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Long Time Waveform as a Tool for Detecting Natural Frequencies

Collecting proper bump test data can often feel like an art form rather than being a true science. Lots of information has to be set up before acquisition, resulting in messages such as "signal overload" and "waiting on trigger" etc. Parameters such as window type, trigger type, and % pre trigger all have to set up in advance. The process can be quite time consuming and frustrating for an inexperienced analyst.

This maintenance tip provides an easier and faster means to capture these important data. We often use the time waveform function to collect transient vibration measurements, where cracked shafts and gears and resonances are being investigated. If a potential resonance condition is suspected, you just need to collect long time waveform data, bumping the machine every five seconds or so. Collection is typically accomplished in less than a minute. By using the partial waveform analysis feature available in most PdM software packages, you can then pan and zoom collected time waveform data and pinpoint natural frequencies that could lead to a resonance condition.

Waiting for triggers and experiencing signal overloading and other warnings are eliminated in using this simple data collection method. The results using time waveform are fast and accurateand involve no guesswork.

Users of this technique say that it is similar to the old method of taking a movie and capturing frame-by-frame snapshots of the desired data.

A screen shot of captured long time waveform data over a 20-second interval is shown below.

commtest fig 1

commtest fig 2

In selecting just one good ring-down effect of the bump in time and converting it to frequency will pinpoint the natural frequency, 765 CPM in this case.

Tip Provided by: Dennis Shreve, CMRP - Customer Support Engineer
Commtest, Inc.
Tel: +1 865.588.2946
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