HYBRID BEARING

When a stray current in an electrical machine uses a bearing as its path to ground, the resulting damage is referred to as “electric arc bearing damage”.

Electric arcing occurs if there is a difference in potential between the shaft and the bearing housing. Even a difference of a few volts in potential can produce the effect. Not only the motor or generator bearings can be affected — a stray current can damage bearings in the machinery directly coupled to the motor or generator. To prevent electric currents from arcing through the bearings, either the housing or bearings should be insulated.

Hybrid bearings have rings of bearing steel and rolling elements of bearing grade silicon nitride (Si3N4). Because silicon nitride has high resistivity, hybrid bearings provide insulation from electric currents in both AC and DC motors.
In addition to being an excellent insulator, hybrid bearings have a higher speed capacity and will provide longer service life than all-steel bearings in most applications.
The density of silicon nitride is only
40 % of the density of bearing steel. Because silicon nitride rolling elements weigh less they have lower inertia. This means less damage to the cage during rapid starts and stops and also significantly lower friction at high speeds. Lower friction means cooler running
and thus longer lubricant life. Hybrid bearings are thus suitable for operating at high speeds.
If the lubrication for some reason becomes insufficient, there is no risk of smearing between silicon nitride and steel. Also, the friction coefficient be- tween steel and silicon nitride is low. This enables hybrid bearings to run cooler and last longer in applications where there is inadequate lubrication. Silicon nitride has higher hardness and higher modulus of elasticity than steel. This means high wear resistance, increased bearing stiffness and longer bearing service life in contaminated environments.
Silicon nitride rolling elements have a lower thermal expansion than steel balls of similar size. This means less sensitivity to temperature gradients at high temperatures for better, more accurate Preloaded control.