Silicon Nitride Si3N4 is the rolling element material — the balls, with the rings left in steel in a hybrid bearing — used as the balls of a hybrid bearing — steel rings, ceramic balls — where the low density cuts centrifugal ball load at high speed and the insulation blocks the shaft currents that erode raceways on inverter-driven motors. On a 6800 series thin section ball bearings that matters because chemically inert — unaffected by water, most acids and alkalis, and it cannot rust. The property data, the comparison against the alternatives and the size table are below.
Why Silicon Nitride Si3N4 for 6800 Series Thin Section Ball Bearings
Used as the balls of a hybrid bearing — steel rings, ceramic balls — where the low density cuts centrifugal ball load at high speed and the insulation blocks the shaft currents that erode raceways on inverter-driven motors. In a 6800 series thin section ball bearings it is the rolling element material — the balls, with the rings left in steel in a hybrid bearing.
Silicon nitride balls weigh less than half what steel balls weigh, so the centrifugal load they impose on the outer raceway at speed is far lower, and they do not conduct: an inverter-driven motor that erodes its raceways with shaft current stops doing so when the current path is broken by the balls.
Material Properties
| Grade designation | hot isostatically pressed silicon nitride to ASTM F2094, density about 3.2 g/cm³ against about 7.8 for bearing steel |
|---|---|
| Magnetic response | Non-magnetic, and an electrical insulator |
| Service temperature | The ceramic itself is stable far beyond any bearing lubricant; the cage and the lubricant set the working limit |
| Corrosion characteristic | Chemically inert — unaffected by water, most acids and alkalis, and it cannot rust |
Service Limits
Ceramic is hard and brittle. It does not tolerate edge loading, impact, or a ring that has been distorted by a bad fit, and a hybrid bearing therefore needs its seats machined properly rather than approximately. It is also more expensive by a wide margin, so it is specified for a reason — speed, insulation, or an environment that will not keep a lubricant — and not as an upgrade.
What the Material Changes About the Bearing
In a hybrid bearing the rings stay steel and the balls become ceramic, so the boundary dimensions and the mounting are unchanged. The differences are all dynamic: lower ball mass and therefore lower centrifugal load at speed, a higher permissible speed, less heat generated, electrical insulation through the ball contact, and a bearing that tolerates marginal lubrication better than an all-steel one.
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 6800 Series Thin Section Ball Bearings Available in Silicon Nitride Si3N4
| Designation | Bore d (mm) | Outer diameter D (mm) | Width B (mm) | Chamfer r min (mm) | Basic dynamic load rating Cr (kN) | Basic static load rating C0r (kN) | Limiting speed grease (rpm) | Mass (kg) |
|---|---|---|---|---|---|---|---|---|
| 6800 | 10 | 19 | 5 | 0.3 | 1.74 | 0.865 | 22000 | 0.0053 |
| 6801 | 12 | 21 | 5 | 0.3 | 1.92 | 1.04 | 20000 | 0.0060 |
| 6802 | 15 | 24 | 5 | 0.3 | 2.08 | 1.27 | 17000 | 0.0071 |
| 6803 | 17 | 26 | 5 | 0.3 | 2.21 | 1.46 | 15000 | 0.0077 |
| 6804 | 20 | 32 | 7 | 0.3 | 4.03 | 2.60 | 13000 | 0.019 |
| 6805 | 25 | 37 | 7 | 0.3 | 4.36 | 3.10 | 11000 | 0.022 |
| 6806 | 30 | 42 | 7 | 0.3 | 4.49 | 3.45 | 9500 | 0.026 |
| 6807 | 35 | 47 | 7 | 0.3 | 4.62 | 3.75 | 8500 | 0.029 |
| 6808 | 40 | 52 | 7 | 0.3 | 4.94 | 4.30 | 7500 | 0.033 |
| 6809 | 45 | 58 | 7 | 0.3 | 6.63 | 6.10 | 6700 | 0.038 |
| 6810 | 50 | 65 | 7 | 0.3 | 6.76 | 6.55 | 6300 | 0.049 |
| 6811 | 55 | 72 | 9 | 0.3 | 9.56 | 9.50 | 5600 | 0.075 |
| 6812 | 60 | 78 | 10 | 0.3 | 11.9 | 11.4 | 5300 | 0.096 |
| 6813 | 65 | 85 | 10 | 0.6 | 12.4 | 12.7 | 5000 | 0.110 |
| 6814 | 70 | 90 | 10 | 0.6 | 12.7 | 13.7 | 4500 | 0.120 |
| 6815 | 75 | 95 | 10 | 0.6 | 13.0 | 14.6 | 4300 | 0.125 |
| 6816 | 80 | 100 | 10 | 0.6 | 13.3 | 15.6 | 4000 | 0.135 |
| 6817 | 85 | 110 | 13 | 1 | 19.5 | 22.4 | 3800 | 0.250 |
| 6818 | 90 | 115 | 13 | 1 | 19.9 | 23.6 | 3600 | 0.265 |
| 6819 | 95 | 120 | 13 | 1 | 20.3 | 24.5 | 3400 | 0.275 |
| 6820 | 100 | 125 | 13 | 1 | 20.8 | 26.0 | 3200 | 0.290 |
