What are the common impact sources
that the enveloping signal and spectrum are helpful in detecting and what
are the frequencies associated with each. Each of the subjects listed below
is discussed more in-depth in the 'Troubleshooting Charts' section.
-
Bearing Defects
- Frequency of peaks will be the specific
bearing defect (impact) frequency.
-
Looseness - Typically
occurring between the shaft and bearing; the housing and bearing; and/or
the internal bearing clearances. The observed frequency on the enveloping
spectrum will be harmonics of running speed (1x, 2x, 3x, etc. x RPM).
-
Electrical Looseness
(ac motors) -
Looseness
in windings, end turns, loose iron, loose connections, etc. Frequency will
be 2x AC line frequency and harmonics. This also applies to Variable Frequency
Drives (VFDs) but the ac frequency must be determined for each case.
-
Lubrication
-
Lack of lubrication will drive up metal
to metal contact (high frequency noise). No specific frequencies are triggered
but a general lifting of floor, or 'carpet' level, will occur.
-
Reciprocating
Equipment - Analyst must determine the specifics
of the machine to determine what frequencies to expect. Running speed harmonics
are common with even numbered harmonics of even higher amplitude (there
are a lot of events at 2x rpm in typical recips), number of pistons x rpm
in some hydraulic pumps. Impacts are normal in equipment such as this and
the analyst should be looking for change from the norm.
-
Gears -
Backlash, other impact sources. Frequencies
typically will be related to the number of teeth.
Note that each
of these 'problems' generates its own, specific frequency(s). Each of these
subjects is covered more extensively in the 'Troubleshooting Charts' section
but there is one common thread to using the enveloping spectrum (a word
of warning, so to speak): |
