“Introduction:

Many machine component failure modes will appear in a spectrum long before they might significantly affect the overall level. Having meaningful Narrowband Alarm Envelopes allows close examination of each spectrum to ensure problems are detected at early stages long before they approach catastrophic proportions (and likewise, damage many other components in the process). Setting appropriate spectral alarm levels for all frequency peaks in an entire set of spectra on a machine is far more complex than determining what overall alarms should be specified for the same measurement positions. A spectrum measurement contains several hundred times as much data as does an overall measurement since a spectrum today normally employs about 400 FFT lines.

Unfortunately, literature or technical papers on how to properly specify Narrowband Alarms for a variety of machine types and operating speeds is very limited in availability. Often, analysts have expressed they feel pretty much “on their own” when the time arrives to specify these Narrowband Envelopes on their machines. And, once they have specified them, these analysts have often confessed to the author they still are often quite uneasy, wondering if these envelopes are properly set throughout the frequency range – low enough in amplitude to detect real problems when they are present (hopefully in incipient stages), but high enough to prevent “false alarms” when no such problems are truly present. It is hoped this paper will go far in addressing these needs and help alleviate much of the continuing “anxiety” being suffered by so many vibration analysts. 

Not only will this article present the theory on how Narrowband Spectral Alarms should be specified, but also several examples using real-world machinery will be presented to illustrate the techniques taught herein and to identify important points (including mistakes) which can be made, and what steps can be taken to ensure one builds meaningful alarm envelopes.

II. Definition Of Narrowband Spectral Alarm Envelopes.

A. What Are Narrowband Spectral Alarm Envelopes?

Reference 1 states that “strictly speaking, Narrowband Spectral Alarms have been described as a system that gives the user almost an infinite number of spectral alarm bands”. That is, if one employs 400 lines in his FFT spectrum, he could, theoretically, specify 400 individual alarms. However, this is not truly practical (and with the Hanning window, is not possible). In essence, in those condition monitoring software systems that offer this capability, Narrowband Envelopes are automatically generated after the user inputs certain data on how he wants each envelope built (see Figure 5 for a sample data input screen used to specify the envelopes to be generated).”