Dimensional and installation data for the 50 mm bore size of NU300 Series Cylindrical Roller Bearings, dimensioned to ISO 15 / DIN 5412-1 / GB/T 283. Selection is by bore diameter — the bearing designation is what the shaft is machined to — so the figures below start with the boundary dimensions and go on to the fits, the clearance and the mounting method that make them work.
50 mm Bore Dimensional Row (ISO 15 / DIN 5412-1 / GB/T 283)
| Designation | Bore d (mm) | Outer diameter D (mm) | Width B (mm) | Chamfer r min (mm) | Rib arrangement |
|---|---|---|---|---|---|
| NU310 | 50 | 110 | 27 | 2 | No axial location, free at both ends |
What a NU300 Series Cylindrical Roller Bearings Does
Line contact between a cylindrical roller and a flat raceway gives the highest radial capacity of any single row rolling bearing of the same envelope, and the rings separate for mounting, so both can be fitted with an interference fit. In this rib arrangement one ring has no ribs at all: the bearing carries no axial load and lets the shaft expand inside itself, which is exactly what is wanted at the non-locating position of a hot shaft.
Pure radial load. With this rib arrangement the bearing carries no axial load at all, and it accommodates axial movement of the shaft inside itself — which is the property that makes it the non-locating bearing on a shaft that grows when it gets hot.
Speed at 50 mm Bore
No limiting speed is tabulated for this product. Where speed matters, the figure to work to is the one the bearing supplier states for the exact design, cage and lubricant supplied — not a family value.
Shaft & Housing Fit for a 50 mm Seat
With the inner ring rotating under a load whose direction is fixed, the inner ring sees a circumferential load and must be an interference fit, or it will creep on the shaft and fret it. The outer ring sees a point load and takes a transition or clearance fit so that it can shift slowly and spread the wear — and so that one of the two bearings on the shaft is free to take up axial expansion.
| Shaft, light load (P ≤ 0.06 C) | j6 |
|---|---|
| Shaft, normal load (P = 0.06–0.12 C) | k5 or k6 |
| Housing — Axial displacement of the outer ring required | H7 |
| Housing — Normal load, outer ring may still shift | J7 |
| Housing — Normal to heavy load, quiet running, outer ring not displaceable | K7 |
ISO 286-2 tolerance grades, applied per the conventional bearing seat recommendations of ISO 1132-2 and DIN 5425-1, for a SOLID STEEL shaft in a cast iron or steel housing. A hollow shaft, a thin-walled housing or a light alloy housing needs a tighter fit than these, because the seat itself deflects.
Radial Internal Clearance at 50 mm Bore
Radial internal clearance is the total distance one ring can be moved relative to the other, across the bearing, before it is mounted. It is not a tolerance and not a quality grade: it is a quantity that gets USED UP on installation. An interference fit on the shaft expands the inner ring; a tight housing fit compresses the outer ring; and a shaft that runs hotter than its housing grows more than the housing does. All three subtract from the clearance the bearing was made with. C3 exists for that reason. Specify it when the shaft fit is heavier than normal, when the inner ring will run 10 °C or more above the outer, when a hollow or heated shaft is involved, or when the bearing sits on an adapter sleeve that is driven up hard. Specify CN — normal, and usually left out of the designation entirely — for the ordinary case of a moderate fit on a shaft close to housing temperature. The failure this prevents is specific and quick: fit a CN bearing with a heavy interference on a shaft that then runs hot, and the residual clearance goes negative. The bearing is now preloaded by its own installation, the rolling elements are squeezed, friction and temperature rise together, and the temperature rise tightens the fit further. It runs hot, then it runs blue, and the raceways spall long before the calculated life.
ISO 5753-1 tabulates clearance separately for each bearing type, and the numbers for this type are not the deep groove ball bearing numbers. Order the group — CN, C3 or C4 — and take the values from the table for this type.
