Electrical corrosion (breakdown) is a common form of early failure of a bearing. This is often seen with electric motors with modern frequency controllers. These cause high peak voltages across the bearing, which can cause currents to flow.
Development of bearing damage
In the first stage, no damage can be seen with the naked eye. This while micro craters have already been created due to the electrical breakdown. What happens is comparable to a welding point. The inner and/or outer ring melts locally. When this happens thousands of times, one gets micro craters in the bearing. The current transfer ensures that small pieces of material are removed from the inner ring, outer ring or the rolling elements.
An example of outer race damage, inflicted by current flow through the bearing, recognizable due to the 'ridges' in the outer race.
The second stage is directly related to micro craters. A ribbed pattern is created on the inner and/or outer ring of the bearing. This is called "fluting." These patterns arise because the material is placed under a higher load due to the elements that roll over the micro craters. Mechanical resonance occurs over time so that the washboard pattern becomes visible. The bearing must then be replaced quickly to prevent consequential damage.
Measurement techniques
The shaft-voltage measurements are carried out with a 200 MHz oscilloscope. In addition to the ability to measure the peak-to-peak value, this also gives the option to visualize the discharge pattern. This pattern is displayed in a time signal.
By placing a carbon brush on the shaft, we are able to measure the discharge with an oscilloscope.
The measurement device is equipped with a carbon brush to establish the connection to the shaft. The carbon brush is arranged isolated and connected to the oscilloscope. The mass is connected to the earth.
The shaft is sanded and the surface is defatted to ensure optimum contact with the carbon brush.
