Case-Hardened 20CrMnTi is the steel of the rings and, in most designs, of the rolling elements — chosen for shock loading and for heavy ring sections, where a tough core resists the cracking a fully through-hardened section would not survive. On a nu300 series cylindrical roller bearings that matters because none of its own; protected in service exactly as through-hardened bearing steel is. The property data, the comparison against the alternatives and the size table are below.
Why Case-Hardened 20CrMnTi for NU300 Series Cylindrical Roller Bearings
Chosen for shock loading and for heavy ring sections, where a tough core resists the cracking a fully through-hardened section would not survive. In a nu300 series cylindrical roller bearings it is the steel of the rings and, in most designs, of the rolling elements.
Carburizing puts the hardness where the rolling contact is and leaves the core tough, so the ring can absorb shock and heavy section stresses that would crack a through-hardened ring. It is the usual choice for large roller bearing rings and for anything that gets hit rather than merely loaded.
Material Properties
| Grade designation | 20CrMnTi (GB/T 3077), of the same class as 20MnCr5 and SAE 8620 — carburized to a hard case over a tough, unhardened core |
|---|---|
| Magnetic response | Magnetic |
| Service temperature | Similar to through-hardened rings — about +120 °C without special stabilization |
| Corrosion characteristic | None of its own; protected in service exactly as through-hardened bearing steel is |
Alternatives and When to Switch
Through-hardened GCr15 is the alternative and the default: it is used wherever the section is modest and the load is steady, which is most of the catalog. Case-hardened rings earn their place under shock and in large sections.
What the Material Changes About the Bearing
The material does not change the envelope: the bore, the outside diameter, the width and the corner radius stay exactly as the boundary dimension standard fixes them, so the bearing is interchangeable with any other bearing of the same designation. What it changes is what happens inside the contact — the fatigue behavior, the tolerance of shock, and the temperature at which the ring stops being dimensionally stable.
Fitting a Steel Bearing in an Aluminum Housing
Aluminum has roughly twice the coefficient of thermal expansion of bearing steel, so a housing bore machined to a normal H7 fit at room temperature grows away from the outer ring as the machine reaches operating temperature. The outer ring then turns in its seat and wears the bore. Where the housing is aluminum and the temperature rise is more than about 30 degrees C, tighten the housing fit by one grade — J7 or K7 in place of H7 — or fit a steel sleeve into the aluminum. Check the resulting internal clearance: a tighter housing fit takes clearance out of the bearing, which is often why C3 is specified alongside it.
Sizes of NU300 Series Cylindrical Roller Bearings Available in Case-Hardened 20CrMnTi
| Designation | Bore d (mm) | Outer diameter D (mm) | Width B (mm) | Chamfer r min (mm) | Rib arrangement |
|---|---|---|---|---|---|
| NU304 | 20 | 52 | 15 | 1.1 | No axial location, free at both ends |
| NU305 | 25 | 62 | 17 | 1.1 | No axial location, free at both ends |
| NU306 | 30 | 72 | 19 | 1.1 | No axial location, free at both ends |
| NU307 | 35 | 80 | 21 | 1.5 | No axial location, free at both ends |
| NU308 | 40 | 90 | 23 | 1.5 | No axial location, free at both ends |
| NU309 | 45 | 100 | 25 | 1.5 | No axial location, free at both ends |
| NU310 | 50 | 110 | 27 | 2 | No axial location, free at both ends |
| NU311 | 55 | 120 | 29 | 2 | No axial location, free at both ends |
| NU312 | 60 | 130 | 31 | 2.1 | No axial location, free at both ends |
| NU313 | 65 | 140 | 33 | 2.1 | No axial location, free at both ends |
| NU314 | 70 | 150 | 35 | 2.1 | No axial location, free at both ends |
| NU315 | 75 | 160 | 37 | 2.1 | No axial location, free at both ends |
| NU316 | 80 | 170 | 39 | 2.1 | No axial location, free at both ends |
| NU317 | 85 | 180 | 41 | 3 | No axial location, free at both ends |
| NU318 | 90 | 190 | 43 | 3 | No axial location, free at both ends |
| NU319 | 95 | 200 | 45 | 3 | No axial location, free at both ends |
| NU320 | 100 | 215 | 47 | 3 | No axial location, free at both ends |
| NU321 | 105 | 225 | 49 | 3 | No axial location, free at both ends |
| NU322 | 110 | 240 | 50 | 3 | No axial location, free at both ends |
