Vibration Analyzer Settings Discussion

I just read this article by Jason Tranter about vibration measurement errors. The topic is very close to what we have been talking about. It is interesting and the advice is very good, however tips like these are rarely followed because so much of this is just arbitrary. The author is not explaining why his recommendations make sense. For example, the requirement to select "Fmax of 70 times the running speed of the shaft". Why? Why not 50 or 100? By the way, in the example that follows the author uses Fmax at 100 times the speed (1800 RPM is 30Hz and he is using Fmax 3000 Hz).

Another arbitrary requirement is that the shaft "should turn at least 50 times during your measurement". Again, why? There must be a reason to it and a vibration analyst will be able to apply this rule more efficiently if he or she knows why the rule exists. And, while we are at it, why not give a simple formula that would help to calculate the number of shaft revolutions during the measurement? The author skips this part. I would use this formula:

revolutions = (lines/Fmax)*((1-overlap/100%)*(averages-1)+1)*RPM/60

Following the example from the article, where lines=3200, Fmax=3000, overlap=67%, averages=5, and RPM=1800, this formula yields 74.24 revolutions. Well, the 75 revolutions sited in the article is close enough.

In vibration analysis, as in many other areas, there is no single answer to how the techniques should be applied. It is true that vibration analyzers today have all these capabilities but using them by following a set of arbitrary recommendations may not work in all situations. It is important to understand the objectives of the measurement and know the machine being tested and its operating characteristics.

There are trade offs in using settings that are not dictated by the task at hand. For example, if unnecessarily long samples are taken, there is a chance that a fluctuation in rotating speed will smear the spectrum peaks at forcing frequencies. If the analysis bandwidth (Fmax) is set too high, there may not be enough resolution to analyze the frequencies of interest. If the resolution is set too high, it might lead to a longer measurement and large file size.