What are shaft currents?

Shaft currents result from the buildup of electric charges on the machine shaft through 3 mechanisms: static discharge resulting from flow of particles in fluids, motor and generator faults, and the motion of magnetized components. The charge buildup creates a potential difference (voltage) between components, which is discharged through sparks. The sparking occurs when the voltage between components exceeds the breakdown voltage of the fluid separating them, typically oil. The voltage at which sparking begins depends on a variety of factors such as the gap between components, oil type, and oil contaminants, and it ranges from 0.5 Volts on roller element bearings to over 10 volts on large fluid bearings. Typically the sparking occurs at the points of lowest clearance between stationary and rotating components.

Spark discharges produce intense heat (over 2500°C ) which creates pitting on both ends of the spark. Depending on the energy level of the sparks, this can lead to a slow and gradual erosion of bearing material sometimes taking years before the damage is manifest in increasing bearing temperatures, or the sparking may be so intense that the machine cannot operate for more than a few hours before it has to be shut down. Shaft current problems can cause significant operating losses.

In some instances, shaft current is inherent to the process, compressors, turbines, and pumps can all experience substantial static discharge loads. Fortunately, static discharges are typically low energy events, and with careful design they can be dissipated to ground. On the other hand magnetized components, and motor/generator faults can produce high energy discharges, and the only effective long term remedy for these is to address the underlying fault. Regardless of the source of the shaft current energy, Sohre brushes can protect vulnerable machine components and keep your machine running.

Shaft-grounding brushes made by Sohre Turbomachinery^® use a proprietary mating of silver and gold fibers, assuring reliable electric contact with the shaft, very low electrical contact resistance, and low contact pressure. Consequently, there is little mechanical wear of either the brush or the shaft surface. Compared to solid brushes made of graphite, carbon, or other materials, Sohre brushes have much lower contact resistance. They eliminate shaft surface deterioration and consequent loss of contact in prolonged service in normal current-passing operation. Solid brushes commonly used with electrical machinery have a considerably higher resistance after wear-in and therefore begin to spark, leading to deterioration of the shaft surface. If brush resistance is higher than resistance within the machine, currents will not flow the brush but rather through the machinery, causing damage not preventing it.

The excellent electrical characteristics of Sohre brushes provide a very low electrical noise level, even at high surface velocity and in aggressive environments. This, a low wear rate, and easy replaceability during operation permit continuous, long-term monitoring and dissipation of electrical discharges during equipment operation.

Sohre brushes are preferred over copper braids, carbon blocks, and other conventional tools because of their superior performance and low maintenance. The latter is an especially important requirement for service in radiation-control areas.

More than 10,000 brushes are now in operation throughout the world – in chemical plants, refineries, paper mills, drilling platforms, oil fields, ships, electric generating plants, cryogenic processes, wind turbines, and many other installations, including on the International Space Station. Several original equipment manufacturers (OEMs) supply our brushes as standard equipment.