ceramic ball bearings

HYBRID BALL BEARINGS
For very high running speeds ball bearings are less suitable because of the additional centrifugal load of the, with cage speed rotating balls. Balls of small diameter or low density are desirable to limit the centrifugal load.
Ball 1: steel D= 8mm m=2.1 gram Ball 2: ceramic D=9.525 mm m=1.5 gram Ball 3: steel D=9.525 mm m=3.5 gram
ceramic balls The centrifugal load is described by F= -mT²R where m is the weight of the ball. Because the density of silicium nitride is 60% of steel, the centrifugal load is reduced by using these balls with 60%. As a consequence the running speed can be increased up to 30%. Because the E-modulus of silicium nitride is about 1.5 times that from steel, a smaller contact surface exists with the consequence of a high contact pressure. So, the benefit of a small centrifugal load is limited by the large E-modulus. From Hertz theory it follows that the maximum load for the combination steel- silicium nitride reduced with 30%. A decrease of 60% centrifugal loading in combination with a 30% decrease in load capacity finally results in an increase of running speed with only 15%. A third effect, that frictional heating between the ceramic balls and the steel rings is less, further positively affect the maximum running speed. small ball diameter A reduction of the ball diameter also results in an increase of the maximum running speed. In the same way as for low density is the advantage of a reduction in centrifugal force limited by a reduction of the contact area. The centrifugal load is with the same cage speed proportional to the ball diameter d³. From Hertz it is known that the maximum load in a point contact is proportional to d². With both effects considered it follows that with the half ball diameter the running speed may increase with 100%. The fact that the external bearing load is carried by more small contacts is not considered here. Environment Ceramic balls in combination with steel steel rings are called hybrid bearings. Hybrid bearings have in aggressive environment else than complete ceramic ball bearings, no advantage because of the steel rings. The same yields for the thermal resistance. Only where abrasive particles can enter the bearing, ceramic balls have the advantage that they roll the particles in the steel ring. For this only one ceramic ball suffices. Lubrication Hybrid bearings need to be oil lubricated just like steel bearings. Lubrication can be omitted with very low running speed and if a short lifetime suffices. In complete ceramic bearings water lubrication or process lubrication suffices.
fys. properties	dim.	Siliciumnitride	100Cr6
density thermal expansion E-modulus poisson number Hardness HV10 heat conduction max. temp.	kg/m3 10E-4 /K GPa - - W/mK °C	3200 3.2 315...320 0.26 1600 30...35 800...1000	7800 11.5 200...210 0.3 700 40...45 150...300
Pot, S., (2001), "Keramische kogellagers (1), a&b, jan/feb. 2001
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