There is a very important aspect concerning the placement of contact sensors. The method of securing the sensor in the machine directly determines the high frequency cutoff, since the sensor-machine contact acts as a mechanical low pass filter. In Table 3.2 are shown the limit frequencies for the different sensor mounting methods displayed in Figure 3.8 and that are grouped as temporary and permanent.
It is preferable to hold the sensor by hand directly on the measurement point rather than using extension rods that act as shock absorbers. The best mounting method is, of course, by screwing the sensor to the surface of the machine at the measurement location, although, due to its cost, this method is only used in permanent monitoring systems. In order to capture signals at very high frequency (from 6 to 8 kHz), the use of silicon-based greases and adhesives is recommended. In general, in industrial machinery it is very difficult to obtain reliable vibration measurements at frequencies above 5 kHz. Modern vibration analyzers feature a linear measurement frequency range that is well above sensors linear response range.
|Highest frequency (Hz)