Electrically insulated bearing and for reducing shaft current2018-05-12
An electrically insulated bearing such as a sleeve or a ball bearing is disclosed for journalling a shaft relative to a stator such that currents in the shaft or in the frame do not flow through the bearing. The insulation is established by plasma or torch spraying an insulator such as a ceramic on either the bearing or the stator surface. The ceramic is impregnated with an insulating varnish to prevent contamination and to provide a smooth surface to aid in mounting the bearing in the stator.
Electric currents in rotating shafts can be either intentionally established or produced by imbalances in inductive rotating machinery. In either case it is often desirable to insulate a shaft from a stator through an insulated bearing. The problem of shaft currents in rotating inductive machinery .has been recognized for decades as a substantial factor limiting bearing life. For example in an electric motor or generator, a shaft current may flow from one end of a shaft through a first bearing to a frame through a second bearing on the other end of the shaft. The current flow through the bearings results in significant degradation of the useful life. If the hear ing is well lubricated and the electromotive force in the shaft is low, then only a potential will be produced across the bearing. Once the electromotive force in the shaft exceeds the dielectric strength of the lubricating film, a current will flow limited only by the resistance of the circuit. In some cases extremely high currents can be encountered causing rapid degradation of the bearing.