Abutment Shoulder & Fillet Radius for 50 mm
The chamfer dimension r min for this size is 2 mm. That figure is a limit on YOUR machining, not just on the bearing: the fillet radius in the corner of the shaft or the housing must be smaller than the bearing's corner radius, or the ring seats on the fillet instead of on the shoulder face and is loaded on its corner.
| Chamfer dimension r min (from the table) | 2 mm |
|---|---|
| Maximum fillet radius on the shaft or housing | 2 mm |
| Shaft shoulder must exceed | 54 mm diameter |
| Housing shoulder bore must be under | 106 mm diameter |
The two shoulder figures are the geometric floor and ceiling: a shoulder outside them fouls the corner radius instead of bearing on the ring face. The practical values sit inside them — the shoulder must contact the ring face over a useful area without touching the seal, the cage or the opposite ring, and it must still leave room for a puller when the bearing is dismounted. Take the working da and Da from the abutment table for the bearing; the ring shoulder height they are built on is not a boundary dimension and is not in this table.
Mounting a 50 mm Bore NU300 Series Cylindrical Roller Bearings
The ring that carries the rotating load takes an interference fit; the ring under a stationary point load takes a transition or clearance fit so that it can creep slowly and spread the wear, and so that one ring is free to take up axial expansion. With a rotating shaft under a fixed load direction, that means an interference fit on the shaft and a looser fit in the housing. Press only against the ring being fitted — force driven through the rolling elements leaves brinell marks in the raceway before the bearing has run a revolution.
At a 50 mm bore the interference is small enough that a cold press or a light hydraulic press is normally enough; heating the bearing in an induction heater or an oil bath to 80–100 °C above the shaft is the alternative, and never above 120 °C for a standard-stabilized bearing. Keep the heated bearing pressed against the shoulder while it cools, because it shrinks axially as well as radially.
This bearing is separable, which makes mounting easier and mis-mounting easier too: fit each ring to its own seat separately, and do not let the two halves be swapped between bearings once they have run — the raceways wear as a matched pair.
Where a 50 mm Bore NU300 Series Cylindrical Roller Bearings Is Used
At a 50 mm bore, nu300 series cylindrical roller bearings are typically found in industrial gearboxes, fans and blowers, conveyor drives, pump and compressor shafts and machine tool spindles, where the load and the heat generated both start to decide the arrangement.
Failure Modes and What They Look Like
- Subsurface fatigue (spalling) — a flake of raceway lifts out and the bearing gets noisy and rough. This is the failure the L10 life calculation predicts — if it arrives near the calculated life the bearing did its job, and if it arrives early the load, the fit or the lubrication was not what the calculation assumed.
- Inadequate lubrication — a polished, then discoloured, then torn raceway, running hot with a darkened grease. It is the most common cause of early failure by a wide margin, and the interval, not the grease, is usually what was wrong.
- Contamination — dents in the raceway with raised edges, then premature spalling from those dents. Wear particles, casting sand and washdown water all do it; sealing and filtration are what prevent it.
- False brinelling and fretting — evenly spaced dull marks at ball or roller pitch, produced by vibration while the shaft is stationary — a machine that travels on a truck, or a standby pump beside a running one. It is not caused by overload and cannot be cured by fitting a bigger bearing.
- Loss of internal clearance — the whole raceway path is loaded, the bearing runs hot from installation and it discolours evenly. The fit was too heavy, the shaft ran hotter than the housing, or a CN bearing was fitted where C3 was needed.
- Edge loading at the roller ends — wear or spalling concentrated in a band at one end of the roller path. It means misalignment or deflection: line contact has no tolerance for angular error, which is what the crowned profile can only partly correct.
Where NOT to Use a NU300 Series Cylindrical Roller Bearings
- Do not put a cylindrical roller bearing where the shaft misaligns or deflects. Line contact has essentially no tolerance for angular error — the roller ends dig in, and the raceway spalls in a band at one end. Use a spherical roller bearing there.
- Do not ask this rib arrangement to locate the shaft. It carries no axial load at all; that is what makes it the correct non-locating bearing, and what makes it the wrong locating one.
- Do not run one under light load at speed. A cylindrical roller bearing needs a minimum load to keep its rollers rolling; below it the unloaded rollers skid and smear the raceway.
Stepping Up or Down from 50 mm
Within the same series, the next bore up in this series is 55 mm and the next bore down is 45 mm. Changing bore changes the shaft, so it is the last thing to change: when the calculated life at 50 mm is short, the first move is to a heavier diameter series at the SAME bore — the 6300 envelope instead of the 6200, for instance — which raises the rating without touching the shaft. The second is to change type: a roller bearing carries far more radial load than a ball bearing of the same envelope.
