900K Series Double Row Cylindrical Roller Bearings (ISO 15 / DIN 5412-9 / GB/T 285) are specified where a position calls for carrying the highest radial load available in a given envelope, on line contact between roller and raceway, and — in the ribless designs — letting the shaft expand axially inside the bearing itself. The roller contacts the raceway along a line rather than at a point, which spreads the load over a far larger area than a ball can, and the ribs — not the series number — decide what the bearing does axially. Below: typical end uses by industry, the specification checklist, mounting and setting practice, and the applications this type is not suited to. This series is catalogued from 100 mm to 180 mm bore; the full table of boundary dimensions is further down the page.
Typical Applications for NNU4900K Series Double Row Cylindrical Roller Bearings
The machine positions and sectors where this type is specified:
- Gearbox floating positions: the non-locating end of a shaft, where a ribless bearing absorbs thermal expansion inside itself and puts no axial force into either bearing.
- Large electric motors and generators: the drive-end position of machines where the shaft grows several tenths of a millimetre as the winding heats.
- Crushers, mills and vibrating plant: heavily loaded, slow to medium speed positions where full complement types add capacity in the same space.
- Pumps and compressors: high radial load with the axial load taken separately by an angular contact or a four-point bearing.
- Maintenance and repair (MRO): replacing a failed bearing where the original designation, clearance group and sealing have to be matched exactly.
How to Specify NNU4900K Series Double Row Cylindrical Roller Bearings for Your Application
- Bore: Start from the bore the shaft or housing gives you — this series is catalogued from 100 mm to 180 mm — then check the outer diameter and width against the space available. The boundary dimensions are fixed by the standard, so they are the same from any manufacturer; the load ratings and limiting speeds are not, and belong to the specific bearing being supplied.
- Tapered bore adjustment: Double row cylindrical bearings are commonly supplied with a tapered bore so the radial clearance can be set by how far the ring is driven onto the spindle taper. That adjustment is the specification — state the target clearance.
- Rib arrangement: This is the type letter and it is not cosmetic: NU and N have a ribless ring, carry no axial load at all and float; NJ has one rib and locates the shaft in one direction; NUP and NF locate in both. NU204, NJ204, NUP204, N204 and NF204 are all 20 × 47 × 14 mm, so the dimension series answers the load question and the type letter answers the location question. State both.
- Located and floating pair: Specify the pair, not the bearing. One position locates the shaft axially and one must be free; two located bearings on a shaft that grows when it warms will load each other axially until something yields.
- Designation and suffixes: State the complete designation including every suffix. The base number gives the type and the boundary dimensions; the suffixes carry the sealing, the cage, the clearance group, the tolerance class and any special execution — and two bearings whose base numbers match but whose suffixes differ are not interchangeable.
- Internal clearance group: State the radial internal clearance: C2 (reduced), CN (normal, and unmarked), C3, C4 or C5. Clearance is consumed after the bearing is fitted — by the interference fit on the shaft, and by the inner ring running hotter than the outer — so a bearing with a tight fit or a hot inner ring is ordered C3 and a bearing mounted lightly at even temperature is left CN. This field is left off orders more often than any other, and the bearing then arrives in the normal group by default.
- Sealing arrangement: This type is supplied open, so the sealing belongs to the housing around it. Specify the shaft seal, flinger or labyrinth that will keep the lubricant in and contamination out — with an open bearing, the sealing arrangement, not the bearing, decides how long it survives its environment.
- Cage material: State the cage. Pressed steel is the default and suits most duties; machined brass takes shock, high temperature and poor lubrication better and is used in large and heavily loaded bearings; polyamide is quiet, light and low-friction but has a temperature limit of about 120 °C and is attacked by aged oil. The cage is not a detail — it is the component that most often sets the real temperature limit of the bearing.
- Shaft and housing fit: Specify the seat tolerances, not just the bearing. The rule is that the ring which rotates relative to the load direction takes the interference fit: for a rotating inner ring under normal radial load the shaft seat is typically k5 or k6 with the housing bore H7, and where the outer ring rotates the interference moves to the housing (N7 or P7) with a clearance fit (g6) on the shaft. Heavier loads and hollow shafts move the fits tighter. A correctly chosen bearing in the wrong fit fails anyway — too loose and the ring creeps and frets its seat, too tight and the interference consumes the internal clearance and the bearing runs preloaded.
- Standard: Boundary dimensions are to ISO 15 / DIN 5412-9 / GB/T 285. Quoting the standard on the order guarantees the mounting dimensions are interchangeable between suppliers — it does not standardize load ratings, limiting speeds or internal design, which remain manufacturer-specific.
Mounting & Assembly Practice
- These bearings are separable, which is why both rings can carry an interference fit at once: fit the ribless ring in the housing and the roller-and-cage assembly on the shaft, then bring the two together.
- Introduce the roller set into the ribless ring squarely and slowly, with the rings concentric. Cocking the roller set on entry scores an axial line across the raceway, and that mark is a fatigue origin from the first revolution.
- A mounting sleeve with a lead chamfer makes that entry repeatable; assembling by feel with the shaft hanging is what produces scored raceways.
- Because both rings are usually a tight fit and the inner ring runs hot, C3 clearance is normal in this family rather than exceptional.
Common Selection Mistakes with NNU4900K Series Double Row Cylindrical Roller Bearings
- Ordering NU where the position has to locate the shaft. It carries no axial load whatsoever, so the shaft is unlocated in both directions.
- Using NJ at both ends of a shaft facing the same way, which locates the shaft in one direction twice and in the other not at all.
- Assuming a full complement version is a free upgrade. Removing the cage adds rollers and radial capacity, but rollers then rub against each other instead of being separated, and the limiting speed falls sharply.
- Sizing this bearing from the boundary dimensions alone. Load ratings and limiting speeds are manufacturer-specific for this series and are deliberately not published here — take them from the catalog of the bearing actually being supplied and calculate the life against them.
Where NNU4900K Series Double Row Cylindrical Roller Bearings Are Not the Right Choice
- Not an axial bearing in any form: the shaft must be located by a separate thrust or four-point contact bearing.
- A cylindrical roller bearing will not tolerate misalignment. The roller contacts the raceway on a line, so a fraction of a degree of tilt moves the entire load onto the roller end, and the stress concentration there is what fails the bearing — use a spherical roller bearing where the seats cannot be held parallel.
- No axial capacity in the NU and N types, at all. Any thrust in the assembly has to be carried by another bearing.
- Full complement (SL, NCF) types have no cage, so roller-to-roller sliding contact sets a much lower limiting speed. They belong in slow, heavily loaded positions, not in a motor.
- No axial capacity: the shaft has to be located by another bearing, a shoulder or a retaining ring.
- This type will not tolerate misalignment. The rolling or sliding contact is a line, so a fraction of a degree of tilt moves the whole load onto the end of the contact and the stress concentration there is what fails the bearing.
