Using demodulation in assessment of the technical condition of bearings

Abstract

The current cases of freight cars uncoupling due to failures of bearing units are too numerous, and to prevent their growth, introduction of modern control and diagnostics of the parts of such unit is required. Car repair enterprises use a wide range of diagnostic equipment to assess the technical condition of the axle box units of freight cars using high frequency instruments detecting any changes inside the part as well as on its surface, allowing under different rotation frequencies to reveal certain defects which may create additional vibrations for the rolling stock dynamics. The information about such defects is transformed into spectral diagrams capable to detect a certain vibration caused by deviations in the part’s technical condition. Among the wide range of vibro-diagnostic instruments with different types of signals, the most frequently used is the equipment based on detection principle, according to which the high-frequency amplitude oscillations are converted into low-frequency ones, which makes it possible to reveal defects in the bearing parts. For this process the basic task is not just converting the high frequency signals, but also reproducing a low-frequency modulating signal which may look like a power surge in the diagram. The basis for the envelope spectrum method is the analysis of high-frequency vibration and the subsequent detection of low-frequency modulated vibration signals. If we extract from the high-frequency signal its low-frequency component, which is called the signal envelope, then it will be possible to judge the place of the defect by the frequency location of the amplitude peaks in the envelope spectrum.