Re-measuring with the sensor mounted at a different location will usually discriminate machine vibration from
mounting resonance. However if a machine signal coincides with the resonance of the mounting configuration,
a greatly increased signal can happen. This can lead to sensor overload which will be manifested with a spec-
trum dominated by a ski slope.
Line frequency harmonics in spectrum
Harmonics of AC line power frequency usually indicate interference from motors, power lines and other emis-
sive equipment. First, ensure that the sensor shield is grounded (at one end only). If the shielding is good,
check the cable routing. A good measurement practice is to run signal cables in cable trays separate from
power line cable trays by at least 12 inches. If a signal cable tray must come near a power cable tray, they
should cross at right angles to reduce the possibility of magnetically coupled signals. Using shielded, twisted
pair cable will also help minimize any magnetically coupled noise into the signal cable. For example, if a power
cable is 440 volts and the vibration signals from the sensor are at the millivolt level, any cross talk will severely
corrupt the data.
Mounting resonance spectrum
Mounting resonance can give false indication of high frequency machinery faults such as gear mesh and bear-
ing problems. The problem is more likely to occur when using probe tips and magnets, and care must be exer-
cised to prevent the measured mounting resonance peaks from being falsely identified as machine produced
peaks. Mounting the sensor on thin plates such as machine guards, can also lower the mounting resonance.
Figure 8 shows the resonance of several common mounting techniques. Mounting resonance plots are shown
for: (a) probe tip; (b) magnet; (c) Wilcoxon’s QuickLINK™; and (d) stud mounted configurations.
Figure 8a: Probe tip mounting resonance
Figure 8b: Magnet mounting resonance
Figure 8c: QuickLINK
TM
mounting resonance
Figure 8d: Stud mounting resonance
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