Page 175 - An Introduction To Predictive Maintenance

Vibration Monitoring and Analysis 165

spectra on all machine-trains on a routine basis. Data management and storage in the
host computer is extremely difficult and costly. Full-range signatures should be col-
lected only if a confirmed problem has been identified on a specific machine-train.
This can be triggered automatically by exceeding a preset alarm limit in the histori-
cal amplitude trends.

Broadband and Full Signature
Systems that use either broadband or full-signature measurements have limitations
that may hamper the program’s usefulness. Broadband measurements usually do not
have enough resolution at running speeds to be effective in early problem diagnos-
tics. Full-signature measurement at every data point requires a massive data acquisi-
tion, handling, and storage system that greatly increases the capital and operating costs
of the program.

Normally, a full-signature spectrum is needed only when an identified machine-train
problem demands further investigation. Please note that although full signatures gen-
erate too much data for routine problem detection, they are essential for root-cause
diagnostics. Therefore, the optimum system includes the capability to use all tech-
niques. This ability optimizes the program’s ability to trend, perform full root-cause
failure analysis, and still maintain minimum data management and storage require-
ments.

Narrowband
Typically, a machine-train’s vibration signature consists of vibration components with
each component associated with one or more of the true running speeds within the
machine-train. Because most machinery problems show up at or near one or more of
the running speeds, the narrowband capability is beneficial in that high-resolution
windows can be preset to monitor the running speeds; however, many of the micro-
processor-based predictive maintenance systems available do not have narrowband
capability. Therefore, care should be taken to ensure that the system used does have
this capability.

APPENDIX 7.1 Abbreviations
A Acceleration
A Zero-to-peak amplitude
Cpm Cycles per minute or cycles/minute
Cps Cycles per second or cycles/second
e.g., For example
F Frequency
f(t) Function of time
F Force
FFT Fast Fourier Transform