A novel shaft-grounding system from Schunk is based on specially-treated carbon fibers and reliably diverts high-frequency current away from sensitive parts in rail vehicles.
Stray or vagabond currents may sound harmless or even romantic to the layperson, but they can cause horrendous damage in transport systems. So a once smooth ball bearing, for instance in the drive system of a tram, might suddenly resemble the surface of a golf ball. The result: extensive repairs and downtime that cost both time and money. But just what are stray currents? AC motors that are powered by inverters are used to drive ball-bearing mounted shafts in rail vehicles. Low-amperage currents are generated in these motor shafts, which, however, exhibit high voltage and high frequency. These currents try to find a path to the vehicle mass and can, as a rule, only flow through the ball bearings, which then suffer irreparable damage through this process. When the current passes through the ball bearings, minute particles are separated from the metallic surface. This process is also known as spark erosion. In the worst-case scenario, the ball bearing fails; this then leads to the breakdown of the transmission and motor. The use of electrically-insulated ball bearings often only provides a local and temporary solution. Since the current can’t leave the shaft through the first ball bearing it then looks for another location, for example the transmission.
Electrical conductors made of carbon fibers reduce wear and maintenance costs
In rail-vehicle applications Schunk uses carbon fibers to divert stray currents out of the rotating shaft, i.e. to ground them. The material prevents damage to ball bearings and thus reduces wear and maintenance costs. With the so-called Carbon-Fiber-Grounding (CFG)-System, a carbon-fiber strand is used. The braid looks like a fine pigtail. The strand is permeated by a carbon-containing atmosphere in the course of a special thermal process carried out at high temperatures in a vacuum furnace. This process is called infiltration or simply “baking”. The result is that a thin layer of carbon is deposited on every individual fiber of the strand. These fibers have only about one tenth the thickness of a human hair.
The strand, with a diameter of only four millimeters, is made up of some 200,000 individual fibers. Due to the larger surface that has been created and the better conductivity of the carbon fibers, the current flows through more easily at high frequencies. A very small amount of contact pressure is sufficient so that the friction wear to the fibers is minimal. “Baking“ also makes it possible to give any desired shape to the fibers. The fiber will retain this form, with a high degree of stability. This allows the compact fibers to even extend into difficult to reach spaces in the drive system and hence to draw off current from the vicinity of the affected ball bearing. Thanks to their compact design, CFG systems can be easily retrofitted thus offering significant savings through relatively low investment requirements.
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