Analyzing A Trend
Analyzing a trend is not typically a difficult thing to do . . . providing you know what it is you are trending. 
  • Are you trending high frequency vibration ? 
  • Not if you only use displacement. 
  • Are you trending the typical, general equipment speed ranges of 1000 - 3600 rpm and associated mechanical problems. 
  • Not with displacement or acceleration - only with velocity.
  • Are you trending very low frequency vibration - below 100 rpm, for instance ?
  • Only with displacement and, if the frequency is low enough, only by changing type of transducer.
With vibration, a trend headed in the wrong direction almost certainly means it is increasing. With a properly set up program, properly collected data and proactive personnel, a program can be successful with collecting little more than trend values. For whatever reason, however, that is rarely the case.
In any event, trending can be useful but it is not usually enough to make specific judgements. A determination of the exact frequencies involved is usually required and should be attempted in any event to avoid a possibly embarassing and expensive mistake. A determination of frequencies can be made two separate ways - viewing the data as a raw time domain plot or putting it through an FFT process to generate a spectrum. An overview of the process of analyzing a spectrum will now be presented followed by an in-depth look at how, in a practtical sense, the FFT process works. Understanding that will help you view the spectrum with an appropriate amount of respect and skepticism.